Laser heated LaB/sub 6/ thermionic cathode on a MV electron beam accelerator

Summary form only given. One of the major problems in high voltage/current and long pulse e-beam generation is plasma diode closure. LaB/sub 6/, thermionic cathodes can eliminate plasma and are more robust from poisoning than oxide cathodes. To heat the LaB/sub 6/ disk on the MELBA cathode stalk at -1 MV we employ a 100-650 W CW Nd:YAG laser. Depending upon the LaB/sub 6/ disk diameter (1 cm or 2.2 cm), temperature, and cathode shape, preliminary analysis of experiments suggests three e-beam modes: 1) Thermionic mode at electron beam current density of 100-150 A/cm/sup 2/, with current shape which matches the voltage flatness, 2) Plasma mode in which 2-6 kA of current is drawn with rapid current ramping, and 3) Combined thermionic and plasma mode with initial thermionic current turn-on which is later overtaken by plasma current. It is believed that the plasma modes may involve explosive edge emission or outgassing from a graphite cathode mounting structure.