Net current generation and beam transport efficiency of grad-B-drift transported relativistic electron beams

Numerical simulations of grad-B drifting, high-current, relativistic electron beams are presented. The simulations use a hybrid fluid/particle-in-cell code to study the net-current and conductivity evolution for 200 to 900 kA, 1.3 MeV annular electron beams in a background gas of nitrogen (N2) at pressures of 1–60 Torr. Optimum guide-wire current and gas pressure for efficient beam transport are found from the simulations to be ∼40 kA and ∼5–15 Torr of N2, respectively, with energy transport efficiencies as high as 80% for transport distances up to 200 cm. For beam currents and/or gas pressures near the high end of the ranges considered, large net currents significantly alter the magnetic-field profile and result in decreased transport efficiency. Transport efficiencies up to 90% are also found for a self-pinched transport mode in the 1–15 Torr N2 range with no wire current.