FEG is typically used under ultra-high vacuum. I'm wondering whether/how it can be used under vacuum levels around 10^(-5) torr but still maintain an appreciable field emission current.
the limiting pressure depends on the mean free path with respect to the vessel dimensions. Lower pressure than 10-5 torr seems to be inadequat. For excellent images you need UHV, but working is possible still for lower pressures.
Rüdiger Mitdank : Thanks. We're looking for such emitter to generate diffracted TEM images at HV level. My concern is that the oxidization of the probe at HV could limit the field emission. But as Ljaz, it could be in a regime of field-thermionic combined emission.