  Field emission or dark current from RF electron guns is problematic for
accelerators for several reasons, and can be more difficult to predict, measure
and understand, compared to the main beam. The work presented here is a study
of the dark current emitted from the CLARA 3GHz 2.5 cell electron gun with
exchangeable photocathode, which is a type of gun used for single pass high
quality low emittance beams for uses such as Free Electron Lasers. The main
purpose of these studies was to identify sources of dark current emission
within the gun, by examining the dark current distribution and patterns on
scintillating screens. It is shown that the features of the patterns can be
reproduced qualitatively, and to some extent quantitatively, by finite element
simulations. Thus, certain surfaces within the gun and cathode apparatus which
contribute most significantly to problematic dark current can be identified,
and further development and preparation of the guns and their cathodes can be
informed to minimise dark current.
