Overview of advances in the basic understanding of dark current and breakdown in RF cavities

In the last decade there have been substantial advances in understanding the nature of field emission through DC high voltage studies that locate emission sites, followed by electron microscopy studies to examine the sites. Emission sites have also been located in superconducting RF (SRF) cavities by temperature mapping. The sites found were also examined in an electron microscope after dissecting the cavities. In many RF tests, the emission current from individual sites was tracked with increasing field until a voltage breakdown occurred. The breakdown event was generally followed by decreased field emission. The RF test was stopped, the cavity dissected and the breakdown site analyzed by microscopy. Scanning electron microscopy, energy dispersive X-ray analysis, and Auger studies were used to obtain topographical and elemental information about the emission and breakdown sites. Although these studies were primarily motivated by the desire to reduce field emission in superconducting niobium cavities, the lessons learnt, and the models that have emerged, apply to all field emitting surfaces.