3D PIC simulations of large-radius, annular E-beams in low-pressure air

Summary form only given. High-current relativistic electron beams (e-beams) produced on the Saturn pulsed power accelerator at Sandia National Laboratories (SNL) can be used to generate high dose bremsstrahlung radiation. Typically, multiple concentric ring diodes on Saturn produce currents up to 10 MA at voltages of about 1.6 MV. To study the beam characteristics and transport efficiencies in gas channels at various pressures ranging from 1 to 20 Torr, two- and three-dimensional PIC simulations using LSP have been performed in air-filled gas channels with R/sub channel/ = 15 cm and L/sub channel/ = 10-15 cm./sub ./ Injected current densities up to 75 kA/cm/sup 2/ with angular half-angle divergence ranging from 5/spl deg/ - 30/spl deg/ were tested. Self-fields, collisions, and ionization / recombination physics were modeled in the gas channel self-consistently using a tenuous medium model. Two-dimensional (2D) simulations demonstrated 90 - 100% transport efficiencies in these channels with gas pressures in the range of 5 - 15 Torr. However, experiments at 5 Torr showed that efficiency degraded after 5 cm of transport, suggesting limitations of the 2D modeling. Here, 3D simulations using LSP suggest the influence of a Weibel-like instability in low-divergence (5/spl deg/ half-angle) injections that leads to filamentation of the e-beams; warmer injections (up to 30/spl deg/ half angle) can help avoid this filamentation instability on the time scale of the simulations. Filamentation of the annular beams and limited current neutralization in low-pressure gases lead to beam-to-beam interaction and subsequent loss of beam transport efficiency. In simulations with injections into un-ionized 5 Torr air, annular beam transport was lost after 5 cm, consistent with experimental observations. Simulated initial "seed" ionization of up to 1% raised plasma conductivity to provide better current neutralization and transport. Transport efficiency and annular beam transport quality versus pre-ionization level, injection divergence half-angle, and pressure will be shown.