A study of low-current-density microsecond electron beam diodes

The performance of several electron sources has been investigated and compared for generating low-current-density microsecond electron beams with applications to gas laser excitation. A brush cathode made with 10-μm-diam carbon filaments gave the best peformance among the various field emitters tested. It emitted well at fields as low as 10 kV/cm. Space-charge-limited flow was established in ∼60 ns and apparent gap closure velocities of 1.5 cm/μs were characteristic of this cathode. Two regimes of plasma injected diodes were investigated. First, substantial control of the diode impedence was obtained when the plasma was allowed to fill the entire anode-cathode volume prior to application of the high-voltage pulse; however the anode current density was found to be nonuniform with poor reproducibility from shot to shot. Second, more predictable behavior was obtained when the plasma was constrained to be on the cathode surface at the time the high-voltage pulse was applied. In this case the time integrated anode current density was found to be uniform within ±15%. Apparent gap closure velocities as low as 2 cm/μs were measured. A description of the high-voltage pulser and of the various plasma sources used for this experiment are also presented.