摘要:

The subsonic limit of the quadratic direct interaction approximation (DIA) applied to the Zakharov equations is compared with the cubic DIA applied to the nonlinear Schro¨dinger equation, which is the subsonic limit of the Zakharov equations. Comparisons with Monte Carlo simulations of a truncated system show that the first theory more accurately describes the regime of stationary turbulence, while the second theory more accurately describes the subsonic evolution of the modulational instability. The weak turbulence limits of the two theories describe the sonic and subsonic regimes, respectively. The addition of vertex corrections to the DIA leads to a hybrid weak turbulence theory that smoothly interpolates between the sonic and subsonic regimes.

ISSN:0031-9171    

DOI:10.1063/1.865181

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

An expression for the ponderomotive potential, which is applicable to the case where the scale length of rf amplitude variation is comparable to the excursion length and the Larmor radius, is presented. This expression is used to explain the empirical scaling law for the potential.

ISSN:0031-9171    

DOI:10.1063/1.865182

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

It is shown that the modified moment method applied to the Vlasov–Landau equation is adequate for accounting for the heat flux inhibition in laser‐driven plasmas, if the temperature profile is properly taken into account in calculating the heat flux. The temperature profiles are obtained by solving the steady‐state mass, momentum, and energy balance equations in which the heat flux is a nonlinear function of the temperature gradient derived from the kinetic equation by means of the modified moment method. There can be two different temperature profiles, depending on the Mach number of the fluid speed. When the temperature profiles thus calculated are self‐consistently used in the heat flux formula, the heat flux limit factors are derived. The factors are of the same order of magnitude as the experimentally inferred value of 0.03. For example, in the case of the hydrogen plasma the factor is less than 0.05 for the supersonic branch.

ISSN:0031-9171    

DOI:10.1063/1.865183

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

The hydrodynamic flow, called the ablative heat wave, from a dense body which is suddenly brought into contact with a thermal bath with a temperature of the order of 105–108°K is investigated theoretically. It is shown that the hydrodynamic equations with nonlinear conduction admit a self‐similar solution of the asymptotic type, i.e., approached by the flow fort→∞. Self‐similarity is a consequence of the fact that the solid may be considered as infinitely dense in this limit, even for a bath temperature varying in time according toT∼t&tgr;(within certain limits for &tgr;). The self‐similar solution is calculated in detail, and scaling relations for the hydrodynamic variables are given. Numerical values are obtained for the radiative heating of gold. The range of validity of the solution is discussed.

ISSN:0031-9171    

DOI:10.1063/1.865184

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

The electron velocity distribution of rf‐heated plasma may be so far from Maxwellian that Spitzer conductivity no longer holds. A new conductivity for such plasmas is derived, and the result can be put in a general form. The new expression should be of practical value in examining schemes for current ramp‐up in tokamaks by means of lower‐hybrid or other waves.

ISSN:0031-9171    

DOI:10.1063/1.865186

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

A theoretical investigation is made of the modulational and filamentation instabilities of laser radiation at the upper‐hybrid frequency in the presence of the self‐generated magnetic field of the order of a few megagauss in a laser‐produced plasma. The nonlinear Vlasov equation is solved to obtain the low‐frequency response of magnetized electrons in the plasma. The present theory is valid for short‐wavelength perturbations and arbitrary values of the wavenumber. It is observed that the growth rates of the instabilities are very sensitive to the plasma parameters near the critical density layer. The growth rate of the modulational instability is always larger than that of the filamentation instability for the same power density of the incident laser radiation. It is further noticed that the modulational instability decreases slowly with the magnetic field, while the filamentation instability remains almost insensitive to the magnetic field.

ISSN:0031-9171    

DOI:10.1063/1.865187

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

The linear and nonlinear evolution of the interchange instability of a resistive plasma (the resistivegmode) in a sheared magnetic field is studied using electrostatic particle simulation methods. Both the fast and slow interchange mode regimes are considered. In both cases the linear growth rates of the modes scale well with the theoretical values. The saturation of the instabilities is caused primarily by convective mixing of pressure over the width of the eigenmode. The saturation levels predicted by mixing length theory are in reasonable agreement with the simulation results.

ISSN:0031-9171    

DOI:10.1063/1.865193

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

The multiple helicity nonlinear interaction of resistive tearing modes is compared for the tokamak and reversed field pinch configurations using the magnetohydrodynamic equations. Unlike the case of the tokamak disruption, for which this interaction is destabilizing when islands overlap, the nonlinear coupling of the dominant helicities is shown to be a stabilizing influence in the reversed field pinch. The behavior of the coupled instabilities in the two configurations can be understood as a consequence of the stability properties of the nonlinearly driven modes. In the case of the tokamak disruption, quasilinear effects linearly destabilize the dominant driven mode, which then feeds energy to the driving mode. For the reversed field pinch the driven modes remain stable, acting as a brake on the growth of the dominant instabilities. Furthermore, for the reversed field pinch configuration, numerical results indicate that nonlinear coupling of different helicities results in noticeably more rapid saturation of the dominant instabilities than was observed in single helicity studies.

ISSN:0031-9171    

DOI:10.1063/1.865194

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

The effect of electron–ion Coulomb collisions on the universal mode is studied using a Fokker–Planck collision operator in the kinetic equation for the electron distributionf∥e(x,v∥,t). The expected smoothing of thev∥structure off∥eis found, and the character of the mode is little changed when &ngr;ei→&ohgr;@B|. It is also confirmed that a collisionality on the order of that of present tokamaks is sufficient to provide for destabilization of the mode even in the absence of nonlinear orbit stochastic diffusion.

ISSN:0031-9171    

DOI:10.1063/1.865196

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

It is shown that diamagnetic propagation of drift‐tearing modes ceases at small island widths as the local density profile is flattened by sound waves. The critical width for density flattening is small compared to island widths typically inferred from observed magnetic fluctuations ofm=2 in tokamak discharges. The rotation of these fluctuations must therefore result from radial electric fields.

ISSN:0031-9171    

DOI:10.1063/1.865197

出版商:AIP    

年代:1985

数据来源: AIP