Emission of an intense large area electron beam from a slab of porous dielectric

Inserting a thick slab of porous dielectric (e.g., ceramic honeycomb) in front of the emitting surface of a large-area planar diode improves the electron beam emission uniformity, decreases the beam current rise and fall times, and maintains a more constant diode impedance. Particle-in-cell simulations of the first few nanoseconds of diode operation show that initially numerous secondary electrons and ions load the ceramic honeycomb. The electrons and ions were confined within the ceramic pores, redistributing the electric field by reducing it within the ceramic pores and increasing it on the cathode surface (by a factor of 2-3). After the initial stage, plasma fills the ceramic pores and the space between the cathode and the ceramic. A space-charge-limited electron beam was then emitted from the ceramic honeycomb. No surface plasma was detected outside the pores inside the diode vacuum. The introduction of dielectric into the diode solves two additional problems associated with large-area planar diodes: (1) Space-charge-limited flow in large-area planar diodes is susceptible to the transit time instability. Feeding the instability are growing transverse electromagnetic (TEM) waves that propagate along the anode-cathode gap. By inserting the slab of honeycomb ceramic in front of the emitting cathode these TEM waves are suppressed. (2) A planar diode emits an electron beam with an enhanced current density at the edges (edge effect). The ceramic slab can be easily machined and contoured so as to reduce this effect. The insertion of ceramic honeycomb into the diode had little affect onthe postshot diode pressure. However, deposition of gamma alumina on the ceramic reduced the postshot diode pressure by 80%. This enables the diode to be repetitively pulsed (rep-rated mode). The modified diode was fielded on Electra, a high-power, rep-rated, electron-beam pumped KrF laser. It operated for 50 000 shots at 1 Hz and 8000 shots at 5 Hz with little or no degradation in the pulse shape and with undetectable loss of cathode material. (C) 2004 American Institute of Physics.