Advanced surface cleaning methods - Three 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 frequencies, which as a final surface cleaning step had been rinsed with high pressure jets of ultrapure water. This treatment resulted in an unprecedented quality and reproducibility of cavity performance. Field emission free surfaces up to peak surface electric fields of E(peak) greater than or equal to 45 MV/m were achieved nearly routinely after buffered chemical polishing of niobium surfaces. In addition, residual surface resistances below R(res) less than or equal to 10 n Ohm and as low as R(res) = 2 n Ohm were not uncommon. In 5-cell production cavities of the Cornell/CEBAF shape gradients as high as E(acc) = 21.5 MV/m corresponding to peak surface fields of E(peak) approximate to 55 MV/m have been measured after post purification with Ti without the need for rf-processing. Several Nb3Sn-cavities exhibited no field emission loading after high pressure ultrapure water rinsing up to the maximum achievable surface fields of E(peak) approximate to 33 MV/m; the field limits were given by the available rf-power. The unprecedented reproducibility of the cavities permitted serial testing of various parameters effecting cavity performance such as the influence of residual gas inside 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 part discusses possibilities for further improvements in cavity cleaning.