Vacuum/volume 41/numbers 4-6/pages 1561 to 1564/1990 O042-207X/90S3.00 + .00 Printed in Great Britain © 1990 Pergamon Press plc Helium cryopumping system in the tandem mirror GAM MA 10 Y Nakashima, K Yatsu, Y Kiwamoto, M Ichimura, K Tsuchiya and S Miyoshi, Plasma Research Center, University of Tsukuba, Tsukuba, Ibaraki 305, Japan and K Masuda, Y Koguchi, T Kai and T Sasaki, Toshiba Corporation, Uchisaiwai-cho, Chiyoda-ku, Tokyo 100, Japan A large-scale cryopumping system using low temperature (3.5K) liquid helium (LHe) was installed in the axisymmetrized tandem mirror GAMMA 10 in order to obtain a clean vacuum condition with a larger pumping speed. Four cryopumps were installed in the end-mirror tanks and in the NBI tanks of the anchor regions on both sides. Performance tests have confirmed that the measured pumping speed agrees well with the designed value. By using this pumping system, the base pressure in the end-mirror region has reached below 1 x 10 -8 torr and improved thermal-barrier experiments were successfully carried out under the upgraded vacuum condition. 1. Introduction It is essential to establish good vacuum conditions with low hydrogen recycling and low impurity concentration for creating high and long-lasting confining potentials in tandem mirror devices ~-4. End-loss particles escaping from the confinement region of tandem mirrors hit the interior wall surface of the vacuum chamber in the end-region and cause the gas desorp- tion of hydrogen .molecules and impurities. The accumulating gas in the vacuum chamber prevents the sustainment of the potentials produced in the plug/barrier region. Therefore, the vacuum pumping is an important ingredient for the plasma confinement and for the particle control in tandem mirrors. Titanium gettering with liquid nitrogen-cooled panels has been widely used as a large-capacity pumping system in the tandem mirror devices4-6. In the tandem mirror GAMMA 10 at the Plasma Research Center, University of Tsukuba, the titanium getter pump was used as a main pumping system until 1987. Recently, in 1988, a large-scale cryopumping system using liquid helium (LHe) was installed in GAMMA 10 to improve vacuum conditions and to obtain a larger pumping speed. This is the first case in tandem mirrors that a large-scale cryopump has been successfully applied to thermal-barrier experiments. In this paper, we describe the design features and the results of the performance tests on the GAMMA 10 cryopumping system. 2. System design Figure 1 shows the schematic view of the GAMMA 10 device and the vacuum system. GAMMA 10 is a minimum B- anchored tandem mirror with a thermal-barrier in each axisym- metric end-mirror region 7. GAMMA 10 consists of a central region, anchor regions, plug/barrier regions, end-regions, which are axially aligned, and NBI and beam dump tanks on sides. Six turbo-molecular pumps are installed as shown in the figure, and provide the base pressure of 4 x 10 -7 Torr without cryo- pumping. The main loads for the cryopumps are the exhaust gas due to the plasma bombardment in the end-tank and the downstream gas from NBIs which are located in the anchor regions and in the end-mirror tanks. The cryopumps are in- stalled in the anchor NBI tanks and the end-mirror tanks on both east and west sides. Table 1 gives several basic parameters for the cryopumping system. The designed temperature of the panel is 3.5 K, as required for obtaining the sufficient pumping speed in the plug/barrier region under the hydrogen atmo- sphere of 10 -7 Torr. A schematic view of the cryopumping system is shown in Figure 2. The He cryogenic system forms a complete closed-cy- cle to maintain a high purity of He gas. The cryogenic system is separated into two sections, in the east and in the west. Both sections are connected to a common buffer tank, for the recovery of He gas, and a He purifier. Liquid helium is supplied to the cryopumps in the end-mirror tank and the NBI tank on each side from each section, which consists of a set of refriger- ators, a compressor and a vacuum pump. 2.1. Cryopump. The cryopump consists of several cryopanel modules made of aluminum alloy, a reservoir tank, entrance and bypass valves, and LN 2 shields. The diameter of the cryopump is about 3 m for the end-mirror tank and about 2 m for the anchor NBI tank. Each panel is screeened with an LN2-cooled louver blind for the radiation shield. The angle and 1561