J. A. Tataronis

Radial plasma transport and toroidal current driven by nonresonant ponderomotive forces

A general form of the time‐averaged ponderomotive force produced by radio frequency waves in magnetized plasma configurations is treated. Included in the ponderomotive force are components that result from plasma flow, the dynamo effect, and wave helicity. A detailed description of effects of individual components in the ponderomotive force on radio frequency (RF) driven transport and current is given.

On the Alfvén resonance and its existence

In this Brief Communication, the continuous spectra of ideal magnetohydrodynamics are revisited in order to point out and discuss equilibrium configurations in which the Alfven continuum is characterized by a nonlogarithmic spatial singularity in the velocity fluctuation of the plasma. This occurs in a compressible sheet pinch with a shearless equilibrium magnetic field directed parallel to a specified magnetic surface wave vector. It is shown that the component of the velocity vector parallel to the direction of inhomogeneity is bounded at the Alfven resonance layers. The velocity vector possesses spatial singularities, but the singularities are restricted to its component perpendicular to both the direction of the spatial inhomogeneity and the equilibrium magnetic field. The singularities are found to be delta functions. Comments are also made on the Alfven resonance in the incompressible plasma and its relationship to the continuous spectra of the compressible plasma.

The influence of multiple ion species on Alfvén wave dispersion and Alfvén wave plasma heating

In this paper, the effects of light impurities, such as deuterium, helium, or carbon, on Alfven wave dispersion characteristics are explored. It is shown that a small population of light impurities in a hydrogen plasma modify the dispersion of the global Alfven waves and the Alfven continuum in such a way that the wave frequency depends weakly on the toroidal wave number. It is also shown that the global Alfven wave enters into the Alfven continuum. Under these conditions, it is possible to heat plasma efficiently by employing an antenna with a broad toroidal wavelength spectrum. The relationship between impurity concentration and the efficiency of Alfven wave heating is explored. Under appropriate conditions, the results indicate that in the presence of impurities, Alfven waves can heat electrons predominantly in the central part of the plasma. This effect is explored via a series of numerical calculations of the heating specifically for the Phaedrus‐T Alfven wave heating experiment [Phys. Fluids B 5, 25...

Finite-amplitude Alfven waves in a dissipative inhomogeneous plasma

The effects of an inhomogeneous background magnetic field and resistivity on finite‐amplitude Alfven waves are studied in terms of the derivative nonlinear Schrodinger equation. It is shown that a weak inhomogeneity in the background magnetic field introduces a perpendicular shear in the resulting waves. This causes the waves to split apart, and a longitudinal current, which increases linearly in time, is produced as a consequence. Resistivity, however, restricts this unbounded growth of current and causes the waves and the current to decay exponentially at later times. The connection of these results to previous results on the Alfven continuum and low‐frequency current drive are discussed in the paper. In the paper the effects of resistivity on the modulational instability of time harmonic solutions of the derivative nonlinear Schrodinger equation are also explored. Both numerical and analytical results indicate that resistivity will decrease the growth rate of envelope perturbations. At large enough val...

EFFECTS OF LINEAR ABSORPTION ON THE STEADY-STATE BEHAVIOR OF A PHOTOREFRACTIVE PHASE-CONJUGATE MIRROR

A photorefractive phase-conjugate mirror with linear absorption is examined. An objective of this study is to identify possible effects of absorption on the self-oscillation conditions of the mirror and the properties of the excited conjugate wave. It has been found that self-oscillation is still possible in the presence of absorption. However, the coupling coefficient required for self-oscillation must increase in magnitude with increasing absorption coefficient to counteract the depressing effects of absorption. A consequence of self-oscillation in the presence of absorption is enhancement of the mirror reflectivity. A parametric study of the absorption-enhanced mirror is presented.

Magnetohydrodynamic ponderomotive forces generated about Alfvén resonance layers

The total ponderomotive force produced by radio frequency electric and magnetic fields about the spatial resonances of the shear Alfven wave is derived. The time‐averaged currents and plasma transport that result from these forces are also derived. The wave analysis is based on a resistive magnetohydrodynamic fluid. A relationship between the time‐averaged rates of change of wave energy dissipation and wave momentum dissipation in the resonant layers is found. Since the deposited wave momentum in the Alfven resonant layers alters the plasma transport locally, control of the density profile is a possible application of this theory.

Response to ‘‘Comment on ‘On the Alfvén resonance and its existence’ ’’ [Phys. Plasmas 2, 340 (1995)]

The spatial singularity of the shear Alfven wave in MHD plasma is a delta function. We show that this delta function solution makes physical sense and explore its characteristics. (AIP)

Enhancement of electron acceleration in laser wakefields by a perpendicularly propagating laser beam

By numerical modeling of the test particle motion of an ensemble of relativistic electrons in a longitudinal laser wakefield, we show in the present contribution that a strong enhancement of electron acceleration and trapping in the wakefield arises in the presence of a perpendicularly propagating transverse laser beam. Ripple of the wakefield amplitude reduces the acceleration enhancement. Effects of phase randomization of the transverse laser field are explored as well.

Random field effects on electron acceleration in crossed laser beams

By numerical modeling of the test particle motion of an ensemble of relativistic electrons in a longitudinal laser wakefield, we show in the present contribution that a strong enhancement of electron acceleration and trapping in the wakefield arises in the presence of a perpendicularly propagating transverse laser beam. Effects of spontaneously arising random fields on the acceleration are explored. Electrons are also strongly accelerated in such wakefields where trapping and acceleration would not be possible without the presence of the transverse laser beam. The maximum energy of accelerated electrons grows almost linearly with time.

Transport of intense LH pulses into a tokamak plasma

Application of lower-hybrid (LH) power in short, intense pulses in the 5–10 GW range should overcome the limiting effects of Landau damping, and thereby permit the penetration of LH power into the interior of large-scale plasmas. We show that, under such very intense LH pulses, wave coupling may deteriorate because of nonlinear density changes due to the ponderomotive force effects in front of the grill. Ponderomotive forces are also likely to induce strong plasma bias and consequent poloidal and toroidal plasma rotation. Although backward electric currents, created in the plasma by intense LH pulses, dissipate a large portion of the radio frequency power absorbed, the current drive efficiency is acceptable. We use a numerical simulation of wave–particle interactions to analyse the applicability of standard quasilinear theory to the case of large energy flux densities. The initial results indicate the existence of important restrictions on the use of the quasilinear approximation. The results of the present paper also indicate that some of the effects considerably alter some ideas of Cohen et al.