  The space charge limit (SCL) of emission from photocathodes sets an upper
limit on the performance of both high- and low-field electron guns. Generally,
one is forced to strike a compromise between the space charge limit and the
cathode's intrinsic emittance (I. Bazarov et al., Phys. Rev. Lett.,
102,104801(2009)). However, it is possible to nearly eliminate the SCL due to
the image charge by engineering the topography of the cathode's surface. A
cathode with a surface plasma frequency below the frequency spectrum of the
accelerating electrons will greatly reduce the bunch's image charge or
polarization of the cathode, resulting in a small image-charge field. Thereby
mitigating the cathode's space charge limit. In the work presented here, a
theory for the image-charge field produced by a disk of charge being
accelerated from the cathode surface is developed to include the
frequency-dependent behavior of surface dielectric function on the fields seen
by the beam. The paper applies this theory to mitigate the SCL of a novel
cathode based upon a wire-array metasurface. It is shown that such
meta-cathodes should have negligible space charge limits for photoelectric,
thermionic and field emission.
