Medium-velocity superconducting cavity for high accelerating gradient continuous-wave hadron linear accelerators

We present the first rf studies of the medium-beta superconducting radio frequency (SRF) elliptical cavities designed for Michigan State University's Facility for Rare Isotope Beams (FRIB) energy upgrade linac. The proposed energy upgrade for this continuous-wave (CW) superconducting linac will double the final beam energy from 200 to 400 MeV/u for the heaviest uranium ions within the 80 m of space available in the FRIB tunnel. Two prototype beta opt = 0.65 644 MHz five-cell elliptical SRF cavities were fabricated and tested to validate the novel cavity design with three conventional rf recipes: (1) Electropolishing (EP-only), (2) EP + 48 h 120 degrees C bake (EP + baking), and (3) Buffered chemical Polishing + 48 h 120 degrees C bake (BCP + baking). The EP-only recipe achieved a 2 K quality factor (Q0) of 2.3 x 1010 at the FRIB energy upgrade design accelerating gradient (Eacc) of 17.5 MV/m, and Q0 of 1.2 x 1010 at a maximum gradient of 26 MV/m, where the gradient was ultimately limited by the available rf amplifier power available for this test. These results validate the potential of the novel 644 MHz medium-beta cavity design and motivate its use in future high-Q0 development work. The multipacting band, which exists at Eacc approximate to 10 MV/m, was completely conditioned in the continuous-wave mode. We also observed that combining the 48 h 120 degrees C baking treatment with EP did not improve the EP-only cavity performance at 2 K due to increased residual resistance (R0) and increased medium-field Q-slope. BCP + baking was also found to produce lower Q0 in this cavity due to increased medium-field Q-slope. The mechanical modes of this cavity were measured at room temperature, verifying that the quality factor of the dominant "accordion" mechanical mode these medium-beta cavities are particularly vulnerable to is close to that of the 1.3 GHz TESLA cavities. Thus, this mode arising from the novel geometry of these cavities is shown to not be of excessive concern for resonance control.