  The present letter extends the prior findings on self-induced heating of
solid state field emission devices. It was found that a vacuum diode (base
pressure $\sim10^{-9}$ Torr), that makes use of graphite-rich polycrystalline
diamond as cathode material, can switch from diode regime to resistor regime,
to glow discharge plasma regime without any external perturbation, i.e. all
transitions are self-induced. Combined results of in situ field emission
microscopy and ex situ electron microscopy and Raman spectroscopy suggested
that the nanodiamond cathode of the diode heated to about 3000 K which caused
self-induced material evaporation, ionization and eventually micro-plasma
formation. Our results confirm that field emission, commonly called cold
emission, is a very complex phenomenon that can cause severe thermal load.
Thermal load and material runaway could be the major factors causing vacuum
diode deterioration, i.e. progressive increase in turn-on field, decrease in
field enhancement factor, and eventual failure.
