Development of a Large Area, Durable Electron Emitter for High Average Power KRF Lasers

The high energy, repetitively pulsed krypton-fluoride (KrF) lasers being developed for use in inertial fusion energy research require large area electron emitters that are both durable and efficient. Specifically, emitters of 1000´s of cm² must reliably produce 100´s of kA at voltages up to 1 MV in flat-topped, sub-microsecond pulses, at pulse repetition frequencies (PRF) of 1-10 Hz for 10⁵ -10⁸ continuous shots, Emission takes place in a vacuum diode that is immersed in an external magnetic field. To achieve the needed durability and efficiency, solutions for gas evolution, gap closure, beam halo formation, current density uniformity, beam current rise time, temperature management, and beam patterning must be found. On-going experiments using the Electra main amplifier at the Naval Research Laboratory have suggested that using a segmented secondary emission cathode may satisfy the above requirements. The secondary emission cathodes initially developed for repetitively pulsed laser operations on Electra consisted of a velvet dielectric primarv emitter coupled with a monolithic ceramic honeycomb secondary emitter. These 3000 cm² cathodes allowed continuous runs of thousands of shots at 1 Hz producing 110 kA peak current at a diode voltage of 500 kV. However, reliability and efficiency were limited at greater PRF due to problems with gas evolution, temperature management, and ultimately gap closure. This paper reports on the experimental development of an improved secondary-emission cathode that largely eliminates these problems and is designed to operate reliably at PRF of 2.5 -5 Hz for extended, continuous runs. Time-resolved electrical and optical measurements of electron emission will be presented and discussed. Further work to convert the monolithic design to a segmented cathode that will allow high efficiencies and high laser outputs will also be presented.