Source-Region Electromagnetic Pulse Associated with High-Energy Electron Beams

Estimates are made of the electromagnetic (EM) fields generated by a highly relativistic beam in two configurations. In the first configuration, the beam impinges on a uranium converter and the subsequent bremsstrahlung radiation produces electric currents in air. The absorbed radiation is derived from previous transport calculations. For the second configuration the beam is directly injected into the ambient air and allowed to propagate in a self-pinched mode. Both calculations assume a beam with an electron energy of 400 MeV, a current of 250 kA, a pulse rise of 2 ns, and duration of 100 ns. The equations governing EM fields, background ionization, and secondary electron currents are solved using finite differencing. The current of the primary electron distribution is found using a prescribed source or macroparticles. For the beam/converter system, fields of 104 V/m and 20 A/m are expected within a 5° cone about the beam axis at ranges up to a few hundred meters. When the beam is directly injected, the primary currents are stronger throughout the simulation volume and conductivity is typically larger by two orders of magnitude. The magnetic field is also increased to 200-300 A/m and the peak electric field is increased by an order of magnitude. In the highly conducting air the electric field decays to an amplitude of the order of 104 V/m while the magnetic field remains large.