  Planar nitrogen-incorporated ultrananocrystalline diamond, (N)UNCD, has
emerged as a unique field emission source attractive for accelerator
applications because of its capability to generate high charge beam and handle
moderate vacuum conditions. Most importantly, (N)UNCD sources are simple to
produce: conventional high aspect ratio isolated emitters are not required to
be formed on the surface, and the actual emitter surface roughness is on the
order of only 100~nm. Careful reliability assessment of (N)UNCD is required
before it may find routine application in accelerator systems. In the present
study using an L-band normal conducting single-cell rf gun, a (N)UNCD cathode
has been conditioned to $\sim$42~MV/m in a well-controlled manner. It reached a
maximum output charge of 15~nC corresponding to an average current of 6~mA
during an emission period of 2.5~$\mu$s. Imaging of emission current revealed a
large number of isolated emitters (density over 100/cm$^{2}$) distributed on
the cathode, which is consistent with previous tests in dc environments. The
performance metrics, the emission imaging, and the systematic study of emission
properties during rf conditioning in a wide gradient range assert (N)UNCD as an
enabling electron source for rf injector designs serving industrial and
scientific applications. These studies also improve the fundamental knowledge
of the practical conditioning procedure via better understanding of emission
mechanisms.
