Experimental studies of long-lifetime cold cathodes for high-power microwave oscillators

Operation of explosive-emission cold cathodes made from various materials was studied at a large number of pulses at current densities of /spl sim/1.0/sup 4/ A/cm/sup 2/. The cathode voltage and the beam current were /spl sim/500 kV and 5 kA, respectively, with a pulsewidth of /spl sim/20 ns. At a small number of pulses (/spl les/10/sup 3/), cathodes of like geometry (even made from different materials) demonstrated similar emission properties. For most of the materials tested, with a large number of pulses (/spl ges/10/sup 3/), the current risetime increased to the fullwidth of the voltage pulse and the maximum current of the vacuum diode decreased. When using a graphite cathode, the maximum current remained invariant until 10/sup 8/ pulses. Mass losses were measured for a series of cathode materials. The results obtained offered the possibility to realize long-lived operation of an X-band relativistic backward-wave oscillator with an almost invariant output power of 350-400 MW during 10/sup 8/ pulses at a pulse repetition rate of 100-150 p.p.s.