Supersonic gas injection for achieving higher density regime in RELAX RFP

RELAX is a low-aspect-ratio (R/a=2=0.5 m/0.25 m) reversed field pinch (RFP) machine to explore the geometrical optimization in the RFP. Typical discharge parameters are I_p~100 kA with flat-topped phase of ~2 ms. The electron density n_e is around 10¹⁹m^-3 and temperature around 100 eV. The electron beta values have reached higher than 10% at the normalized density n/n_G~0.3 where n_G is the Greenwald density. Our scenario to achieve higher density at high current is the use of supersonic gas injection after achieving high current with low density. We have developed a gas injection system consisting of a fast-acting electromagnetic valve combined with a simulated Laval nozzle. The gas flow velocities were estimated by using the time-of-flight technique. We will report initial experimental results, basic data of the gas injection system, and detailed discussion on our supersonic gas injection scenario.