Table of contents

It is shown that a circularly polarized electromagnetic wave with frequency omega ₀ near the electron plasma frequency propagating along a DC magnetic field decays into a Langmuir wave and a circularly polarized Alfven wave. It is found that the pump and Alfven waves have the same helicity. Explicit expressions for the growth rate and threshold are obtained.

The divertor experiment ASDEX has been equipped with a Thomson scattering system, designed to measure radially resolved electron temperature and density profiles in the boundary layer between the hot plasma core and the wall. Thus, it is possible to obtain detailed information about plasma edge phenomena as a basis of divertor physics. The measured electron temperature at the magnetic separatrix can be described by the Coppi-Mazzucato (1979) scaling law for the electron heat conductivity in ohmically heated plasmas. This is analyzed over a wide range of ASDEX plasma parameters and discharge conditions. During additional heating by neutral injection the T_e-profiles show a steepening in the scrape-off area, which can be a consequence of a local decrease in thermal conductivity responsible for the good energy confinement observed for 'H'-regime discharges.

The beta effects on the plasma equilibrium in Modified Bumby Torus (MBT) sector, which is an asymmetric closed line system with l=0 and fairly large l=+or-1 field distortions, are studied. For this purpose, the equilibrium of high beta plasma produced by theta-pinch is compared with that of betaless plasma numerically calculated from the measured magnetic field profiles in device. The equilibrium condition depends weakly on beta value, but the plasma cross-section is vertically elongated as the beta value increases. The m=1 long wavelength MHD instability is not observed during the observation time of approximately 15 mu s. These experimental results are compared with MHD theory based on the new ordering taking the finiteness of l=+or-1 field distortion ( delta _+or-1>or approximately=1) into account, which suggest significant stabilizing effects due to self formation of magnetic well and also due to the conducting wall.

A motion of ions perpendicular to the magnetic field is observed experimentally during a large amplitude potential oscillation caused by the formation and disruption of moving double layers in a plasma. This radial motion is closely related to an appearance of the strong radial electric field formed between a high potential plasma composing the double layer and a wall surrounding the plasma column. The field accelerates the ions outwards from the plasma and gives rise to an ion transport towards the wall.

Current disruptions may be excited spontaneously in a laboratory plasma when the electric current density along the magnetic field is large. This phenomenon is investigated in the plasma column of a triple plasma device with an inductive external electric circuit. Each current disruption then gives an inductive voltage pulse of several hundred volts over the plasma, and this potential drop is found to be concentrated at an electric double layer which forms in the plasma during the pulse. Most of the magnetic energy, initially stored in the circuit inductance, is transferred to the double layer during the period of current decrease and released there as particle energy. Simultaneously as the current decreases the potential level at a local potential minimum in the plasma is found to decrease. Reflection of electrons at this minimum is conjectured to cause the decrease of the current during a disruption.

In this paper, an attempt is made to develop an analytical density correction method for cylindrical Langmuir probes displaying end effects caused by finite probe length and sheath expansion as well as potential inhomogeneities within the plasma. The correction depends on probe as well as plasma parameters such as the Debye length, the probe radius, the probe length, and the applied potential, and is based on the deviation of the measured characteristics from the ideal form according to orbit-limited conditions. A comparison with real data was carried out and showed the applicability of the method. Based on this correction, accurate Langmuir-probe electron-density measurements are possible in the very low density ionospheric plasma (n_e>or approximately=10³ cm^-3) as well as in the disturbed regions around rockets or satellites, especially in the wake. The range of application of the conventional Langmuir-probe method has been extended.

Experimental results on the formation of ion acoustic double layers resembling asymmetric ion-holes are presented. In a double plasma device, modified suitably to inject electron beam into the target plasma, modulation of the beam through step potential leads to excitation of ion-acoustic fluctuation. The ion-acoustic fluctuation, growing away from the grids separating source and target plasmas, developed into weak asymmetric ion-acoustic double layer. The observations are in qualitative agreement with theoretical models and computer simulations.

The authors derive the nonlinear gyrokinetic equation for axisymmetric plasmas in its most general form, i.e. including finite beta , collisions, driving toroidal electric field, and, accordingly, non-Maxwellian, poloidally dependent unperturbed distribution functions. The authors note the relation between the assumption of radial periodicity of the linear eigenfunction-the period being a multiple of the distance between rational surfaces-and the ballooning representation. They remark that this assumption is not justified in the presence of nonlinear interactions, and, from there, suggest a possible saturation mechanism of the potentially dangerous toroidicity induced modes.

The connection between the slab-like and the curvature-induced electrostatic drift modes in the collisionless regime of a plasma in an axisymmetric toroidal confinement geometry is discussed.

The dispersion relation of the nonlinear anti-symmetric surface mode (in the whole electromagnetic region of its existence) in a plasma slab bounded by a linear dielectric is derived for strong striction nonlinearity. The phase velocity is expressed in terms of the wave electric field amplitudes at the boundary and at the axis of the slab. Using the same method, the authors obtain the linear dispersion relation of the wave in a new form. The approximation of a weak nonlinearity is also mentioned.

It is shown that one-dimensional high-frequency magnetoacoustic solitons are unstable with respect to three-dimensional perturbations. The instability is due to the vector nonlinearity.