Use of combined sputter ion and NEG pumps in the heavy ion medical machine

The Heavy Ion Medical Machine (HIMM) is the first medical accelerator being built in China using heavy ion beams for cancer therapy. The accelerator complex, developed by IMP (the Institute of Modern Physics, Chinese Academy of Sciences), has several innovative features, including the use of a synchrotron ring as the main ion accelerator. To guarantee a sufficient storage lifetime of the heavy ions, the average pressure for the entire synchrotron ring must be lower than 5 × 10-7 Pa. This has been achieved integrating a sputter ion and a NEG pump in a suitably designed combination pump. The compact size of the combination pump (CP) allows its easy mounting in spite of the severe space limitations of the machine. The overall CP pumping speed exceeds 1000 l/s (H2), a factor 2 larger than a standard ion pump of comparable size. This feature allows achieving lower ultimate pressure in shorter time, this being beneficial to increase the beam time for patient treatment at HIMM. The CP was also tested under very high gas loads to assess its usability at higher pressure (10-5-10-6 Pa). Preliminary data indicate that long term stable pumping performances of the CP can be achieved also under these more demanding conditions. The result is of more general interest for compact accelerators operating in this pressure range and requiring very compact pump design solutions.