Long pulse electron beams produced from carbon fiber cathodes

Carbon fiber cathodes have been used to produce electron beams from 10's of nanoseconds to 10's of microseconds. Cathode areas can vary from a single fiber (/spl sim/10 micron diameter) to many hundreds of square cm. The micron size diameter of the fiber is responsible for the high electric field produced at the tips which allows operation of these cathodes at average fields as low as 10 kV/cm. During operation the emitting tips heat up and some plasma forms and erosion takes place. However, carbon has the highest sublimation energy per unit volume among all the elements. It does not melt and it sublimation temperature is high (4000 K). Therefore, no rounding of the tips take place (metals always melt and form a small sphere which turns off the emission). These properties allow producing cathodes with very million shot lifetimes at current densities of 10's to 1000's of A/cm/sup 2/. We describe our work on carbon fiber cathodes which range in operation from 6 kV to 500 kV anode-cathode voltage and up to a few microseconds pulse duration.