High-current-density operation of RFP with newly developed program

A new mode of external-field programming for setting up a reversed-field pinch (RFP) is tested by using a small toroidal-pinch apparatus, STP-1. It involves creating an initial plasma with a screw pinch followed by external-field reversal. The program is done carefully so as to satisfy the equilibrium relation with respect to the minor radius throughout the setting-up phase. The screw pinch heats up the plasma effectively, and a consequent increase of the trapped flux in the plasma by a factor of two is attained, as compared with the previous programming mode. Actually, at a plasma current of 58 kA, a stable classical operation time of 13 μs is achieved with a density of 3.5 × 1015 and a temperature of 20 eV. Higher-current operation at 110 kA, which gives a sufficient plasma current density of 4 kA · cm−2 for a compact RFP reactor, results in higher plasma temperature and density of 40 eV and 4.5 × 1015 cm−3, respectively. The duration of the stable discharge, however, is limited to about 10 μs, in spite of the expected longer confinement time at the higher temperature.