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  • 1990-1994  (13)
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Material
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Year
  • 1
    Electronic Resource
    Electronic Resource
    Analytic model for the runaway distribution function in the presence of spatial diffusion (1994)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 1 (1994), S. 684-690 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A steady-state kinetic model for runaway electrons in the presence of radial diffusion in a stochastic magnetic field is adopted and solved for a constant magnetic diffusivity. The model is constructed to recover the correct runaway production rate in the absence of spatial diffusion. The parallel energetic electron distribution function f is found by matching the solutions from three regions in parallel velocity space and is employed to form moments of f. Upper and lower bounds on the spatial diffusion are obtained by using these moments and the model exhibits the strong sensitivity to collisionality needed to explain the difference between similar plasmas with little or no hard x-ray signal and those with significant hard x-ray signals.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.870814
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    Paper (German National Licenses)
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  • 2
    Electronic Resource
    Electronic Resource
    A quasilinear, Fokker–Planck description of fast wave minority heating permitting off-axis tangency interactions (1994)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 1 (1994), S. 52-63 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The off-axis quasilinear fast wave minority heating description of Catto and Myra [Phys. Fluids B 4, 187 (1992)] has been improved and implemented in a code which solves the combined quasilinear and collision operator equation for the minority distribution function. Geometrical complications of a minority resonance nearly tangent to a flux surface in the presence of trapped as well as passing particles are retained. The tangency interactions alter the moments and the fusion reaction rate parameter in a model which explores heating on a single flux surface. The strong tangency interactions enhance the more familiar interactions due to trapped particles turning in the vicinity of the minority resonance. An asymmetry in off-axis heating effects occurs because heating on the low field side of the magnetic axis heats more trapped particles than high field side heating. This asymmetry is responsible for the better performance of the low field side case relative to the high and on-axis cases and provides some control over the power absorbed by and the energy stored in the trapped particles.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.870558
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    Paper (German National Licenses)
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  • 3
    Electronic Resource
    Electronic Resource
    Far field sheaths from waves in the ion cyclotron range of frequencies (1994)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 1 (1994), S. 2890-2900 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Experimental evidence suggests that unabsorbed wave energy in ion cyclotron range of frequency fast wave (FW) experiments can result in deleterious edge interactions. A model describing the formation of far field sheaths due to FW interaction with material surfaces is presented. Near conductors that do not conform to flux surfaces, an incoming FW causes the generation of a slow wave (SW) component. The E(parallel) of the SW drives an RF sheath, in a manner similar to what has been previously discussed for antenna (near field) sheaths. To assess the importance of the proposed mechanism, a heuristic scaling model of the resultant sheath voltage V is developed and compared with a numerical code. The model illustrates the important dependencies of V on the single pass absorption, edge density, FW frequency, FW cutoff location, and limiter/wall geometries and yields qualitative agreement with the experimental observations.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.870529
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    Paper (German National Licenses)
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  • 4
    Electronic Resource
    Electronic Resource
    Quasilinear diffusion in stochastic magnetic fields: Reconciliation of drift-orbit modification calculations (1993)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 5 (1993), S. 1160-1163 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: An apparent difference between two calculations of drift-orbit modifications to the spatial diffusion rate of particles in stochastic magnetic fields is addressed [H. E. Mynick and J. A. Krommes, Phys. Rev. Lett. 43, 1506 (1979); J. R. Myra and P. J. Catto, Phys. Fluids B 4, 176 (1992)]. The calculations are reconciled by noting the relevance of an inequality which was not discussed in these studies. It is shown, both analytically and by Monte Carlo simulation, that the diffusion coefficient can be sensitive to the spectral width of the magnetic turbulence relative to a finite Larmor radius parameter.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.860906
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    Paper (German National Licenses)
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  • 5
    Electronic Resource
    Electronic Resource
    Radio-frequency-sheath-driven edge plasma convection and interaction with the H mode (1993)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 5 (1993), S. 3603-3617 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: It is shown that radio-frequency (rf) antenna sheaths can bias the edge plasma potential and drive steady-state convective cells in the scrape-off layer (SOL). The resulting E×B convective flow opposes the direction of the sheared flow in the SOL induced by the radially decaying Bohm sheath potential. A two-dimensional fluid simulation shows that the interaction of the opposing poloidal flows produces secondary vortices, which connect the edge of the confined plasma to the antenna limiters, when the antenna–plasma separation is typically of order a few times the local electron skin depth at the antenna. Estimates for typical tokamak edge parameters suggest that the transit time of particles and energy across these vortices is rapid enough to cause the broadening of SOL density and temperature profiles observed during high-power heating with ion cyclotron range of frequency (ICRF) antennas in monopole phasing. Radio-frequency-sheath-driven convection is also a good candidate to explain the phasing dependence of the global confinement properties of ICRF H modes on the Joint European Torus (JET) [Fusion Technol. 11, 13 (1987)]. A comparison of the JET H-mode data with the theoretical modeling supports this idea and suggests that ICRF convection may be a useful tool to spread the heat deposition in the divertor and to extend the lifetime of the H mode.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.860832
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    Paper (German National Licenses)
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  • 6
    Electronic Resource
    Electronic Resource
    Effect of drifts on the diffusion of runaway electrons in tokamak stochastic magnetic fields (1992)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 4 (1992), S. 176-186 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The quasilinear diffusion of runaway electrons in tokamak stochastic magnetic fields is examined. Previous models are generalized with respect to the spatial location and coherency of the perturbing magnetic fields, treatment of the radial as well as poloidal drift motion of the electrons, and the role of sidebands that arise from the beating of the electron drift motion with the applied perturbing fields. It is found that drift effects act to reduce the level of quasilinear diffusion by an amount that depends on the poloidal distribution of the magnetic turbulence. The results are employed to estimate the internal magnetic fluctuation levels at the edge during recent experiments on the TEXT tokamak [Phys. Fluids B 3, 2038 (1991)], where the drift modification effects are shown to be small. It is inferred that intrinsic magnetic turbulence controls runaway diffusion, but not the thermal diffusivity of the background electrons.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.860431
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    Paper (German National Licenses)
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  • 7
    Electronic Resource
    Electronic Resource
    A quiver kinetic formulation and evaluation of fluxes in the long mean-free path limit (1990)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 2 (1990), S. 2395-2407 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Intense, applied radio frequency (rf) fields can cause one or more charged particle species to oscillate with a quiver speed comparable to its thermal speed. When collisional nonresonant wave particle processes dominate over collisionless resonant interactions, the quiver kinetic formalism [Phys. Fluids B 1, 1193 (1989)] may be employed. The intense wave fields in the edge plasma of an rf heated tokamak satisfy these criteria and are investigated in this paper. Previous work is extended to permit an evaluation of particle fluxes near the last closed flux surface in the long mean-free path limit. Two types of convective fluxes are found, one local to the region of intense fields and one nonlocal. The magnetization flux of electrons (which is local) is shown to be the dominant convective flux in a simple illustrative model for the case of fast wave ion cyclotron heating.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.859505
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    Paper (German National Licenses)
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  • 8
    Electronic Resource
    Electronic Resource
    Runaway electrons as a diagnostic of magnetic fluctuations in the edge plasma of the Texas Experimental Tokamak (1992)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 4 (1992), S. 2092-2097 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Recent experimental and theoretical results are combined to explore the energy dependence of runaway and suprathermal electron diffusion. It is shown that a quasilinear theory containing both magnetic and electrostatic E×B-driven radial transport is consistent with the available experimental data, and supports the notion that magnetic turbulence dominates runaway diffusion but electrostatic turbulence dominates thermal diffusion in the edge plasma of the Texas Experimental Tokamak (TEXT) [Nucl. Technol./Fusion 1, 479 (1981)]. These observations further expand the possibilities for employing runaway or suprathermal electrons as a diagnostic of the underlying transport mechanisms in a tokamak plasma.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.860016
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    Paper (German National Licenses)
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  • 9
    Electronic Resource
    Electronic Resource
    A quasilinear description for fast-wave minority heating permitting off-magnetic axis heating in a tokamak (1992)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 4 (1992), S. 187-199 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: By keeping only the essential physical effects the simplest as possible, experimentally relevant quasilinear description of fast-wave minority heating in a tokamak is derived in a tractable form suitable for convenient numerical implementation. The resulting quasilinear operator retains all trapped and passing particle effects and permits the minority heating resonance line to be offset from the magnetic axis. Rather than averaging the infinite, homogeneous quasilinear operator, the derivation is carried out in tokamak geometry to properly treat the merging of distinct rf–particle interaction regions. Merging occurs when the banana tips of trapped particles are in the vicinity of the minority resonance or, for both trapped and passing particles, when the minority resonance is tangent or nearly tangent to a flux surface. A phenomenological model of the influence of collisions on merging is also presented.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.860432
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    Paper (German National Licenses)
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  • 10
    Electronic Resource
    Electronic Resource
    Modeling and simultaneous sawtooth measurements of the thermal and energetic electron diffusivities from the Texas Experimental Tokamak (1993)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 5 (1993), S. 125-137 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Convenient mathematical models appropriate for simultaneously measuring the thermal and energetic electron diffusivities by their responses to internal disruptions are presented and compared to simultaneous soft and hard x-ray sawtooth data from the Texas Experimental Tokamak (TEXT) [Nucl. Technol./Fusion 1, 479 (1981)]. The eigenfunction expansion technique employed is first illustrated on a constant diffusivity, single sawtooth crash model that is unable to give a satisfactory fit to the TEXT soft x-ray data at disparate radii for an initial condition of a flattened temperature profile out to the mixing radius. A more sophisticated single crash model having a parabolic radial dependence for the thermal diffusivity and a flattened profile initial condition substantially improves the fit to the TEXT soft x-ray data. Simultaneous measurements of the sawtooth oscillations on the hard x-ray signal caused by runaway electrons hitting the limiter are interpreted to obtain a measure of the diffusivity of the energetic electrons in TEXT following the same crash used to measure the thermal diffusivity. A single sawtooth crash model is no longer adequate because the diffusivity of the energetic electrons may be substantially less than the thermal value. Therefore the eigenfunction expansion technique is extended to a periodic sawtooth crash model. Measurements of diffusivities far smaller than those that can be measured by a single crash model are possible and more than one value of diffusivity can fit the simultaneous TEXT hard x-ray data. The lower values of diffusivity could not have been obtained by a single crash model. Additional hard x-ray measurements having differing sawtooth periods are needed to remove the ambiguity. As more extensive hard x-ray data become available on TEXT, it may become possible to simultaneously and unambiguously measure the thermal and energetic electron diffusivities during the same crash.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.860860
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    Paper (German National Licenses)
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