A cryopump which has an expansion engine is designed and developed as the first machine in Japan for the nuclear fusion program in the Electrotechnical Laboratory. The flow diagram and the photograph are shown in Fig. 1 and 2 respectively. The cryogenerator consists of a single stage plunger type expansion engine which drives an oil pump or an induction motor as its load, several heat exchangers and a liquid nitrogen container which is set for pre-cooling helium gas. The whole system of the cryogeneratot is set in a vacuum vessel which is evacuated by an oil diffusion pump to 10^-5 Torr range. The pump head is connected with the cryogenerator by a vacuum cylinder which contains two cold helium pipes, going to and back from the cryo-panel. The pump head is designed for the experiment of pumping performance as shown in Fig. 3 and 4 and the surface area of the cryopanel is 2, 500 cm^-2. The temperature of cryopanel can be reduced to 9°K within a few hours after starting the expansion engine. The refrigeration capacity at 9°K is 90 watts and the heat loss of cryopump head is very small. The ultimate pressure at the test dome is 1.5 × 10^-8 Torr after outgassing of 150°C for 20 hours. From mass spectrometrice analysis, it is supposed that the hydrogen partial pressure has the direct influence on the ultimate pressure. The measured pumping speed is nearly equal to the theoretical one and remains constant even after the condensation of large amount of air, 160cm³ (N. T. P.) /an² on the cold surface. The pumping speed increases slightly as the pressure becomes higher, and the increase is larger as the temperature of the cryo-panel is lower. The whole performance of this cryopump is satisfactory for our purpose. We have a future plan to apply a larger cryopump of analogous design to a space simulation chamber.

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