Experimental Study of the Effect of Radial Electric Fields on the Stability of a Magnetically Confined Plasma

Low-frequency, spontaneous, electrostatic oscillations propagating azimuthally in low-β, magnetically confined plasma columns, have been studied in many devices. In most of these experiments, the peak amplitude of the observed oscillations has been in a region where both radial density gradients and radial electric fields existed. To separate these two effects we studied the behaviour of the coherent oscillations of an alkali plasma in conditions where the radial electric field could be externally controlled. The experiments were performed in a single-ended, large-diameter (8 cm), alkali plasma (Q-machine). To achieve an adjustable electric field, the hot end-plate was split into two concentric parts separated by a small gap. By applying a steady voltage between these sections, an adjustable electric field is observed down the 90-cm length of the plasma column. For low applied voltages the plasma was quiescent. When the voltage was increased above a threshold value (between 0.05 and 0.5 V), low-frequency (1-10 kHz) coherent oscillations were observed. In this case, the electric-field drift velocity was much greater than the density-gradient drift velocity. The oscillations propagated azimuthally in the E x B direction and formed a standing wave in the radial direction. Fora given azimuthal mode number, the frequency was proportional to the applied voltage. Potential and density fluctuations were out of phase, suggesting that this is a flute-type-mode, perhaps driven by shear in azimuthal velocity. At higher applied voltages (1-3 V), the density gradient in the centre part of the plasma increased noticeably. In this region there was a strong density gradient, but a comparatively small electric field. Here, a different kind of azimuthally propagating, coherent, low-frequency (1-3 kHz) oscillation was observed. In this case, density and potential oscillations were in phase, suggesting some form of drift wave. (author)