Superconducting Triple-Spoke Cavity for β =0.5 Ions

This paper reports results of cold tests of a 345 MHz, three-spoke-loaded TEM-class superconducting niobium cavity being developed for the RIA driver linac and for other high-intensity ion linac applications. The cavity has a beam aperture of 4 cm diameter, an interior length of 67 cm, and the transit-time factor peaks at = v/c = 0.5. In tests at 4.2 k, the cavity could be operated cw above the nominal design accelerating gradient of 9.9 MV/m, which corresponds to peak surface fields of 27.5 MV/m electric and 850 gauss magnetic. At this gradient the cavity provides more than 6 MV of accelerating potential. Cavity Q at 9.3 MV/m exceeded the nominal performance goal of 7.3×10⁸. Operation at the design gradient at 4.2 K causes substantial boiling and two-phase flow in the liquid helium coolant, with the potential for microphonic-induced fluctuations of the rf frequency. Total microphonic rf frequency fluctuations were less than 1 Hz RMS operating cw at 9.7 MV/m at 4.2 K