A vacuum-to-air interface for the advanced test accelerator

Abstract

The operation of some future accelerators will require that a pulsed particle beam exit a region which is at a pressure of 10−5 Torr or less into a region which is at atmospheric pressure. The separation between atmosphere and vacuum could be achieved with small apertures or differential pumping stages. In this paper we report some of the results of tests carried out on a component which could be used as the first stage of a differentially pumped system. The component employs an expendable foil and foil advancement mechanism which serve as both an orifice and a fast-acting vacuum valve. In operation, the particle beam penetrates the thin foil, thereby creating an aperture of minimum geometry between the atmosphere and the vacuum system. The beam duration is on the order of 70 to 100 ns. During the balance of the (1-s) cycle, the foil is advanced to seal the opening and recover the loss in vacuum. A series of differentially pumped stages will be used to further separate the two pressure regimes. The design goal for this first stage was to achieve a pressure reduction of 760:1. Our test results show that for a 1-mm-diam orifice, undergoing multiple cycles, the system pressure rise is approximately 1.7 Torr/cycle and for a 10.0-mm-diam orifice, the pressure rise is approximately 2.9 Torr/cycle. The gas throughput for the 10-mm orifice was approximately 9700 Torr l/s. Future objectives include the further reduction of the system pressure rise in order to maintain 1.0 Torr or less during the 1-s cycle time.