  High current bright sources are needed to power the next generation of
compact rf and microwave systems. A major requirement is that such a source
could be sustainably operated at high frequencies, well above 1 GHz, and high
gradients, well above 100 MV/m. Field emission sources offer simplicity and
scalability in a high frequency era of the injector design, but the output rf
cycle charge and high gradient operation remain a great and largely unaddressed
challenge. Here, a field emission cathode based on ultra-nano-crystalline
diamond or UNCD, an efficient planar field emission material, was tested at 100
MV/m in an L-band injector. A very high charge of 38 pC per rf cycle (300 nC
per rf pulse corresponding to rf pulse current of 120 mA) was demonstrated.
This operating condition revealed a space charge dominated emission from the
cathode and revealed a condition under which the 1D Child Langmuir limit was
surpassed. Finally, a beam brightness of $\sim 10^{14}-10^{15}$
A/m$^2\times$rad$^2$ was estimated.
