Simulation, generation, and characterization of high brightness electron source at 1 GV/M gradient

This paper describes computer simulations and measurements on an electron bunch from a pulsed, high gradient gap. MAFIA and PBGUNS were used to calculate the emittance, brightness and energy spread of the electron beam for peak currents ranging from 10 A to 1 kA and pulse durations ranging from 0.3 ps to 10 ps. Under optimum conditions, normalized emittance of 10^-7 pi m-rad, beam brightness of 3×10¹⁵ A/(m-rad)² and energy spread of 0.15% were obtained. A pulsed high voltage with 1 MV amplitude, and approximately 1 ns duration was applied to the diode with an interelectrode gap ranging from 2 mm to 0.5 mm. Copper cathodes with three different surface preparations, diamond polished, diamond cut and chemically cleaned, have been tested for their voltage hold-off properties under this high gradient and the Fowler-Nordheim plots were generated. The diamond polished OFC class II copper was shown to consistently produce lower dark current and higher hold-off voltage. Photoemission studies have been made using light from a KrF excimer. The field enhancement factor for photoemission was calculated to be 5, an order of magnitude smaller than the dark current beta for a similar surface.