Radiographic paraxial diode experiments on RITS-6

Intense electron beam-driven diodes are being developed for flash x-ray radiography at 7.5 - 12 MV on the RITS-6 accelerator at Sandia National Laboratories. One of several diodes under investigation is the paraxial diode in which a 40-kA electron beam is formed in vacuum, propagated through a thin anode foil, and focused inside a gas-filled transport cell onto a high-atomic-number target to generate x-rays. Two key objectives are to produce a small spot size, <;5 mm, and high forward-directed dose, >;600 R@1m. Particle-in-cell (PIC) simulations have shown that the primary limitation in spot size is the finite decay of the plasma return current which causes the beam focal position to sweep axially during the pulse, hence leading to an increased time-integrated spot size [1]. Recently obtained time-resolved measurements of the spot size which support this hypothesis are reported. Interpretation of these results was aided by PIC simulations in which additional physical processes were included, such as stimulated ion emission from the foil and increasing area of cathode emission [2]. Other outstanding issues for the paraxial diode on RITS-6 are also presented which include electron power-flow coupling to the diode, cavity modes which may affect focusing, and gas-breakdown physics models. Measurements of dose, dose rate, time-integrated (and time-resolved) spot size, and current are reported from experimental results primarily at 7.5 MV. Time-resolved images of the cathode emission are also presented.