  The complete design, fabrication, and performance evaluation of a compact,
single cell, $X$-band ($\sim$9 GHz) electron injector based on a field emission
cathode (FEC) are presented. A pulsed electron beam is generated by a 10's of
kW radiofrequency (RF) magnetron signal from a plug-in thin film
nitrogen-incorporated ultrananocrystalline diamond (N)UNCD FEC cartridge.
Testing of the $X$-band injector with the (N)UNCD FEC was conducted in a
beamline equipped with a solenoid, Faraday cup and imaging screen. The results
show that typically the (N)UNCD FEC cartridge produces $\gtrsim$1 mA/cm$^2$ at
a surface electric field of 28 MV/m. The diameter of the output beam generated
from the 4.4 mm diameter (N)UNCD cartridge can be as small as 1 mm. In terms of
its practical applications, the demonstrated $X$-band electron injector with
the (N)UNCD plug-in FEC can serve as a source for X-ray generation, materials
processing, travelling-wave tubes (including GHz and THz backward wave
oscillators), or can be used to drive slow-wave accelerating structures. The
results presented also suggest that this field emitter technology based on
planar (N)UNCD thin films, which are simply grown on the surface of optically
polished stainless steel, can enable a vast number of device configurations
that are efficient, flexible in design, and can be packaged with ease.
