Role In Wave Propagation Research Articles

Recent progress in lower hybrid current drive theory and experiments

In this paper lower hybrid current drive (LHCD) experimental milestones paving the way for future experiments are briefly summarized. The current drive efficiency scaling with the electron temperature is discussed. The role of wave propagation in determining the power deposition profile is stressed and methods are discussed to control the current density profile. Modelling of negative central shear configurations, experimentally obtained by LHCD, are reported. A good agreement is found between the modelling results and the experimental findings, thus showing that a good degree of understanding has been achieved in LHCD theory.

Wave Activity Diagnostics Applied to Baroclinic Wave Life Cycles

Wave activity diagnostics are calculated for four different baroclinic wave life cycles, including the LC1 and LC2 cases studied by Thorncroft, Hoskins, and McIntyre. The wave activity is a measure of the disturbance relative to some zonally symmetric, time-independent basic state, which need not be the initial zonally averaged state and which satisfies a finite-amplitude conservation relation. The wave activity density and fluxes may be calculated in terms of Eulerian variables provided that the potential vorticity is a monotonic function of latitude on isentropic surfaces in the basic state. The LC1 and LC2 experiments used initial states in which the potential vorticity (PV) did not satisfy this monotonicity condition. Therefore two approaches are taken. The first is to define a basic state that is not the initial state and use this to calculate the wave activity diagnostics. The second is to carry out new LC1- and LC2-type experiments on initial states in which the monotonicity condition is satisfied. New basic states are generated by PV rearrangement and inversion. The results allow quantification of the difference between LC1- and LC2-type life cycles. They also show that LC1- and LC2-type behavior occurs for different initial states other than those used by Thorncroft, Hoskins, and McIntyre and that the classification is therefore robust in terms of the potential vorticity field and wave activity diagnostics. If one were to consider only eddy kinetic energy, the distinction is no longer clear. In fact, in the evolution of eddy kinetic energy the modified LC1-type life cycle resembles LC2 and the modified LC2 more than it resembles LC1. The results also shed new light on the role of wave propagation in baroclinic life cycles. In particular, it is found that during the later stages of the life cycle the pattern of equatorward wave activity flux that has often been interpreted as associated with equatorward wave propagation in the subtropical upper troposphere is in fact associated primarily with advective transport of wave activity. New finite-amplitude expressions are presented for the wave activity associated with potential temperature gradients on the lower boundary. Problems with using PV rearrangement techniques are discussed.

Wave dispersion theory in a plasma column bounded by a cylindrical waveguide

A unified theory of the electromagnetic wave propagation in a plasma column immersed in an axial magnetic field is developed, including the important influence of finite geometrical effects on wave dispersion properties. The analysis is carried out within the framework of a macroscopic cold fluid model. Coupled eigenvalue equations for the electromagnetic perturbations are obtained for an arbitrary density profile. For a flat-top density, a closed algebraic dispersion relation of the electromagnetic wave is obtained without any prior approximation. This transcendental dispersion relation is analytically solved in special cases of (a) uniform plasma, (b) infinite magnetic field, (c) zero magnetic field, (d) electrostatic perturbations, and (e) the low-frequency whistlerlike mode. In order to demonstrate the important influence of finite geometrical effects, the low-frequency whistlerlike mode is rigorously investigated for axisymmetric waves in a completely filled waveguide. From the analysis, it is shown that the whistlerlike mode propagates in the axial direction only when the total electron line density exceeds a critical line density. Moreover, the conventional dispersion relation of the low-frequency whistler mode is recovered only for a large plasma radius. Otherwise, the plasma geometrical effects play a pivotal role in wave propagation.

Small-amplitude waves and screening in dense degenerate plasma

The propagation of electro-acoustic waves in a dense degenerate plasma is considered, for the density range (106÷109) g/cm3 and at temperatures up to (108÷109) °K. Appropriate dispersion relations are obtained, for longitudinal and transverse waves, including the effects of ionic mobility. It is found that although the ion gas plays only a small role in wave-propagation, its role in screening cannot be neglected.