Design Considerations for Very High Voltage DC Electron Guns and Industrial Tubes

Many modern electron beam devices for particle accelerators and industrial applications (such as non-destructive evaluation, or NDE) require very high DC voltages (200 kV to > 1 MV). In high-power guns for electron accelerators, high voltages are necessary to overcome the space-charge forces that exist in a tightly bunched beam. For NDE, very high voltage beams are needed to produce energetic x-rays to penetrate thick, dense or high-Z materials. Considerably more is written about pulsed power systems than DC or quasi-DC systems. Since the design considerations for DC and pulsed electron sources only partially overlap, designers must often rely on practices that are inappropriate or insufficient for DC work. In this paper, we will discuss the overall system design of very high voltage DC guns, including vacuum properties, insulators, power supplies, electrode materials and preparation, and electrode conditioning. Particular emphasis will be placed on photoemission type sources, which have extremely demanding requirements.