In Situ Observation of Dark Current Emission in a High Gradient rf Photocathode Gun.

Undesirable electron field emission (also known as dark current) in high gradient rf photocathode guns deteriorates the quality of the photoemission current and limits the operational gradient. To improve the understanding of dark current emission, a high-resolution (similar to 100 mu m) dark current imaging experiment has been performed in an L-band photocathode gun operating at similar to 100 MV/m of surface gradient. Scattered strong emission areas with high current have been observed on the cathode. The field enhancement factor beta of selected regions on the cathode has been measured. The postexaminations with scanning electron microscopy and white light interferometry reveal the origins of similar to 75% strong emission areas overlap with the spots where rf breakdown has occurred.