New developments in paraxial radiographic diode technology for focusing intense relativistic electron beams

The Pulsed Power Group at the Atomic Weapons Establishment (AWE) is interested in the generation of small, intense, multimegavolt X-ray sources for radiographic applications. For this paper, research has been carried out into electron beam transport and focusing studies on one of our single pulse forming line flash X-ray machines. Experiments were undertaken on E Minor to improve the radiographic spot size generated by our "paraxial diode". This is a novel anode/cathode assembly designed to produce a small-diameter electron beam focused onto a high-Z target. Electrons are propagated through a gas-filled cone such that their electrostatic repulsive and magnetic attractive forces are almost cancelled. The cone's length and gas pressure were initially optimized and then specific gas species applied to the drift cone. Helium gas improved spot size reproducibility with respect to variations in drift gas pressure when compared with dry air. The addition of a "secondary cell" to increase the neutralization of the electron beam's self-electric forces yielded a 30% improvement in measured radiographic spot size.