Plasma-filled focusing cell for radiographic paraxial diodes on RITS

Paraxial diodes have been a stronghold for high-brightness, flash X-ray radiography. In its traditional configuration, an electron beam impinges onto an anode foil, entering a gas-filled transport cell. Within the cell, the beam is focused into a small spot onto a high-Z target to generate X-rays for the radiographic utility. Simulations using Lsp, a particle-in-cell code, have shown that within the gas-filled focusing cell the electron beam spot location sweeps axially during the course of the beam pulse. The result is a larger radiographic spot than is desirable. Lsp has also shown that replacing the gas-filled cell with a fully ionized plasma on the order of 10¹⁶ cm^-3 will prevent the spot from significant beam sweeping, thus resulting in a smaller, more stable radiographic spot size. Sandia National Laboratories (SNL) is developing a plasma-filled focusing cell for future paraxial diode experiments. A z-discharge in a hydrogen fill is used to generate a uniform, highly ionized plasma. Laser interferometry is the key diagnostic to determine electron density in a light lab setting and during future paraxial diode shots on SNL´s RITS-3 accelerator. A time-resolved spot diagnostic will also be implemented during diode shots to measure the change in spot size during the course of the pulse.