Analysis of the Gas Puffing Performance for Improving the Repeatability of Ohmic Discharges in the SUNIST Spherical Tokama

The gas puffing performance plays a key role in repeatable discharges in the Sino- UNIted Spherical Tokamak (SUNIST) experiments. In this paper, temporal evolution of the gas pressure in the vacuum vessel and the dependence of the repeatability of plasma discharges on different timing arrangements between the gas puffing pulse and the Ohmic field have been experimentally investigated. The results show that, after a fast rising phase, the gas pressure becomes quasi-stationary. In the regime of the discharges being started up when the gas pressure has already reached the quasi-stationary state for about 37 ms, an improved repeatability of the plasma discharges is achieved.