Study of dark current phenomena in a superconducting accelerating cavity at the S-DALINAC

We report on a study of field emission accompanied by emission of light in a 20 cell superconducting niobium 3 GHz cavity of the S-DALINAC. The spectral power of the observed light could be interpreted as black body radiation at about 1500 K. The maximum energies of dark current electrons accelerated across the whole cavity were determined from bremsstrahlung spectra and compared with the results obtained by numerical simulations of electron trajectories utilizing a code based on the Leap-Frog method. Explanation of the experimentally observed energies was only possible by assuming a cavity with a strongly deteriorated field profile of the accelerating π-mode caused by detuning of an end cell by 5.4 MHz. The consistency of this hypothesis was experimentally confirmed by comparing the present passband frequencies of this particular cavity with both the calculated ones from a simple lumped circuit model and the eigenfrequencies of other S-DALINAC cavities.