DESIGN AND TEST OF A CRYOGENIC SEAL FOR RECTANGULAR WAVEGUIDE USING VATSEAL TECHNOLOGY

Vacuum sealing technology for superconducting rf applications must possess certain characteristics: reliability, cleanability, low-particulation, and ultra-high-vacuumcompatible seal quality. These requirements most often lead to the use of CF-type flanges with copper gaskets, aluminum-magnesium diamond seals, or formed indium wire. The use of superconducting rf technology in high-current accelerators and the associated high-power waveguides leads to an additional requirement: minimization of trapped field losses in the seal and beam impedance by minimizing the width and depth of the pocket created by the seal. CF-type flanges and diamond seals must be made circular and have a minimum stand-off distance from the inner diameter of the sealing flanges, making them especially unsuited for rectangular waveguides. Indium wire can be made in any shape, but the appropriate line loading must be established, and the spreading of the indium during sealing is challenging to control. Disassembly of the indium joint is also quite messy and risks contamination of any nearby niobium surfaces. A commercial sealing technology known as VATSEAL may neatly address all of the above requirements. Elsewhere, its feasibility was studied using finite element modeling. In this paper, we installed the VATSEAL to our mock-up flanges, measured vacuum leak rate in a pumped helium bath, and confirmed the seal is high vacuum tight even at 1.65 K.