X-ray spectroscopy-a new method to investigate field emission in superconducting accelerating cavities

We report on a study of field emission accompanied by emission of visible light and by emission of characteristic X-rays in a 5 and a 20 cell superconducting niobium 3 GHz accelerating cavity of the S-DALINAC injector. The nearly absorption free measurement of characteristic niobium X-rays inside the beam pipe allowed for the first time an estimate of the field emission current during the normal operation of superconducting accelerating cavities within their cryostat. The field emission current for different accelerating field strengths is well described by the law of Fowler and Nordheim. Hence the field enhancement factors for the observed field emission sites could be determined. For electron tracking calculations based on the Leap-Frog method the field strength distribution of the 5 cell cavity was determined. Due to these calculations it can be assumed that light emitting spots are electron emitters too and that the characteristic niobium X-rays are emitted from the same areas. We additionally report on the cleaning method and heat treatment of the superconducting niobium accelerating cavities at the S-DALINAC with particular application to the 5 cell cavity. After that, up to the maximum applied field strength of Eacc=6.5MV/m field emission as well as the emission of visible light and characteristic X-rays were not observed anymore.