DC HELICITY INJECTION AND MHD RELAXATION STUDIES IN AN RFP BY MAKING USE OF INSULATED SEGMENTS OF DISCHARGE CHAMBER

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It is one of the objectives of this research project to search for the possibility of DC helicity injection scheme in an RFP by taking advantage of the insulated segments of the discharge chamder of the STE-2 RFP.The other includes the MHD relaxation process associated with the helicity injection. Experiments in STE-2 with standard RFP and ultra-low-q (ULQ) plasmas have provided the following results.(1) DC helicity injection was performed with the use of metal electrodes inserted at the edge. The electrodes were made of stainlewss steel with surface area of 0.8 cm2. These two electrodes were inserted at the same toroidal location, separated by 270ﾟ poloidally. The maximum voltage applied was 400 V with a current a current of 100 A.The injected current density was roughly equal to the average toroidal current density. A slight (5 %) increase of the toroidal magnetic flux was observed in the vicinity of the electrodes. This toroidally localized flux increase did not depend upon the pola … More rity of the applied voltage, which was contradictory to the expectation based on the helicity balance relation. No appreciable increase was observed either in the edge poloidal field or in the edge toroidal field. The results may indicate that increased local beta value due to localized Joule heatinh plays important role in plasma response.(2) DC helicity injection was made by applying voltage between the insulated segments of the chamber and simultaneously by spatially modulating the toroidal field so as to produce the magnetic flux penetrating these segments. This method was applied only to medium-current (20 kA) ULQ plasmas because of the deleterious effect of the toroidal field modulation on RFP discharges. The plasma current measured at the chamber poloidal gap has increased by 20 % with the applied voltage of 1.3 kV.A further optimization of RFP discharge is in progress by reducing field errors at the gap with the help of localized conducting shell. The new scheme DC helicity injection will be applied to the improved RFP discharges. Less