High Current Beam Hollowing Instability Study.

Abstract : High current relativistic electron beams propagating in air are subject to a variety of macroscopic resistive instabilities, including hollowing, hose, and filamentation. Although the hose instability typically dominates experiments, computer simulations indicate that hollowing is potentially very disruptive to fast current risetime beams, especially at low air density. There have, been at least three experiments which exhibited the hollowing instability clearly. Results from the last of these, performed using the 4 MeV, 70 kA IBEX beam generator, are analysed. The instability occurs routinely in computer simulations for beam currents above 17 kA and risetimes less than a few ns in full density air, and over broader parameter ranges at reduced air densities. It develops rapidly in the beam nose and is highly disruptive. Although emittance tailoring and electron energy spread can raise the threshold for the onset of hollowing, the stability of high current beams against hollowing can be assured only by maintaining a sufficiently long current risetime. A series of three high current beam experiments have been performed to confirm this theoretical picture of the hollowing instability.