Microscopic examination of field emission areas in superconducting Nb cavities

A superconducting cavity has been developed in which it is possible to expose a small region of the cavity, called the dimple, to very high surface RF electric fields. In a break-apart version of this cavity, the dimple is part of a demountable niobium end-plate which fits into the SEM/EDS system. Surface RF electric fields between 50 and 70 MV/m have been supported by the dimple in several tests. On subsequent SEM examination of the end-plates, many interesting features associated with emission were discovered. Each emission site is located at the center of a starburst shaped region of reduced secondary emission coefficient. At the center of these sites craters of molten niobium have often been found. Sometimes the sites contain features which look like nodules or ripples on the niobium surface. >