The IPCR 160 cm cyclotron has a vacuum chamber, i. e the accelerating chamber, of which the volume is 1×10⁴l and the surface areas in vacuum is about 140m². Many parts, such as dees, ion source and surface shim coils and other undesirable outgassing sources are included in the accelerating chamber. Therefore, vacuum pumps are required to obtain high vacuum in the presence of much outgassing and to have small pump-down time from atmosphere, and also to be firm from troubles.
The pumping system is composed with 32″, 14″ and 6″oil diffusion pumps in series connection aims to promote self purification of oil. Experiments were carried out to confirm this purpose. Several data with regard to 32″ oil diffusion pump, including pumping speed for varying jet gaps and heater wattage, are given in this paper. In normal operation the pumping speeds of 32″ oil diffusion pump are 2×10⁴l/sec and at the connection between the pumping system and the accelerating chamber are about 1×10⁴ l/sec with the baffle. Rough pumping system consists of 1500l/min rotary pump and 300M³/h roots pump with 650l/min rotary backing pump. When there are much outgassing, the roots pump is connected with the 14″ oil diffusion pump in order to evacuate with large throughput. Outgassing throughput during pumping down was measured under several conditions.
In early stage, the base pressure in the accelerating chamber was 1×10^-5 Torr and operating pressure 12×10^-5 Torr. But its base pressure has been improved to 12×10^-6 Torr and operating pressure to 25×10^-6 Torr after replacing rubber water pipes in vacuum with metallic flexible one.
The amount of leakage of the accelerating chamber is less than 0.3 lus and total throughput about 2 lus including outgassing.

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