摘要:

The electron‐cyclotron maser instability is studied for the case of an anisotropic electron velocity distribution in the regime where the relativistic corrections to the wave dispersion are significant. Solution of the linear dispersion relation reveals that when the plasma frequency–gyrofrequency ratio &ohgr;pe/&OHgr;e<vte/c, the instability is localized just belowk⊥c/&OHgr;e=1. The growth rate is then strongly peaked for emission at 90° to the magnetic field and is considerably larger than would be the case if the cold‐plasma dispersion theory were valid. These features are confirmed by electromagnetic particle simulations. The simulations also show that saturation results from perpendicular diffusion in velocity space and that the saturation level increases as &ohgr;pe/&OHgr;eis decreased. A quasilinear analysis predicts that the saturation level scales as (&OHgr;e/&ohgr;pe)2&ohgr;maxI, where &ohgr;maxIis the maximum linear growth rate. Applications of the maser instability to the generation of the Earth’s auroral kilometric radiation are discussed.

ISSN:0031-9171    

DOI:10.1063/1.865492

出版商:AIP    

年代:1986

数据来源: AIP

摘要:

The region of velocity space is determined in which electron runaway occurs because of a dc electric field. Phase‐space analysis of the relaxation equations describing test electrons, corroborated by two‐dimensional (2‐D) numerical integration of the Fokker–Planck equation, reveals that the Dreicer condition for runawayv2∥≥(2+Zi)Ec/Eis only sufficient. A weaker conditionv2∥≥(2+Zi)1/2Ec/Eis established, and it is shown, in general, that runaway in velocity space only occurs for those electrons that are outside one of the separatrices of the relaxation equations. The scaling withv∥of the parallel distribution function and of the perpendicular temperature is also derived.

ISSN:0031-9171    

DOI:10.1063/1.865493

出版商:AIP    

年代:1986

数据来源: AIP

摘要:

A simulation model for steady‐state, lower hybrid current drive is described which incorporates a relativistic, one‐dimensional Fokker–Planck calculation and a toroidal ray tracing code. Two‐dimensional (v⊥) effects are included in the Fokker–Planck analysis in the form of a large perpendicular electron temperature that results from pitch angle scattering. An increase in the parallel refractive index of the lower hybrid waves, arising from toroidal geometry effects, is proposed as a physical mechanism whereby injected rf waves at high phase velocity (ve≪v∥≲c) can interact via the Landau resonance with electrons at low phase velocity (v∥≲3ve). Numerical results relevant to the Alcator C [Phys. Rev. Lett.53, 450 (1984)] and PLT [Phys. Rev. Lett.49, 1255 (1982)] experiments are presented which demonstrate the dependency of the current drive efficiency on various plasma parameters.

ISSN:0031-9171    

DOI:10.1063/1.865494

出版商:AIP    

年代:1986

数据来源: AIP

摘要:

A variational principle is developed for computing accurate values of local plasma transport coefficients in nonsymmetric toroidal confinement configurations. Numerical solutions of the linearized drift Fokker–Planck equation are used to obtain the thermodynamic fluxes as functions of collision frequency and the radial electric field. Effects resulting from the variation of the longitudinal adiabatic invariantJalong an orbit (resulting from particle transitions from helically trapped to toroidally trapped orbits) are treated. The velocity‐space distribution resulting from trapped, circulating, and transition particle orbits is well represented by a Legendre polynomial expansion in the pitch angle coordinate. The computational effort is significantly reduced from that required with Monte Carlo methods through use of an efficient treatment of the disparity between the time scales of collisionless and collisional particle dynamics. Numerical computations for a stellarator configuration are presented.

ISSN:0031-9171    

DOI:10.1063/1.865495

出版商:AIP    

年代:1986

数据来源: AIP

摘要:

A theory describing the influence of energetic trapped particles on resistive‐interchange‐ballooning modes in tokamaks is presented. It is shown that a population of hot particles trapped in the region of adverse curvature can resonantly interact with and destabilize the resistive interchange mode, which is stable in their absence because of favorable average curvature. The mode is different from the usual resistive interchange mode not only in its destablization mechanism, but also in that it has a real component to its frequency comparable to the precessional drift frequency of the rapidly circulating energetic species. Corresponding growth rate and threshold conditions for this trapped‐particle‐driven instability are derived and finite banana width effects are shown to have a stabilizing effect on the mode. Finally, the ballooning/tearing dispersion relation is generalized to include hot particles, so that both the ideal and the resistive modes are derivable in the appropriate limits.

ISSN:0031-9171    

DOI:10.1063/1.865496

出版商:AIP    

年代:1986

数据来源: AIP

摘要:

The resistive Alfve´n spectrum for tokamaklike configurations with one or two singular surfaces is analyzed by combined WKBJ treatment and numerical solution using a spectral code. It is found that the ideal continua disappear; these are approximated only at the end points, at the singular surfaces, and at selected interior points (given by extrema of &ohgr;A(r)=‖k ⋅ B0‖/(&rgr;0)1/2). The normal modes are damped and the eigenvalues lie on specific curves that become independent of resistivity for small &eegr;.

ISSN:0031-9171    

DOI:10.1063/1.866002

出版商:AIP    

年代:1986

数据来源: AIP

摘要:

A fully three‐dimensional ray tracing code that includes a realistic antenna pattern, toroidal effects, and refraction is used to calculate the absorption by non‐Maxwellian distributions of the extraordinary mode in the nonrelativistic limit away from normal incidence. The ray tracing approach extends results obtained by previous authors who used a relativistic treatment in slab geometry to a more realistic configuration and a broader range of parameters. A monotonically decreasing superthermal tail in the distribution of electron velocities parallel to the tokamak magnetic field is represented by adding a drifted Maxwellian distribution to a bulk Maxwellian distribution. This combines a high degree of flexibility in the set of tail parameters available with the possibility of obtaining exact analytic expressions for the dielectric tensor elements. At low densities, small tails (tail fraction &bartil;0.5%) can significantly alter the wave absorption characteristics, including the integrated absorption, spatial deposition profile, and velocity space deposition profile. Because of the presence of tail electrons, significant heating is found for high field side launching in a high‐density plasma where the fundamental cyclotron resonance is cut off.

ISSN:0031-9171    

DOI:10.1063/1.865497

出版商:AIP    

年代:1986

数据来源: AIP

摘要:

A combined numerical and analytical study is presented of the linear stability problem for resistive tearing modes in the presence of perpendicular and parallel equilibrium flows. Detailed information is given about growth rates and the shape of the marginal stability curve. In the case of purely perpendicular flows, a region in parameter space is found where the growth rate is complex, and the region of instability is larger than previously recognized. An exact analytic solution is found for parallel flows when the plasma response is dominated by viscous forces.

ISSN:0031-9171    

DOI:10.1063/1.865498

出版商:AIP    

年代:1986

数据来源: AIP

摘要:

Beta is computed self‐consistently in reversed field pinches, using reduced magnetohydrodynamics with transport. Qualitative agreement with the OHTE experiment [Bull. Am. Phys. Soc.29, 330 (1984)] is found. A derivation of the finite beta reduced equations is presented. The relation of the dynamo effect to anomalous transport is described. The scaling of the &dgr;B/BwithSis obtained.

ISSN:0031-9171    

DOI:10.1063/1.865499

出版商:AIP    

年代:1986

数据来源: AIP

摘要:

The issue of proper boundary conditions when performing magnetohydrodynamic simulations of the reversed field pinch is examined. Of particular concern is the choice of constant current, which when combined with other commonly used boundary conditions may, under careless implementation, lead to an inconsistency. It is shown that this may cause erroneous results. Cases both with and without Hall terms are presented.

ISSN:0031-9171    

DOI:10.1063/1.865461

出版商:AIP    

年代:1986

数据来源: AIP