摘要:

Cartesian velocity moments of the Boltzmann equation are evaluated using modal solutions to the spherical harmonic oscillator as a basis set. The nonlinear collision matrix describing the interaction between any two modes is evaluated analytically for the Landau collision operator, and matrix elements describing collisions between identical particles are calculated for some pairs of azimuthally symmetric modes. First‐order linear transport coefficients calculated directly from collision matrix elements are shown to agree exactly with previously published results; coefficients of thermal conductivity and viscosity are computed much more accurately by trivially extending this calculation. Relaxation times for self‐collisions in a two‐dimensional linearized plasma are also computed, indicating that the plasma equilibrates in roughly one to ten times the Spitzer self‐collision time. The results obtained in this paper are useful for both analytic and numerical simulations of nonequilibrium plasmas and an explicit six‐moment model for a one‐component azimuthally symmetric plasma is given.

ISSN:0899-8221    

DOI:10.1063/1.860885

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The stochastic transport of MeV ions induced by low‐nmagnetic perturbations is studied, focusing chiefly on the stochastic mechanism operative forpassingparticles inlow‐frequency perturbations. Beginning with a single‐harmonic form for the perturbing field, it is first shown numerically and analytically that the stochastic threshold of energetic particles can be substantially lower than that of the magnetic field, contrary to earlier expectations, so that magnetic perturbations could cause appreciable loss of energetic ions without destroying the bulk confinement. The analytic theory is then extended in a number of directions, to clarify the relation of the present stochastic mechanism to instances already found, to allow for more complex perturbations, and to consider the more general relationship between the stochasticity of magnetic fields and that of particles of differing energies (and pitch angles) moving in those fields. It is shown that the stochastic threshold is, in general, a nonmonotonic function of energy, whose form can to some extent be tailored to achieve desired goals (e.g., burn control or ash removal) by a judicious choice of the perturbation. Illustrative perturbations are exhibited that are stochastic for low‐ but not for high‐energy ions, for high‐ but not for low‐energy ions, and for intermediate‐energy ions, but not for low or high energy. The second possibility is the behavior needed for burn control; the third provides a possible mechanism for ash removal.

ISSN:0899-8221    

DOI:10.1063/1.860886

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

In a turbulent magnetized plasma a relationship between velocity distribution of particles and its stochastic electric field is studied by solving the equation of motion both analytically and numerically. The distribution of velocity perpendicular to magnetic field direction is shown to be determined by the power spectral density of the stochastic electric field at the cyclotron frequency and the energy dissipation rate. This result will present a phenomenological understanding of high ion temperatures in several kinds of turbulent magnetized plasmas.

ISSN:0899-8221    

DOI:10.1063/1.860887

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

A recently obtained nonlocal expression for the electron heat flux valid for arbitrary ionization numbersZis used to study the structure of a plane shock wave in a fully ionized plasma. Nonlocal effects are only important in the foot of the electronic preheating region, where the electron temperature gradient is the steepest. The results are quantified as a function of a characteristic Knudsen number of that region. This work also generalizes to arbitrary values ofZprevious results on plasma shock wave structure.

ISSN:0899-8221    

DOI:10.1063/1.860888

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Resistive pressure‐gradient‐driven turbulence causes a modification of the average poloidal flow profile through Reynolds stress. The poloidal flow profile modification can be induced through a diamagnetic term and/or ‘‘seed’’ flow. This leads to several possible results for the steady‐state poloidal flow profile. If this profile is such thatV‘&thgr;≠0 at the singular surface, there is a reduction in linear growth rate and suppression of the fluctuation. WhenV‘&thgr;= 0 andV’&thgr;≠0, there is linear stabilization but the fluctuation suppression is weak.

ISSN:0899-8221    

DOI:10.1063/1.860889

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

A map model has been developed for studying the nonlinear interaction of alpha particles with the toroidal Alfve´n eigenmodes. The map is constructed by assuming a linear interaction during a single poloidal transit, which allows the study of the nonlinear interaction over many transits. By using this map, analytic expressions are obtained for the particle nonlinear bounce frequency, and the wave amplitude threshold for the onset of particle orbit stochasticity. The map model can also facilitate self‐consistent simulations which incorporate the time variation of the waves.

ISSN:0899-8221    

DOI:10.1063/1.860890

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

A new set of equations appropriate for the study of the turbulence, due to the ion temperature gradient in the slab in the fluid description, is proposed. This model is similar to many existing models including the one used in the work of Hamaguchi and Horton (HH) [S. Hamaguchi and W. Horton, Phys. Fluids B2, 1834 (1990)]. The main difference is that in this model the ion diamagnetic drift contributes to the kinetic energy in the energy balance relation. It is achieved through more complete analysis for the polarization drift due to the finite‐Larmor‐radius effects. The linear growth rate in the model is found to be smaller and the numerical results show that the heat transport is smaller by an order of magnitude when compared to HH.

ISSN:0899-8221    

DOI:10.1063/1.860891

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Positive/negative ion plasmas, composed of Ba+, SF6−, and residual electrons, were observed to display characteristics of strong turbulence. Experiments on the UCI Q machine linked the presence of negative ions (and the depletion of electrons) with large density fluctuations (&dgr;n/n&bartil;1), large‐amplitude, low‐frequency electrostatic noise (f≤20 kHz), and rapid transport of ions across magnetic field lines (D⊥&bartil;104cm2/sec). Ion velocity distributions were heated parallel to and cooled perpendicular to the confining magnetic field. The partial pressure of gaseous SF6was shown to serve as a regulator of plasma turbulence. Turbulence levels could be smoothly varied from quiescent states (&dgr;n/n&bartil;0.01) to strongly turbulent states (&dgr;n/n&bartil;1).

ISSN:0899-8221    

DOI:10.1063/1.860892

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The two‐nonlinearity model of dissipative trapped‐electron drift wave turbulence [Y.‐M. Liangetal., Phys. Fluids B5, 1128 (1993)] is generalized to include the effects of a sheared magnetic field. Because of the coupling of drift waves with the damped ion‐acoustic modes, the eigenmodekhas a radial structure centered on its rational surface characterized by a mode width &Dgr;k. In this work, both the linear properties of the model, and the multiple‐helicity nonlinear dynamics, including the interactions of the two nonlinearities, i.e., theE×Bdrift and the polarization drift nonlinearities, are analyzed in detail. In particular, a one‐point renormalization is performed to investigate the nonlinear eigenmode properties and the saturation of the system in the multiple‐helicity limit. It is shown that, for the ultrastrong magnetic shear limit where &Dgr;k≤2&rgr;s, the polarization drift nonlinearity is the dominant nonlinear transfer mechanism. However, for the moderate‐to‐weak magnetic shear limit where &Dgr;k≫2&rgr;s, both nonlinearities and cross‐coupling effects will affect the nonlinear transfer processes. An analytic explanation of previous computational observation of the suppression of magnetic shear damping by turbulence [D. Biskamp and M. Walter, Phys. Lett. A109, 34 (1985)] is also given.

ISSN:0899-8221    

DOI:10.1063/1.860893

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

There exist a number of observations of magnetohydrodynamic (MHD) activity that can be related to resistive MHD modes localized near the plasma boundary. To study the stability of these modes, a free boundary description of the plasma is essential. The resistive plasma–vacuum boundary conditions have been implemented in the fully toroidal resistive spectral codecastor(Complex Alfve´n Spectrum in Toroidal Geometry) [Proceedingsofthe18thConferenceonControlledFusionandPlasmaPhysics, Berlin, edited by P. Bachmann and D. C. Robinson (European Physical Society, Petit‐Lancy, Switzerland, 1991), p. 89]. The influence of a free boundary, as compared to a fixed boundary on the stability of low‐mtearing modes, is studied. It is found that the stabilizing (toroidal) effect of a finite pressure due the plasma compression is lost in the free boundary case for modes localized near the boundary. Since the stabilization due to the favorable average curvature in combination with a pressure gradient near the boundary is small, the influence of the pressure on the stability is much less important for free boundary modes than for fixed boundary modes.

ISSN:0899-8221    

DOI:10.1063/1.860894

出版商:AIP    

年代:1993

数据来源: AIP