Particle simulations of relativistic electron beam injection from spacecraft

Abstract

Linear accelerators producing relativistic (5 MeV) electron beams are now down to a size that allows them to be flown on spacecraft and sounding rockets. This opens up new opportunities for atmospheric/ionospheric modification experiments where the mesosphere and lower thermosphere regions can be perturbed down to 40‐km altitude. In this paper the relativistic electron beam injection process is investigated by means of three‐dimensional particle‐in‐cell simulations to determine the initial interaction of the beam with the spacecraft and the ambient plasma. The results indicate that relativistic beams are more stable than keV‐energy beams investigated in the past, allowing the injection and propagation of beams with currents several orders of magnitude higher than those for keV‐energy beams. The superior stability of relativistic beams is the result of a combination of effects including the higher relativistic electron mass, a lower beam density, and a smaller effect from spacecraft charging. Relativistic beams injected downward from spacecraft are therefore expected to deposit a large fraction of the energy in the middle atmosphere. In the high‐current limit (I > 100 A) the beam self‐fields are strong. In this regime a beam may propagate in the ion‐focused regime, where beam electrons expel ambient electrons to create a channel of ambient ions that space charge neutralize the beam. The establishment of the ion channel, however, creates significant turbulence and scattering.