Beam test of niobium sputtered QWRs and upgrading of ALPI medium β cavities

Four new super-conducting Quarter Wave Resonators (QWR, β=0.13), obtained by niobium sputtered on an OFHC copper substrate, were recently installed and tested on the ALPI post-accelerator (1). The resonators substituted four similar ones that were operating at 4 MV/m at 7 W, because two of their couplers jammed. Now the cavities have a new coupler design and show an acceleration field, measured with different ion species, in excess of 6 MV/m at 7 W (about 1 MeV energy gain per cavity per charge unit). The resonators show excellent stability on long time runs even at such a high accelerating field. The high accelerating fields, together with the good stability and the low costs, made this technology almost ideal for heavy ion acceleration. Moreover the niobium sputtering process was applied to substitute the electroplated lead coating in four medium β(β=0.11) resonators after only minor changes in the resonator shape. Accelerating fields in excess of 4.5 MV/m a 7 W could be obtained in laboratory even thou...