  Reported here is the first demonstration of electron beam generation in an
SRF TESLA 1.3 GHz gun equipped with field emission cathode when operated at 2
Kelvin. The cathode is submicron film of nitrogen-incorporated
ultrananocrystalline diamond [(N)UNCD] deposited atop a Nb RRR300 cathode plug.
The output current was measured to increase exponentially as a function of the
cavity gradient. Our results demonstrate a feasible path toward simplified
fully cryogenic SRF injector technology. One important finding is that the
electron emitter made of (N)UNCD, a material long been known as a highly
efficient field emission material, demonstrated a record low turn-on gradient
of 0.6 MV/m. A hypothesis explaining this behavior is proposed.
