Bibliothek

Notizen: Thomson scattering measurements performed in the divertor of the DIII-D tokamak [Plasma Physics Controlled Nuclear Fusion Research, 1986 (International Atomic Energy Agency, Vienna, 1987), Vol. I, p. 159] during detached operation show that the electron temperatures are typically between 0.8 and 2.0 eV throughout a region which may extend several centimeters above the target plate. At such low temperatures the excitation of recycling deuterium atoms or impurities should be weak. Nevertheless, significant radiation is frequently detected in these locations. It has been suggested that recombination processes, which become important only below about 1.5 eV for deuterium, are responsible for the observed emission. This hypothesis has been investigated by comparing ratios of deuterium lines from attached and detached plasmas with theoretical ratios expected for ionizing or recombining conditions. The analysis of several discharges indicates that the mechanism for production of the emission changes from being collisional excitation of atomic deuterium to a mixture of collisional-radiative recombination and collisional excitation as plasmas evolve from attached to detached states. Localization of D-α emission to low-temperature regions using tangentially viewing camera data together with Thomson scattering results and measurements of deuterium atom temperatures are consistent with these conclusions. © 1997 American Institute of Physics.

Notizen: In an experiment relevant to the fast ignitor fusion concept, a preformed plasma simulating the corona of a fusion target has been probed with laser intensities exceeding 1018 W/cm2. It is seen that the f number of the probe beam is increased as it propagates through the target plasma—a channeling of the power. The transmission and backscatter of the probe beam have been examined, and the analysis suggests anomalous absorption of laser light in shorter scale length plasmas. © 1997 American Institute of Physics.

Notizen: Absorption-spectroscopy measurements using an embedded titanium layer have been performed on laser implosions of spherical shells. Predicted absorption features formed by the titanium layer were observed: absorption lines due to 1s–2p transitions in titanium ions of incomplete L shell, as well as K-edge absorption in cold titanium. Also observed were oscillations due to EXAFS (Extended X-Ray Absorption Fine Structure) above the Ti K edge. The core temperature and shell areal density and temperature are derived from the measurements and are found to fall short of the values predicted by one-dimensional code simulations. This shortfall is attributed to the lack of irradiation symmetry, which leads to hydrodynamic instability and mixing (smoothing by phase plates was not used in this experiment). Signatures of mixing include the emission of the He-α line of Ti+20 due to titanium migrating to the core, as well as the EXAFS spectrum indicating cold titanium close to peak compression.

Notizen: Extensive simulations are performed to investigate effects of electron cyclotron instabilities on the gyrotron beam quality, using two-dimensional axisymmetric particle-in-cell (PIC) codes. Both electrostatic and electromagnetic models, as well as realistic geometries of the gyrotron, are considered. It is found that a large beam density can lead to an electrostatic-instability-induced energy spread which substantially degrades the gyrotron efficiency. © 1997 American Institute of Physics.

Notizen: A self-consistent, two-dimensional analysis is presented on confining a region of neutral plasma with a Penning/Malmberg type plasma trap using a nested well configuration. It is found that a neutral plasma region having disparate electron and ion temperatures or having high charge state ions can be confined with static fields. For confining a neutral region comprised of electrons and equal temperature low charge state ions, a quasistatic approach appears promising. © 1997 American Institute of Physics.

Notizen: The Vlasov fluid model is used to study the m=0 and m=1 internal and free boundary modes in a collisionless, large Larmor radius Z pinch. Two methods (initial value and variational) are employed, and give good agreement. The growth rate can be reduced from its zero Larmor radius value by a factor of up to 10 for m=1, and up to 3 for m=0. Stability thresholds and the role of resonant ions are discussed. © 1997 American Institute of Physics.

Notizen: Based on the resolution of the partial differential equation describing the external excitation of the Bohm–Gross longitudinal wave, analytical expressions for causal responses of a dissipative macroscopic plasma are derived. Both impulsive and harmonic solutions representing the spatial Green's functions of the radiation problem are given. These exact responses of the plasma, expressed in terms of two-variable Lommel functions, are then used to gain some better understanding of the excitation and dynamics of the well-known thermal wave. Special attention is paid to the resonant excitation case. Intrinsic characteristics of the secular behavior of the radiated signal are illustrated and analyzed. It is shown that the proffered algebraic solutions constitute a generalization of previous results inferred from an asymptotic representation of the Green's functions, or from the familiar steady state harmonic approach. © 1997 American Institute of Physics.

Notizen: Experiments on the excitation of ion acoustic solitons using a fine mesh grid in a normal two component plasma are described. The excitation is novel in that a modulated high-frequency sinusoidal wave voltage signal is applied to the grid. The carrier frequency of the high-frequency sinusoidal wave is above the ion plasma frequency. An interpretation of the velocity modulation and bunching of free streaming ions that pass through the grid to which the signal is applied is given. © 1997 American Institute of Physics.

Notizen: An enhancement of the velocity diffusion over the quasilinear value is observed in the regime where the autocorrelation time is much smaller than the linear growth time or resonance broadening time. The diffusion enhancement occurs when the resonance broadening time is small compared with the linear growth time. These simulations are self consistent and have enough modes to be in the continuous spectrum limit. That is, even at the initial amplitudes the intermode spacing is sufficiently small that the resonance overlap parameter is large. A possible mechanism for the enhanced diffusion (spontaneous spectrum discretization) is discussed. © 1997 American Institute of Physics.

Notizen: Significant recombination of the majority ion species has been observed in the divertor region of Alcator C-Mod [I. H. Hutchinson et al., Phys. Plasmas 1, 1511 (1994)] under detached conditions. This determination is made by analysis of the visible spectrum from the divertor, in particular the Balmer series line emission and the observed recombination continuum, including an apparent recombination edge at ∼375 nm. The analysis shows that the electron temperature in the recombining plasma is 0.8–1.5 eV. The measured volume recombination rate is comparable to the rate of ion collection at the divertor plates. The dominant recombination mechanism is three-body recombination into excited states (e+e+D+⇒D0+e), although radiative recombination (e+D+⇒D0+hν) contributes ∼5% to the total rate. Analysis of the Balmer series line intensities (from nupper=3 through 10) shows that the upper levels of these transitions are populated primarily by recombination. Thus the brightnesses of the Balmer series (and Lyman series) are directly related to the recombination rate. © 1997 American Institute of Physics.

Notizen: It is argued that the equilibrium state of a wiggler background magnetoplasma corresponds to a zero momentum rather than to a plasma acquiring a non-zero velocity. (AIP)

Notizen: Jung's analysis of dynamics screening appears to be flawed by attempting to patch many-body effects into a single-body theory and a more rigorous foundation is necessary. Jung's result and dynamical screening have been previously analyzed many times.(AIP)

Notizen: W. M. Stacey presents stability criteria for the thermal instability based on a linear one-dimensional (1D) slab model including the effects of ionization recombination [Phys. Plasmas 3, 2673 (1996)], large radial gradients and rotation [Phys. Plasmas 3, 3032 (1996)] and external edge heating [Phys. Plasmas 4, 134 (1997)]. The second paper is incorrectly presented as a two-dimensional (2D) analysis. The hypothesis for the poloidal asymmetry of the multifaceted asymmetric radiation from the edge (MARFE) is based on a misinterpretation of the linear results and contradicted by our own linear analyses in a 1D cylindrical [Phys. Plasmas 1, 2623 (1994)] and a 2D tokamak model [Plasma Phys. Controlled Fusion 39, 423 (1997)]. The explanation for the evolution into a detached plasma and for MARFE suppression by external edge heating are interesting hypotheses, supported by experimental observations. However, a 1D linear analysis does not offer the basis to support these hypotheses theoretically.© 1997 American Institute of Physics.

Notizen: An analysis of ion cyclotron resonance heating in a plasma containing two ion species is given. Particular emphasis is placed on the important case of comparable concentrations of the two species. The properties of ion-Bernstein waves in such a plasma are discussed. A summary of the cutoff–resonance (Budden) and cutoff–resonance–cutoff (triplet) configurations is given and an analytic expression obtained for the Budden transmission coefficient (relevant also to the triplet case), which is valid for any concentration ratio of the two-ion species. A formula for the damping coefficient of the ion hybrid wave is obtained and a ray tracing code is used to illustrate the absorption of this mode by electrons close to the two-ion hybrid resonance. The reduced, second-order fast wave equation is solved numerically, illustrating the transition from strong electron heating through mode conversion at the two-ion hybrid resonance to direct electron damping of the fast wave in a high beta plasma.

Notizen: This is an extension of a study of electric currents and force balance orthogonal to a unidirectional magnetic field of induction (B) in a collisionless plasma [K. D. Cole, Phys. Plasmas 3, 2717 (1996)]. All quantities are assumed to vary only in one direction orthogonal to B. Two new concepts are introduced, which are not in conventional plasma physics. First a magnetic polarization effect corresponding to electric polarization is found. This defines a contribution to the magnetic moment per unit volume of −ρmc2E2/B3. Second the conduction current is defined in terms of the curl of the magnetic field intensity H, in accordance with Maxwell. In this scheme, when there is no electric field, (1/4π)(∂H/∂z)B+(∂p⊥/∂z)=0, instead of the conventional equation with B/μ0 in place of H. In both schemes, j×B−(bold del)p⊥=0. Though this makes no difference to the electric current intensity calculated in the two schemes, and no difference to the force balance equation, it makes a significant difference to the relationship between B and p⊥ in many plasma conditions of interest. When the electric field is nonzero, the situation is more complex. It is shown that the new diamagnetism of this paper and the dielectric current of the earlier one are consistent with special relativity. Some applications of the theory are discussed. © 1997 American Institute of Physics.

Notizen: A three-dimensional nonlinear Kadomtsev–Petviashvili (K–P) wave equation is derived to discuss the solitons in multicomponent plasma contaminated with negative ions. A new formalism of the simple wave solution method is developed for finding the soliton behaviors caused by the presence of negative ions in plasmas. It is seen that the nonlinear wave equation leads, in some cases, to an ordinary differential equation and a straight way for solving the soliton propagation in plasmas. The overall observations describe the natures of compressive and rarefactive solitons along with the shock-like wave caused by the interaction of negative ions. Also discussed are the possible controls of the scenarios of soliton behaviors. Moreover it is believed, from present investigations, that the observations of collapses or explosions in solitons could enhance the understanding of the soliton phenomena in laboratory and space plasmas. © 1997 American Institute of Physics.

Notizen: Resistive drift-wave turbulence in a slab geometry is studied by statistical closure methods and direct numerical simulations. The two-field Hasegawa–Wakatani (HW) fluid model, which evolves the electrostatic potential and plasma density self-consistently, is a paradigm for understanding the generic nonlinear behavior of multiple-field plasma turbulence. A gyrokinetic derivation of the HW model is sketched. The recently developed Realizable Markovian Closure (RMC) is applied to the HW model; spectral properties, nonlinear energy transfers, and turbulent transport calculations are discussed. The closure results are also compared to direct numerical simulation results; excellent agreement is found. The transport scaling with the adiabaticity parameter, which measures the strength of the parallel electron resistivity, is analytically derived and understood through weak- and strong-turbulence analyses. No evidence is found to support previous suggestions that coherent structures cause a large depression of saturated transport from its quasilinear value in the hydrodynamic regime of the HW model. Instead, the depression of transport is well explained by the spectral balance equation of the (second-order) statistical closure when account is taken of incoherent noise. © 1997 American Institute of Physics.

Notizen: Explicit results pertaining to nonlinear oscillations in a magnetoactive beam–plasma system that shows transient or streaming phenomena are analytically presented. The nonlinear differential equation is formulated and solved in terms of a modified elliptic integral of motion. The phase-space mappings, and their striking features both in strong streaming and nonstreaming situations, are shown. The new features concerning streaming plasma analysis are illustrated in the form of frequency modulation, especially at the onset of chaos. The diminishing trend of modulated frequency in the strong streaming regions is concluded. The findings may have important implications for magnetospheric and cosmic plasma settings where beam plasma interactions exist. © 1997 American Institute of Physics.

Notizen: The influence of energetic ions on the stability of ideal double kink modes in a tokamak plasma with negative magnetic shear is investigated. It is found that the fast ions play a similar role as for the ordinary m=n=1 internal kink. In particular, phenomena analogous to sawtooth stabilization and fishbone excitation are possible.

Notizen: Variations in the plasma parameters of a large volume, helicon source as a function of applied rf power (0–2 kW), driving frequency (8–18 MHz), magnetic field (0–1.4 kG) and fill pressure (2–10 mTorr) have been studied. The transitions between the capacitive, inductive, and resonant helicon mode are consistent with previous experiments. Our data indicate that the transition to the helicon mode occurs at a unique magnetic field, independent of the driving frequency. Based on the helicon wave dispersion relation, from which helicon wavelengths can be calculated, the observed variations in plasma density as a function of driving frequency suggest that the wavelength of the helicon wave is a weak function of driving frequency. Calculation of the electron energies which correspond to the phase velocity of the driving wave (i.e., Landau damping) suggest that either Landau damping cannot be responsible for the efficient ionization of helicon sources, or that the helicon portion of the discharge does not extend over the entire radius of the apparatus. © 1997 American Institute of Physics.

Notizen: The response of the electron distribution function in one dimension to a traveling wave electric field is modeled for parameters relevant to a low-pressure helicon wave plasma source, and the resulting change in the ionization rate calculated. This is done by calculating the trajectories of individual electrons in a given wave field and assuming no collisions to build up the distribution function as the distance from the antenna is increased. The ionization rate is calculated for argon by considering the ionization cross section and electron flux at a specified position and time relative to the left-hand boundary, where the distribution function is assumed to be Maxwellian and the wave travels to the right. The simulation shows pulses in the ionization rate that move away from the antenna at the phase velocity of the wave, demonstrating the effect of resonant electrons trapped in the wave's frame of reference. It is found that the ionization rate is highest when the phase velocity of the wave is between 2 and 3×106 m/s, where the electrons interacting strongly with the wave (i.e., electrons with velocities inside the wave's "trapping width") have initial energies just below the ionization threshold. Results from the model are compared with experimental data and show reasonable qualitative agreement. © 1997 American Institute of Physics.

Notizen: In dense plasmas, dynamic plasma screening effects are investigated on electron capture from hydrogenic ions by past fully stripped ions. The classical Bohr–Lindhard model has been applied to obtain the electron capture probability. The interaction potential in dense plasmas is represented in terms of the longitudinal dielectric function. The classical straight-line trajectory approximation is applied to the motion of the projectile ion in order to visualize the electron capture probability as a function of the impact parameter, projectile energy, and plasma parameters. The electron capture probability including the dynamic plasma screening effect is always greater than that including the static plasma screening effect. When the projectile velocity is smaller than the electron thermal velocity, the dynamic polarization screening effect becomes the static plasma screening effect. When the projectile velocity is greater than the plasma electron thermal velocity, the interaction potential is almost unshielded. The difference between the dynamic and static plasma screening effects is more significant for low energy projectiles. It is found that the static screening formula obtained by the Debye–Hückel model overestimates the plasma screening effects on the electron capture processes in dense plasmas. © 1997 American Institute of Physics.

Notizen: A modified Penning trap is described with an annular confinement region and a toroidal magnetic field. A non-neutral electron plasma is confined axially by an electrostatic field and, in the radial direction, particles are constrained to lie within a small drift distance of a cylindrical flux surface. Drift orbits of all particles are banana-shaped and collisions cause neoclassical transport. © 1997 American Institute of Physics.

Notizen: It is argued that a wiggler plasma may acquire a non-zero velocity in the equilibrium and that it should be somehow consistent with the conservation of energy. (AIP)

Notizen: We used a semiclassical technique to calculate the interaction energy between the electron (velocity v) and the target electron by including the dynamic screening effect and effects on atomic transition processes. This differs from previous work. (AIP)

Notizen: Dust particles appear on the edge of tokamak plasmas as a result of disruptions and plasma interactions with divertor plates, limiters, or blankets, as well as of pellet injections. It is shown that the presence of the dust impurities can lead to an effective decrease of the slow Alfvén wave absorption in the region. These waves are used for heating, current drive, and control of the low-to-high (L–H) confinement transition. Radio-frequency forces can also be strongly changed in the presence of dust particles. © 1997 American Institute of Physics.

Notizen: A simplified linear analysis for resistive-viscous magnetic helical instabilities of arc discharges in a cylindrical plasma is developed. Based on a set of electrostatic magnetohydrodynamic (MHD) equations, resistive-viscous m=1 modes with an external axial magnetic field are studied. Explicit analytic results are obtained, from which the growth rate and the stability criterion can be shown, and the electrostatic assumption can be justified. In comparison with the previous channel model calculations, this analytic treatment can provide a simplified model for instability estimates, while avoiding artificial assumptions and misorderings in the energy equation. © 1997 American Institute of Physics.

Notizen: In the electron beam–plasma interaction at an electric double layer the beam density is much higher than in the classical beam–plasma experiments. The wave propagation takes place along the density gradient that is present at the high potential side of the double layer. Such a case is studied experimentally by injecting the electron beam from a plane cathode, without any grids suppressing the gradient, and by particle simulations. The high frequency field concentrates in a sharp "spike" with a half width of the order of one wavelength. The spike is found to be a standing wave surrounded by regions dominated by propagating waves. It forms at a position where its frequency is close to the local plasma frequency. The spike forms also when the electric field is well below the threshold for modulational instability, and long before a density cavity is formed in the simulations. Particle simulations reveal that, at the spike, there is a backward traveling wave that, when it is strongly damped, accelerates electrons back towards the cathode. In a simulation of a homogeneous plasma without the density gradient no spike is seen, and the wave is purely travelling instead of standing. © 1997 American Institute of Physics.

Notizen: Variations of plasma density are investigated as a function of frequency of rf power in a helicon plasma source. Abrupt, almost step-like changes in the plasma density are observed during the frequency scans under various conditions of the input rf power, the argon gas pressure, and the magnetic field. It is found that the transition frequencies shift to the lower value region as the input rf power and/or the argon gas pressure is increased, and to the higher value region as the magnetic field is increased. The observed density transitions are compared with semianalytical calculations based on the power balance relation and it has been shown that the results are in good agreement. © 1997 American Institute of Physics.

Notizen: Due to the presence of a population of heavy, negatively charged vortex elements, plasma becomes birefringent to a low frequency circularly polarized wave of infinitesimally small amplitude. The right circular polarization part remains dispersive whereas the left circular polarization part decays if the rotation associated with the vorticity is anticlockwise as seen by an observer looking antiparallel to the direction of the axis of rotation. © 1997 American Institute of Physics.

Notizen: Recent successes in confining antimatter in the form of positron and antiproton plasmas have created new scientific and technological opportunities. Plasma techniques have been the cornerstone of experimental work in this area, and this is likely to be true for the foreseeable future. Work by a number of groups on trapping antimatter plasmas is summarized, and an overview of the promises and challenges in this field is presented. Topics relating to positron plasmas include the use of positrons to study the unique properties of electron–positron plasmas, the interaction between positrons and ordinary matter, and the laboratory modeling of positron-annihilation processes in interstellar media. The availability of cold, trapped antiprotons and positrons makes possible the production of neutral antimatter in the form of antihydrogen. This is expected to enable precise comparisons of the properties of matter and antimatter, including tests of fundamental symmetries and the measurement of the interaction of antimatter with gravity. © 1997 American Institute of Physics.

Notizen: Without invoking any kind of "loss of memory" hypothesis, a diffusion equation is derived for the Hamiltonian dynamics defined by H=p2/2+(K/4π2)∑m=−MM cos(q−mt+cursive-phim), where the cursive-phims are fixed random phases. The key point of the derivation is a property of locality for the waves inducing transport. Using perturbation theory, it is shown that only waves whose phase velocities satisfy |m−p(t)|≤α(K/4π2)2/3, where α is a constant, approximately 5, play a relevant role for the statistical properties of the dynamics. This implies scaling properties for the dynamics, and leads to the understanding of the origin of force decorrelation and of diffusion, and to the prediction of their occurrence in time. Moreover, the convergence of the diffusion coefficient to its quasilinear value when K→∞ is shown, and is interpreted as the consequence of the crossover between two regimes of decorrelation that are of different natures. © 1997 American Institute of Physics.

Notizen: Enhanced cross-magnetic-field diffusion of test particles in pure ion plasmas has been measured. The ion plasma is contained in a Penning-Malmberg trap for weeks near thermodynamic equilibrium, characterized by rigid rotation and uniform density and temperature. Plasma expansion and loss is suppressed by a "rotating wall" technique, i.e., a weak electrostatic potential rotating faster than the plasma. Test particle transport is then measured even though there is zero net transport, in a regime where neutral collisions are negligible. The observed test particle transport is diffusive, i.e., proportional to the gradient of the test particle concentration. The measured diffusion coefficients scale as nT−1/2B−2 over a range of 40 in density, 50 in temperature, and 5 in magnetic field. This diffusion is about ten times greater than predicted by classical collisional theory, which describes velocity-scattering collisions with impact parameters ρ(approximately-less-than)rc. The enhanced transport is thought to be due to non-velocity-scattering "E×B drift" collisions with rc〈ρ(approximately-less-than)λD. Initial estimates of diffusion due to these long-range collisions are three times less than the measurements, and substantial theory questions remain. © 1997 American Institute of Physics.

Notizen: The performance of the Tokamak Fusion Test Reactor [D. Meade and the TFTR Group, in Plasma Physics and Controlled Nuclear Fusion Research, Washington, D.C., 1990 (International Atomic Energy Agency, Vienna, 1991), Vol. I, pp. 9–24], as defined by the maximum fusion power production, has been limited, not by confinement, but by stability to pressure-driven modes. Two classes of current profile modification have been investigated to overcome this limit. A new technique has been developed to increase the internal inductance of low-q (q(approximate)4), high-current (Ip〉2MA) plasmas. As was the case at higher edge q, the disruptive β limit has been found to increase roughly linearly with the internal inductance, li. Plasmas with hollow current profiles, i.e., reversed shear, are also predicted and experimentally observed to have increased stability in the negative shear region to ballooning and kink modes. However, performance of these plasmas is still limited by pressure-driven modes in the normal shear region. © 1997 American Institute of Physics.

Notizen: To evaluate stellarators as power plants, a helias configuration was developed by running high performance computer codes written at New York University. A modular coil set was found that had ample spacing for a reactor and took advantage of the concept of quasihelical symmetry. Since that time, the design has been further optimized and a similar stellarator has been discovered that has just two field periods and a plasma with the low aspect ratio 3.5. Its principal advantage is that in a flux coordinate system its magnetic-field structure comes close to having axial symmetry, so transport is predicted to be almost as good as that in a tokamak at high temperatures. Only 16 moderately twisted coils are required to produce the external magnetic field, and the configuration might well be viewed as a steady-state tokamak rather than an alternate concept. © 1997 American Institute of Physics.

Notizen: Current profile control is employed in the Madison Symmetric Torus [R. N. Dexter et al., Fusion Technol. 19, 131 (1991)] reversed field pinch to reduce the magnetic fluctuations responsible for anomalous transport. An inductive poloidal electric-field pulse is applied in the sense to flatten the parallel current profile, reducing the dynamo fluctuation amplitude required to sustain the equilibrium. This technique demonstrates a substantial reduction in fluctuation amplitude (as much as 50%), and improvement in energy confinement (from 1 to 5 ms); a record low fluctuation (0.8%) and record high temperature (615 eV) for this device were observed simultaneously during current drive experiments. Plasma beta increases by 50% and the Ohmic input power is three times lower. Particle confinement improves and plasma impurity contamination is reduced. The results of the transient current drive experiments provide motivation for continuing development of steady-state current profile control strategies for the reversed field pinch. © 1997 American Institute of Physics.

Notizen: The world's first high-power auxiliary heating experiments in a tight aspect ratio (or spherical) tokamak have been performed on the Small Tight Aspect Ratio Tokomak (START) device [Sykes et al., Nucl. Fusion 32, 694 (1992)] at Culham Laboratory, using the 40 keV, 0.5 MW Neutral Beam Injector loaned by the Oak Ridge National Laboratory. Injection has been mainly of hydrogen into hydrogen or deuterium target plasmas, with a one-day campaign to explore D→D operation. In each case injection provides a combination of higher density operation and effective heating of both ions and electrons. The highest β values achieved to date in START are volume average βT∼11.5% and central beta βO∼50%. Already high, these values are expected to increase further with the use of higher beam power. © 1997 American Institute of Physics.

Notizen: A series of experiments has been carried out to determine the relative screening of the low recycling gaseous impurities, nitrogen and methane, in Alcator C-Mod tokamak [Phys. Plasmas 1, 1511 (1994)] discharges. The impurity density in the core plasma is directly proportional to the rate of impurity injection. Screening was found to vary weakly with plasma density but to be markedly dependent on the poloidal position of injection. Detachment of the plasma results in significantly less screening both in Ohmic and in high confinement (H-mode) discharges, particularly when the gas is injected into the divertor or at the inner vessel wall. Evidence of impurity transport from the scrape-off layer into the divertor using bolometry and visible spectroscopy is presented and modeling of the transport using a two-dimensional Monte Carlo code is described. © 1997 American Institute of Physics.

Notizen: Planned experiments in the Joint European Torus [Plasma Physics and Controlled Fusion Research, Proceedings, 13th International Conference, Washington, D.C., 1990 (International Atomic Energy Agency, Vienna, 1991), Vol. 1, p. 27] (JET) with deuterium–tritium (D–T) plasmas require high fusion performance for α-particle heating studies and for investigation of isotope dependence in conditions relevant to the International Thermonuclear Experimental Reactor [Plasma Phys. Controlled Fusion 37, A19 (1995)]. In deuterium plasmas, the highest neutron rates have been obtained in the hot-ion high-confinement mode (H mode) which is ultimately limited by magnetohydrodynamic (MHD) phenomena when the pressure gradient approaches ideal ballooning and kink stability limits in the vicinity of the edge transport barrier. Results are reported confirming the MkII divertor's increased closure and pumping in this regime, progress in understanding the MHD-related termination is discussed, and the use of ion cyclotron resonance heating (ICRH) in combination with high-power neutral beams to increase the neutron yield is described. In separate experiments internal transport barriers have been established through careful programming of the current ramp and heating waveforms, and neutron emission comparable with the best hot-ion H-modes achieved. Steady-state H-mode discharges exhibiting edge localized modes (ELMs) in reactor-like configurations and conditions have been demonstrated, including cases in which relevant dimensionless parameter values are preserved, ready also for testing in D–T. © 1997 American Institute of Physics.

Notizen: A series of experiments was conducted on the DIII-D tokamak [J. L. Luxon and L. G. Davis, Fusion Technol. 8, 441 (1985)] to investigate the physical processes which limit density in high confinement mode (H-mode) discharges. The typical H-mode to low confinement mode (L-mode) transition limit at high density near the empirical Greenwald density limit [M. Greenwald et al., Nucl. Fusion 28, 2199 (1988)] was avoided by divertor pumping, which reduced divertor neutral pressure and prevented formation of a high density, intense radiation zone (MARFE) near the X-point. It was determined that the density decay time after pellet injection was independent of density relative to the Greenwald limit and increased nonlinearly with the plasma current. Magnetohydrodynamic (MHD) activity in pellet-fueled plasmas was observed at all power levels, and often caused unacceptable confinement degradation, except when the neutral beam injected (NBI) power was ≤3 MW. Formation of MARFEs on closed field lines was avoided with low safety factor (q) operation but was observed at high q, qualitatively consistent with theory. By using pellet fueling and optimizing discharge parameters to avoid each of these limits, an operational space was accessed in which density ∼1.5×Greenwald limit was achieved for 600 ms, and good H-mode confinement was maintained for 300 ms of the density flat-top. More significantly, the density was successfully increased to the limit where a central radiative collapse was observed, the most fundamental density limit in tokamaks. © 1997 American Institute of Physics.

Notizen: The dynamics of a relativistic plasma wave (RPW) resonantly excited by a two frequency CO2 laser pulse and the effects of this wave on a co-propagating relativistic electron beam were studied through experiments and supporting simulations. The amplitude of the RPW and its harmonics were resolved in time and space with a Thomson scattering diagnostic. In addition, the plasma wave amplitude-length product and temporal duration were independently measured through time and frequency resolved forward scattering. The transverse electric and magnetic fields associated with the RPW were studied by the scattering of a 2 MeV electron beam, and the eventual heating of the plasma after the breakup of the RPW was measured from the x-ray radiation spectrum. The experiments and simulations show that the RPW reaches a peak amplitude of approximately 30%, with the amplitude limited by plasma blowout driven by the radial ponderomotive forces of the plasma wave. © 1997 American Institute of Physics.

Notizen: The essence of the Bernstein–Landau paradox is that in a stable unmagnetized plasma electrostatic waves exhibit collisionless Landau damping, while in a magnetized plasma the Bernstein modes, perpendicular to the magnetic field, are exactly undamped, independent of the strength of the magnetic field. This problem is the subject of the present study. An analytical solution of the initial value problem for perturbations perpendicular to the magnetic field is given, which is a generalization of the well-known Landau work to magnetized plasmas. By introducing, according to Plemelj's prescription, plus- and minus-functions, having unique analytical properties, the character of the short-term and long-term plasma response is revealed, showing in the small magnetic field limit Landau damping in the first gyroperiod, followed by recurrence, and exhibiting irregular behavior with no damping at large times. The initial damping rate is seen to be close to the commonly used Landau damping rate for unmagnetized plasmas, however with a significant systematic deviation. A corrected expression for the Landau damping rate is found which yields a perfect description of the initial damping of oscillations perpendicular to a weak magnetic field. An alternative approach, expansion over Bernstein modes, is also employed. It is found that a zero-frequency (convective) mode, revealed earlier in particle simulations, is included in the complete linear treatment. © 1997 American Institute of Physics.

Notizen: The maximum normalized beta achieved in long-pulse tokamak discharges at low collisionality falls significantly below both that observed in short pulse discharges and that predicted by the ideal MHD theory. Recent long-pulse experiments, in particular those simulating the International Thermonuclear Experimental Reactor (ITER) [M. Rosenbluth et al., Plasma Physics and Controlled Nuclear Fusion (International Atomic Energy Agency, Vienna, 1995), Vol. 2, p. 517] scenarios with low collisionality νe*, are often limited by low-m/n nonideal magnetohydrodynamic (MHD) modes. The effect of saturated MHD modes is a reduction of the confinement time by 10%–20%, depending on the island size and location, and can lead to a disruption. Recent theories on neoclassical destabilization of tearing modes, including the effects of a perturbed helical bootstrap current, are successful in explaining the qualitative behavior of the resistive modes and recent results are consistent with the size of the saturated islands. Also, a strong correlation is observed between the onset of these low-m/n modes with sawteeth, edge localized modes (ELM), or fishbone events, consistent with the seed island required by the theory. We will focus on a quantitative comparison between both the conventional resistive and neoclassical theories, and the experimental results of several machines, which have all observed these low-m/n nonideal modes. This enables us to single out the key issues in projecting the long-pulse beta limits of ITER-size tokamaks and also to discuss possible plasma control methods that can increase the soft β limit, decrease the seed perturbations, and/or diminish the effects on confinement. © 1997 American Institute of Physics.

Notizen: Scalings for the stored energy and neutron yield, determined from experimental data, are applied to both deuterium-only and deuterium–tritium plasmas in different neutral-beam-heated operational domains in the Tokamak Fusion Test Reactor [Nucl. Fusion 25, 1167 (1985)]. The domain of the data considered includes the Supershot, high poloidal beta, low-mode, and limiter high-mode operational regimes, as well as discharges with a reversed magnetic shear configuration. The new important parameter in the present scaling is the peakedness of the heating beam fueling profile shape. Ion energy confinement and neutron production are relatively insensitive to other plasma parameters compared to the beam fueling peakedness parameter and the heating beam power when considering plasmas that are stable to magnetohydrodynamic modes. However, the stored energy of the electrons is independent of the beam fueling peakedness. The implication of the scalings based on this parameter is related to theoretical transport models such as radial electric field shear and ion temperature gradient marginality models. Similar physics interpretation is provided for beam heated discharges on other major tokamaks.

Notizen: Neoclassical transport in the presence of large ion orbits is investigated. The study is motivated by the recent experimental results that ion thermal transport levels in enhanced confinement tokamak plasmas fall below the "irreducible minimum level" predicted by standard neoclassical theory. This apparent contradiction is resolved in the present analysis by relaxing the basic neoclassical assumption that the ions orbital excursions are much smaller than the local toroidal minor radius and the equilibrium scale lengths of the system. Analytical and simulation results are in agreement with trends from experiments. The development of a general formalism for neoclassical transport theory with finite orbit width is also discussed. © 1997 American Institute of Physics.

Notizen: Experiments in the Tokamak Fusion Test Reactor (TFTR) [Phys. Plasmas 2, 2176 (1995)] have explored several novel regimes of improved tokamak confinement in deuterium–tritium (D–T) plasmas, including plasmas with reduced or reversed magnetic shear in the core and high-current plasmas with increased shear in the outer region (high li). New techniques have also been developed to enhance the confinement in these regimes by modifying the plasma-limiter interaction through in situ deposition of lithium. In reversed-shear plasmas, transitions to enhanced confinement have been observed at plasma currents up to 2.2 MA (qa(approximate)4.3), accompanied by the formation of internal transport barriers, where large radial gradients develop in the temperature and density profiles. Experiments have been performed to elucidate the mechanism of the barrier formation and its relationship with the magnetic configuration and with the heating characteristics. The increased stability of high-current, high-li plasmas produced by rapid expansion of the minor cross section, coupled with improvement in the confinement by lithium deposition has enabled the achievement of high fusion power, up to 8.7 MW, with D–T neutral beam heating. The physics of fusion alpha-particle confinement has been investigated in these regimes, including the interactions of the alphas with endogenous plasma instabilities and externally applied waves in the ion cyclotron range of frequencies. In D–T plasmas with q0〉1 and weak magnetic shear in the central region, a toroidal Alfvén eigenmode instability driven purely by the alpha particles has been observed for the first time. The interactions of energetic ions with ion Bernstein waves produced by mode conversion from fast waves in mixed-species plasmas have been studied as a possible mechanism for transferring the energy of the alphas to fuel ions. © 1997 American Institute of Physics.

Notizen: Recent measurements of the two-dimensional (2-D) spatial profiles of divertor plasma density, temperature, and emissivity in the DIII-D tokamak [J. Luxon et al., in Proceedings of the 11th International Conference on Plasma Physics and Controlled Nuclear Fusion (International Atomic Energy Agency, Vienna, 1987), p. 159] under highly radiating conditions are presented. Data are obtained using a divertor Thomson scattering system and other diagnostics optimized for measuring the high electron densities and low temperatures in these detached divertor plasmas (ne≤1021 m−3, 0.5 eV≤Te). D2 gas injection in the divertor increases the plasma radiation and lowers Te to less than 2 eV in most of the divertor volume. Modeling shows that this temperature is low enough to allow ion–neutral collisions, charge exchange, and volume recombination to play significant roles in reducing the plasma pressure along the magnetic separatrix by a factor of 3–5, consistent with the measurements. Absolutely calibrated vacuum ultraviolet spectroscopy and 2-D images of impurity emission show that carbon radiation near the X-point, and deuterium radiation near the target plates contribute to the reduction in Te. Uniformity of radiated power (Prad) (within a factor of 2) along the outer divertor leg, with peak heat flux on the divertor target reduced fourfold, was obtained. A comparison with 2-D fluid simulations shows good agreement when physical sputtering and an ad hoc chemical sputtering source (0.5%) from the private flux region surface are used. © 1997 American Institute of Physics.

Notizen: The confinement improvement in reversed-shear experiments on the Tokamak Fusion Test Reactor [Plasma Phys. Controlled Fusion 26, 11 (1984)] is investigated using nonlinear gyrofluid simulations including a bounce-averaged trapped electron fluid model. This model includes important kinetic effects for both ions and electrons, and agrees well with linear kinetic theory. Both reversed shear and the Shafranov shift reverse the precession drifts of a large fraction of the trapped electrons, which significantly reduces the growth rate of the trapped electron mode, found to be the dominant instability in the core. Two positive feedback transition mechanisms for the sudden improvement in core confinement are discussed: (1) Shafranov shift suppression of the trapped electron mode, and (2) turbulence suppression by radially sheared E×B flows. While both effects appear to be playing roles in the transition dynamics in most experiments, we show that Shafranov shift stabilization alone can cause a transition. © 1997 American Institute of Physics.

Notizen: The paper analyzes low-frequency waves in a plasma model that is made up of two thermally anisotropic magnetohydrodynamic components, by means of wave-front diagrams, a useful tool that has not attracted the desired attention. The wave-front diagrams of the fast propagating suprathermal mode, besides the usual fast, slow, and Alfvén modes, have been plotted for a variety of situations. These diagrams are used to bring out the physical significance of the anisotropic model vis-à-vis the isotropic model. The question of stability that has been completely ignored so far in the plasma models based on two magnetohydrodynamic components has also been addressed. Analogues of the firehose and mirror instabilities, which are supported by this model, are examined. Their comparison with single-component anisotropic plasma results suggests the possibility of suppressing the mirror instability. © 1997 American Institute of Physics.

Notizen: Measurements have been made of nonlinear sawtooth oscillations of the displacement of a magnetized electron column in a cryogenic, cylindrical trap. First reported 7 years ago, these oscillations occur when the displacement is destabilized by a resistive wall and damped by a temperature-dependent collisional viscosity. A typical evolution can last for thousands of seconds. Measurements show that oscillations of the plasma displacement are accompanied by oscillations in the plasma temperature. A simple predator-and-prey model of the temperature and displacement gives rise to a limit cycle solution due to the nonmonotonic dependence of the viscosity on temperature. These limit cycles are in good quantitative agreement with the measured sawtooth oscillations. © 1997 American Institute of Physics.

Notizen: For a single particle species confined by a uniform magnetic field B, the Bogema–Davidson inversion theorem determines a self-consistent rigid-rotor equilibrium distribution function F(H⊥−ωrPθ) as the derivative of the given radial density profile n(r) with respect to the corresponding generalized potential ψ(r). Aside from the quadratic density profile, most forms one can choose for n(r), even if integrable, yield potentials ψ(r) that cannot be inverted. The function F can be found parametrically, however, by writing the derivative of n with respect to ψ as the ratio of the radial derivatives of n and ψ. Several nontrivial examples of physical interest are studied. Numerical methods for calculating F are presented, its asymptotic form and integration are investigated, and applications to experiment are discussed. © 1997 American Institute of Physics.

Notizen: The diffusion of particles in the presence of randomly distributed trapping centres is examined. An analytical approach is developed for three simple models of the trap-release processes. It is shown that the particle motion remains diffusive on the average, but the diffusion coefficient can have large fluctuations. The results of the numerical simulations confirm the main qualitative trends found in the analytical study. Although they are very simple, the models can be useful for the examination of the diffusion in tokamak plasma in the presence of quasi-coherent structures which act as trapping centres. © 1997 American Institute of Physics.

Notizen: It is observed that, there exist certain forbidden regions in ion temperatures, for a compressive solitary wave, which do not have any solitary wave solution. These regions exist for a two- as well as a single-electron temperature plasma. A physical model is proposed to explain this phenomenon on the basis of the energy relation. It is revealed that a one-dimensional single warm ion fluid consists of two distinct energy components. A simple algebraic condition has been derived which expresses the balance between nonlinearity and dispersion for a fully nonlinear solitary wave. It shows that both the nonlinearity and dispersion decrease with increasing ion temperature and that the balance condition is well maintained for the whole spectrum of solitary wave solutions. © 1997 American Institute of Physics.

Notizen: A robustly accurate and effective method is presented to solve Laplace's equation in general azimuthally symmetric geometry for the magnetic scalar potential in the region surrounding a plasma discharge which may or may not contain external conductors. These conductors can be topologically toroidal or spherical, and may have toroidal gaps in them. The solution is incorporated into the various magnetohydrodynamic stability codes either through the volume integrated perturbed magnetic energy in the vacuum region or through the continuity requirements for the normal component of the perturbed magnetic field and the total perturbed pressure across the unperturbed plasma–vacuum boundary. The method is based upon using Green's second identity and the method of collocation. As useful by-products, the eddy currents and the simulation of Mirnov loop measurements are calculated.

Notizen: Annular Al-wire Z-pinch implosions on the Saturn accelerator [D. D. Bloomquist et al., Proceedings, 6th Pulsed Power Conference (Institute of Electrical and Electronics Engineers, New York, 1987), p. 310] that have high azimuthal symmetry exhibit both a strong first and weaker second x-ray burst that correlate with strong and weaker radial compressions, respectively. Measurements suggest that the observed magnetic Rayleigh–Taylor (RT) instability prior to the first compression seeds an m=0 instability observed later. Analyses of axially averaged spectral data imply that, during the first compression, the plasma is composed of a hot core surrounded by a cooler plasma halo. Two-dimensional (2-D) radiation magnetohydrodynamic computer simulations show that a RT instability grows to the classic bubble and spike structure during the course of the implosion. The main radiation pulse begins when the bubble reaches the axis and ends when the spike finishes stagnating on axis and the first compression ends. These simulations agree qualitatively with the measured characteristics of the first x-ray pulse and the overall energetics, and they provide a 2-D view into the plasma hydrodynamics of the implosion.

Notizen: The experiments to characterize the modification of edge fluctuations using electrostatic probes have been performed on Keda Tokamak-5C (KT-5C) [World Survey of Activities in Controlled Fusion Research, Nuclear Fusion Special Supplement (International Atomic Energy Agency, Vienna, 1991), p. 190]. The results show that the externally excited perturbation propagates in the direction of electron directional movement. The observed phenomena are mainly related to the electrons, and can be well explained by the proposed dynamic theory of the ballistic mode. The experimental correlation coefficient and the longitudinal dispersion relation are in good agreement with the proposed ballistic model. © 1997 American Institute of Physics.

Notizen: Adiabatic acceleration of charged particles along magnetic field lines originates from the coupling between the electric drift and longitudinal motion in a nonunidirectional magnetic field. As a result, initially slow particles entering the reconnection site of an X-type magnetic geometry can leave the latter as substantially accelerated jets directed along the magnetic separatrices. The corresponding energy spectrum has a power-law form, with the spectral index depending on the angle between the separatrices. © 1997 American Institute of Physics.

Notizen: The results of a numerical simulation of anomalous electron precipitation in the Earth's auroral zone induced by waves are discussed. In order to study the plasma electron–wave interaction, a two-dimensional quasilinear code has been used. A quasilinear operator models the electron Landau damping of any plasma eigenmodes: kinetic Alfvén waves, whistlers, lower hybrid waves, etc. The distribution function structure was analyzed. The electron current induced by waves, the density, and the plasma energy decay, which stimulates the electron precipitation, have also been investigated. © 1997 American Institute of Physics.

Notizen: A new dispersion relation for low-frequency magnetic electron drift modes (MEDV) in a nonuniform plasma is derived by incorporating the ion dynamics. It is shown that the ion dynamics introduces new classes of instabilities when the wave frequency either lies between the electron and ion plasmas frequencies or is close to the ion plasma frequency. The modified MEDV modes involving the ion motion can cause nonthermal particle as well as electron heat transports. © 1997 American Institute of Physics.

Notizen: The temporal and spatial evolution of paired luminous rings is observed in pulsed capacitive radio-frequency (rf) hydrogen discharges. The time-resolved axial profile of the light intensity indicates that the outermost ring pairs near the electrodes start to appear earlier than the inner ones, and that only the left-side (right-side) rings of ring pairs turn on when the rf voltage applied to the left-side (right-side) electrode is positive. The physical mechanism to create the paired rings seems to be similar to that of the standing striations in dc glow discharges. © 1997 American Institute of Physics.

Notizen: The linear and nonlinear evolution of the plane current–vortex sheet, with a basic magnetic field given by B0(y)=tanh yêz, and a basic velocity field given by W0(y)=V tanh Ryêz, is examined. The discovery of an ideal instability in a large region of parameter space previously found to be stable is reported. In this paper numerical evidence is presented that this parameter regime is in fact highly unstable, with growth rates exceeding those of the modes existing in the region of parameter space previously found to be unstable. An examination of the perturbation energy balance indicates that enhanced energy transfer from the basic velocity field to the perturbed velocity and magnetic fields is responsible for the enhanced growth rate. This occurs due to processes absent from both the resistive and Kelvin–Helmholtz instabilities. Nonlinearly it is found that magnetic reconnection can occur on an ideal time scale in certain cases. These faster instabilities lead to a more violent cascade of excitation in the streamwise direction, as evidenced by the rapid formation of higher harmonics of the initial disturbance. A nonlinear saturation due to increased correlation of the perturbed velocity and magnetic field occurs for all cases. © 1997 American Institute of Physics.

Notizen: The quasilinear fluctuation integral is calculated for a two-dimensional, unmagnetized plasma (composed of charged rods), and is expressed in terms of Fokker–Planck coefficients. It is found that in two dimensions, the enhanced fluctuations generated by fast electrons lead to anomalously large transport coefficients. In particular, the effect of a small population of fast electrons is only weakly dependent on their density. In three dimensions, the effect of fast electrons is masked by the dominant approximation, but higher-order terms describe processes similar to those in two dimensions, and these terms can become significant for weakly stable plasmas. The differences between two and three dimensions arise from the fact that both emission and damping of plasma waves are retained to lowest order in two dimensions, while the three-dimensional dominant approximation effectively includes only wave emission by test particles. An understanding of the differences between two and three dimensions is crucial to the interpretation of two-dimensional particle simulations. © 1997 American Institute of Physics.

Notizen: The effect of the electron pressure gradient in Ohm's law on the nonlinear development of the internal kink mode is investigated. While pressure fluctuations have a destabilizing effect, the average pressure gradient giving rise to diamagnetic flows is stabilizing. If the latter is strong enough, it leads to saturation at finite island size. The relevance of the results for the sawtooth phenomenon in tokamak plasmas is discussed. © 1997 American Institute of Physics.

Notizen: A model of interactions between ions and neutrals in the high electric field is proposed and the exact solution to the Boltzmann equation with the interaction terms is derived. It is shown that the solution produces the known dependence of the ion drift velocity and the mobility on the parameters of the plasma and neutrals in the high electric field. Experimental results on mobilities of N2+ ions in the parent gas are studied by using the formula of the drift velocity obtained in our model. As an application, the presheath formation at the plasma–wall boundary is discussed and a comparison with experimental measurements of sheath potential profiles has been performed. © 1997 American Institute of Physics.

Notizen: The possibility that linearly stable ("submarginal") profiles can support turbulent transport in the absence of external noise is considered in the context of a one-dimensional, realizable stochastic model of "almost passive" advection that includes a stability threshold dependent on a critical gradient. The important limits of zero and infinite Kubo number (dimensionless autocorrelation time) are solved analytically. When the stability threshold is linear, it is proven rigorously that, within the context of the model, submarginal profiles do not carry any turbulent flux. A generalization to include a nonlinear stability threshold reminiscent of a subcritical bifurcation supports submarginal profiles. © 1997 American Institute of Physics.

Notizen: Theoretical predictions of ion and electron thermal diffusivities are tested by comparing calculated and measured temperatures in low (L) mode plasmas from the Tokamak Fusion Test Reactor [D. J. Grove and D. M. Meade, Nucl. Fusion 25, 1167 (1985)] nondimensional scaling experiments. The DIII-D [J. L. Luxon and L. G. Davis, Fusion Technol. 8, 441 (1985)] L-mode ρ* scalings, the transport models of Rebut-Lallia-Watkins (RLW), Boucher's modification of RLW, and the Institute for Fusion Studies-Princeton Plasma Physics Laboratory (IFS-PPPL) model for transport due to ion temperature gradient modes are tested. The predictions use the measured densities in order to include the effects of density profile shape variations on the transport models. The uncertainties in the measured and predicted temperatures are discussed. The predictions based on the DIII-D scalings are within the measurement uncertainties. All the theoretical models predict a more favorable ρ* dependence for the ion temperatures than is seen. Preliminary estimates indicate that sheared flow stabilization is important for some discharges, and that inclusion of its effects may bring the predictions of the IFS-PPPL model into agreement with the experiments. © 1997 American Institute of Physics.

Notizen: A linear theory of resonant backward-wave oscillators (BWOs) that takes into account the zero spatial harmonic is developed. It is shown that, depending on the phase shift, this harmonic can either increase or decrease the starting current of the device. The nonlinear theory shows the same effect of the phase shift on the maximum efficiency of the BWO. The numerical analysis of nonstationary processes has also demonstrated the effect of the phase shift on the range of beam current values in which the BWO operates in a steady-state single-frequency regime. The effect of boundary conditions at the cathode end of the slow-wave structure on the phase shift between harmonics is also analyzed. © 1997 American Institute of Physics.

Notizen: The theory describing the effect of fluctuations in technical parameters (voltages, beam current, external magnetic fields) on the linewidth of gyrotron radiation is presented. Results of the analysis of free-running gyrotron oscillators are given. The effect of technical fluctuations on the spectrum density of the frequency noise is considered and some estimates are compared with available experimental data. © 1997 American Institute of Physics.

Notizen: As the multipactor charge builds up, the mutual repulsion overtakes the focusing effect of the rf interaction, and the thin charge ribbon formed during the initial buildup expands into a wide band. Two synergistic effects lead to saturation (a) first, spreading of the impact phases into the region where the vacuum rf retards emission (b) later, and field reversal in the front end of the bunch due to the space-charge field, regardless of the rf phase. The effective secondary yield under opposing field is zero regardless of impact energy. Steady state is achieved when the ensemble-average yield δ drops to unity. The spreads in the impact energy are less important; impacts under reversed field cause saturation even at constant (independent of impact energy) secondary yield. The bunch breakup threshold is identified from bunch stability theory. It is found that near saturation both one-surface and two-surface cascades take place simultaneously; particle orbits alternate between one- and two-surface multipactors until they are finally lost to unfavorable phase/reversed field impacts. The possibility of saturation with a tight bunch, before breakup, is also discussed. © 1997 American Institute of Physics.

Notizen: Inverse bremsstrahlung heating of laser-produced plasmas can result in a non-Maxwellian electron distribution function. This leads to a modification of Langmuir wave collisional and Landau damping coefficients and may result in changes in the stimulated Raman scattering threshold and scattered light frequency spectra. The importance of these effects for current laser–plasma interaction experiments is discussed. © 1997 American Institute of Physics.

Notizen: A general analytical framework is developed for the nonlinear dispersion relations of a class of large amplitude one-dimensional isolated envelope solitons for modulated light pulse coupled to electron plasma waves, previously investigated numerically [Kozlov et al., Zh. Eksp. Teor. Fiz. 76, 148 (1979); Kaw et al., Phys. Rev. Lett. 68, 3172 (1992)]. The analytical treatment of weakly nonlinear solitons [Kuehl and Zhang, Phys. Rev. E 48, 1316 (1993)] is extended to the strongly nonlinear limit. © 1997 American Institute of Physics.

Notizen: It is proved that (a) the solutions of the ideal magnetohydrodynamic equation, which describes the equilibrium states of a cylindrical plasma with purely poloidal flow and arbitrary cross-sectional shape [G. N. Throumoulopoulos and G. Pantis, Plasma Phys. Controlled Fusion 38, 1817 (1996)], are also valid for incompressible equilibrium flows with the axial velocity component being a free surface quantity and that (b) for the case of isothermal incompressible equilibria the magnetic surfaces necessarily have a circular cross-section. © 1997 American Institute of Physics.

Notizen: One of the scientific success stories of fusion research over the past decade is the development of the E×B shear stabilization model to explain the formation of transport barriers in magnetic confinement devices. This model was originally developed to explain the transport barrier formed at the plasma edge in tokamaks after the L (low) to H (high) transition. This concept has the universality needed to explain the edge transport barriers seen in limiter and divertor tokamaks, stellarators, and mirror machines. More recently, this model has been applied to explain the further confinement improvement from H (high) mode to VH (very high) mode seen in some tokamaks, where the edge transport barrier becomes wider. Most recently, this paradigm has been applied to the core transport barriers formed in plasmas with negative or low magnetic shear in the plasma core. These examples of confinement improvement are of considerable physical interest; it is not often that a system self-organizes to a higher energy state with reduced turbulence and transport when an additional source of free energy is applied to it. The transport decrease that is associated with E×B velocity shear effects also has significant practical consequences for fusion research. The fundamental physics involved in transport reduction is the effect of E×B shear on the growth, radial extent, and phase correlation of turbulent eddies in the plasma. The same fundamental transport reduction process can be operational in various portions of the plasma because there are a number of ways to change the radial electric field Er. An important theme in this area is the synergistic effect of E×B velocity shear and magnetic shear. Although the E×B velocity shear appears to have an effect on broader classes of microturbulence, magnetic shear can mitigate some potentially harmful effects of E×B velocity shear and facilitate turbulence stabilization. Considerable experimental work has been done to test this picture of E×B velocity shear effects on turbulence; the experimental results are generally consistent with the basic theoretical models. © 1997 American Institute of Physics.

Notizen: The stability of a magnetized plasma that includes a sheared transverse flow is analyzed by using both fluid and kinetic formalisms. In addition to the well known Kelvin–Helmholtz modes it is found that another branch of oscillation exists which can dominate the collective effects in a plasma if the magnitude of shear in the transverse flow is sufficiently strong. The source of free energy for the new branch is an inhomogeneity in the energy density caused by the velocity shear. Kelvin–Helmholtz modes, when examined with a fluid theory, are found to be robust and therefore have dominated the analysis of plasma systems with velocity shear in both laboratory and space plasmas. However, when a kinetic formalism is applied to Kelvin–Helmholtz modes it is found that these modes are strongly Landau damped especially when the ion temperature is comparable to or larger than the electron temperature. In addition, since the Kelvin–Helmholtz mode is dependent explicitly on the second derivative of the flow it is sensitive to the profile of the flow. On the other hand, the new branch is dependent on the localized nature of the flow and hence it is less sensitive to the details of the flow profile. The two branches of oscillation are compared using both fluid and kinetic theories and their regimes of dominance discussed.

Notizen: The stability of tearing modes in the presence of a rigid toroidal rotation of the plasma is examined. The effect of rotation is modeled by a centrifugal force, which in the cylindrical limit reduces to a uniform force transverse to the plasma column. The influence of this centrifugal force on the outer region matching data Δ, which provides a measure of the energy available for driving the tearing mode, is numerically investigated. It is found that the stability properties of the mode in the presence of the flow are sensitive to the equilibrium density profile. © 1997 American Institute of Physics.

Notizen: In a magnetized laboratory plasma described in the companion paper [Stenzel and Urrutia, Phys. Plasmas 4, 26 (1997)], a large positive voltage step (V(very-much-greater-than)kTe/e) is applied to electrodes. The current front propagates in the whistler mode in the parameter regime of electron magnetohydrodynamics. The topology of the current density is that of nested helices. Large transient currents in excess of the electron saturation current can be drawn. A transient radial electric field associated with the current rise, excites a compressional, large amplitude, radially outgoing sound wave, which leaves the current channel depleted of plasma. The current collapses due to the density erosion. Electric field reversal excites a rarefaction wave which leads to a partial density and current recovery. Periodic plasma inflow and outflow cause the current to undergo strong relaxation oscillations at a frequency determined by the electrode diameter and the sound speed. In addition, a broad spectrum of microinstabilities is observed in regions of high current density. For drift velocities approaching the thermal speed, the spectrum extends beyond the ion plasma frequency (ωpi) up to the electron plasma frequency (ωpe). Correlation measurements above ωpi reveal modes propagating along the electron drift at speeds above the sound speed but well below the electron drift speed. © 1997 American Institute of Physics.

Notizen: A new map called the symmetric simple map is introduced to represent the chaotic trajectories of magnetic field lines in the scrape-off layer of a single-null divertor tokamak. Good surfaces of this map are very nearly axisymmetric. Therefore it gives a far better representation of the magnetic topology of a single-null divertor tokamak. The map is investigated in detail and used to analyze the generic features of the field line trajectories and their footprint on the divertor plate. The map is employed to calculate the variations in the fraction of magnetic flux from the stochastic layer diverted onto plate, in the footprint and in related parameters as the map parameter is varied. The Lyapunov exponents and the field diffusion coefficients are calculated. The low mode number map and the dipole map are introduced to include the effects of low and high mode number perturbations in the new map. © 1997 American Institute of Physics.

Notizen: The energy spectra of energetic confined alpha particles are being measured using the pellet charge exchange method [R. K. Fisher, J. S. Leffler, A. M. Howald, and P. B. Parks, Fusion Technol. 13, 536 (1988)]. The technique uses the dense ablation cloud surrounding an injected impurity pellet to neutralize a fraction of the incident alpha particles, allowing them to escape from the plasma where their energy spectrum can be measured using a neutral particle analyzer. The signal calculations given in the above-mentioned reference disregarded the effects of the alpha particles' helical Larmor orbits, which causes the alphas to make multiple passes through the cloud. Other effects such as electron ionization by plasma and ablation cloud electrons and the effect of the charge state composition of the cloud, were also neglected. This report considers these issues, reformulates the signal level calculation, and uses a Monte-Carlo approach to calculate the neutralization fractions. The possible effects of energy loss and pitch angle scattering of the alphas are also considered. © 1997 American Institute of Physics.

Notizen: The nonlinear behavior of the toroidal Alfvén eigenmode (TAE) driven unstable by energetic ions in the Tokamak Fusion Test Reactor (TFTR) [Phys. Plasmas 1, 1560 (1994)] is studied. The evolution of instabilities can take on several scenarios: a single mode or several modes can be driven unstable at the same time, the spectrum can be steady or pulsating, and there can be negligible or anomalous loss associated with the instability. This paper presents a comparison between experimental results and recently developed nonlinear theory. Many features observed in experiment are compatible with the consequences of the nonlinear theory. Examples include the structure of the saturated pulse that emerges from the onset of instability of a single mode, and the decrease, but persistence, of TAE signals when the applied rf power is reduced or shut off. © 1997 American Institute of Physics.

Notizen: A new gyrokinetic equation is derived for rotating plasmas with large flow velocities on the order of the ion thermal speed. Neoclassical and anomalous transport of particles, energy, and toroidal momentum are systematically formulated from the ensemble-averaged kinetic equation with the gyrokinetic equation. As a conjugate pair of the thermodynamic force and the transport flux, the shear of the toroidal flow, which is caused by the radial electric field shear, and the toroidal viscosity enter both the neoclassical and anomalous entropy production. The interaction between the fluctuations and the sheared toroidal flow is self-consistently described by the gyrokinetic equation containing the flow shear as the thermodynamic force and by the toroidal momentum balance equation including the anomalous viscosity. Effects of the toroidal flow shear on the toroidal ion temperature gradient driven modes are investigated. Linear and quasilinear analyses of the modes show that the toroidal flow shear decreases the growth rates and reduces the anomalous toroidal viscosity. © 1997 American Institute of Physics.

Notizen: In the presence of a high-frequency intense uniform electric field, the collisions of electrons with ions can be made more frequent or less frequent, depending on the polarization of the hf field, the direction and magnitude of particle velocity, and the ratio of the plasma Debye length to the size of the electron oscillation in the hf field. The stimulated bremsstrahlung emission is calculated for both circularly and linearly polarized fields. © 1997 American Institute of Physics.

Notizen: Scattering instabilities, such as stimulated Brillouin scatter (SBS) and stimulated Raman scatter (SRS), transfer momentum to the plasma which leads to flow inhomogeneities in laser hot spots that may significantly reduce the level of SBS. Simple estimates and simulations shown that the magnitude of flow fluctuations can reach Mach numbers of order unity in a time scale of hundreds of picoseconds. © 1997 American Institute of Physics.

Notizen: The nonlinear destabilization of pressure driven (ballooning) modes by internal kink modes during sawtooth cycles was found in the Tokamak Fusion Test Reactor (TFTR) [Park et al., Phys. Rev. Lett. 75, 1763 (1995)]. It is shown by a numerical parameter study, including aspect ratio and magnetic shear profile effects, that in ideal magneto-hydrodynamics (MHD), this nonlinear destabilization occurs in plasma equilibria close to marginal stability to linear sub-harmonics of the internal kink with toroidal mode number n≥3. © 1997 American Institute of Physics.

Notizen: A theory of plasma response to electromagnetic perturbations has been developed based on the full solution to the Fokker–Planck equation in high Z plasmas without electron–electron collisions. For the first time the transverse electron susceptibility has been calculated for the entire range of frequencies and wave numbers including the quantitative description of the weakly collisional regime where the wave frequency is comparable to the electron–ion collision frequency and the wave number is comparable to the inverse electron mean free path. The results have been compared to approximate expressions for the electron conductivity based on the Drude model showing discrepancy by a factor of few in regions where the spatial dispersion is important. The theory is applied to the calculation of laser light absorption in solid density plasmas. © 1997 American Institute of Physics.

Notizen: The kinetic theory of transport in an impure, partially ionized, edge plasma is developed. It is found that the thermal force between bulk ions and impurities is enhanced by the presence of neutral atoms, but the dynamical friction is not affected by the neutrals. When the neutral viscosity is large, an additional force on the impurities also arises. This force is parallel to the magnetic field, and is proportional to the shear of the parallel plasma velocity and the perpendicular ion density and temperature gradients. © 1997 American Institute of Physics.

Notizen: For an arbitrary monotonic charging function, the dynamics of a dust grain is dissipative and energy is a Liapunov function. In an arbitrary external potential two types of equilibria exist. The first type, with uncharged grain, is always unstable. The second type of equilibrium, admitting states of both positive and negative charge, can be marginally stable; stability depends on the local potential. Under spatially uniform (constant or time-dependent) potentials, motion is free while the charge adapts to the potential. For a spatially oscillating potential, the phase space is that of the simple pendulum with one additional degree of freedom, the charge. Dissipation in the charging process forbids periodic behavior and ensures the existence of attractors: A grain is at stable equilibrium only when charged positively and trapped in a potential well, or when charged negatively on top of a hill. The small oscillations near a stable equilibrium decay weakly, and the grain charge oscillates at twice the oscillation frequency. © 1997 American Institute of Physics.

Notizen: The effect of ion and electron drifts on the existence of arbitrary amplitude solitary waves is studied using Sagdeev's pseudopotential method. It is found that if the electron drift velocity u0 is finite, solitary waves may exist for relatively large values of v0/c, where v0 is the ion drift velocity and c is the velocity of light. © 1997 American Institute of Physics.

Notizen: The expansion into vacuum of a one-dimensional, collisionless, negative ion plasma is investigated in the framework of the Vlasov–Poisson model. The basic equations are written in a "new time space" by use of a rescaling transformation and, subsequently, solved numerically through a fully Eulerian code. As in the case of a two species plasma, the time-asymptotic regime is found to be self-similar with the temperature decreasing as t−2. The numerical results exhibit clearly the physically expected effects produced by the variation of parameters such as initial temperatures, mass ratios and charge of the negative ions. © 1997 American Institute of Physics.

Notizen: The spatiotemporal evolution of near-forward stimulated Brillouin scattering (SBS) is studied in detail. For large scattering angles SBS grows and saturates as a three-wave instability. For small scattering angles SBS begins to grow as a three-wave instability, then continues to grow and saturates as a four-wave instability. Expressions for the saturation time and steady-state gain exponent of SBS are derived for large and small scattering angles. © 1997 American Institute of Physics.

Notizen: The problem of nonlinear Burger equation in a plasma contaminated with heavy dust grains has been revisited. As discussed earlier [C. B. Dwivedi and B. P. Pandey, Phys. Plasmas 2, 9 (1995)], the Burger equation originates due to dust charge fluctuation dynamics. A new alternate mathematical approach based on a simple traveling wave formalism has been applied to find out the solution of the derived Burger equation, and the method recovers the known shock-wave solution. This technique, although having its own limitation, predicts successfully the salient features of the weak shock-wave structure in a dusty plasma with dust charge fluctuation dynamics. It is emphasized that this approach of the traveling wave formalism is being applied for the first time to solve the nonlinear wave equation in plasmas. © 1997 American Institute of Physics.

Notizen: Eikonal approximation is applied to investigate elastic electron-ion collisions in weakly coupled plasmas. Plasma screening effects on eikonal phase are investigated for eikonal differential elastic scattering cross sections. The electron-ion interaction potential in weakly coupled plasmas has been obtained by the introduction of the longitudinal plasma dielectric function. The semiclassical straight-line trajectory method is applied to the path of the projectile electron in order to investigate the variation of the eikonal phase as a function of the impact parameter and the plasma parameters. In the first-order eikonal approximation, the dynamic plasma screening effect is identical to the static screening effect obtained by the Debye–Hückel potential. The eikonal differential elastic cross section substantially decreases with an increase in the projectile energy and increases as the plasma screening effect decreases through the Debye length. The plasma screening effects are more significant for large impact parameters. © 1997 American Institute of Physics.

Notizen: The flow resonance is concealed when the linearized equation of motion in magnetohydrodynamics is written in terms of the Lagrangian displacement vector. Its contribution to the dispersion equation of a rotating cylindrical plasma column, surrounded by a thin resistive wall, is clarified by a simple model calculation, relevant for axisymmetric modes, in the Eulerian representation. Under certain assumptions the flow resonance is shown to damp the resistive wall mode, with the damping proportional to the square of the gradient of vorticity at the resonant surface. It is shown that a flow resonance can stabilize a slightly elliptical plasma, surrounded by a thin resistive wall, against axisymmetric modes. © 1997 American Institute of Physics.

Notizen: The Alfvén continuum of an asymmetric plasma configuration that is an exact solution of the equilibrium equations of ideal magnetohydrodynamics is treated. The equilibrium is parallel to an infinite straight magnetic axis about which the magnetic lines of force form closed magnetic surfaces. Properties of the governing continuum equation for small plasma beta are explored, and the associated spectrum is determined. The continuum has a localized component characterized by modes that decay to zero along magnetic field lines, and a non-localized component characterized by modes that approach non-zero constants at the plasma ends. The localized component is restricted to real frequencies, while the non-localized component covers the entire complex frequency plane. The non-localized modes can be joined generally with those on neighboring field lines to form modes that smoothly cover the entire magnetic surface. The localized modes do not occur in symmetric plasmas. © 1997 American Institute of Physics.

Notizen: A variational formalism is introduced in the theory of three-wave parametric instabilities in inhomogeneous plasmas. This minimum pump strength principle (MPSP) is then applied to two model problems, the first being the Rosenbluth model equations [Phys. Rev. Lett. 29, 565 (1972)]. By choosing appropriate trial functions, the MPSP is used to solve for the complex eigenfrequency of the most unstable mode. The wave vector mismatch is assumed to be of the form κ(x)=κ(n)(0)xn/n!, where n is any positive integer. The results are compared to numerical solutions of the same eigenvalue problem. The second problem is the Liu, Rosenbluth, and White Raman sidescattering model [Phys. Fluids 17, 1211 (1974)], which is treated for any positive-integer power law density profile. The choice of trial functions, the role of symmetry, and various useful approximations are discussed.

Notizen: The theory of the two-plasmon decay instability is recast in the form of a variational principle for the pump strength or intensity of the incident laser. This allows the calculation of growth rates, frequency shifts, and threshold conditions for two-plasmon decay modes that occur in integer power law density profiles, including the effects of oblique incidence of the laser and both S and P polarizations. The transition between parabolic profiles occurring at the peak of an exploding foil target and linear profiles on its flanks is treated as well.

Notizen: The theory of temporally unstable modes or bound states associated with stimulated Raman scattering in inhomogeneous plasmas is recast in the form of a variational principle for the pump strength or intensity of the incident laser. This Minimum Pump Strength–Variational Principle (MPS–VP) formalism allows the unification of disparate results on growth rates, frequency shifts, and threshold conditions, which in the past relied on specific and restrictive assumptions on the density profile, scattering geometry, temperature, and damping rates. The variational approach leads to generalizations of these results in a uniform manner. Various levels of sophistication in the choice of trial and dual functions are explored. Simplifications and short cuts that will be used throughout this series of papers are tested here, and their regions of validity explored. The principle new result of some practical interest is the growth rate of Raman sidescattering occurring anywhere at the peak or on the flanks of a parabolic density profile such as can be found in an exploding foil experiment. This new calculation includes any density at or below quarter-critical, at any temperature, and at low or high damping rate limits.

Notizen: The theories of the two plasmon decay (TPD) instability and stimulated Raman scattering (SRS) are unified and presented using a variational formulation. The mixed polarization high-frequency instability is the resulting generalization which has SRS and TPD as special cases. The most unstable mode's properties are derived and shown as a function of perpendicular wave number. For vanishing wave number we recover the Raman backscattering result and for wave numbers a few times larger than the square root of the homogeneous plasma growth rate normalized to the pump frequency, the two plasmon decay instability modes are recovered. This transition is accompanied by a change of polarization from a purely electromagnetic transversely polarized wave in the case of Raman to a strictly electrostatic or longitudinally polarized wave in the case of TPD, and admixtures in between. Experimental signatures of these modes are given and a method is proposed by which the density scale length and electron temperature would be simultaneously inferred from the width and height of the spectral signature corresponding to these hybrid modes in an ideal experiment.