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  • 1
    Electronic Resource
    Electronic Resource
    Discrete symmetries in axisymmetric toroidal plasma confinement (2001)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 8 (2001), S. 1451-1454 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Several discrete symmetry parameters characterizing axisymmetric toroidal plasmas have been introduced. For a device without up–down symmetry such parameters are S1, related to the handedness of the toroidal current, S2, related to the direction of the toroidal velocity, and S3, characterizing the handedness of the toroidal magnetic field. All these parameters can acquire values ±1, thereby making eight different combinations. For a device with up–down symmetry only the mutual orientation of the toroidal current, toroidal velocity, and toroidal magnetic field remain important. Based on the general expression for the Lagrangian for charged particles in an external magnetic field, we identify the invariance properties of this Lagrangian (and, accordingly, of plasma behavior) with respect to these transformations. Reduced plasma models, based on the Maxwell–Boltzmann equations, and on the magnetohydrodynamic equations, are also considered. An analysis of the plasma behavior in the context of the symmetry properties may be helpful in identifying the most probable theoretical models of plasma transport. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1357217
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  • 2
    Electronic Resource
    Electronic Resource
    Low-to-high confinement transition simulations in divertor geometry (2000)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 7 (2000), S. 1951-1958 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Recent results are presented for turbulence in tokamak boundary plasmas and its relationship to the low-to-high confinement (L–H) transition in a realistic divertor geometry. These results are obtained from a three-dimensional (3D) nonlocal electromagnetic turbulence code, which models the boundary plasma using fluid equations for plasma vorticity, density, electron and ion temperatures and parallel momenta. With sources added in the core-edge region and sinks in the scrape-off layer (SOL), the code follows the self-consistent profile evolution together with turbulence. Under DIII-D [Luxon et al., International Conference on Plasma Physics and Controlled Nuclear Fusion (International Atomic Energy Agency, Vienna, 1986), p. 159] tokamak L-mode conditions, the dominant source of turbulence is pressure-gradient-driven resistive X-point modes. These modes are electromagnetic and curvature-driven at the outside mid-plane region but become electrostatic near X-points due to magnetic shear and collisionality. Classical resistive ballooning modes at high toroidal mode number, n, coexist with these modes but are sub-dominant. Results indicate that, as the power is increased, these modes are stabilized by increased turbulence-generated velocity shear, resulting in an abrupt suppression of high-n turbulence and the formation of a pedestal in density and temperature, as is characteristic of the H-mode transition. The sensitivity of the boundary turbulence to the direction of the toroidal field Bt is discussed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.874044
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  • 3
    Electronic Resource
    Electronic Resource
    Particle trajectories in a sheath in a strongly tilted magnetic field (1998)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 5 (1998), S. 808-817 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: In this paper we provide a systematic analysis of particle motion in a sheath under the condition that the magnetic field intersects the wall at a small angle. A general qualitative classification of particle trajectories is presented. It is shown that the ion motion in the ion subsheath can be described in terms of an adiabatic invariant, despite the fact that the electrostatic potential varies on a scale comparable with the ion gyroradius. Trajectories of heavy impurity ions for the cases of both low and high ionization states have been found. Electron motion is considered, and it is shown that, if the electron gyroradius is much less than the electron Debye radius, electrons hit the wall at shallow angles. The possible role of an electrostatic field directed along the wall is briefly discussed. The results obtained in the paper can be used for evaluating the sputtering rate and the secondary emission coefficient, as well as for analyzing sheath stability and for formulating the boundary conditions for the bulk of the plasma. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.872764
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  • 4
    Electronic Resource
    Electronic Resource
    Kinetic effects on particle and heat fluxes in detached plasmas (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 3386-3396 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A model of high recycling scrape-off layer plasmas in tokamaks is presented where both ion and electron species are described by nonlinear kinetic equations. Coulomb and charged-neutral particle collisions are included. The ambipolar electric field and electrostatic sheath potential are evaluated self-consistently. Two models of fluid neutral transport are used to distinguish the neutral density variation for different tokamak divertor geometries. These models are incorporated into a comprehensive three-dimensional (1-D, 2 V) hybrid collisional particle-in-cell–Monte Carlo code W1 [Contrib. Plasma Phys. 34, 436 (1994)]. This code is used to investigate the effects of neutrals on divertor plasma detachment phenomena and on parallel heat and particle fluxes in the presence of strong gradients where fluid descriptions break down. Results are given for simulations of detached and attached divertor plasmas, and comparisons are made with solutions from a one-dimensional fluid model and with experimental observations. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.871615
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  • 5
    Electronic Resource
    Electronic Resource
    Kinetic effects in tokamak scrape-off layer plasmas : The 38th annual meeting of the Division of Plasma Physics (DPP) of the American Physical Society (1997)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 4 (1997), S. 1672-1680 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The short mean-free path expansion used in fluid modeling of scrape-off layer plasmas is often violated for typical discharge parameters, especially by the superthermal particles, which carry most of the heat flux. Thus, the tail of the distribution function can strongly depart from Maxwellian due to nonlocal mean-free path effects, which can modify plasma transport, impurity radiation, and plasma–neutral gas interactions. These nonlocal effects become particularly pronounced for detached plasma conditions that are characterized by sharp gradients in the plasma parameters along the magnetic field. These problems are being addressed by developing one spatial dimension and two velocity variables, fully kinetic, collisional, and time-dependent particle-in-cell code, W1 [Contrib. Plasma Phys. 34, 436 (1994)], and its parallel-computer version, PW1 [Contrib. Plasma Phys. 34, 424 (1996)]. Comparisons are made with the Fokker–Planck code ALLA [Phys. Plasmas 3, 1634 (1996)] and with experimental results. Kinetic effects on probe measurement interpretation, impurity radiation, and parallel heat conductivity due to non-Maxwellian features in scrape-off layer plasmas are considered. Heat conductivity is compared with ad hoc heat flux limit models. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.872280
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  • 6
    Electronic Resource
    Electronic Resource
    Fluid simulations of nonlocal dissipative drift-wave turbulence (1995)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 2 (1995), S. 686-701 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A two-dimensional [2d(x,y)] fluid code has been developed to explore nonlocal dissipative drift-wave turbulence and anomalous transport. In order to obtain steady-state turbulence, the y-averaged fluctuating density 〈n〉 has been forced to be zero in simulations, thus the difficulty of choosing proper sources and sinks in turbulence simulation codes has been avoided. If Ln(very-much-greater-than)Lc or Lαlc(very-much-greater-than)Lc, where Ln is the density gradient scale length, Lc the turbulence correlation length Lc, and Lαlc the adiabaticity-layer width, it has been shown that "local'' turbulence simulations give reasonable results. However, for Ln∼Lc, or Lαlc∼Lc "local'' turbulence codes are found to overestimate the flux. For a family of hyperbolic tangent background density profiles, n0(x)=nm−n1 tanh[(2x−Lx)/2Δn] with n1〈0.5nm, it has been demonstrated that the nonlocality of the turbulence leads to a transition from local gyro-Bohm (Dlocal(approximately-equal-to)7.6(Te/eB)[ρs/Ln(x)] [αlc(x)/0.01]−1/3), where αlc(x)=α(x)/κ(x)〈1, to nonlocal gyro-Bohm transport scaling [Dnonlocal(approximately-equal-to)7.6(Te/eB)(n1ρs/nmΔn) (αnlc/0.01)−1/3(Δn/40ρs)2/5 for αnlc(x)=α/κmax〈1, κ(x)=ρs/Ln(x) and α=k2(parallel)χe]. For the case Φ0(x)=−n0(x) with the model hyperbolic tangent density profiles n0(x), velocity shear increases the turbulence flux by 230% and the root-mean-square (RMS) fluctuating density by 36%. Otherwise, for Φ0(x)=n0(x), the turbulence flux is reduced by 71% and the RMS value of fluctuating density is decreased by 31% by velocity shear effects. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.871420
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  • 7
    Electronic Resource
    Electronic Resource
    Velocity diffusion in two-frequency electron cyclotron resonance heating (1986)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Fluids 29 (1986), S. 1061-1075 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Two-frequency electron cyclotron resonance heating (ECRH) is modeled by a four-dimensional mapping derived from the nonrelativistic single particle equations of motion. The model includes changes in parallel energy due to the spatially separate resonance zones. The intrinsic diffusion in parallel energy leads to axial losses in a magnetic trap which can rival those induced by collisions. An analytical model for this process gives a loss rate in good agreement with a numerical simulation. The quasilinear diffusion rate into the loss cone and the rate of Arnold diffusion through the adiabatic barrier are calculated for practical ECRH experiments, and compared with similar effects arising from interparticle collisions.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.865904
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  • 8
    Electronic Resource
    Electronic Resource
    Dissipative trapped particle modes in tandem mirrors (1985)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Fluids 28 (1985), S. 2824-2837 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Dissipative trapped particle modes are studied in tandem mirrors by including electron collisions and ion Landau damping. A variational approach is used to obtain a collisional response in terms of the collisionless result plus a collisional term. The collisional term is then self-consistently solved in all collision frequency regimes. When ν/ω(very-much-less-than)1 (ν≡electron collision frequency) and the dissipationless mode is stable through a positive (negative) charge uncovering mechanism, there are two stable waves with phase velocities in the ion (electron) diamagnetic direction. The higher frequency wave with ω∼ω@B|i(ω*e) is destabilized by ion (electron) dissipation while the lower frequency wave is destabilized by electron (ion) dissipation. At high collision frequency (ν〉ω@B|e), the only unstable trapped particle wave has ω∼ω*e, with electron collisions being destabilizing.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.865202
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  • 9
    Electronic Resource
    Electronic Resource
    Transient electrostatic potentials driven by short-pulse electron cyclotron heating (1987)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Fluids 30 (1987), S. 3540-3547 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Electrons heated in a local resonance zone of a magnetic well created either by a linear mirror or a finite aspect ratio torus are considered. If the heating has sufficient strength to drive a significant nonisotropy in the electron distribution function, an electrostatic potential variation along a field line is developed to maintain charge neutrality. The buildup of this space varying potential for times shorter than ion-transit times is determined analytically, and a limit to the number of electrons within the well that can be heated is found. The subsequent evolution of the potential on ion-transit and collision time scales is also determined. The theory is applied both to large mirror ratio devices characteristic of magnetic mirror confinement and to small mirror ratio devices characteristic of tokamaks. In the former configuration the theory is compared to experimental observations of potential buildup and decay in the multiple-mirror experiment (MMX) device [Phys. Fluids 29, 1208 (1986)] and shown to be consistent with the experiments. In the latter configuration, using the expected parameters of the Microwave Tokamak Experiment (MTX) [Proceedings of the 14th Annual Conference on Plasma Physics (Pergamon, New York, in press)], it is found that ΔΦ∼0.3Te is built up on the hot-electron-transit time scale, dropping to about 0.2Te on the ion-transit time scale, and then decaying further on a collisional time scale. Here Te is the electron temperature before heating. The potential has both poloidal and toroidal variation. Possible consequences include enhanced neoclassical transport and ion heating, and parametric excitation of low-frequency modes.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.866436
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  • 10
    Electronic Resource
    Electronic Resource
    Nonambipolar radial particle transport in a tandem mirror (1985)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Fluids 28 (1985), S. 3609-3618 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Nonambipolar transport has been measured in the tandem mirror TMX-U [Phys. Rev. Lett. 53, 783 (1984)] by applying charge conservation to the measured electron currents to the end walls. The resulting confinement time τ⊥ is found to depend upon the central-cell potential φ approximately as τ⊥(msec) =3φ(kV)−2. The transport rate, deduced from the data, agrees to within a factor of 1–5 with resonant-transport theory applied to the measured plasma parameters. Attempts to include radial effects by modeling the plasma self-consistently using resonant transport are less successful; near the axis the transport coefficients become too small to explain the equilibrium. Modeling using an ad hoc φ−2 law for the transport coefficients is more successful.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.865317
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