Plasma dynamics in relativistic electron beam diodes for flash x-ray radiography

Experiments have been conducted at Sandia National Laboratories´ RITS-6 accelerator facility (operating at 7.5 MV and 180 kA) investigating plasma formation and propagation in relativistic electron beam diodes used for flash X-ray radiography. High resolution, visible and ultraviolet spectra were collected in the A-K gap of the self-magnetic pinch (SMP) diode. Time and space resolved spectra are compared with time-dependent, collisional-radiative (CR) calculations and Lsp, hybrid particle-in-cell code simulations. Results indicate the presence of a dense (1times10¹⁷cm^-3), low temperature (few eV), on-axis plasma, composed primarily of protons from electrode surface contaminants, which rapidly expands (10- 30cm/mus) from the anode to the cathode. In addition, a cathode plasma sheath is observed which extends several millimeters into the A-K gap. It is believed that the interaction of these electrode plasmas causes a premature impedance collapse of the diode and subsequent reduction in radiation output. Diagnostics include high speed imaging and spectroscopy using nanosecond gated ICCD cameras, streak cameras, and photodiode arrays.