GAMMA 10 vacuum system and study of wall conditions

Abstract

GAMMA 10 is an axisymmetrized tandem mirror with thermal barriers located at the Plasma Research Center, University of Tsukuba. Good vacuum conditions required for the formation and sustainment of the plug‐thermal‐barrier potentials are obtained with Ti‐deposited liquid‐nitrogen‐cooled panels with a pumping speed of 1.9×106 l/s and a careful wall conditioning with ion cyclotron resonance heating plasma shots. One characteristic of the pumping system is that the titanium gettering is used not in the plasma confinement regions but in separate vacuum regions such as the end regions. More than 300 wall conditioning shots are required to obtain good wall conditions. The progress of the wall conditioning is monitored by comparing the pressure at 40 ms after the start time of the plasma discharge for cases with and without plasma shots. Studies of gas dynamics indicate a long sticking time of hydrogen of the order of 10 ms on the wall surfaces. The speed of the plasma pumping in the central region is estimated to be 4–6.5×105 l/s. Because of the plasma pumping, the required vacuum conditions are attained without large‐pumping‐speed pumps in the central region.