Paraxial gas-cell focusing of relativistic electron beams for radiography

Summary form only given. A quantitative description of the underlying physics governing the operation of pulsed-power driven, gas-filled, paraxial diodes is presented. Gas-filled focusing cells are routinely used to transport high energy density electron beams for use in flash X-ray radiography experiments. Intense X-rays are generated when the high current (10-30 kA) and high voltage (3-10 MeV) electron beam stops in a high atomic number target and creates bremsstrahlung radiation. The radiation spot and dose are critically dependent on the beam characteristics at the target. The paraxial diode acts as a 1/4 betatron focusing lens with a focal length F proportional to the square root of the beam energy and inversely proportional to the square root of the net current (beam+plasma) in the gas cell, F/spl prop/(/spl gamma//I/sub net/)/sup 1/2/. The time integrated radiation spot is determined both by finite beam emittance and by focal sweeping due to time variations in net current. The beam radius, focal plane, emittance, paraxial dose and time-integrated spot are calculated via particle-in-cell simulations with the Lsp code. The calculated radiation focal plane, spot and dose are compared to data obtained by the Atomic Weapons Establishment from a variety of experimental configurations. Suggestions to improve the X-ray intensity are presented.