Field emission in superconducting RF cavities

Broad area rf field emission from superconducting Niobium surfaces was investigated. A specially designed superconducting resonator (reentrant cavity) allows direct measurement of the field emission current. Experiments were performed at four cavity modes between 500 MHz and 3.5 GHz in order to study the frequency dependence of rf field emission. Details for the cavity design, cavity treatment, experimental set-up, and measurement technique are given. Field emission currents in the range 10^-15A to 10^-6A were detected at macroscopic electric surface fields of 10 to 50 MV/m at the emitter. The data were analyzed with the Fowler-Nord-heim theory, modified for rf fields. The microscopic field enhancement factor B was in the range of 40 to 150 for different surface conditions and showed no significant frequency dependence. These observations are in agreement with the predictions of dc field emission, but in contradiction to the not yet understood field emission phenomena in superconducting accelerating cavities. The hypothesis of field emission from hot emitters, probably consisting of dust particles heated by the rf fields, can qualitatively explain this discrepancy. Experimental observations on superconducting accelerating cavities support this model.