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

Field-emission loading has become the dominant limitation for the achievable accelerating gradients in superconducting cavities built from high-purity Nb since quenching caused by small defects can be suppressed by the improved thermal conductivity of the cavity wall. The rf power absorbed by free electrons contributes significantly to the total losses and therefore to the effective quality factor of superconducting cavities at high field levels. The energy gained by the electrons is converted at the cavity wall mainly into heat and partially into X-radiation both of which can be detected locally by scanning systems of resistor thermometers and photodiodes. The authors report on investigations on 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 enhancement factors obtained from integrating quantities like the measured Q degradation and electron probe current are compared with those from local detectors. First 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 degrees C under exposure to titanium vapor with controlled dust free assembly of the cavity has resulted in a field emissions free Nb surface up to a peak electric surface field of 64 MV/m