摘要:

Bifurcation theory techniques are used for the quantitative investigation of a plane current sheet in plasmas with electric current as the control parameter. The existence of a bifurcation point is shown and the bifurcating solution is calculated. Stability results are achieved for the plane sheet and the bifurcating branches. The results allow conclusions to be drawn about the dynamic evolution of the system.

ISSN:0031-9171    

DOI:10.1063/1.865542

出版商:AIP    

年代:1986

数据来源: AIP

摘要:

A nonlinear dispersion relation for a circularly polarized electromagnetic wave of a finite wavelength propagating along a constant magnetic field is derived by a kinetic theory. The dispersion relation includes an important correction term to the dispersion relation previously obtained for the ion cyclotron range frequency and may be applied to problems in any range of frequency. Using this dispersion relation, parametric instabilities relevant in a finite‐amplitude right circularly polarized wave (rwave), that is, excitations of newly found branches of a strongly dampedrwave, the Raman scattering to a dampedrwave, and the Brillouin scattering, have been studied.

ISSN:0031-9171    

DOI:10.1063/1.865543

出版商:AIP    

年代:1986

数据来源: AIP

摘要:

The resistivity fluctuation correlation function and electrostatic potential spectrum of resistivity‐gradient‐driven turbulence are calculated analytically and compared to the results of three‐dimensional numerical calculations. Resistivity‐gradient‐driven turbulence is characterized by effective Reynolds’ numbers of order unity. Steady‐state solution of the renormalized spectrum equation yields an electrostatic potential spectrum ⟨&fgr;ˆ2⟩ky∼k−3.25y. Agreement of the analytically calculated potential spectrum and mean‐square radial velocity with the results of multiple helicity numerical calculations is good. This comparison constitutes a quantitative test of the analytical turbulence theory used. The spectrum of magnetic fluctuations is also calculated and agrees well with that obtained from the numerical computations.

ISSN:0031-9171    

DOI:10.1063/1.865544

出版商:AIP    

年代:1986

数据来源: AIP

摘要:

The effects of turbulence on magnetic reconnection are investigated by two‐dimensional spectral method magnetohydrodynamic computations. The nonlinear evolution of the periodic sheet pinch configuration is studied as an initial value problem. Turbulence is initiated by including a low level of broadband fluctuations in the initial data. Nonlinear features of the evolution, appropriately described as turbulence, are seen early in the solutions and persist throughout the runs. Small‐scale, unsteady coherent electric current and vorticity structures develop in the reconnection zone, resulting in enhanced viscous and resistive dissipation. Unsteady and often spatially asymmetric fluid flow develops. Large‐scale magnetic islands produced by reconnection activity, undergo internal pulsations. Small‐scale magnetic islands, or ‘‘bubbles’’ develop near the reconnection zone, prodcing multiple X points. Large‐amplitude electric field fluctuations, often several times larger than the reconnection electric field, are produced by large island pulsations and by motion of magnetic bubbles. Spectral analysis of the fluctuations show development of broad band excitations, reminiscent of inertial and dissipation range spectra in homogeneous turbulence. Two‐dimensional spectra indicate that the turbulence is broadband in both spatial directions. It is suggested that the turbulence that develops from the randomly perturbed sheet pinchbears a strong resemblence to homogeneous magnetohydrodynamic turbulence, and that analytical theories of reconnection must incorporate these effects.

ISSN:0031-9171    

DOI:10.1063/1.866004

出版商:AIP    

年代:1986

数据来源: AIP

摘要:

Results of a numerical study of Alfve´n waves are presented subject to nonlinearity, dispersion, growth, and damping. The model presented is the derivative nonlinear Schro¨dinger equation, modified to include linear growth and damping processes. The processes that are considered are wave amplification by streaming particle distributions, and damping resulting from ion‐cyclotron resonance absorption. These growth and damping mechanisms are dominant in different portions of wavenumber space. The primary role of nonlinearity is the transfer of wave energy from growing or amplified wavenumbers to those which are damped. A nonlinear saturation mechanism thereby results, in which instability of low wavenumber modes may be quenched. A simple phenomenological model is developed, which accounts for many of the salient features of the numerical calculations. The application of these results to observations of Alfve´n waves upstream of the Earth’s bow shock is briefly considered. It is suggested that the short wavelength ‘‘shocklet’’ structures resemble the soliton‐like pulses that emerge from the driven derivative nonlinear Schro¨dinger equation. However, the nonlinear effects discussed in this paper do not seem responsible for limiting the amplitude of the ‘‘low‐frequency’’ waves in the foreshock region.

ISSN:0031-9171    

DOI:10.1063/1.865545

出版商:AIP    

年代:1986

数据来源: AIP

摘要:

Analytic expressions for the parallel viscosity‐driven neoclassical fluxes in the banana regime in arbitrary nonsymmetric toroidal plasmas are derived without considering boundary layer effects. They can be evaluated numerically if the Fourier decomposition of the strength of the magnetic field ‖B‖ is known. Since no averaging over the high‐frequency component of the Fourier decomposition of ‖B‖ has been employed, these expressions can reproduce all the known results in symmetric systems and predict the possibility of reversing the direction of the bootstrap current and Ware pinch flux in a stellarator.

ISSN:0031-9171    

DOI:10.1063/1.865546

出版商:AIP    

年代:1986

数据来源: AIP

摘要:

A modified quasistatic theory that incorporates inductive effects in earlier electrostatic models connects the conventional electrostatic and magnetohydrodynamic (MHD) pictures of line tying by cold plasma. The modified theory predicts that curvature‐driven flute modes in mirror‐confined plasmas can be stabilized by moderate concentrations of cold plasma if the beta of the hot, mirror‐confined plasma is less than a critical value. The maximum stable beta for an idealized stratified model of a hot‐ion plasma, separated from conducting end walls by cold plasma, is given approximately by &bgr;crit&bartil;(4RpRc/LhLc) ×[&ohgr;2pe(cold)/ (k2⊥c2+&ohgr;2pe(cold))]. For &ohgr;2pe(cold)≪k2⊥c2, this result is identical with the predictions of electrostatic models. In the opposite limit, the result resembles the predictions of ideal MHD models, namely, that unstable curvature‐driven modes can occur even in the presence of dense cold plasmas if the hot‐plasma beta exceeds a limiting value estimated to be &pgr;2RpRc/L2.

ISSN:0031-9171    

DOI:10.1063/1.865547

出版商:AIP    

年代:1986

数据来源: AIP

摘要:

The properties of resistive instabilities developing in equilibrium configurations in which flows are present are studied. The linearized magnetohydrodynamics (MHD) equations are solved numerically, taking into account the effects of resistivity, adopting equilibria with magnetic and velocity fields in the ( y‐z) plane, and varying thexdirection with two different scale lengths. The results show that the stability properties of the system are strongly influenced by the profile of the velocity field considered. The static tearing mode is recovered in the limit of small amplitude of the velocity with respect to the Alfve´n velocity and/or small values of the ratio between magnetic and velocity scale lengths. In the opposite limit we obtain a purely ideal behavior in which the Kelvin–Helmholtz instability appears to be modified by the presence of a shear magnetic field. In between those limits the effect of the velocity is to change the dependence of the growth rate on the relevant parameters and to distort the spatial profile of the perturbations with respect to the static case.

ISSN:0031-9171    

DOI:10.1063/1.865548

出版商:AIP    

年代:1986

数据来源: AIP

摘要:

Electron instabilities of magnetized spherical shell distributions in velocity space with a colder Maxwellian background are investigated analytically with simulations using electrostatic particle codes. The resonant and nonresonant instabilities observed in the particle simulations are in agreement with zeros of the dielectric function, as found from the resonant approximation for waves with an electric field component along the magnetic field or by computation with a root solver code in the case of perpendicular propagation. Saturation of the instabilities is by nonlinear cyclotron resonance with the cold background in the resonant case or by nonstochastic cyclotron harmonic damping by the cold background in the nonresonant case. Instabilities invariably lead to perpendicular acceleration and heating of the cold background to velocities sometimes exceeding the shell velocity.

ISSN:0031-9171    

DOI:10.1063/1.865549

出版商:AIP    

年代:1986

数据来源: AIP

摘要:

By means of the two‐fluid energy principle, the kink stability boundary of a field‐reversed ion layer of arbitrary thickness immersed in a dense low‐temperature background plasma is investigated theoretically. This system is found to have a stability window against kinks. The dependence of the kink stability regime on the equilibrium properties of the system is also shown. In the thin layer limit, a comparison is made between the previous theories and the present theory.

ISSN:0031-9171    

DOI:10.1063/1.865550

出版商:AIP    

年代:1986

数据来源: AIP