Field-emission loading of superconducting accelerator cavities at L- and S-band frequencies

The authors report on investigations of the number, location, strength and stability of electron emitters found in superconducting single-cell cavities at L- and S-band frequencies. The applied experimental techniques as well as the results of electron trajectory calculations are described. Field-enhanced factors obtained from integrating quantities like the measured Q degradation and electron probe current are compared with those from local detectors. Initial results on the effect of different preparation techniques and of RF and helium processing are presented. In one case the combination of high-temperature annealing at 1300°C under exposure to titanium vapor with controlled dust-free assembly of the cavity has resulted in a field-emission-free Nb surface up to a peak electric surface field of 64 MV/m