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  • 201
    Digitale Medien
    Digitale Medien
    Spherical stellarator with plasma current (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 3055-3064 
    Details
    ISSN: 1089-7674
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: Recently proposed novel concept of a spherical stellarator (P. E. Moroz, "Spherical stellarator configuration,'' to appear in Phys. Rev. Lett) is enhanced by adding the plasma current to the otherwise pure stellarator system. The coil configuration of this ultra low aspect ratio system differs from that of a spherical tokamak by inclination of external parts of the toroidal field coils. It is shown that the configuration considered possesses many attractive properties, including: wide flexibility of operating regimes, compact design and coil simplicity, good access to the plasma, closed vacuum flux surfaces with large enclosed volume, significant external rotational transform, strong magnetic well, and a high plasma β [β(0) in excess of 30%] equilibrium. It is shown that the bootstrap effect in a spherical stellarator, in principle, can supply the full plasma current required for the high-β equilibrium. © 1996 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.871653
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  • 202
    Digitale Medien
    Digitale Medien
    Effect of negative ions on filamentation instability in collisionless three-fluid theory of plasmas with cold ion species (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 3182-3184 
    Details
    ISSN: 1089-7674
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: The growth rate of filamentation is examined analytically, as a function of the number density and the particle mass of negative ions, in the collisionless three-fluid theory of a plasma with cold ions. The growth rate dependence on the number density shows a qualitative change according to whether the pump intensity is below or above a critical level. In all cases of the pump intensity, the higher particle mass of negative ions means the weaker growth of filamentation. © 1996 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.871621
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  • 203
    Digitale Medien
    Digitale Medien
    Nonlinear saturation of the parallel velocity shear instability (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 3185-3187 
    Details
    ISSN: 1089-7674
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: The linear stability and eventual nonlinear saturation of electrostatic instabilities due to the shear in a plasma flow in the direction parallel to the magnetic field are described, analytically, via collisional fluid equations. Saturation is provided by coupling the linearly unstable eigenmode to a linearly damped nonlinear harmonic (double wave number). These modes are shown to saturate only when the linear instability driving term is below a critical value. The saturation amplitude is a function of the ratio of the "ideal'' growth rate to the diamagnetic frequency and can easily reach levels of 25% or more. © 1996 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.871622
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  • 204
    Digitale Medien
    Digitale Medien
    Comment on "Parametric excitation of Bernstein modes in laser-produced plasma'' [Phys. Plasmas 2, 3832 (1995)] (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 3194-3196 
    Details
    ISSN: 1089-7674
    Quelle: AIP Digital Archive
    Thema: Physik
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.871625
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  • 205
    Digitale Medien
    Digitale Medien
    Response to "Comment on ‘Parametric excitation of Bernstein modes in laser-produced plasma' '' [Phys. Plasmas 2, 3832 (1995)] (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 3197-3197 
    Details
    ISSN: 1089-7674
    Quelle: AIP Digital Archive
    Thema: Physik
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.871626
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  • 206
    Digitale Medien
    Digitale Medien
    Plasma response to ultrarelativistic beam propagation (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 2753-2759 
    Details
    ISSN: 1089-7674
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: Plasma response to the ultrarelativistic beam of an arbitrary density is considered assuming a slow beam current growth. The problem is reduced to a single second order differential equation, which is numerically solved for Gaussian beams of various radii and densities. Graphs obtained clarify the structure of the ion channel, give insight into charge and current neutralizations of the dense beam, and permit the analysis of the focusing force acting on the beam in a plasma. © 1996 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.872081
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  • 207
    Digitale Medien
    Digitale Medien
    Two-body similarity and its violation in tokamak edge plasmas (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 3191-3193 
    Details
    ISSN: 1089-7674
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: Scaling laws found under the assumption that two-body collisions dominate can be effectively used to benchmark complex multi-dimensional codes dedicated to investigating tokamak edge plasmas. The applicability of such scaling laws to the interpretation of experimental data, however, is found to be restricted to the relatively low plasma densities (〈1019 m−3) at which multistep processes, which break the two-body collision approximation, are unimportant. © 1996 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.871624
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  • 208
    Digitale Medien
    Digitale Medien
    Theory of the collisional presheath in a magnetic field parallel to the wall (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 2486-2495 
    Details
    ISSN: 1089-7674
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: In the limit of a small Debye length (λD→0) the plasma boundary layer in front of a negative absorbing wall is split up into a collision-free planar space charge sheath and a quasineutral presheath, where the ions are accelerated to fulfill the Bohm criterion. Apart from ion inertia, the mechanism of the presheath depends on an additional effect controlling the ion field acceleration. The present paper considers a stationary presheath dominated by the deflection of the ion orbits in a magnetic field parallel to the wall. The ion transport is provided by charge exchange collisions with cold neutrals. The potential profiles and ion distributions resulting from the self-consistent kinetic analysis are compared with expectations based on a previous hydrodynamic model. The transition from "closed'' to "open'' ion orbits results in typical deviations near the sheath edge. The kinetic Bohm criterion is found to be fulfilled marginally. © 1996 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.871966
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  • 209
    Digitale Medien
    Digitale Medien
    Evaluation of the modified plasma dispersion function for half-integral indices (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 2496-2501 
    Details
    ISSN: 1089-7674
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: Space and astrophysical plasmas typically possess particle distribution functions with a power-law tail (in energy) that are well modeled by generalized Lorentzian distributions with an associated spectral index κ. Dispersion equations for linear waves of any mode in a plasma described by a Lorentzian-type particle distribution involve the modified plasma dispersion function Zκ*, a special function analogous to the plasma dispersion function Z that arises when the particle distribution is Maxwellian. The function Zκ*, originally defined by Summers and Thorne [Phys. Fluids B 3, 1835 (1991)] for integral values of κ, was recently generalized to real values of κ by Mace and Hellberg [Phys. Plasmas 2, 2098 (1995)]. In the present paper, a general formula is derived for the modified plasma dispersion function Zκ* corresponding to half-integral values of κ, and simple, explicit closed-form expressions are given for the functions Z3/2*, Z5/2*, Z7/2*, Z9/2*, and Z11/2*. These results complement the simple, closed-form expressions for the functions Zκ*, for κ=1, 2, 3,..., that already exist in the literature. © 1996 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.871967
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  • 210
    Digitale Medien
    Digitale Medien
    Measurement of the full polarimetric transition matrix of a magnetized plasma (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 2555-2558 
    Details
    ISSN: 1089-7674
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: It is shown that the full polarimetric transition matrix of a magnetized plasma can be determined with a good time resolution (down to 100 ns) and, furthermore, it is possible to make use mainly of phase measurements, which are less sensitive to noise than amplitude measurements. The simultaneous measurement of the nine elements of the absolute transition matrix (or of the eight elements of the normalized transition matrix) allows a significant enhancement of the information obtained for each probing beam. © 1996 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.871977
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  • 211
    Digitale Medien
    Digitale Medien
    Rapid generation of Langmuir wave packets during electron beam–plasma instabilities (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 2559-2563 
    Details
    ISSN: 1089-7674
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: A new mechanism for Langmuir wave localization is investigated. Typical computer simulation of weak electron beam–plasma instabilities proceeds as follows: As Langmuir waves grow out of the noise, they react back on the beam particles by trapping. The subsequent phase shift puts the electrons out of phase with the waves. Then the instability is locally quenched, resulting in strong wave modulation, even before saturation of the instabilities. The typical size of wave packets is given by 2πvp/ω¯t, where vp and ω¯t denote the phase speed of the dominant wave and the average bounce frequency, respectively. The time scale for this process is considerably shorter than that of parametric instabilities. © 1996 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.871963
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  • 212
    Digitale Medien
    Digitale Medien
    Pulsed currents carried by whistlers. VII. Helicity and transport in heat pulses (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 2599-2609 
    Details
    ISSN: 1089-7674
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: In a uniform magnetoplasma (n(approximately-equal-to)1011 cm−3, kTe≥0.5 eV, B0≥15 G, 1 m × 2.5 m), electrons are heated locally and temporally by applying a short current pulse to a loop antenna or disk electrode. Electron magnetohydrodynamics characterize the experimental conditions. After the end of the applied current pulse and whistler wave transients, a current system driven by temperature gradients remains embedded in the plasma. The current system exhibits helicity. The associated electron drifts convect heat out of the flux tube. From diamagnetic field measurements, the decay of the electron temperature is obtained with high sensitivity (ΔkTe(approximately-equal-to)0.001 eV). The heat transport is inferred from the space–time dependence of the electron temperature. The temperature enhancement is confined to a channel whose length depends on heat input since the transport coefficients are temperature-dependent. © 1996 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.872079
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  • 213
    Digitale Medien
    Digitale Medien
    Nonlinear expansion and heating of a nonneutral electron plasma due to elastic collisions with background neutral gas (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 2615-2619 
    Details
    ISSN: 1089-7674
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: This paper investigates theoretically the heating and nonlinear expansion of a nonneutral electron plasma due to elastic collisions with constant collision frequency νen between the plasma electrons and a background neutral gas. The model treats the electrons as a strongly magnetized fluid (ωpe2/ωce2(very-much-less-than)1) immersed in a uniform magnetic field B0eˆz. The model also assumes an axisymmetric plasma column (∂/∂θ=0) with negligible axial variation (∂/∂z=0), and that the process of heat conduction is sufficiently fast that the electrons have relaxed through electron-electron collisions to a quasi-equilibrium state with scalar pressure P(r,t)=n(r,t)T, and isothermal temperature T. Assuming that the electrons undergo elastic collisions with infinitely massive background gas atoms, global energy conservation is used to calculate the electron heating rate, dT(t)/dt, as the plasma column expands on a time scale τdiff∼(ωpe2νen/ωce2 )−1, and the electrostatic potential energy decreases. Coupled dynamical equations that describe the nonlinear evolution of the mean-square column radius r20(t) and electron temperature T(t) are derived and solved numerically. © 1996 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.871974
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  • 214
    Digitale Medien
    Digitale Medien
    Tomography of (2, 1) and (3, 2) magnetic island structures on Tokamak Fusion Test Reactor (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 2631-2640 
    Details
    ISSN: 1089-7674
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: High-resolution electron cyclotron emission (ECE) image reconstruction has been used to observe (m,n)=(2,1) and (3, 2) island structures on Tokamak Fusion Test Reactor [Plasma Phys. Controlled. Fusion 33, 1509 (1991)], where m and n are the poloidal and the toroidal mode number, respectively. The observed island structure is compared with other diagnostics, such as soft x-ray tomography and magnetic measurements. A cold elliptic island is observed after lithium pellet injection. Evidence for the enhancement of the heat transfer due to the island is observed. A relaxation phenomenon due to the m=2 mode is newly observed in Ohmic plasmas. © 1996 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.871534
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  • 215
    Digitale Medien
    Digitale Medien
    Explanation for MARFE formation and subsequent evolution into a detached symmetric plasma edge (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 2673-2678 
    Details
    ISSN: 1089-7674
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: An explanation is proposed for the experimentally observed tokamak phenomenon of a symmetric radiating edge evolving with increasing density into a multifaceted asymmetric radiation from the edge (MARFE) condition and then cooling into a detached plasma with a symmetric edge. The thermal stability impurity density limits increase with the poloidal mode number, m, of the edge plasma distribution. When the edge impurity density exceeds the m=0 mode density limit, the edge plasma evolves into a mixture of m(approximately-greater-than)0 modes which becomes more highly asymmetric as the density increases further and exceeds the density limits of successively higher modes, forcing the plasma into the remaining stable higher-m modes—the stable MARFE. If the radiative cooling of the edge is sufficient to reduce the temperature to Te∼10 eV, the various thermal modes are stabilized by the strong positive temperature dependence of the ionization cooling in the presence of neutral atoms, the impurity density limits are thereby increased, and the plasma evolves back into a stable m=0 mode. On the other hand, if the plasma becomes highly enough asymmetric before sufficient cooling takes place, nonlinear effects drive a disruptive collapse of the radial temperature distribution. © 1996 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.871524
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  • 216
    Digitale Medien
    Digitale Medien
    Fluctuation-induced flux at the plasma edge in toroidal devices (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 2664-2672 
    Details
    ISSN: 1089-7674
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: Fluctuation-induced fluxes have a bursty character. As a consequence, a significant part of the total particle flux is carried out by sporadic, large transport bursts. The local flux distribution function is consistent with a near-Gaussian character of the fluctuations. The radial dependence of the statistical properties of plasma fluctuations and induced fluxes have been investigated in the plasma boundary region of the TJ-I tokamak [I. García-Cortés et al., Phys. Fluids B 4, 4007 (1992)] and the TJ-IU torsatron [E. Ascasibar et al., in Plasma Physics and Controlled Fusion Research, Proceedings of the 15th Conference on Plasma Physics and Controlled Nuclear Fusion Research, Seville (International Atomic Energy Agency, Vienna, in press)]. There is a striking similarity between the statistical properties of turbulent transport in both devices. © 1996 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.871523
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  • 217
    Digitale Medien
    Digitale Medien
    Mechanisms for collisionless absorption of light waves obliquely incident on overdense plasmas with steep density gradients (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 2702-2709 
    Details
    ISSN: 1089-7674
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: For p-polarized laser light obliquely incident on overdense plasmas with steep density gradients, a new collisionless absorption mechanism (sheath-transit absorption) is studied analytically and numerically. Complementary to Brunel's "not-so-resonant'' resonant absorption, and to the conventional resonant absorption, the sheath-transit absorption is most effective for steep density gradients and when the light pressure is less than the plasma pressure. It is also shown that the assumption of instantaneous particle reflection, usually a reasonable assumption for the normal incidence case, is invalid for the p-polarized oblique incident case. A test-particle model which provides a simple physical picture of the sheath-transit absorption is presented. Absorption coefficients obtained from the test-particle model agree reasonably well with those from particle-in-cell (PIC) simulations. The transition from the resonant absorption to the sheath-transit absorption as the density gradient steepens is demonstrated by PIC simulations with a wide range of density gradients. © 1996 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.871527
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  • 218
    Digitale Medien
    Digitale Medien
    Rapid reconnection in compressible plasma (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 2725-2741 
    Details
    ISSN: 1089-7674
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: A study of set-up, propagation, and interaction of non-linear and linear magnetohydrodynamic waves driven by magnetic reconnection is presented. The source term of the waves generated by magnetic reconnection is obtained explicitly in terms of the initial background conditions and the local reconnection electric field. The non-linear solution of the problem found earlier, serves as a basis for formulation and extensive investigation of the corresponding linear initial-boundary value problem of compressible magnetohydrodynamics. In plane geometry, the Green's function of the problem is obtained and its properties are discussed. For the numerical evaluation it turns out that a specific choice of the integration contour in the complex plane of phase velocities is much more effective than the convolution with the real Green's function. Many complex effects like intrinsic wave coupling, anisotropic propagation characteristics, generation of surface and side wave modes in a finite beta plasma are retained in this analysis. © 1996 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.871723
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  • 219
    Digitale Medien
    Digitale Medien
    Kinetic description of the influence of electron-beam misalignment on the performance of a coaxial-cavity gyrotron (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 2760-2765 
    Details
    ISSN: 1089-7674
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: Based on the gyrokinetics of free-electron masers, the influence of the electron-beam misalignment on the energy-transfer rate and starting current of a coaxial cavity gyrotron oscillator is studied. As a practical example, a numerical analysis is given to the 1.5 MW/140 GHz coaxial-cavity gyrotron that is being constructed at the Forschungszentrum Karlsruhe (FZK) [M. Thumm, Fusion Eng. Design 30, 139 (1995)]. Results show that the electron-beam misalignment will decrease the energy-transfer rate and increase the starting current, and the dependence of the output power on the magnetic field may become more sensitive due to a poor alignment of the electron-beam. © 1996 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.871531
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  • 220
    Digitale Medien
    Digitale Medien
    Interaction of crossed laser beams with plasmas (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 2215-2217 
    Details
    ISSN: 1089-7674
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: The parametric interaction of two crossed collimated laser beams with ion plasma modes has been studied. The underlying process is a density grating that is created by the two laser beams. The Bragg diffraction that is produced enhances forward stimulated Brillouin scattering (SBS) which results in a time dependent energy exchange between the two beams. A diversity of other SBS processes which depend on the symmetry of driving laser beams are also discussed. © 1996 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.871703
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  • 221
    Digitale Medien
    Digitale Medien
    Theory of drift waves in the presence of parallel and perpendicular flow curvature. I. Slab model (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 2224-2228 
    Details
    ISSN: 1089-7674
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: It has recently [S. Sen and M. G. Rusbridge, Phys. Plasmas 2, 2705 (1995)] been shown that, contrary to the usual belief, parallel flow curvature (V(parallel)″) can stabilize drift-like microinstabilities. Here the earlier work is extended to include the effect of the perpendicular flow curvature (V⊥″), which is known to have a stabilizing role on the drift-like microinstabilities. The full analytic stability analysis shows that the ratio of the stabilizing influences of the perpendicular to the parallel flow curvature scales as Ls/2Ln, where Ls and Ln are the magnetic shear and the density variation scale length, respectively. Thus, at the plasma edge (since Ls(very-much-greater-than)Ln) the perpendicular flow may play a crucial role in stabilizing microinstabilities and turbulence in the improved regimes of confinement [like the high (H) modes]. However, in the core confinement improvement [like the very high (VH) mode] both the parallel and the perpendicular flow curvatures are important, since Ls∼2Ln. Furthermore, as the confinement improvement in the core is usually related to the toroidal velocity and since V(parallel) coming from the toroidal flow is much more than V⊥ (V⊥∼εV(parallel), here ε is the inverse aspect ratio), this implies that it is the parallel component of the toroidal flow and not the perpendicular component, as is usually thought, which is responsible for the core confinement improvement. © 1996 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.871929
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  • 222
    Digitale Medien
    Digitale Medien
    Plasma–neutral interaction in tokamak divertor for "gas box'' neutral model (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 2280-2285 
    Details
    ISSN: 1089-7674
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: Plasma flow through the gas cloud in a tokamak divertor for "gas box'' divertor geometry and Knudsen regime of neutral transport is investigated. It is shown that similar to the neutral models that have considered previously, (i) plasma parameters near the target is sensitive to the energy flux into the hydrogen recycling region and can change rapidly, resulting in bifurcation-like behavior, which might be interpreted as a transition to detached regime, (ii) plasma flux onto the target starts to decrease at a very low plasma temperature near the target, while a strong pressure drop already occurs. At low plasma temperature near the target the recombination processes can significantly alter the plasma flux onto the target. © 1996 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.871910
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    Transform-limited coherent synchrotron radiation wavepackets in a chirped pulse free-electron laser (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 2446-2456 
    Details
    ISSN: 1089-7674
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: A novel source of transform-limited pulses of coherent electromagnetic radiation relying on the synchrotron radiation process in a fast wave guiding structure is investigated theoretically. An ultrashort electron bunch transversally accelerated by a periodic external field is considered. At grazing, where the bunch and group velocities are matched, the duration of the resulting ultrawideband chirped pulse is governed by group velocity dispersion instead of slippage. Because of the intimate connection between the rate of chirping and the bandwidth, the corresponding pulse duration is shown to be very close to the Fourier transform limit. In addition, the propagation of such chirped pulses through a guiding structure with negative group velocity dispersion is investigated both theoretically and computationally. The spectral and temporal characteristics of the chirped and compressed pulses are derived analytically. Detailed computer calculations complement this theoretical analysis. © 1996 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.871706
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  • 224
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    Parallel transport in ideal magnetohydrodynamics and applications to resistive wall modes (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 2469-2471 
    Details
    ISSN: 1089-7674
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: It is shown that in magnetohydrodynamics (MHD) with an ideal Ohm's law, in the presence of parallel heat flux, density gradient, temperature gradient, and parallel compression, but in the absence of perpendicular compressibility, there is an exact cancellation of the parallel transport terms. This cancellation is due to the fact that magnetic flux is advected in the presence of an ideal Ohm's law, and therefore parallel transport of temperature and density gives the same result as perpendicular advection of the same quantities. Discussions are also presented regarding parallel viscosity and parallel velocity shear, and the generalization to toroidal geometry. These results suggest that a correct generalization of the Hammett–Perkins fluid operator [G. W. Hammett and F. W. Perkins, Phys. Rev. Lett. 64, 3019 (1990)] to simulate Landau damping for electromagnetic modes must give an operator that acts on the dynamics parallel to the perturbed magnetic field lines. © 1996 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.871709
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  • 225
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    A novel phase focusing mechanism in multipactor discharge (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 1481-1483 
    Details
    ISSN: 1089-7674
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: In spite of the mutual repulsion among the space charges, a new phase-focusing mechanism is discovered whereby the leading edge of the multipactor discharge in an rf circuit grows at the expense of the trailing edge. This effect arises from the different impact energies, and hence different secondary electron yields, experienced by different portions of the discharge. This phase focusing mechanism may shape the steady-state multipactor discharge in the form of a very tight bunch of electrons. © 1996 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.872027
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  • 226
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    Attenuation of energetic helium beams in fusion plasmas (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 1512-1523 
    Details
    ISSN: 1089-7674
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: The efficiency of plasma heating and diagnostics based on using atomic beams or neutralized ion fluxes depends crucially on the atomic processes involved in the stopping of the atoms. These processes have been studied in detail for helium atoms in the energy range from 25 keV/u to 2.5 MeV/u. The processes considered include both the ground state and the excited states of the atoms, thus allowing for the multistep electron loss in collisions with the plasma constituents. Special attention is paid to electron loss from the atoms in metastable states. The effective beam stopping cross section and attenuation factor have been calculated for a wide variety of beam and plasma parameters. The atomic database necessary for these calculations has been documented using the best data available at present. The results are also given in terms of convenient analytic fits. Special calculations have been carried out for the helium probing beam and neutralized alpha particles in International Thermonuclear Experimental Reactor (ITER) [Kikuchi et al., Fusion Eng. Design 16, 253 (1991)] plasmas. © 1996 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.872012
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  • 227
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    Dynamic and Debye shielding and anti-shielding (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 1820-1826 
    Details
    ISSN: 1089-7674
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: While shielding in collisional plasmas obeys the standard Debye result, shielding in collisionless plasmas is far more complex than commonly believed. For example, a one-dimensional (highly magnetized), immobile-ion plasma can, in some circumstances, anti-shield a positive test charge; i.e. the plasma becomes more positive in the vicinity of the test charge. When shielding does occur, it results from electrons dynamically trapped in the neighborhood of the test charge. A new theory of collisionless (Dynamic) shielding in one, two and three dimensions is presented here, and is in excellent agreement with experiments in pure electron plasmas. Because the distribution functions found in Dynamic shielding are highly non-Maxwellian in the non-linear regime, collisionless Dynamic shielding can be substantially less efficacious than collisional Debye shielding. © 1996 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.871685
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  • 228
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    Nonlinear mean field electrodynamics of turbulent dynamos (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 1853-1857 
    Details
    ISSN: 1089-7674
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: Mean field electrodynamics of turbulent dynamos taking into account Lorentz forces of the generated magnetic fields is derived and studied. Small-scale magnetic fields that are much stronger than the mean field B0 modify the fluid turbulence in such a way as to suppress the α effect: α=α0(1+R)−1, where α0 is the kinematic value, and the reduction factor is proportional to the magnetic Reynolds number Rm, R=RmB20/(4πρv2), and v is the characteristic turbulent velocity. In two dimensions the analog is turbulent magnetic diffusivity suppression. Suppression becomes noticeable at very low values of the mean magnetic field, B20∼ρv2/Rm. The modification of turbulence by the small-scale magnetic fields is discussed. © 1996 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.871981
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  • 229
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    Alpha particle losses from Tokamak Fusion Test Reactor deuterium–tritium plasmas (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 1875-1880 
    Details
    ISSN: 1089-7674
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: Because alpha particle losses can have a significant influence on tokamak reactor viability, the loss of deuterium–tritium alpha particles from the Tokamak Fusion Test Reactor (TFTR) [K. M. McGuire et al., Phys. Plasmas 2, 2176 (1995)] has been measured under a wide range of conditions. In TFTR, first orbit loss and stochastic toroidal field ripple diffusion are always present. Other losses can arise due to magnetohydrodynamic instabilities or due to waves in the ion cyclotron range of frequencies. No alpha particle losses have yet been seen due to collective instabilities driven by alphas. Ion Bernstein waves can drive large losses of fast ions from TFTR, and details of those losses support one element of the alpha energy channeling scenario. © 1996 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.871983
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  • 230
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    Comparison of detached and radiative divertor operation in Alcator C-Mod (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 1908-1915 
    Details
    ISSN: 1089-7674
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: The divertor of the Alcator C-Mod tokamak [Phys. Plasmas 1, 1511 (1994)] routinely radiates a large fraction of the power entering the scrape-off layer. This dissipative divertor operation occurs whether the divertor is detached or not, and large volumetric radiative emissivities, up to 60 MW m−3 in ion cyclotron range of frequency (ICRF) heated discharges, have been measured using bolometer arrays. An analysis of both Ohmic and ICRF-heated discharges has demonstrated some of the relative merits of detached divertor operation versus high-recycling divertor operation. An advantage of detached divertor operation is that the power flux to the divertor plates is decreased even further than its already low value. Some disadvantages are that volumetric losses outside the separatrix in the divertor region are decreased, the neutral compression ratio is decreased, and the penetration efficiency of impurities increases. © 1996 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.871986
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  • 231
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    Radially global gyrokinetic simulation studies of transport barriers (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 1959-1966 
    Details
    ISSN: 1089-7674
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: Improvements in tokamak transport have recently been obtained in a variety of operational modes through the formation of transport barriers, or good confinement radial zones. Here global nonlinear three-dimensional toroidal gyrokinetic simulation is used to study three effects that are linearly stabilizing and may cause the formation of transport barriers, namely, sheared toroidal rotation, reversed magnetic shear, and peaked density profiles. The effect of toroidal shear flow on ion heat diffusivity is found to be relatively weak compared to mixing-length expectations based on linear calculations. In contrast, it is found that weak or negative magnetic shear (s〈1/2) in combination with a peaked density profile relative to the temperature profile greatly suppresses ion-temperature-gradient-driven turbulence in the central region of global nonlinear simulations. Similar features are seen experimentally in reversed magnetic shear tokamak plasmas. There is some nonlocal penetration (∼20–30ρi) of the turbulence into the subcritical core region. © 1996 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.871992
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  • 232
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    Demonstration of high-performance negative central magnetic shear discharges in the DIII-D tokamak (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 1983-1991 
    Details
    ISSN: 1089-7674
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: Reliable operation of discharges with negative central magnetic shear has led to significant increases in plasma performance and reactivity in both low confinement, L-mode, and high confinement, H-mode, regimes in the DIII-D tokamak [Plasma Physics and Controlled Nuclear Fusion Research 1986 (International Atomic Energy Agency, Vienna, 1987), Vol. 1, p. 159]. Using neutral beam injection early in the initial current ramp, a large range of negative shear discharges have been produced with durations lasting up to 3.2 s. The total noninductive current (beam plus bootstrap) ranges from 50% to 80% in these discharges. In the region of shear reversal, significant peaking of the toroidal rotation [fφ(0)∼30–60 kHz] and ion temperature [Ti(0)∼15–22 keV] profiles are observed. In high-power discharges with an L-mode edge, peaked density profiles are also observed. Confinement enhancement factors up to H≡τE/τITER-89P∼2.5 with an L-mode edge, and H∼3.3 in an edge localized mode (ELM)-free H mode, are obtained. Transport analysis shows both ion thermal diffusivity and particle diffusivity to be near or below standard neoclassical values in the core. Large pressure peaking in the L mode leads to high disruptivity with βN≡βT/(I/aB)≤2.3, while broader pressure profiles in the H mode gives low disruptivity with βN≤4.2. © 1996 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.871994
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  • 233
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    Characteristics of high-confinement modes in Alcator C Mod (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 1992-1998 
    Details
    ISSN: 1089-7674
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: The regime of high particle and energy confinement known as the H mode [Phys. Rev. Lett. 49, 1408 (1982)] has been extended to a unique range of operation for divertor tokamaks up to toroidal fields of nearly 8 T, line-averaged electron densities of 3×1020 m−3, and surface power densities of nearly 0.6 MW/m2 in the compact high-field tokamak Alcator C Mod [Phys. Plasmas 1, 1511 (1994)]. H modes are achieved in Alcator C Mod with Ion Cyclotron Resonant Frequency (ICRF) heating and with Ohmic heating alone without boronization of the all molybdenum tiled first wall. Large increases in charge exchange flux are observed during the H mode over the entire range of energies from 2 to 10 keV. There appears to be an upper limit to the midplane neutral pressure, of about 0.08 Pa above which no H modes have been observed. The plasmas with the best energy confinement have the lowest midplane neutral pressures, below 0.01 Pa. There is an edge electron temperature threshold such that Te≥280 eV ±40 eV for sustaining the H mode, which is equal at L–H and H–L transitions. The hysteresis in the threshold power between L–H and H–L transitions is less than 25% on average. Both core and edge particle confinement improve by a factor of 2–4 from L mode to H mode. Energy confinement also improves by up to a factor of 2 over L mode. © 1996 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.871995
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  • 234
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    Laser–plasma interactions in ignition-scale hohlraum plasmas (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 2029-2040 
    Details
    ISSN: 1089-7674
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: Scattering of laser light by stimulated Brillouin scattering (SBS) and stimulated Raman scattering (SRS) is a concern for indirect drive inertial confinement fusion (ICF). The hohlraum designs for the National Ignition Facility (NIF) raise particular concerns due to the large scale and homogeneity of the plasmas within them. Experiments at Nova have studied laser–plasma interactions within large scale length plasmas that mimic many of the characteristics of the NIF hohlraum plasmas. Filamentation and scattering of laser light by SBS and SRS have been investigated as a function of beam smoothing and plasma conditions. Narrowly collimated SRS backscatter has been observed from low density, low-Z, plasmas, which are representative of the plasma filling most of the NIF hohlraum. SBS backscatter is found to occur in the high-Z plasma of gold ablated from the wall. Both SBS and SRS are observed to be at acceptable levels in experiments using smoothing by spectral dispersion (SSD). © 1996 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.872000
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  • 235
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    Digitale Medien
    Two-dimensional radiation-magnetohydrodynamic simulations of SATURN imploding Z pinches (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 2063-2069 
    Details
    ISSN: 1089-7674
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: Z-pinch implosions driven by the SATURN device [D. D. Bloomquist et al., Proceedings of the 6th Institute of Electrical and Electronics Engineers (IEEE) Pulsed Power Conference, Arlington, VA, edited by P. J. Turchi and B. H. Bernstein (IEEE, New York, 1987), p. 310] at Sandia National Laboratory are modeled with a two-dimensional radiation magnetohydrodynamic (MHD) code, showing strong growth of the magneto-Rayleigh–Taylor (MRT) instability. Modeling of the linear and nonlinear development of MRT modes predicts growth of bubble-spike structures that increase the time span of stagnation and the resulting x-ray pulse width. Radiation is important in the pinch dynamics, keeping the sheath relatively cool during the run-in and releasing most of the stagnation energy. The calculations give x-ray pulse widths and magnitudes in reasonable agreement with experiments, but predict a radiating region that is too dense and radially localized at stagnation. We also consider peaked initial density profiles with constant imploding sheath velocity that should reduce MRT instability and improve performance. Krypton simulations show an output x-ray power (approximately-greater-than)80 TW for the peaked profile. © 1996 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.872003
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  • 236
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    Digitale Medien
    Effects of variable x-ray preheat shielding in indirectly driven implosions (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 2094-2097 
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    ISSN: 1089-7674
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: The performance of indirectly driven fusion capsules has been improved by mid Z doping of the plastic capsule ablator. The doping increases x-ray preheat shielding leading to a more isentropic compression, higher convergence, and higher neutron yield. A 4× increase in neutron yield is both calculated and observed as the Ge doping level is increased from 0% to 3% by atomic fraction. A predicted 40% decrease in x-ray image core size with increasing Ge content is confirmed. © 1996 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.872007
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  • 237
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    Digitale Medien
    Complexity in plasma: From self-organization to geodynamo (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 2135-2142 
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    ISSN: 1089-7674
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: A central theme of "Complexity'' is the question of the creation of ordered structure in nature (self-organization). The assertion is made that self-organization is governed by three key processes, i.e., energy pumping, entropy expulsion and nonlinearity. Extensive efforts have been done to confirm this assertion through computer simulations of plasmas. A system exhibits markedly different features in self-organization, depending on whether the energy pumping is instantaneous or continuous, or whether the produced entropy is expulsed or reserved. The nonlinearity acts to bring a nonequilibrium state into a bifurcation, thus resulting in a new structure along with an anomalous entropy production. As a practical application of our grand view of self-organization a preferential generation of a dipole magnetic field is successfully demonstrated. © 1996 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.871666
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  • 238
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    Development and applications of a plasma waveguide for intense laser pulses (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 2149-2155 
    Details
    ISSN: 1089-7674
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: The demonstration of optical guiding of high-intensity laser pulses in a plasma fiber waveguide [C. G. Durfee III and H. M. Milchberg, Phys. Rev. Lett. 71, 2409 (1993)] has opened the way to new advances in the development of nonlinear optics-based short-wavelength light sources, soft x-ray lasers, and compact laser-driven charged particle accelerators, and offers a new practical realm in which to study, control, and apply nonperturbative laser–matter interactions at ultrahigh intensity. An overview of selected experimental and theoretical results and their applications is presented. © 1996 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.871668
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  • 239
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    Multimegawatt cyclotron autoresonance accelerator (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 2163-2168 
    Details
    ISSN: 1089-7674
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: Means are discussed for generation of high-quality multimegawatt gyrating electron beams using rf gyroresonant acceleration. TE111-mode cylindrical cavities in a uniform axial magnetic field have been employed for beam acceleration since 1968; such beams have more recently been employed for generation of radiation at harmonics of the gyration frequency. Use of a TE11-mode waveguide for acceleration, rather than a cavity, is discussed. It is shown that the applied magnetic field and group velocity axial tapers allow resonance to be maintained along a waveguide, but that this is impractical in a cavity. In consequence, a waveguide cyclotron autoresonance accelerator (CARA) can operate with near-100% efficiency in power transfer from rf source to beam, while cavity accelerators will, in practice, have efficiency values limited to about 40%. CARA experiments are described in which an injected beam of up to 25 A, 95 kV has had up to 7.2 MW of rf power added, with efficiencies of up to 96%. Such levels of efficiency are higher than observed previously in any fast-wave interaction, and are competitive with efficiency values in industrial linear accelerators. Scaling arguments suggest that good quality gyrating megavolt beams with peak and average powers of 100 MW and 100 kW can be produced using an advanced CARA, with applications in the generation of high-power microwaves and for possible remediation of flue gas pollutants. © 1996 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.871670
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  • 240
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    Does high density–low pressure etching depend on the type of plasma source? (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 2197-2202 
    Details
    ISSN: 1089-7674
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: Etching of SiO2 with CF4 in three types of high density–low pressure (5×1011 cm−3, 1–10 mTorr) etch tools: electron cyclotron resonance (ECR), inductively coupled (ICP), and helicon (HRF) is described. Although the physical processes that produce the plasma in the three types of sources are quite different, the etch rate processes are identical when viewed from the wafer sheath boundary. Measurements demonstrate that if sufficient fluorine is present, the etch rate limiting step depends only on the ion energy flux to the wafer, rather than on the details of the chemical species. Etch rate control depends only on the wafer bias power. Experimental results are device independent so the etch rate in high density–low pressure plasma sources does not depend on the plasma source power. Major differences in tool etch rate characteristics are more likely determined by tool wall material (and wall chemistry) and tool geometry rather than the physical process that is used to produce the plasma. © 1996 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.871675
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  • 241
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    Influence of trapped electrons on the time-independent states of a negatively biased single-ended Q machine (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 1192-1201 
    Details
    ISSN: 1089-7674
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: Measurements of basic plasma parameters in a low-density single-ended Q machine with a negatively biased cold plate are compared with pertinent predictions of time-independent collisionless plane-diode theory. As was also found in particle simulations [Phys. Fluids B 3, 244 (1991)] and previous experiments, theory and experiment agree well if space-charge conditions near the hot-plate surface are electron rich, in which case the potential distributions are monotonically decreasing. For ion-rich conditions, however, where the potential distributions exhibit a single maximum located in front of the hot plate, the plasma properties are found to depend very sensitively on the amount and distribution of the electrons trapped in the region of positive potential. In this regime, the experimental findings agree very well with a theoretical model in which a certain amount of trapped electrons is assumed. © 1996 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.871743
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  • 242
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    Nonlocal electron transport in spherical plasmas (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 1280-1283 
    Details
    ISSN: 1089-7674
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: The influence of spherical geometry on nonlocal radial electron heat transport has been studied using perturbation analysis . The nonlocal expression for the radial heat flux is obtained in the limit of large ion charge. The deviation of the spherical nonlocal heat transport from the planar theory has been investigated and it has been found that the space curvature can significantly modify the heat flux compared to the planar result when the delocalization length (that associated with the faster electrons which dominate energy transport) is comparable to the radius. © 1996 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.871752
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    A physical mechanism for the onset of radial electric fields in magnetically confined plasmas (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 1284-1288 
    Details
    ISSN: 1089-7674
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: A simple physical mechanism is described, which could trigger the Low-mode to High-mode (L–H) transition. The instantaneous ion density profile is significantly modified by a sudden temperature increase, because Larmor radii and banana orbit widths are proportional to thermal velocity. The electric fields that are observed in H-mode plasmas could be produced in the radial region where a large second derivative of the density profile exists, either by strong additional heating or by the heat pulse associated to a sawtooth crash. The L–H transition threshold for the time derivative of the ion temperature is of the order of magnitude of the values that are measured in the outer part of the plasma by electron temperature fast diagnostics at sawtooth crashes. This model agrees with the experimental evidence that L–H transitions are often triggered by a sawtooth crash, and the predicted dependence of the threshold on plasma parameters is fairly consistent with available data. © 1996 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.871753
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  • 244
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    Alfvén ion–ion hybrid wave heating in the Phaedrus-T tokamak (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 1331-1339 
    Details
    ISSN: 1089-7674
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: In the Phaedrus-T tokamak [R. A. Breun et al., Fusion Technol. 19, 1327 (1991)], Alfvén waves are indirectly driven by a fast wave antenna array. Small fractions of minority ions are shown to have a large effect on the Alfvén spectrum, as measured at the edge. An ion–ion hybrid Alfvén mode has been identified by measuring dispersion properties. Landau damping is predicted to be large and spatially localized. These Alfvénic waves are experimentally shown to generate correlated electron heating and changes in density near the core of the tokamak plasma. Fast wave antenna fields can mode convert at a hybrid Alfvén resonance and provide a promising route to spatially localized tokamak heating and current drive, even for low effective ionic charge Zeff≈1.3–2. © 1996 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.871786
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  • 245
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    Observation of wave-induced chaotic radial transport in a laboratory terrella experiment (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 2143-2148 
    Details
    ISSN: 1089-7674
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: The wave-induced chaotic radial transport of energetic electrons has been observed in a laboratory terrella, the Collisionless Terrella Experiment (CTX) [H. P. Warren and M. E. Mauel, Phys. Plasmas 2, 4185 (1995)]. In the experiment electron-cyclotron-resonance heating (ECRH) is used to create a localized population of trapped energetic electrons which excite drift-resonant electrostatic fluctuations. Measurements with multiple high-impedance floating potential probes are used to determine the amplitude, frequency, and mode structure of the observed fluctuations. Energetic electron transport is observed with gridded particle detectors. Poincaré surfaces of section indicate that increases in the flux of energetic electrons to the detectors occur only when fluctuations which meet the conditions for global chaos are present. Quasilinear transport simulations do not reproduce several important features of the experimental measurements. In contrast, Hamiltonian simulations reproduce many of the salient temporal characteristics of the experimental measurements and indicate that the persistence of phase-space correlations plays an important role in the energetic electron transport observed in the experiment. © 1996 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.871667
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  • 246
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    A study on ion energy distribution functions and plasma potentials in helicon wave plasmas (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 1462-1469 
    Details
    ISSN: 1089-7674
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: The characteristics of high-density helicon wave plasma generated in a quartz tube of 10 cm in diameter have been studied. The optimum conditions for efficiently exciting helicon wave plasma have been investigated. It is also observed that plasma fully ionized by helicon waves has ion pumping effect. Whether the discharge is inductive or capacitive, the plasma potential is important, since it determines the energy of the ion incident on the wall. It is investigated theoretically and experimentally that the fluctuation of the plasma potentials influences the ion energy distribution functions monitored by the retarding field energy analyzer. It is also observed that the plasma potential fluctuates with peak-to-peak voltage Vp–p in the low-density mode. The radio-frequency modulation to the plasma potential is weaker in the helicon mode than in the low mode. © 1996 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.871737
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  • 247
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    Digitale Medien
    Radial thermal stability of the radiative mantle (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 1012-1021 
    Details
    ISSN: 1089-7674
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: Sufficient conditions for the thermal stability against radial excursions of a cylindrical equilibrium temperature distribution in a plasma with a strongly radiative edge, or mantle, are established in terms of the minimum allowable ratio of the core heating power density to the edge radiation power density. A positive dependence of the thermal conductivity upon temperature is found to be stabilizing, and a divertor separatrix boundary condition for the temperature is found to be destabilizing relative to a fixed separatrix temperature condition. The stability conditions are consistent with the conditions extant in a stable radiative mantle experiment and in a stable radiative mantle simulation. Expressions are developed for a radiation edge density limit and for the maximum fraction of the plasma heating power which can be radiated from the mantle. © 1996 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.871812
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  • 248
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    Magnetohydrodynamic simulations of direct current helicity injection for current drive in tokamaks (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 1038-1046 
    Details
    ISSN: 1089-7674
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: Magnetohydrodynamic (MHD) computations of direct current (DC) helicity injection for tokamak-like configurations show current drive with no "loop voltage'' in a resistive, pressureless plasma. Self-consistently induced, hollow current profiles are unstable to resistive modes that partially relax the profile through the MHD dynamo mechanism. The resulting current profiles remain quite hollow, however, and tokamaks are not generated. The current driven by the fluctuations leads to closed contours of average poloidal flux, but the 1% fluctuation level is large enough to produce a region of stochastic magnetic field. A limited Lundquist number (S) scan from 2.5×103 to 2×104 indicates that both the fluctuation level and current profile relaxation increase with S. A simple quasilinear power scaling is consistent with these results at low S and suggests that at larger S, the fluctuation level decreases. © 1996 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.871759
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  • 249
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    Digitale Medien
    Dependence of stimulated Brillouin scattering on focusing optic F number in long scale-length plasmas (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 1091-1095 
    Details
    ISSN: 1089-7674
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: The onset of stimulated Brillouin scattering (SBS) has been examined by varying the effective F number (lens focal length/incident beam diameter) of the optical system used to irradiate a performed 1 mm long, longitudinally uniform hydrocarbon (CH) plasma. SBS onset occurs at higher average laser intensity for smaller F, in qualitative agreement with theory. Quantitative comparison between theory and experiment, and implications for the National Ignition Facility [Phys. Plasmas 2, 2480 (1995)] are discussed. © 1996 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.871764
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  • 250
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    Digitale Medien
    Whistler waves produced by a modulated spiraling electron beam: Linear approach (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 1120-1129 
    Details
    ISSN: 1089-7674
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: The theory of whistler wave interaction with a modulated electron beam injected obliquely to the magnetic field in an unbounded space plasma is considered. The study of the energy transfer between the spiraling beam and the whistler wave is done in the case of sheared whistlers, when the parallel wave number is very small compared to the perpendicular one. In the linear regime, structures of potentials and electromagnetic fields in the beam vicinity and in the wave zone are obtained analytically: Simple expressions for field components are provided for Cerenkov and Doppler resonances, in double and single pole cases. Emitted power is calculated as a function of beam parameters. © 1996 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.871768
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  • 251
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    Digitale Medien
    Analytic expressions for polarimetry in plasma with large Cotton–Mouton or Faraday effects (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 1182-1183 
    Details
    ISSN: 1089-7674
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: Analytic expressions for plasma polarimetry are derived for the case when either the Cotton–Mouton effect or the Faraday effect is large while the other effect is small. © 1996 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.871774
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  • 252
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    Digitale Medien
    Variational structure for dissipationless linear drift-wave equations (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 744-748 
    Details
    ISSN: 1089-7674
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: A derivation from first principles of the Lagrangian density for the dissipationless linear drift- wave equation recently introduced by Mattor and Diamond [Phys. Plasmas 1, 4002 (1994)] is presented. An exact wave-action conservation law for linear drift waves propagating in a rotating magnetized plasma is then derived from the Lagrangian density without requiring the use of the eikonal representation for the wave fields. © 1996 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.871808
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  • 253
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    The structure of three-dimensional magnetic neutral points (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 759-770 
    Details
    ISSN: 1089-7674
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: The local configurations of three-dimensional magnetic neutral points are investigated by a linear analysis about the null. It is found that the number of free parameters determining the arrangement of field lines is four. The configurations are first classified as either potential or non-potential. Then the non-potential cases are subdivided into three cases depending on whether the component of current parallel to the spine is less than, equal to or greater than a threshold current; therefore there are three types of linear non-potential null configurations (a radial null, a critical spiral and a spiral). The effect of the four free parameters on the system is examined and it is found that only one parameter categorizes the potential configurations, whilst two parameters are required if current is parallel to the spine. However, all four parameters are needed if there is current both parallel and perpendicular to the spine axis. The magnitude of the current parallel to the spine determines whether the null has spiral, critical spiral or radial field lines whilst the current perpendicular to the spine affects the inclination of the fan plane to the spine. A simple method is given to determine the basic structure of a null given M the matrix which describes the local linear structure about a null point. © 1996 American Institute of Physics. [S1070-6634X(96)03803-4]
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.871810
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  • 254
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    Digitale Medien
    Relativistic theory of cross-field transport and diffusion in a plasma (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 804-814 
    Details
    ISSN: 1089-7674
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: A modified Chapman–Enskog analysis is employed to solve the relativistic Boltzmann transport equation. This is in the context of binary Coulomb encounter in a weakly coupled and fully ionized nondegenerate electron–ion plasma across a magnetic field. Specific formulas concerning transport coefficients are derived analytically in the various limits of temperatures, viz., nonrelativistic, moderately relativistic, and ultrarelativistic temperatures. Early classical results are equally recovered. The dependence of coefficients on thermal regimes is quantitatively discussed and their markedly diminishing trends are analyzed. © 1996 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.871781
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  • 255
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    Digitale Medien
    Computer studies on the spontaneous fast reconnection mechanism in three dimensions (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 853-862 
    Details
    ISSN: 1089-7674
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: The spontaneous evolution of fast reconnection is studied in three dimensions by extending (in the z direction) the previous two-dimensional model that considered only the x-y plane [M. Ugai, Phys. Fluids B 4, 2953 (1992)]. It is demonstrated that the reconnection development strongly depends on three-dimensional effects; only when the central current sheet is sufficiently long in the z direction, say more than a few times larger than the current sheet width, the fast reconnection mechanism fully develops by the self-consistent coupling between the global reconnection flow and the current-driven anomalous resistivity. In this case, the reconnection flow can grow so powerfully as to enhance the current density (the current-driven resistivity) locally near an X line; otherwise, such a vital reconnection flow cannot be caused. The resulting quasisteady fast reconnection mechanism is significantly confined in the z direction, where a strong (Alfvénic) plasma jet results from standing switch-off shocks; accordingly, a large-scale plasmoid is formed and propagates in the middle of the system. It is concluded that the well-known two-dimensional spontaneous fast reconnection model can reasonably be extended to three dimensions. © 1996 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.871789
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  • 256
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    Digitale Medien
    A Vlasov moment description of cyclotron wave–particle interactions (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 1437-1446 
    Details
    ISSN: 1089-7674
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: A quasifluid formalism designed to capture some effects of cyclotron interactions is presented. Starting from the contractions of exact moments of the Vlasov equation, a closure for cyclotron interactions is achieved by using kinetic information directly. This nonperturbative approach does not require a priori assumptions about zeroth-order particle velocity distributions. The nonlinear coupling between field-aligned particle thermal velocities and transverse cyclotron wave and thermal motions are described by off-diagonal elements of the pressure tensor. These elements are related to the growth and damping of cyclotron wave energy. A functional form for an effective wave–particle momentum transport coefficient is derived from the requirement of consistency between the energy and momentum moment equations, but its specific magnitude and sign, determined by threshold temperature anisotropy levels, must be input from kinetic theory. This effective transport coefficient has a nondefinite sign, reminiscent of the gyroviscous coefficients of classical transport, and is consistent with the time reversibility of the Vlasov equation. A coupled set of cyclotron equations of state for the evolution of the parallel and perpendicular pressures are derived. This formalism provides a connection between known kinetic solutions of cyclotron interactions and fluid plasma equations. © 1996 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.871733
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  • 257
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    Digitale Medien
    Anisotropy effects on toroidal ηi stability from collisionless two-fluid theory (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 939-955 
    Details
    ISSN: 1089-7674
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: Ion temperature and temperature gradient anisotropy effects on the toroidal branch of the local ηi instability are analyzed using collisionless two-fluid theory. For isotropic gradients there is good agreement with the Braginskii model for the thresholds even for anisotropic ion temperature, if one averages over the degrees of freedom. But during rapid parallel or perpendicular heating anisotropy in both Ti and ηi is expected to occur. For parallel heating, upper ηi(parallel) thresholds several times the lower threshold occur at moderate Ti(parallel)/Ti⊥ for moderate to large εn, with low growth rates in the intermediate regime. For perpendicular heating lower thresholds are found but are readily finite-Larmor-radius stabilized. Comparison with kinetic theory shows a sizable ηi range for which the instability is non-reactive with low growth rate. Growth rates and thresholds tend to agree better with kinetic theory for k(square root of)Ti⊥/(miωci2)〈0.5 than the Braginskii model. The perturbo-theoretic limitations of both two-fluid theories are delineated. © 1996 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.871799
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  • 258
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    Digitale Medien
    Temperature fluctuations and plasma edge turbulence in the Brazilian tokamak TBR (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 971-977 
    Details
    ISSN: 1089-7674
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: To investigate the tokamak turbulence, a set of Langmuir probes and a triple probe have been designed and used in the TBR [J. Fusion Energy 12, 529 (1993)] to measure average and fluctuating values of density, potential, and temperature of the plasma edge. The obtained results showed a significant influence of the temperature fluctuations in the transport parameters. Namely, taking into account this influence, the density and plasma potential power spectra were obtained, and the turbulence parameters reevaluated. Furthermore, the computed cross-power-spectra showed appreciable linear correlation, and the cross-bispectra showed a quadratic mode coupling between temperature fluctuations and other quantities. Significant bicoherence between these fluctuations was observed. Finally, for a fluctuation monitored at two probe points, no preferential direction for energy cascading was detected. © 1996 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.871802
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  • 259
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    Digitale Medien
    Tokamak configurations with reversed magnetic shear and a peaked current density profile (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 441-443 
    Details
    ISSN: 1089-7674
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: Recent experiments suggest that the reversal of magnetic shear in the central region of a tokamak plasma has beneficial effects on plasma transport. In this Letter, it is shown that a hollow toroidal current density distribution is not a necessary condition for the existence of tokamak equilibria with reversed magnetic shear. © 1996 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.871894
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  • 260
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    Digitale Medien
    Streaming instability in the grain charge fluctuations (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 457-460 
    Details
    ISSN: 1089-7674
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: Highly charged massive dust grains present in a dusty plasma may exhibit charge fluctuations in response to certain types of oscillations incorporated in the plasma. In this situation the grain charge becomes a time-dependent and self-consistent dynamic variable. The consequent modifications in the collective properties of a dusty plasma in response to longitudinal space-charge waves are investigated. It is shown this new effect gives rise to purely damped modes, in addition to causing a collisionless damping of the normal modes. © 1996 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.871897
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  • 261
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    Digitale Medien
    The resistive interchange mode in the presence of equilibrium flow (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 473-481 
    Details
    ISSN: 1089-7674
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: The influence of a sub-Alfvénic equilibrium flow along the magnetic field and a weak fluid viscosity on the resistive gravitational interchange mode is investigated in plane slab geometry. With a uniform resistivity, the current layer is subject to both tearing and interchange so that a degree of mode-mixing is inevitable. In contrast to the tearing mode which is destabilized, sub-Alfvénic flow is shown to have a stabilizing effect on the linear resistive interchange mode. © 1996 American Institute of Physics. [S1070-664(96)00702-9]
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.871873
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  • 262
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    Digitale Medien
    Magnetization and other characteristics of exact relativistic dispersion of circularly polarized radiation in a magnetized plasma (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 468-472 
    Details
    ISSN: 1089-7674
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: A relativistic determination has been made of the zero frequency magnetic moment field [the inverse Faraday effect (IFE)], synchrotron radiation, and other consequences of the exact dispersion relation of a circularly polarized wave propagating in a two-component, cold, magnetized plasma. The relativistically correct Faraday angle of rotation of the plane of polarization of the electromagnetic (EM) wave due to the variation of the dispersion rates of the right- and left-circularly polarized components of the wave has been obtained. For Alfvén wave frequencies (wave frequency less than the ion gyrofrequency), a relativistically correct analysis of the interesting new features which appear due to the predominance of the wave-induced ion dynamics over electron motion is also presented. © 1996 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.871870
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  • 263
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    Digitale Medien
    Transit-time scattering and heating of a relativistic electron beam in strong Langmuir turbulence (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 498-510 
    Details
    ISSN: 1089-7674
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: A Fokker–Planck theory is developed to describe the diffusion in momentum space of a beam of relativistic electrons due to multiple transit-time interactions with an ensemble of coherent Langmuir wave packets. The theory incorporates two ingredients: a perturbed-orbit calculation of the momentum change of a test particle during a single transit-time interaction, and an ensemble average of the resulting Fokker–Planck coefficients based on the statistical properties of strong Langmuir turbulence. An approximate analytic solution of the Fokker–Planck equation is obtained for the case of a strongly collimated beam, and is used to interpret measurements of energy and pitch-angle scattering in relativistic-electron-beam (REB) experiments. Fokker–Planck coefficients are also calculated for a weakly collimated beam. It is shown that the theory correctly predicts the amount of energy scattering in REB experiments, but underestimates the pitch-angle scattering regardless of the distribution of wave packet orientations and the degree of collimation of the beam. This discrepancy may be a product of the approximate wave-packet structure assumed in the analysis, or of systematic errors in the experimental data; alternatively, it may imply that a non-transit-time process is responsible for part of the pitch-angle scattering observed. © 1996 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.871876
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  • 264
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    Thomas–Fermi-like and average atom models for dense and hot matter (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 714-730 
    Details
    ISSN: 1089-7674
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: All the Thomas–Fermi approaches to the thermodynamics and atomic physics properties of dense and ionized matter consisting of a single element are systematically derived and compared within a density-functional theoretical framework. The corresponding results are contrasted to those of the average atom model by using similar approximations for exchange, correlation, and gradient corrections. Emphasis is led on equations of state, ionization, level shifts, and radial moments. The same numerical algorithms are used to unravel similar trends or identify specific ones, in terms of density and temperature variations. The most sophisticated Thomas–Fermi–Dirac–Weizäcker method yields the closest results to the hybrid average atom model using quantized bound states. Parameters ranges of potential interest for inertially confined thermonuclear fusion stress out density in the 0.1–10 times the solid, and temperature up to 10 keV. © 1996 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.871806
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    Creation of a resonant diocotron mode (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 771-780 
    Details
    ISSN: 1089-7674
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: The modal structure for linearized waves in a magnetron or a crossed-field amplifier (CFA) is shown to be very sensitive to the profile of the electron density at the edge of the electron sheath. As is well known, with the classical Brillouin density profile, no propagating diocotron mode can exist. Furthermore, in the parameter regime at which these devices operate (ω=kvd where ω is the frequency, k is the wave vector and vd is the drift velocity at the top of the sheath), there are no unstable modes. However if we replace the Brillouin density profile with a "ramped'' density profile (where the discontinuity is replaced by a finite, but large, negative density gradient), then we can show that any mode in the operating regime will become weakly unstable. However these weakly unstable modes in the presence of the strong density gradient at the edge of the ramped density profile combine to generate a quasilinear diffusion of order unity. Thus, after a time on the order of a few cyclotron periods, the original density profile will become modified. This modification is expected to generate a plateau on the edge of the sheath at which time a resonant propagating diocotron mode can exist. Implications of these results and their predictions for magnetron and CFA operation are discussed. © 1996 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.871777
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  • 266
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    On the conical refraction of hydromagnetic waves in plasma with anisotropic thermal pressure (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 800-803 
    Details
    ISSN: 1089-7674
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: A phenomenon analogous to the conical refraction widely known in crystalo-optics and crystalo-acoustics is discovered for the magnetohydrodynamical waves in a collisionless plasma with anisotropic thermal pressure. Angle of the conical refraction is calculated for the medium under study which is predicted to be 18°26′. Possible experimental corroboration of the discovered phenomenon is discussed. © 1996 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.871780
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  • 267
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    Coalescence of two parallel current loops in a nonrelativistic electron–positron plasma (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 844-852 
    Details
    ISSN: 1089-7674
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: The coalescence of two parallel current loops in an electron–positron plasma is investigated by a three-dimensional electromagnetic relativistic particle code. Instead of mixing uniformly in the dissipation region as observed for current coalescence in an electron–ion plasma, electrons and positrons initially in the loops are driven to move separately by the magnetic gradient drift. Redistribution of the current-carrying electrons and positrons creates new current loops, which coalesce again, if the initial drift velocities remain greater than a critical value after coalescence. It was found that the energy stored in the current loops dissipates gradually through several coalescences. Consequently, the electrons and positrons near the current loops are heated through the coalescence. This process is qualitatively different from the explosive energy release during coalescence in an electron–ion plasma. © 1996 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.871784
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  • 268
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    Compressibility and rotation effects on transport suppression in magnetohydrodynamic turbulence (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 889-900 
    Details
    ISSN: 1089-7674
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: Compressibility and rotation effects on turbulent transports in magnetohydrodynamic (MHD) flows under arbitrary mean field are investigated using a Markovianized two-scale statistical approach. Some new aspects of MHD turbulence are pointed out in close relation to plasma compressibility. Special attention is paid to the turbulent electromotive force, which plays a central role in the generation of magnetic and velocity fluctuations. In addition to plasma rotation, the interaction between compressibility and magnetic fields is shown to bring a few factors suppressing MHD fluctuations and, eventually, density and temperature transports, even in the presence of steep mean density and temperature gradients. This finding is discussed in the context of the turbulence-suppression mechanism in the tokamak's high-confinement modes. © 1996 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.871792
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  • 269
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    Solitary kinetic Alfvén waves on the ion-acoustic velocity branch in a low-β plasma (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 4304-4306 
    Details
    ISSN: 1089-7674
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: Starting with an exact nonlinear equation governing solitary kinetic Alfvén waves (SKAWs), a new type of SKAW on the ion-acoustic velocity branch is obtained in a low-β plasma. It propagates in a direction oblique to the ambient magnetic field, has a dispersion originating from the finite perpendicular wavelength, and is accompanied by a density hump soliton similar to that of the SKAWs on the Alfvén velocity branch in the case of 1(very-much-greater-than)β(very-much-greater-than)me/mi. It has a perturbed electric field with the same order as that of the latter SKAWs, but its perturbed magnetic field is weaker by a factor of (β/2)1/2, and in the small amplitude limit, the analytical result also leads to the Korteweg–de Vries soliton. © 1996 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.872047
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  • 270
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    A three-circulation theorem for relativistic plasmas (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 482-489 
    Details
    ISSN: 1089-7674
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: It is shown that the sum of three circulations (hydrodynamic, magnetic, and thermal) is constant in time for each component of a perfect relativistic plasma. A constraint for the initial configuration ("Weber/Ertel case,'' the lines of canonical vorticity lie on surfaces of constant entropy) leads to zero potential vorticity also for later times; this is generally realized in inhomogeneous flow equilibria for which the explicit time-dependence of Clebsch potentials is given. Equilibrium states of warm relativistic plasmas with vanishing thermal circulation are discussed. © 1996 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.871874
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  • 271
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    Interpretation of ion cyclotron emission from sub-Alfvénic fusion products in the Tokamak Fusion Test Reactor (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 543-553 
    Details
    ISSN: 1089-7674
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: Ion cyclotron emission (ICE) has been observed during neutral beam-heated supershots in the Tokamak Fusion Test Reactor (TFTR) [Phys. Rev. Lett. 72, 3526 (1994)] deuterium–tritium campaign at fusion product cyclotron harmonics. The emission originates from the outer midplane edge plasma, where fusion products initially have an anisotropic velocity distribution, sharply peaked at a sub-Alfvénic speed. It is shown that the magnetoacoustic cyclotron instability, resulting in the generation of obliquely propagating fast Alfvén waves at fusion product cyclotron harmonics, can occur under such conditions. The time evolution of the growth rate closely follows that of the observed ICE amplitude. Instability is suppressed if the fusion products undergo a moderate degree of thermalization, or are isotropic. In contrast, the super-Alfvénic fusion products present in the outer midplane of the Joint European Torus (JET) [Nucl. Fusion 33, 1365 (1993)] can drive the instability if they are isotropic or have a broad speed distribution. This may help to account for the observation that fusion product-driven ICE in JET persists for longer than fusion product-driven ICE in TFTR supershots.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.871881
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  • 272
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    Detection of fusion neutrons from a deuterium-proton mixed plasma in the GAMMA 10 Tandem Mirror (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 578-583 
    Details
    ISSN: 1089-7674
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: Neutron diagnostics are applied to a deuterium plasma mixed with protons in the central cell of the GAMMA 10 tandem mirror [Inutake et al., Phys. Rev. Lett. 55, 939 (1985)]. The deuterium ions are heated with a slow ion cyclotron wave tuned to the fundamental deuterium resonance near the mid-plane of the central cell while the plasma is sustained with the fundamental resonance heating of protons in the minimum-B anchor cells. The measurement is based on in situ calibration to determine the transmission efficiency through machine walls and the counting efficiency of the detection system for neutrons emitted from 252Cf with energy close to that in the deuterium–deuterium (D–D) fusion reaction. The observation shows that the count rate of neutrons increases with diamagnetism, and this relation is accounted for in terms of fusion reaction between deuterium ions with a transverse temperature exceeding 10 keV. Discrepancies among ion temperatures determined with different diagnostics are mostly attributed to insufficient knowledge of the profiles of plasma parameters. The results indicate that the neutron measurement can be added as a powerful diagnostic tool for hot ions if combined with more detailed profile measurements. © 1996 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.871885
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  • 273
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    High-frequency core localized modes in neutral beam heated plasmas on TFTR (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 593-605 
    Details
    ISSN: 1089-7674
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: A band of high-frequency modes in the range 50–150 kHz with intermediate toroidal mode numbers 4〈n〈10 are commonly observed in the core of supershot plasmas on TFTR [R. Hawryluk, Plasma Phys. Controlled Fusion 33, 1509 (1991)]. Two distinct varieties of magnetohydrodynamic (MHD) modes are identified, corresponding to a flute-like mode predominantly appearing around the q=1 surface and an outward ballooning mode for q(approximately-greater-than)1. The flute-like modes have nearly equal amplitude on the high-field and low-field side of the magnetic axis, and are mostly observed in moderate performance supershot plasmas with τE〈2τL, while the ballooning-like modes have enhanced amplitude on the low-field side of the magnetic axis and tend to appear in higher performance supershot plasmas with τE(approximately-greater-than)2τL, where τL is the equivalent L-mode confinement time. Both modes appear to propagate in the ion diamagnetic drift direction and are highly localized with radial widths Δr∼5–10 cm, fluctuation levels ñ/n, T˜e/Te〈0.01, and radial displacements ξr∼0.1 cm. Unlike the toroidally localized high-n activity observed just prior to major and minor disruptions on TFTR [E. D. Fredrickson et al., Proceedings of the 15th International Conference on Plasma Physics and Controlled Nuclear Fusion Research, Seville, Spain (International Atomic Energy Agency, Vienna, 1995), No. IAEA-CN-60/A-2-II-5], these modes are typically more benign and may be indicative of MHD activity excited by resonant circulating beam ions. © 1996 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.871887
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  • 274
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    A hybrid Eulerian Vlasov code. I. Study of high-frequency beatwave experiment and Manley–Rowe action evolution in a finite causal system (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 650-668 
    Details
    ISSN: 1089-7674
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: For the first time, beatwave simulations relevant to the UCLA experiment (University of California at Los Angeles) [see Clayton et al., Phys. Rev. Lett. 70, 37 (1993), and also Phys. Plasmas 1, 1753 (1994)] have been made with a relativistic Eulerian Hilbert–Vlasov code for a realistically high ratio of driver frequency to plasma wave frequency (ω0/ωp≈30). Some of the more striking features that have emerged from the Hilbert–Vlasov simulations are discussed in this paper, with particular emphasis on particle dynamics in phase space with beam injection, and action transfer results obtained from the derivation of the integrated Manley–Rowe relations derived for a finite causal system. © 1996 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.871892
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  • 275
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    Lower-hybrid-like wave in a dusty plasma with charge fluctuation (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 694-695 
    Details
    ISSN: 1089-7674
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: The dispersion relation for the case of a lower-hybrid-like wave in a dusty plasma is obtained taking into account the charge fluctuation. It is found that charge fluctuation terms lead to a novel dissipation of the lower-hybrid-like modes in a dusty plasma. © 1996 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.871903
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  • 276
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    Scrape-off layer profile modifications by convective cells (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 699-701 
    Details
    ISSN: 1089-7674
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: Convective cells (CC's) are important in understanding density profile modifications induced by ion cyclotron range of frequencies (ICRF) antennas. This has motivated the present work in which the effect of CC's on transport in the scrape-off layer is studied, in the regime where the density gradient scale length Ln and the cell size L are comparable. Monte Carlo simulations show that closed cell convection acts to flatten the density profile, and that open cells enhance the particle flow to the wall, depleting the density and yielding profiles similar to those measured near ICRF antennas. A new one-dimensional, two-branch model of CC transport is shown to agree well with the simulations. The model gives rise to two characteristic scale lengths, only one of which is retained in the enhanced diffusion models that are applicable for Ln(very-much-greater-than)L. The two-branch model is expected to be useful in analyzing ICRF experiments. © 1996 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.871871
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  • 277
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    Tearing mode with strong flow shear in the viscosity-dominated limit (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 4301-4303 
    Details
    ISSN: 1089-7674
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: Using a standard boundary layer approach, the tearing mode with shear flow comparable with shear magnetic field and fluid viscosity much larger than resistivity has been studied analytically. The results show that, the growth rate in this case scales as ν2/3ν, where νν is the normalized viscosity, in agreement with the numerical results of Einaudi and Rubini [Phys. Fluids B 1, 2224 (1989)]. It is found analytically that large viscosity may have a destabilizing effect on the instability. © 1996 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.872045
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  • 278
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    Shock waves in plasmas containing variable-charge impurities (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 4313-4315 
    Details
    ISSN: 1089-7674
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: Shock structures in plasmas containing variable-charge macro particles are shown to exist because of an effective dissipation associated with charging of the latter. © 1996 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.872048
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  • 279
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    Fast surface waves in an ideal Hall-magnetohydrodynamic plasma slab (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 4346-4354 
    Details
    ISSN: 1089-7674
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: The propagation of fast sausage and kink magnetohydrodynamic (MHD) surface waves in an ideal magnetized plasma slab is studied taking into account the Hall term in the generalized Ohm's law. It is found that the Hall effect modifies the dispersion characteristics of MHD surface modes when the Hall term scaling length is not negligible (less than, but comparable to the slab thickness). The dispersion relations for both modes have been derived for parallel propagation (along the ambient equilibrium magnetic field lines).The Hall term imposes some limits on the possible wave number range. It turns out that the space distribution of almost all perturbed quantities in sausage and kink surface waves with Hall effect is rather complicated as compared to that of usual fast MHD surface waves. The applicability to solar wind aspects of the results obtained, is briefly discussed. © 1996 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.872050
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  • 280
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    Ion extraction from plasma by using a radio frequency resonant electric field (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 4367-4375 
    Details
    ISSN: 1089-7674
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: In order to raise an ion extraction efficiency from a plasma, a new method using an rf field has been proposed and demonstrated. The resonant frequencies of the rf field were theoretically evaluated to excite the eigenwave of the plasma. The lower frequency of the two plasma-sheath resonances under the magnetic field was selected because it has hardly any dependence on the plasma density when the density is over a critical value. Verification of this method was carried out using Xe discharge plasma (electron density, 1×1016 m−3; electron temperature, 8 eV) between the parallel plate electrodes (length, 0.5 m). The resonance was found at about 10 MHz, which agreed with the theoretical result. The ion current at the resonance was anisotropic and was twice as large as the ion saturated current, which is the limiting value of the conventional electrostatic method. © 1996 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.872053
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  • 281
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    Cross field diffusion and transport in a relativistic and beam-driven magnetoplasma: Collisional kinetics (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 4410-4414 
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    ISSN: 1089-7674
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: Streaming or beam-driven kinetic theory is formulated for a relativistic and collisional plasma diffusing across magnetic field lines, including the small density and temperature gradient. Explicit formulas for modified transport coefficients are presented in various thermal regimes of interest. Their dependence on streaming velocity (V0) is discussed qualitatively. © 1996 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.872058
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  • 282
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    Localized whistler eigenmodes in tokamaks (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 4102-4105 
    Details
    ISSN: 1089-7674
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: The existence of radially well-localized whistler eigenmodes (WE) is shown. The location of WE modes may vary from the region close to the plasma edge to the central region, depending on the ratio of poloidal and toroidal wave numbers and on the plasma density profile. © 1996 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.871542
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  • 283
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    Role of neutrals in the phase transition model (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 4106-4114 
    Details
    ISSN: 1089-7674
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: The phase transition model for the transition from the low-confinement regime (L mode) to high-confinement regime (H mode) is extended to incorporate the effect of the neutral particles on the transition threshold. For usual edge plasma parameters, the increase of effective poloidal viscosity through charge exchange damping and reduction of the effective fluxes by both ionization and charge exchange increases the threshold power required for the transition. For plasmas with the effective energy flux smaller than the convective energy flux, the transition may be triggered by a neutral influx. © 1996 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.871543
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  • 284
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    Modeling of parasitic current collection by solar arrays in low-earth orbit (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 4181-4189 
    Details
    ISSN: 1089-7674
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: In this paper we describe the development of a model of the electron current collected by solar arrays from the ionospheric plasma. This model will assist spacecraft designers in minimizing the impact of plasma interactions on spacecraft operations as they move to higher-voltage solar arrays. The model was developed by first examining in detail the physical processes of importance and then finding an analytic fit to the results over the parameter range of interest. The analytic model is validated by comparison with flight data from the Photovoltaic Array for Space Power Plus diagnostics (PASP Plus) flight experiment [D. A. Guidice, 34th Aerospace Sciences Meeting and Exhibit, Reno, NV, 1996, AIAA 96-0926 (American Institute of Aeronautics and Astronautics, Washington, DC, 1996)]. © 1996 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.871550
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  • 285
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    Dynamic modeling of magnetically insulated transmission line systems (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 4207-4219 
    Details
    ISSN: 1089-7674
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: Negative conductors in vacuum transmission lines used in multiterrawatt applications emit electrons freely. These lines are efficient only because the self-magnetic field of the power flow forces the electrons to flow parallel to the electrodes. Excepting numerical simulations, dynamic modeling of systems of these transmission lines has generally either ignored electron flow, or has included only those electrons that cross immediately to the anode at the front of the forward wave. In this paper we describe an analytic model that includes flowing electrons and the effects of these flows on line voltage and on the reduction of magnetic flux. Axial electron currents are modeled using simple, measurable, and calculable parameters. Transverse electron currents are modeled using general patterns found empirically from simulation data. These currents are in turn related by an expanded set of Telegrapher equations. An example of the use of the model is compared to two-dimensional, time-dependent particle-in-cell simulations. © 1996 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.871553
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  • 286
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    Comparisons of experimental measurements and two-dimensional plasma-fluid numerical simulations of a magnetized plasma column (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 4250-4267 
    Details
    ISSN: 1089-7674
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: The density, the electron, ion, and neutral temperatures in dense (ne(approximately-greater-than)1013 cm−3) and cool (Te〈8 eV) magnetized helium plasma columns submerged in a cool neutral gas (n0∼3×1015 cm−3) environment have been measured. These plasmas, of similar collisionality to those expected in the scrape-off layer of ITER (International Thermonuclear Experimental Reactor) [R. Parker, Contrib. Plasma Phys. 34, 422 (1994)], are found to obey the low-recycling behavior of approximate plasma pressure balance along the column axis. Density decreases by a factor of about 2 and Te remains isothermal. Computer simulations using the B2 [R. Schneider et al., J. Nucl. Mater. 196–198, 810 (1992)] plasma-fluid code have been performed and are in good agreement with experimental results. Measured ion temperatures are consistent with classical energy transfer from the electrons. Neutrals within the plasma are heated by elastic and charge-exchange collisions. This set of measurements represents the most detailed comparisons between experimental results and B2 predictions. © 1996 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.871555
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  • 287
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    Drift-orbit-driven flux and plasma viscosity in asymmetric toroidal plasmas (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 4276-4277 
    Details
    ISSN: 1089-7674
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: It is shown, based on the kinetic definition of the drift-orbit-driven flux, that the drift-orbit-driven flux is related to either the toroidal viscosity, or the poloidal viscosity, or a combination of both. This result is the same as that obtained from the less direct moment equation approach [Phys. Fluids 26, 3315 (1983)]. © 1996 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.871557
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  • 288
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    A Lagrangian study of dynamics and singularity formation at magnetic null points in ideal three-dimensional magnetohydrodynamics (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 4281-4283 
    Details
    ISSN: 1089-7674
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: The ideal three-dimensional incompressible magnetohydrodynamics equations are analyzed at magnetic null points using a generalization of a method from fluid dynamics. A closed system of ordinary differential equations governing the evolution of traces of matrices associated with the fluid velocity and magnetic field gradients are derived using a model for the pressure Hessian. It is shown rigorously that the eigenvalues of the magnetic field gradient matrix are constant in time and that, in the model, a finite time singularity occurs with characteristics similar to the magnetic field-free case. © 1996 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.871559
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  • 289
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    Comment on "Three-dimensional stability of drift vortices'' [Phys. Plasmas 3, 160 (1996)] (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 4290-4291 
    Details
    ISSN: 1089-7674
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: Three-dimensional stability of drift vortices, in which the three-dimensionality is caused by a coupling of the vortex to ion-acoustic waves that propagate along the magnetic field, has recently been studied [Phys. Plasmas 3, 160 (1996)]. It is the purpose of the present Comment to draw attention to an important body of numerical results on this problem published in Plasma Physics 18, 187 (1976). © 1996 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.871562
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  • 290
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    Leaky electromagnetic wave resonances of a plasma sphere (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 3540-3544 
    Details
    ISSN: 1089-7674
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: In the paper we consider theoretically electromagnetic modes in a homogeneous cold plasma sphere. We find, in addition to the surface transverse magnetic modes, known from the electrostatic analysis of the same structure, two new families of weakly leaky modes (one of them consisting of transverse electric modes). A classification of the modes is proposed. An approximate analytic expression for the wave damping rate of the surface modes due to energy leakage is given, making it possible to draw conclusions on the behavior of more complex three-dimensional plasmas. © 1996 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.871944
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  • 291
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    Particle-in-cell simulations of fast magnetic field penetration into plasmas due to the Hall electric field (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 3556-3563 
    Details
    ISSN: 1089-7674
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: Particle-in-cell (PIC) simulations are used to study the penetration of magnetic field into plasmas in the electron-magnetohydrodynamic (EMHD) regime. These simulations represent the first definitive verification of EMHD with a PIC code. When ions are immobile, the PIC results reproduce many aspects of fluid treatments of the problem. However, the PIC results show a speed of penetration that is between 10% and 50% slower than predicted by one-dimensional fluid treatments. In addition, the PIC simulations show the formation of vortices in the electron flow behind the EMHD shock front. The size of these vortices is on the order of the collisionless electron skin depth and is closely coupled to the effects of electron inertia. An energy analysis shows that one-half the energy entering the plasma is stored as magnetic field energy while the other half is shared between internal plasma energy (thermal motion and electron vortices) and electron kinetic energy loss from the volume to the boundaries. The amount of internal plasma energy saturates after an initial transient phase so that late in time the rate that magnetic energy increases in the plasma is the same as the rate at which kinetic energy flows out through the boundaries. When ions are mobile it is observed that axial magnetic field penetration is followed by localized thinning in the ion density. The density thinning is produced by the large electrostatic fields that exist inside the electron vortices which act to reduce the space-charge imbalance necessary to support the vortices. This mechanism may play a role during the opening process of a plasma opening switch. © 1996 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.871946
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  • 292
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    Stability of magnetic vortices with flow in anisotropic magnetohydrodynamics (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 3583-3590 
    Details
    ISSN: 1089-7674
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: The eigenvalue problem for linear stability of concentric radial profiles of current and vorticity in reduced forms of three-dimensional magnetohydrodynamics is solved numerically. Arbitrary relative amplitudes of the velocity and magnetic fields are considered. Vorticity profiles are unstable if nonmonotonic, but are stabilized by a poloidal magnetic field when the on-axis vertical current is at least as large as the on-axis vertical vorticity. Nonmonotonic current profiles are less efficient at stabilization. When the neutral modes have vertical structure, an added poloidal magnetic field does not stabilize the mode unless the vertical field is also moderately strong. Current profiles in which the integrated current changes sign, although spectrally stable, are shown to be nonlinearly unstable via both numerical solution and Lyapunov techniques. © 1996 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.871949
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  • 293
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    A self-consistent model for low-high transitions in tokamaks (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 3701-3712 
    Details
    ISSN: 1089-7674
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: A system of equations that couples the rapidly varying fluctuations of resistive ballooning modes to the slowly varying transport of the density, vorticity and parallel momentum have been derived and solved numerically. Only a single toroidal mode number is retained in the present work. The low-mode (L-mode) phase consists of strong poloidally asymmetric particle transport driven by resistive ballooning modes, with larger flux on the outboard side compared to the inboard side. With the onset of shear flow driven by a combination of toroidal drive mechanisms as well as the Reynolds stress, the fluctuations associated with the resistive ballooning modes are attenuated leading to a strong reduction in the particle transport. The drop in the particle transport results in steepening of the density profile leading to the high-mode (H-mode). © 1996 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.871520
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  • 294
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    Electron temperature measurements from optically gray third harmonic electron cyclotron emission in the DIII-D tokamak (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 3725-3731 
    Details
    ISSN: 1089-7674
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: In many high-performance discharges in the DIII-D tokamak [J. L. Luxon and L. G. Davis, Fusion Technol. 8, 441 (1985)] high electron density prevents determining the electron temperature from the second harmonic electron cyclotron emission (ECE). A technique for obtaining central electron temperatures from optically gray third harmonic ECE is presented that does not require knowledge of the reflective properties of the vessel wall. The temperature values derived from ECE spectra measured with an absolutely calibrated Michelson interferometer agree with independent measurements by Thomson scattering. As part of this work, a method of determining the optical depth of third harmonic frequencies in a low aspect ratio tokamak is also demonstrated. The optical depth measurements are in agreement with calculations correct to first order in the Larmor radius. © 1996 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.871506
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  • 295
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    Analysis of the beam bunching produced by a free electron laser (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 4197-4206 
    Details
    ISSN: 1089-7674
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: A high-power microwave Free Electron Laser (FEL) experiment has been performed to study the inherent electron beam bunching that occurs in the FEL interaction. An induction linac delivered a 1 kA, 2.2 MeV electron beam to a FEL amplifier. In the process of amplifying the input signal of frequency 35 GHz to power levels of the order of 10 MW, the electron beam was bunched by the ponderomotive force. In this paper we describe the FEL experiment and the optical technique that was used to observe the bunches. Their properties as functions of the basic FEL parameters are discussed. © 1996 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.871552
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  • 296
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    Effects of secondary electrons due to ionization on model predictions of discharge-pumped XeCl laser (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 4225-4228 
    Details
    ISSN: 1089-7674
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: A self-consistent code has been utilized in modeling a discharge-pumped XeCl laser. The electron energy distribution function (EEDF) has been calculated using a time-dependent Boltzmann equation. The effects of the secondary electrons produced by ionization on EEDF, rate coefficients, species densities, laser energy, and breakdown delay time have been examined by comparing different models in a wide range of discharge parameters. Efforts have been made at explaining the discrepancy in the breakdown delay time between simulation and experiment. It is found that the secondary electrons due to ionization play an important role in determining the breakdown delay time. Furthermore, the measured breakdown delay time can be well reproduced using the model presented in this work instead of increasing the total excitation cross sections of xenon. © 1996 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.871517
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  • 297
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    An investigation into the role that a transverse magnetic field plays in the formation of large anode sheath potentials (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 4239-4249 
    Details
    ISSN: 1089-7674
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: A 9.25 A low-pressure (45–55 mTorr) hollow cathode arc discharge has been used to simulate plasma processes that occur at the anode of magnetoplasmadynamic accelerators used for space propulsion applications. The interest in the near-anode region is related to findings of past research, which indicate that large anode sheath potentials can drive as much as 70% of the input electrical power into the anode, thus degrading thrust efficiency. Presented here are results that essentially characterize the behavior of the near-anode plasma as a function of a transverse magnetic field. Plasma diagnostics included single Langmuir probe techniques, emission spectroscopy, and water calorimetry for anode heat flux measurements. Phenomenological arguments based on measurements taken suggest that observed changes in anode fall voltage are related to variations in the measured local electron number density as the magnetic field is varied. This behavior is attributed to the variations in the measured ionization rate, which is shown to be a nonlinear function of transverse magnetic field. © 1996 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.871519
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  • 298
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    Lower-hybrid wave collapse (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 133-144 
    Details
    ISSN: 1089-7674
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: The formation, collapse and arrest of lower-hybrid wave packets are investigated analytically. The three-dimensional structure of the wave packet is incorporated in the analysis and its polarization is studied for the first time. Nonlinear collapse thresholds are obtained via a Hamiltonian formulation and are used in calculating the probability distribution of collapsing wave packet structures as a function of their polarization. Transit-time interaction theory is then used to calculate the arrest scale at which collapse is halted as the waves are damped. It is found that collapse thresholds are lowest for circularly polarized packets, but that nearly linearly polarized ones predominate in collapse because of their greater numbers in the linear phase of the evolution. It is argued that subsonic collapse persists until very near arrest, in accord with recent numerical simulations. Time scale analysis shows that the parallel field structure has difficulty in attaining its self-similar form in the available collapse time, also in accord with simulations. Transit-time theory implies that electrons travelling roughly parallel to the ambient magnetic field can arrest collapse at a scale comparable to that previously estimated for ions; which process dominates depends on the electron and ion temperatures and packet geometry. The resulting arrest scales are found to be in accord with the simulations. © 1996 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.871839
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  • 299
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    Three-dimensional stability of drift vortices (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 160-167 
    Details
    ISSN: 1089-7674
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: The three-dimensional stability of drift vortices that are two-dimensionally stable in the framework of the Hasegawa–Mima equation is studied. The three-dimensionality is caused by a coupling of the vortex to ion-acoustic waves that propagate along the magnetic field. It is concluded that this coupling does not significantly destabilize the vortex. A general stability criterion is derived that guarantees stability if the ratio between the parallel wave number k and the azimuthal mode number m is large enough. It is also found that no instability exists in the limit k→0 if the potential vorticity is a strictly decreasing function of r. Such profiles are typical for vortices that are two-dimensionally stable. Some particular profiles where the potential vorticity vanishes outside some radius are also examined. In one case an instability with m=1 is found, but a numerical solution of the eigenvalue problem shows that the maximum growth rate is very small, three orders of magnitude smaller than the angular velocity of the vortex flow. © 1996 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.871842
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  • 300
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    Weakly relativistic solitons in a cold plasma with electron inertia (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 145-148 
    Details
    ISSN: 1089-7674
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: Ion-acoustic solitons have been investigated in a cold plasma in the presence of electron inertia through the derivation of the Korteweg–de Vries (KdV) equation taking into account of weakly relativistic effects. Interestingly, relativistic solitons of both compressive and rarefactive characters are found to exist at the negligible difference of u0/c and v0/c (u0, v0 being the initial speeds of streaming electrons and ions respectively, and c, the velocity of light) of the order 1×10−7. © 1996 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.871840
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