Investigation of Plasma Interaction with a High-Frequency Magnetic Field Increasing Toward the Periphery

Experimental investigation of the method of stabilizing a straight plasma column with a current by a high-frequency magnetic multipole field has demonstrated that a stabilized pinch may be observed only with a sufficiently high rate of increase in the current through the plasma. The rapid increase in current leads to the fact that the stable pinch exists for only a short time (up to 10 microsec ). Further investigations have basically been aimed at discovering the conditions of formation of the pinch in a high-frequency field and the cause for its decay. The observed stabilization effect was studied both in systems with a self excited oscillator and in systems with an impulse excited oscillator. In both cases experience indicates essential dependence of the observed effect on the highfrequency discharge stage with which the discharge forming the plasma column begins. Obviously, the processes which take place in the high-frequency discharge excited by the currents in the stabilizing rods also have a noticeable effect on constriction of the plasma and on confining the plasma column. The research which has been performed has been partially devoted to probe type measurements of the magnetic field and spectral measurements of the density of the high-frequency discharge plasma. It has been demonstrated that in the absence of a discharge current in sufficiently powerful high-frequency discharge the plasma is concentrated near the axis. A longitudinal quasistationary magnetic field also appears in the high-frequency discharge. In the previous research the results of the observations were quantitatively compared with the theory of the dynamic stabilization technique under the assumption that the most dangerous deformation of the plasma column is the deformation with the wavelength, which is the maximum possible in the device; that is, it is twice the electrode spacing ℓe. The investigations have demonstrated that the observed stabilization effect does not depend on ℓe . This is evidence of the necessity of further development of the theoretical description of the interaction of a plasma column with a high-frequency multipole field. All this research was performed on devices in which the selfconstricted discharge was created in straight tubes 6.0 - 20.0 cm in diameter with an electrode spacing of 30-80 cm, and the stabilizing field was excited by currents in rods located outside the discharge tube. The ''Volna''(''Wave'') system is a toroidal chamber in which the plasma is confined and heated by a magnetic field wave traveling along the axis. To compensate for the Hall drift current and the possible entrainment of the ions in the system into drift motion, two opposing traveling waves with different frequencies but moving with the same phase velocities have been investigated. A set of comparable measurements has been taken with both one and two waves. The effect of compensation for the drift current and its influence on the plasma confinement and stability has been demonstrated. Plasma stability is studied by using waves of different spatial length (8,12 and 16 wave length around the perimeter of the toroidal discharge chamber) each of which is characterized by a certain well depth (min B) for which the shorter the wavelength, the stronger the min B. The results which were obtained indicate that with a decrease in the spatial wavelength the plasma stability is increased. The experimental research which has been performed indicates that the depth of the skin layer is apparently not defined by the classical relations. (author)