Radiographic paraxial diode investigations on RITS-6

Presently, investigations of intense electron beam-driven diodes for flash x-ray radiography are being conducted on the upgraded RITS-6 accelerator, 7-12 MV, at Sandia National Laboratories. One of several diodes under investigation is the paraxial diode, which employs a gas-filled transport cell to focus the beam 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, ∼ 600-1000+ Rads@1m. LSP 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 drift axially during the timescale of the pulse, hence leading to an increased spot size [1]. Replacing the gas with a preionized plasma has shown potential for stabilizing the beam focus by completely neutralizing the beam current [2]. Plans for the incorporation of a novel plasma-filled version of the gas-filled paraxial are presented. Another outstanding issue for the paraxial diode on RITS-6 concerns the powerflow coupling from the magnetically insulated transmission line to the diode. The radiation pulse is dominated by large amplitude oscillations, ∼125 MHz, which are correlated with the drive pulse. LSP simulations have shown that higher frequency modes, ∼ 220 – 365 MHz, are excited in the diode region as well. Efforts to mitigate these oscillations are presented. Forward-directed dose as well as time-integrated and time-resolved radiation spot measurements are reported.