Tests of a niobium split-ring superconducting heavy-ion accelerating structure

A niobium split-ring accelerating structure designed for use in the Argonne superconducting heavy-ion energy booster has been successfully tested. The superconducting resonator has a resonant frequency of 97 MHz and an optimum particle velocity β = 0.11. Ultimate performance is expected to be limited by peak surface fields, which in this structure are 4.7 Eaelectric and 170 Ea(Gauss) magnetic, where Eais the effective accelerating gradient in MV/m. RF losses in two demountable superconducting joints severely limited performance in initial tests. Following independent measurements of the rf loss properties of several types of demountable joints, one demountable joint was eliminated and the other modified. Subsequently, the resonator could be operated continuously at Ea= 3.6 MV/m (corresponding to an energy gain of 1.3 MeV per charge) with 10w rf input power. Maximum field level was limited by electron loading. The mechanical stability of the resonator under operating conditions is excellent: vibration induced eigenfrequency noise is less than 120 Hz peak to peak, and the radiation pressure induced frequency shift is Δf/f = 1.6 × 10^-6Ea².