Electron beam hollowing instability simulations

Intense electron beam hollowing instabilities, although routinely observed in axisymmetric computer simulations of beam propagation in air, are not often seen in experiments because of competing nonaxisymmetric effects. Recent experiments with the IBEX accelerator 4 MeV, 70 kA electron beam [R. B. Miller, M. G. Mazarakis, J. W. Poukey, and R. J. Adler, IEEE Trans. Nucl. Sci. 30, 2722 (1983)] provided detailed measurements of hollowing instability scaling over the pressure range of 10--80 Torr. Simulations performed in support of those experiments are reported here. Both experiment and simulation show large amplitude hollowing at the lowest pressures, decreasing in magnitude as the pressure is increased. The instability is suppressed above about 120 Torr. In addition, the simulations indicate that hollowing can be reduced by matching the beam radius at injection with its equilibrium value in air. The good agreement between experiment and simulation validates existing hollowing instability scaling laws