摘要:

Subgrid modeling for magnetohydrodynamic (MHD) turbulent shear flows is studied with the aid of a statistical result from a multiscale direct‐interaction theory. The ensemble‐mean MHD correlation function is combined with a filtering procedure to give a subgrid‐scale MHD stress. This result leads to a subgrid model of Smagorinsky type, which is applicable to large eddy simulations of earth’s magnetic dynamo and reversed‐field pinches of plasma.

ISSN:0031-9171    

DOI:10.1063/1.866306

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

Two models of Langmuir turbulence, the nonlinear Schro¨dinger equation and the Zakharov equations, are solved numerically for an initial value problem in which the electric field evolves from an almost flat initial condition via the modulational instability and finally saturates into a set of solitons. The two models agree well with each other only when the initial dimensionless electric field has an amplitude less than unity. An analytic soliton gas model consisting of equal‐amplitude, randomly spaced, zero‐speed solitons is remarkably good at reproducing the time‐averaged Fourier spectra in both cases.

ISSN:0031-9171    

DOI:10.1063/1.866307

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

The generation of plasma density fluctuations by nonlinear Alfve´n waves is considered. Of particular interest is the modification to the ‘‘static approximation,’’ describing the relationship between wave intensity and plasma density, when the Alfve´n wave packet is rapidly steepening. The study concentrates on a forced wave equation for the plasma density, which yields the static approximation in a select approximation. An analytic solution to this equation is obtained for a reasonable model of a steeping wave packet. The principal results are (a) the static approximation should give a surprisingly robust description of the coupling between Alfve´n wave intensity and plasma density, (b) departures of the plasma density perturbation from the static approximation result are determined by a parameter that is a simple function of the wave amplitude and plasma &bgr;, (c) for rapidly evolving wave packets, the position of maximum density change is displaced from that of maximum wave intensity, and (d) the magnitude of the density fluctuation will not be much different than the static approximation estimate of (b2/B20)n0.

ISSN:0031-9171    

DOI:10.1063/1.866308

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

Electron and ion dynamics are investigated through particle simulation of a supercritical oblique collisionless shock. As &thgr; deviates from 90°, ions are accelerated and trapped in the electrostatic wells and later become detrapped; this results in strong ion heating perpendicular toB0. Below a critical angle &thgr;teelectrons are strongly energized alongB0, and heated. A large parallel electron current builds up and induces new transverse electromagnetic components in the ramp of the shock. For weaker angles, ion heating vanishes below a second critical angle &thgr;ti.

ISSN:0031-9171    

DOI:10.1063/1.866309

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

In carrying out a nonlocal analysis of drift cyclotron loss cone (DCLC) modes in a cylindrically symmetric plasma, a new dispersion relation branch not described by local theories has been found. The model assumes a low‐&bgr; plasma column with a Gaussian density profile in a uniform magnetic field. The exactk‐space integral equation is solved using a Laguerre polynomial decomposition of the eigenfunctions. It is these new modes that couple with the diamagnetic drift wave to form unstable DCLC modes, a phenomenon not predicted by local theory. The existence of these modes requires a proper treatment of the spatial variation of the equilibrium distribution function describing the ions.

ISSN:0031-9171    

DOI:10.1063/1.866310

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

The stability of a hot particle annulus in a mirror cell at arbitrary drift reversal is analyzed by using a cylindrical model with embedded currents that simulate the role of field line curvature in actual mirror experiments. New effects are included by taking into account the Landau damping arising from the spread of hot particle drift velocities. It is found that the ‘‘precessional layer’’ mode is destabilized near the pressure gradient value at which drift reversal is achieved. Its growth rate is proportional toN−4, whereN−2is a coupling constant &ggr;2MHD/ &ohgr;˜2&kgr;[&ohgr;˜&kgr;is the curvature frequency, &ggr;MHDis the typical magnetohydrodynamic (MHD) growth rate]. In addition the unstable interaction of the precessional layer mode with surface waves that can propagate below and above the ion‐cyclotron frequency is described. Below the ion‐cyclotron frequency the surface wave resembles the Alfve´n wave, while above the ion‐cyclotron frequency the surface wave resembles a whistler wave.

ISSN:0031-9171    

DOI:10.1063/1.866311

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

A limited asymmetric distortion of the distribution of electron velocities parallel to the magnetic field, sufficient to support the plasma current, is considered. Its effects on second harmonic electron cyclotron absorption of the extraordinary mode propagating away from normal incidence are quantified using a fully three‐dimensional ray‐tracing code. Significant deviations from the zero current power deposition profiles occur, both in real space and in velocity space. At higher densities, the effects of the extraordinary mode cutoff are softened. These considerations should be taken into account in future studies of temperature and current profile modification using second harmonic electron cyclotron absorption on COMPASS [ProceedingsoftheCourseandWorkshoponApplicationsofrfWavestoTokamakPlasmas, Varenna (Monotypia Franchi, Perugia, 1985), Vol. II, p. 797].

ISSN:0031-9171    

DOI:10.1063/1.866312

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

The hot electron buildup by second harmonic electron cyclotron resonance heating in the thermal barrier region of a tandem mirror has been studied by using a Fokker–Planck code with a self‐consistent potential profile taken into account. It has been found that the evolution of the hot electron population and the potential profile consists of two phases. In the first phase the rf diffusion is dominant and a quick increase in both the hot electron density and the mean energy is observed. No further increase in the mean energy is observed thereafter. The potential is deepest during the first phase. The second phase starts in the mean free time of the pitch angle scattering of hot electrons on cold electrons and ions. In this phase the hot electron population increases at the rate of the pitch angle scattering. The potential dip becomes shallow because of the accumulation of pitch‐angle‐scattered passing ions.

ISSN:0031-9171    

DOI:10.1063/1.866313

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

Numerically calculated tokamak equilibria are used to compute banana‐plateau transport coefficients for finite‐aspect‐ratio, finite‐beta plasmas. Calculations are presented for the Spherical Torus Experiment (STX) (NTIS Document No. DE 86004663) and the Princeton Beta Experiment (PBX) (NTIS Document No. DE 86011173). In STX, the poloidal variation ofB≡‖B‖ over a magnetic surface tends to be reduced in regions of large major radiusR. The reduction of radial transport caused by this quasiomnigeneous condition is offset by increased drifts and trapping probabilities for smallerR. Thus the modulation &Dgr;=(Bmax−Bmin)/(Bmax+Bmin) on a magnetic surface becomes the critical parameter determining neoclassical transport. In PBX, the bean‐shaped topology of the magnetic surfaces leads to the presence of multiple magnetic wells. Numerical calculations confirm that analytic calculations of neoclassical transport based on the total fraction of circulating particles are valid even when geometrically distinct classes of trapped particles are present.

ISSN:0031-9171    

DOI:10.1063/1.866314

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

Two‐dimensional numerical plasma simulations have been carried out in a uniform magnetic field to study the effects of neutral beam injection on plasma diffusion. Neutral beams injected across a magnetic field are assumed to be ionized by various ionization processes in a plasma. It is found that the suprathermal convective motion of a plasma generated by the injection of neutral beams is dissipated via anomalous viscosity, leading to enhanced cross‐field diffusion. The diffusion coefficient depends weakly on the magnetic field and plasma density, similar to the diffusion caused by thermally excited convective cells. The magnitude of the diffusion increases with the injection energy and is much larger than the thermal diffusion because of the presence of suprathermal plasma convection. It is shown that a similar anomalous plasma diffusion may occur in a plasma subject to radio frequency (rf) wave heating where only a localized region of a plasma across the magnetic field is heated to a temperature much higher than the surrounding temperature. Theoretical investigations are described on the scaling of enhanced plasma diffusion.

ISSN:0031-9171    

DOI:10.1063/1.866315

出版商:AIP    

年代:1987

数据来源: AIP