Recent paraxial diode experiments on RITS-6

Summary form only given. Development of intense electron beam-driven diodes for flash X-ray radiography is being carried out at 7.5-12 MV on the RITS-6 accelerator at Sandia National Laboratories. One of several diodes under investigation is the paraxial diode, which employs a gas-filled transport cell to focus an electron beam onto a high-atomic-number target to generate X-rays. Three key objectives are to produce a small spot size, <5 mm, high forward-directed dose, >600 rads at 1 m, and efficiently couple this relatively high-impedance diode to the lower-impedance RITS-6 accelerator. Particle-in-cell (PIC) simulations have shown that the primary limitation in spot size is due to the finite decay of the plasma return current which causes the beam focal location to sweep axially during the timescale of the pulse, hence leading to an increasing spot size. Time-resolved measurements of the spot size which convey this trend are reported. Interpretation of these results was aided by PIC simulations in which additional physics was included. Other outstanding issues for the paraxial diode on RITS-6 are also presented which include electron powerflow coupling to the diode, cavity modes which may affect focusing, and gas-breakdown models. Measurements of dose, dose rate, time-integrated (and time-resolved) spot size, and current are reported for experimental results at 7.5 and 10.5 MV.