Spectroscopic investigations of gas and plasma-filled paraxial diodes for x-ray radiography

Summary form only given. Experiments were conducted at SNL on the RITS-3 accelerator (5.5 MV and 120 kA), and are continuing on the next generation RITS-6 accelerator (10 MV and 120 kA) to study the role of plasmas in electron beam driven flash X-ray radiographic diodes. Plasmas formed by interactions with a relativistic electron beam can affect the impedance behavior of the diodes and contribute to variations in beam focusing. Initial studies investigated beam transport through a preionized background hydrogen plasma. Visible spectroscopy was used to spatially and temporally map the plasma conditions in the focusing cell during the time of the electron beam propagation. Results indicate that the electron density, temperature, and charge state distributions change with the presence of the beam, and can thus be used as quantitative measures of beam location and quality. Studies are now underway to look inside the transport region of neutral gas-filled paraxial diodes to gain similar insights. Experiments looking at plasma formation on the vacuum side of the thin metal anode foils used with paraxial diodes were also conducted. The presence of plasmas in the vacuum gap during the pulse can lead to impedance collapse and pulse shortening. Initial studies on RITS-3 showed foil-plasma formation occurring on the timescale of the beam pulse. These experiments are now being repeated at higher voltages on RITS-6. Diagnostics include a gated, intensified multichannel plate camera with a multifiber input, and a multichannel plate intensified streak camera, each combined with a 1 meter Czerny-Turner monochromator. Recent results are presented