  In this paper, we propose a masked photocathode inside a photoinjector for
generating high brightness electron beam. Instead of mounting the photocathode
onto an electrode, an electrode with small hole is used as a mask to shield the
photocathode from the accelerating vacuum chamber. Using such a masked
photocathode will make the replacement of photocathode material easy by
rotating the photocathode behind the electrode into the hole. Furthermore, this
helps reduce the dark current or secondary electron emission from the
photocathode material. The masked photocathode also provides transverse cut-off
to a Gaussian laser beam that reduces electron beam emittance growth from
nonlinear space-charge effects.
