The axisymmetric hollowing instability of an intense relativistic electron beam propagating in air

A space-charge neutralized electron beam propagating through an ionizable gas can be destablized as a result of magnetic repulsion of the beam electrons by induced return currents. The hollowing instability has recently been studied through the use of axially-symmetric particle simulation codes, which have shown that it is the result of avalanche ionization near the beam axis. The resulting highly concentrated return current weakens the self-pinching force near the axis and triggers the hollowing. The criteria that E/p must be greater than ≅130 kV cm/sup -1/ Atm/sup -1/ and current neutralization greater than ≅50% are required for avalanche initiated hollowing have been deduced from these computer experiments. The authors have performed experiments with the beam produced by the IBEX accelerator (E≅4 MeV, I≅70 kA and >4 kA/ns) that have verified the existence of the hollowing instability on this beam as predicted when the air pressure was reduced below the threshold value. The most persuasive observations were time-resolved radiographs of the beam striking a thin tantalum target foil that were made with an X-ray framing camera. These radiographs also revealed a tendency of the beam to tear into filaments after hollowing