Movable limiter experiment of TPE-1RM 15 reversed field pinch machine

Two movable limiters with a graphite head (35 mmφ and 40 mm high) were installed in TPE-1RM 15 reversed field pinch (RFP) machine. Measurement of heat flux input to the movable limiters, effect of the insertion of the limiter on plasma properties and surface analyses of the graphite head after the exposure were conducted. The heat flux into the lower half of the graphite head (mainly electron drift side) exceeded that from the upper half (mainly ion drift side) by factor of 4∼6 at the maximum insertion of the limiters (10 mm inward from the shadow of the fixed limiters). This factor increased as the movable limiter protruded into the plasma, and this profile is explained by the change of pitch profile of the magnetic field line at the plasma periphery. At the maximum insertion of the two movable limiters, the energy input into a graphite head was about 10 % of the joule input energy during the current sustainment phase. Heat flux carried by electron was about 2.2∼2.3 mJ/m2 which is much higher than the averaged thermal wall flux of about 3 kJ/m2 during the current sustainment phase. The one turn loop voltage and plasma resistance increased when the movable limiters were inserted beyond the shadow of the fixed limiters, and the increment of the joule input power is roughly explained by the increment of the loss power into the protruded movable limiters. Unbalanced position scanning showed that relative distance of a movable limiter from the plasma column was not affected by another movable limiter installed 180deg toroidally away from the former limiter. Fundamental surface analyses (SEM, EPMA) were tried to the graphite head, and they showed that deposition of metal impurities (Fe and Cr) was higher at the corner of the ion drift side than that of electron drift side, and that the corner of the electron drift side was more roughened than the ion drift side. (author)