  A case performance study of a planar field emission cathode (FEC) based on
nitrogen-incorporated ultrananocrystalline diamond, (N)UNCD, was carried out in
an RF 1.3 GHz electron gun. The FEC was a 100 nm (N)UNCD film grown on a 20 mm
diameter stainless steel disk with a Mo buffer layer. At surface gradients
45-65 MV/m, peak currents of 1-80 mA (equivalent to 0.3-25 mA/cm$^2$) were
achieved. Imaging with two YAG screens confirmed emission from the (N)UNCD
surface with (1) the beam emittance of 1.5 mm$\times$mrad/mm-rms, and (2)
longitudinal FWHM and rms energy spread of 0.7% and 11% at an electron energy
of 2 MeV. Current stability was tested over the course of 36$\times$10$^3$ RF
pulses (equivalent to 288$\times$10$^6$ GHz oscillations).
