摘要:

Nonlinear evolution of drift instabilities in the presence of electron–ion collisions in a shear‐free slab has been studied by using gyrokinetic particle simulation techniques as well as by solving, both numerically and analytically, model mode‐coupling equations. The purpose of the investigation is to determine the mechanisms responsible for the nonlinear saturation of the instability and for the ensuing steady‐state transport. Such an insight is very valuable for understanding drift‐wave problems in more complicated geometries. The results indicate that the electronE×Bconvection is the dominant mechanism for saturation. It is also found that the saturation amplitude and the associated quasilinear diffusion are greatly enhanced over their collisionless values as a result of weak collisions. In the highly collisional (fluid) limit, there is an upper bound for saturation withe&fgr;/Te&bartil;(&ohgr;l/&OHgr;i)/(k⊥  &rgr;s)2. The associated quasilinear diffusion, which increases with collisionality, takes the form ofDql&bartil;&ggr;l/k2⊥, where &ohgr;land &ggr;lare the linear frequency and growth rate, respectively. In the steady state, the diffusion process becomes stochastic in nature. The relevant mechanisms here are related to the velocity‐space nonlinearities and background fluctuations. The magnitude of the diffusion at this stage can be comparable to that of quasilinear diffusion in the presence of collisions, and it seems to remain finite even in the collisionless limit.

ISSN:0031-9171    

DOI:10.1063/1.866393

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

The power dependence of the wave damping in a tokamak plasma for arbitrary direction of propagation, mode of polarization, and wave frequency is investigated. Using a 2‐D Fokker–Planck code, it is shown that the wave damping increases or decreases with the rf power depending on the velocity range of the absorbing electrons.

ISSN:0031-9171    

DOI:10.1063/1.866394

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

The stellarator neoclassical transport caused by particles trapped in local helical wells is calculated in the low‐collisionality regime using a systematic expansion. The behavior of electron transport is found to be the same over a wide range of energies but the behavior of ion transport for low‐energy ions is found to be different than that for high‐energy ions. Furthermore, the electron fluxes do not vary with the change in the radial ambipolar electric field nearly as much as do the ion fluxes. Thus the particle diffusion is controlled by the electrons. A nonradial ambipolar electric field is induced by ion drift. This electric field enhances the transport by about 15%–20%. A convenient graphical method that allows one to determine the magnitude of the radial ambipolar field for machines with different parameters is presented. Numerical examples show that electron energy confinement time is comparable to the ion energy confinement time for all the different size stellarators studied. Although the neoclassical losses are large, it is shown that ignition can be achieved in a reasonably sized stellarator reactor. Finally, fron the standpoint of reactor economics, the confinement scaling law shows that in order to increasen&tgr;, it is better to increase the aspect ratio than the overall dimensions of the reactor.

ISSN:0031-9171    

DOI:10.1063/1.866395

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

An analytic calculation of the neoclassical fluxes in the plateau regime in the absence of momentum and energy sources is carried out with a general model field,B=B0[1+∑i&egr;i cos(li&thgr;−mi&zgr;)], following the moment‐equation approach. It is shown that the radial particle and heat fluxes resulting from each harmonic (&egr;i) are additive and have the same sign. Consequently, the neoclassical fluxes in a stellarator in the plateau regime are larger than those in the corresponding ‘‘equivalent’’ tokamak (same aspect ratio and rotational transform). Numerical calculations performed with a drift kinetic equation solver agree to within 10% with the analytic results for the examples considered.

ISSN:0031-9171    

DOI:10.1063/1.866396

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

Differential rotation characteristic of the tandem mirror central cell is shown to have a significant stabilizing effect on the rotation and unfavorable curvature driven drift modes. Stability boundaries and parametric dependence of the growth rate and frequency of them=1 andm=2 modes are given as a function of the passing electron density fraction, wall‐to‐plasma radius, and rotation speed.

ISSN:0031-9171    

DOI:10.1063/1.866397

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

It is shown that for trapped particle modes in tandem mirrors, the pressure of the passing particles in the anchor region introduces a stabilizing term proportional to the sum of the anchor’s field line curvature and total diamagnetic pressure gradient. The theory is applied to the proposed gas dynamic trap experiment.

ISSN:0031-9171    

DOI:10.1063/1.866398

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

A variety of passive stabilization techniques compatible with S‐1 inductive spheromak formation [Phys. Rev. Lett.46, 188 (1981)] has been evaluated for effectiveness against the magnetohydrodynamic (MHD) tilting instability. A line‐tying stabilization effect has been previously identified theoretically. In the present work, the effects have been experimentally identified and investigated. The perturbed current patterns within passive stabilizing structures have been measured, and line‐tying currents have been identified. The simple ring model of tilting has been expanded to include line tying, and has provided estimates for growth rates which agree well with the measured values.

ISSN:0031-9171    

DOI:10.1063/1.866399

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

A three‐level atomic model is used for determining the steady‐state collisional‐radiative coefficients in nonequilibrium partially ionized argon plasmas. Rate equations for populations of the atomic levels are coupled to an electron Boltzmann equation that includes inelastic processes. The solution of these coupled equations yields an analytical form for the non‐Maxwellian electron distribution function. The reabsorption of radiation in the plasma is included through the use of Holstein radiation escape factors [Phys. Rev.83, 1159 (1951)]. The cross sections in this model are included in the form of simple analytical expressions obtained from a numerical fitting of available experimental data. The results and limitations of the approach are discussed in a broad range of plasma conditions (electron temperature below 25 000 °K and atomic density of 1014–1018cm−3).

ISSN:0031-9171    

DOI:10.1063/1.866400

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

The energy radiated by laser‐produced Al plasmas is calculated using a one‐dimensional Lagrangian code and a collisional radiative equilibrium model. The predicted spectra are used to calculate the energy transmitted through Al foils of various thicknesses, and comparisons are made with experimental data. The sensitivity of the transmitted energies to the flux limiterf, used in the simulation is assessed, and it is suggested that this could be a useful independent technique for testing heat transport models in simulations.

ISSN:0031-9171    

DOI:10.1063/1.866401

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

The preheat of the cold substrate by the x rays generated in the coronal region of a laser produced aluminum plasma was calculated for a trapezoidal laser pulse in the intensity rangeI=1013–8×1014W/cm2. The computer code incorporates nonlocal thermodynamic equilibrium (non‐LTE) radiation emission rates in the hot region and photoionization as the main photoabsorbing process in the cold compressed region. The temperature and the density variations of the photoionization cross section were accounted for. Black‐body radiation diffusion was also included. The results indicate the formation of a well‐defined radiation heated zone in the back portion (close to the ablation surface) of the shock wave. Comparisons were made on the effects of the x‐ray radiation and the black‐body diffusion on the shock wave propagation. The similarities and differences between x‐ray and fast electron preheat are also discussed. In a different set of computations an attempt was made to simulate the measurements of McLeanetal. [Phys. Rev. Lett.45, 1246 (1980)] of the backsurface temperature of laser irradiated aluminum foils. The computational results agree favorably with those of the experiments.

ISSN:0031-9171    

DOI:10.1063/1.866348

出版商:AIP    

年代:1987

数据来源: AIP