  Undesirable electron field emission (a.k.a. dark current) in high gradient RF
photocathode guns deteriorates the quality of photoemission current and limits
the operational gradient. To improve the understanding of dark current
emission, a high-resolution (~100 um) dark current imaging experiment has been
performed in an L-band photocathode gun operating at ~100 MV/m of surface
gradient. Dark current from the cathode has been observed to be dominated by
several separated strong emitters. The field enhancement factor, beta, of
selected regions on the cathode has been measured. The post scanning electron
microscopy (SEM) and white light interferometer (WLI) surface examinations
reveal the origins of ~75% strong emitters overlap with the spots where rf
breakdown have occurred.
