Development of an alternative differential pumping system by virtual vacuum interface plasma window

Plasma window (PW) is an innovative technology, by which atmospheric pressure and vacuum can be separated. The PW is expected to be useful for an alternative differential pump. Thus, we focus on the reduction of pumps of He gas cell charge stripper in the RIKEN heavy ion beam accelerator, where the gas cell is kept to be 7 kPa while beam transport line is 10^-6 Pa by using 5 stage differential pumps installed both sides. Our goal is to remove two differential pump stages by PWs with an opening of 8 mm.

The apparatus we developed is a cascade arc He discharge source. Although we have succeeded in separating 2.2 kPa and 50 Pa by using our PW without large pumps, we have not achieved the requirement for the differential pumps for ion accelerator. Therefore, we improve the electrode structure, especially, LaB₆ cathode heated by a C/C heater is employed. Cylindrical LaB₆ rings yield lots of electrons due to hollow cathode effect. We expect that this structure contributes to a high-performance PW. In this presentation, we will introduce the new electrode in detail, comparing with the previous apparatus.