摘要:

Application to a tokamak of electron‐cyclotron resonant heating (ECRH) injected at a given poloidal angle will result in a small but important variation of average electron magnetic moment with poloidal angle. This, coupled with the dependence of magnetic field strength on poloidal angle, will result in the transmission of a net toroidal force to the electrons, the direction and magnitude of which depend on the poloidal angle at which the ECRH is injected and the direction of velocity of the electrons by which it is absorbed. The geometries for which this mechanism adds to the Fisch–Boozer mechanism [Phys. Rev. Lett.45, 720 (1980)] are described, as are those for which this mechanism is in opposition. Taking this mechanism into account, it is demonstrated that electrons at large minor radii can be driven with significantly greater efficiency if resonance is on the inboard side of the tokamak than if it is on the outboard side, and the ratio of the two efficiencies is calculated for the case of Coulomb collisionality in the limit of large Larmor radius. The case of turbulence‐dominated collisionality is considered, and a general prediction of the ratio of the efficiency of this drive mechanism to that of the Fisch–Boozer mechanism is presented, in which the ratio of the efficiencies is given in terms of the dependence of the slowing‐down time on velocity. The mechanism described herein provides a significant correction to predictions of current drive efficiency for infinite Larmor radius Coulomb collisionality, and is likely to provide a more significant correction yet for more realistic models of collisionality.

ISSN:0031-9171    

DOI:10.1063/1.865982

出版商:AIP    

年代:1986

数据来源: AIP

摘要:

The stability of high‐beta plasmas with helical magnetic axis is studied by investigating the linearized magnetohydrodynamic equations. It is shown that the stability of shearless configurations is determined by an ordinary differential equation that describes modes with large variations across the flux surfaces.

ISSN:0031-9171    

DOI:10.1063/1.865983

出版商:AIP    

年代:1986

数据来源: AIP

摘要:

Pinch‐point instability analysis is used to study the effects of finite temperature on the time‐asymptotic pulse shapes of electrostatic and electromagnetic two‐stream instabilities. Their absolute or convective instability nature is established over the entire regime from instability threshold at finite temperatures to the cold‐plasma hydrodynamic limit. The analysis is based upon nonrelativistic Vlasov theory dispersion relations for streams and plasmas with Maxwellian thermal distributions.

ISSN:0031-9171    

DOI:10.1063/1.865984

出版商:AIP    

年代:1986

数据来源: AIP

摘要:

The motion of a tenuous bunch of fast electrons through an isotropic plasma has been studied using a one‐dimensional Vlasov code [J. Comp. Phys.27, 315 (1978)]. Given a fairly large initial perturbation from equilibrium, the moving bunch sheds a train of nonlinear plasma oscillations, which appear to be sustained by a mild instability.

ISSN:0031-9171    

DOI:10.1063/1.865991

出版商:AIP    

年代:1986

数据来源: AIP

摘要:

The linearized Vlasov–Maxwell equations are used to investigate harmonic stability properties for a planar wiggler free‐electron laser (FEL). The analysis is carried out in the Compton regime for a tenuous electron beam propagating in thezdirection through the constant‐amplitude planar wiggler magnetic fieldB0=−Bw cos k0zeˆx. Transverse spatial variations are neglected (∂/∂x=0=∂/∂y), and the case of an FEL oscillator (temporal growth) is considered. Assuming ultrarelativistic electrons and &kgr;2=a2w/(&ggr;20−1) ≪1, wherea2w=e2B2w/m2c4k20and &ggr;0mc2is the electron energy, the kinetic dispersion relation is derived in the diagonal approximation for perturbations about general beam equilibrium distribution functionG+0(&ggr;0). Because of the wiggler modulation of the axial electron orbits, strong wave–particle interaction can occur for &ohgr;≊[k+k0(1+2l)] &bgr;Fc, where &bgr;Fcis the axial velocity, &ohgr; andkare the wave oscillation frequency and wavenumber, respectively, andl=0, 1, 2, . . . are harmonic numbers corresponding to an upshift in frequency. The strength of thelth harmonic wave–particle coupling is proportional toKl(b1) =[Jl(b1)−Jl+1(b1)]2, whereb1=(k/8k0)&kgr;2. Assuming thatG+0(&ggr;0) is strongly peaked around &ggr;0=&ggr;ˆ≫1, detailedlth harmonic stability properties are investigated for (a) strong FEL instability corresponding to monoenergetic electrons (&Dgr;&ggr;=0), and (b) weak resonant FEL instability corresponding to a sufficiently large energy spread that ‖Im &ohgr;/[k+k0(1+2l)] &Dgr;vz‖≪1.For monoenergetic electrons the characteristic maximum growth rate scales as [Kl(bˆ1)(1+2l)]1/3, which exhibits a relatively weak dependence on harmonic numberl. Here,bˆ1= 1/2 [a2w/(2+a2w)] (1+2l). On the other hand, for weak resonant FEL instability, the growth rate scales asKl(bˆ1)/(1+2l), which decreases rapidly for harmonic numbersl≥1.

ISSN:0031-9171    

DOI:10.1063/1.865992

出版商:AIP    

年代:1986

数据来源: AIP

摘要:

Analytic and numerical solutions for the problem of the interaction of intense ion beams with matter in the form of plane targets are considered in this paper. The theory of the interaction of protons with matter at low energies is discussed and calculations are presented for the energy loss of protons in aluminum and gold. Zero‐ and one‐dimensional models are developed and the results are compared to numerical simulations carried out with the one‐dimensional Lagrangian hydrodynamic codemedusa[Comp. Phys. Comm.1, 271 (1974)], which has been extended to include the various physical effects needed to carry out realistic simulations of the interaction of ion beams with matter. The theory and simulation of the acceleration of foils by intense ion beams is also considered and representative results are given. The theoretical results are used to investigate the optimum conditions in which to carry out stopping power experiments for ions in hot, dense plasmas, so that the theory can be tested. These results are needed in order to perform more realistic pellet calculations for inertial fusion.

ISSN:0031-9171    

DOI:10.1063/1.865993

出版商:AIP    

年代:1986

数据来源: AIP

摘要:

Experiments on the expansion of a plasma into the wake region behind an object inserted in a flowing plasma are described. The dominant expansion mechanism that fills in the wake is determined to be a self‐similar expansion. Accelerated ions with a velocity greater than the local ion‐acoustic velocity are detected ahead of the self‐similar expansion front. In a plasma with two species, two groups of accelerated ions are detected.

ISSN:0031-9171    

DOI:10.1063/1.865994

出版商:AIP    

年代:1986

数据来源: AIP

摘要:

Experiments on the propagation of an ion‐acoustic soliton into a region of inhomogeneous plasma density are described. It is found that the local amplitude and velocity decrease and the width increases as the soliton propagates into a region of lower density. The results are in good agreement with a model based on the KdV equation and with the recent study of Kuehl and Imen [Phys. Fluids28, 2375 (1985)].

ISSN:0031-9171    

DOI:10.1063/1.865995

出版商:AIP    

年代:1986

数据来源: AIP

摘要:

A detailed experimental study of the bootstrap and Pfirsch–Schlu¨ter currents that are predicted by neoclassical transport theory is presented. In a toroidal octupole, on magnetic surfaces within the separatrix, the observed parallel plasma currents are in excellent quantitative agreement with neoclassical theory with regard to the spatial structure (along a magnetic surface), collisionality dependence, and toroidal magnetic field dependence. On magnetic surfaces outside the separatrix, the ion portion of the parallel current is in agreement with neoclassical theory, but the electron parallel current is observed to obtain a unidirectional component that deviates from and exceeds the theoretical prediction.

ISSN:0031-9171    

DOI:10.1063/1.865996

出版商:AIP    

年代:1986

数据来源: AIP

摘要:

Electron temperature fluctuations have been investigated in the edge region of the Caltech research tokamak [S. J. Zweben and R. W. Gould, Nucl. Fusion25, 171 (1985)], and an upper limit to this fluctuation level was found atT˜e/Te≲15%. This measurement, together with previous measurements of density and electric and magnetic field fluctuations, allows a unique comparison of the heat transport resulting from three basic turbulent mechanisms: (1) heat flux from the particle flux resulting from microscopic density and electric field fluctuations; (2) thermal conduction resulting from microscopic temperature and electric field fluctuations; and (3) thermal conduction resulting from microscopic magnetic field fluctuations. The measurements indicate that, in the edge regions, the electron heat transport caused by the measured turbulence‐induced particle flux is comparable to or greater than that caused by the thermal conduction associated with the electron temperature and electric field fluctuations, and is significantly greater than that resulting from the measured magnetic fluctuations. This electron heat loss caused by the plasma turbulence is found to be an important electron energy loss mechanism in the edge regions.

ISSN:0031-9171    

DOI:10.1063/1.865942

出版商:AIP    

年代:1986

数据来源: AIP