Pulsed electron field emission from prepared conductors

The electron emission characteristics of metal cathodes subjected to pulsed electric fields in the absence of insulating magnetic fields has been investigated experimentally. Uniform electric fields in the range of 0.2 to 0.8 MV/cm were applied to 50 cm2 surfaces under vacuum in single pulses of approx. 60 ns duration at a voltage of approx. 0.5 MV. Bare metals and metals coated with dielectric materials were studied. Results show that bare metals with freshly prepared surfaces can withstand fields of approx. = but > 300 kV/cm for approx. = but > 40 ns without significant emission. Emission-induced discharges degrade the surfaces such that full space-charge-limited current densities (100 to 250 A/cm2 for this experiment) are obtained at fields as low as 200 kV/cm on subsequent pulses. In the case of coated surfaces, it was found that dielectrics could occasionally suppress emission completely up to approx. 300 to 400 kV/cm, and unlike bare metals, could partially suppress emission after having passed significant current at fields up to 0.6 MV/cm