Niobium coaxial quarter-wave cavities

摘要

This paper reports the development status of a superconducting niobium coaxial quarter-wave line accelerating structure to be used in a heavy-ion booster linac following the 16UD pelletron electrostatic accelerator at the Nuclear Science Center, New Delhi. The 97 MHz, single drift-tube niobium structure is housed in an integral stainless-steel jacket, to permit a common beam-line and cryogenic vacuum in the linac, but using only niobium (rather than niobium-copper composite material) in the outer rf cavity wall. The cavity is tunable over a range of several hundred KHz by deforming a section of the cavity wall by as much as 2 mm. The tuning section is a niobium (open quotes)bellows(close quotes) assembly pneumatically extended with helium gas up to one atmosphere in pressure. The coaxial-line cavity is designed to permit strongly-coupling cavities in pairs, thus reducing the number of independently-phased elements required for a heavy-ion linac. Results of initial cold tests are discussed.