Current densities and closure rates in diodes containing laser-driven, cesium-coated thermionic cathodes

Publisher Website
Google Scholar
Abstract
Cathodes emitting electron current pulses of 102–103 A/cm2 for durations of longer than 1 μs, with very low emittance, are desirable for free electron laser and high-power microwave tube applications. Not all of these criteria can be satisfied by either dispenser thermionic cathodes or plasma forming field emitters. However, pulse-laser heated smooth cesiated refractory metal surfaces have the potential to achieve these conditions. In this paper, test results from low-voltage (2–20 kV) diodes containing cesiated tungsten cathodes are discussed. Space-charge-limited current densities of up to 30 A/cm2 were obtained in fields with maximum strengths of 40 kV/cm. Plasma closure rates at these currents were measured to be 0.17–0.4 cm/μs. Such slow velocity fronts are postualted to be due to the domination of the plasma by massive cesium ions generated from species desorbed from the cathode, rather than to light hydrogen ions (as in the case of field-emission-type cathodes).