The LLNL tandem mirror experiment (TMX) upgrade vacuum system

The tandem mirror experiment (TMX) upgrade is a large, tandem, magnetic‐mirror fusion experiment with stringent requirements on base pressure (10−8 Torr), low H reflux from the first walls, and peak gas pressure (5×10−7 Torr) due to neutral beam gas during plasma operation. The 225 m3 vacuum vessel is initially evacuated by turbopumps. Cryopumps provide a continuous sink for gases other than helium, deuterium, and hydrogen. The neutral beam system introduces up to 480 l/s of H or D. The hydrogen isotopes are pumped at very high speed by titanium sublimed onto two cylindrical radially separated stainless steel quilted liners with a total surface area of 540 m2. These surfaces (when cooled to about 80 K) provide a pumping speed of 6×107 l/s for hydrogen. The titanium getter system is programmable and is used for heating as well as gettering. The inner plasma liner can be operated at elevated temperatures to enhance migration of gases away from the surfaces close to the plasma. Glow discharge cleaning is par...