Plasma-filled paraxial diode operation on RITS-3

Summary form only given. In electron beam driven flash X-ray radiography, an electron beam is focused onto a high atomic number target to generate high dose X-rays while achieving small, <5 mm, spot sizes. The paraxial diode, which utilizes a gas-filled transport cell to focus the beam into a small spot, has been implemented for several decades. Recent particle-in-cell (PIC) simulations have identified key performance limitations which might prevent this diode from meeting future design requirements. The primary limitation to achieving a small, stable spot is the resistive decay of the plasma return current within the gas-filled transport cell. The time-dependent net currents cause the beam focal plane to shift axially during the timescale of the pulse. This axial movement, referred to as "beam sweep," smears out the radiographic spot, leading to a larger spot size than is desired. By replacing the neutral gas with a highly conductive, pre-ionized plasma with density of order 10¹⁶ cm^-3, simulations show that the net currents are significantly reduced so that little beam sweep occurs, thereby achieving a smaller time-integrated spot. A plasma-filled version of the paraxial diode was devised and fielded on the RITS-3 accelerator at Sandia National Laboratories, operating at 4 MV. Several RITS shots were fired with plasma densities typically in the range of 10¹⁵-10 ¹⁶ cm^-3, as well as with initially neutral gas (0.5 torr hydrogen and 1-5 torr air). Stable spot sizes were measured for several plasma-filled paraxial diode shots. Highlights from the spectroscopic survey are reported. Plans for future plasma-filled paraxial diode investigations are also presented