Axisymmetric Hollowing Instability of an Intense Relativistic Electron Beam Propagating in Air

Simulations of relativistic electron beams propagating in air show axisymmetric hollowing instabilities of the beam, caused by avalanche ionization of the air near the beam axis. This produces an induced return current peaked on axis, which weakens the pinch force there and triggers the instability for E/p>130 kV cm^-1 atm^-1 and current neutralization >50%. We performed experiments verifying the existence of this instability on a 6-kA/ns, 70-kA, 4-MeV beam for air pressures less than {~}80 Torr, the predicted threshold.