ADVANCED SURFACE CLEANING METHODS - 3 YEARS OF EXPERIENCE WITH HIGH-PRESSURE ULTRAPURE WATER RINSING OF SUPERCONDUCTING CAVITIES

In the last three years we have carried out a large number of tests on single cell and multi-cell niobium and Nb3Sn cavities at L-band frequ encies, which as a final surface cleaning step had been rinsed with hi gh pressure jets of ultrapure water. This treatment resulted in an unp recedented quality and reproducibility of cavity performance. Field em ission free surfaces up to peak surface electric fields of E(peak) gre ater than or equal to 45 MV/m were achieved nearly routinely after buf fered chemical polishing of niobium surfaces. In addition, residual su rface resistances below R(res) less than or equal to 10 n Ohm and as l ow as R(res) = 2 n Ohm were not uncommon. In 5-cell production cavitie s of the Cornell/CEBAF shape gradients as high as E(acc) = 21.5 MV/m c orresponding to peak surface fields of E(peak) approximate to 55 MV/m have been measured after post purification with Ti without the need fo r rf-processing. Several Nb3Sn-cavities exhibited no field emission lo ading after high pressure ultrapure water rinsing up to the maximum ac hievable surface fields of E(peak) approximate to 33 MV/m; the field l imits were given by the available rf-power. The unprecedented reproduc ibility of the cavities permitted serial testing of various parameters effecting cavity performance such as the influence of residual gas in side the cavities prior to cooldown, the removal of the surface damage layer or the impact of peripheral parts such as rf-windows. The major portion of this paper summarizes several of the results obtained from investigations carried out during the last three years. The second pa rt discusses possibilities for further improvements in cavity cleaning .