Propagation of a mildly relativistic electron beam at sub-Torr pressures

This report presents observations on propagation of a mildly relativistic electron beam (γ = 2.4) in several different gases (air, He, N2, and Ar) at low pressure (10--320 mTorr). At such low pressures, and with beam currents of several kiloamperes, beam-induced ionization may result in self-focusing of an electron beam. Beams propagating under this condition are said to be in the ion-focusing regime of propagation. The injected beam has an rms radius of 0.7 cm and a transverse temperature of 35 keV. Beam transport efficiency was measured for He, N2, and Ar. A simple calculation was performed to determine the space-charge neutralization fraction as a function of time. Using the results of this calculation, the charge-transport efficiency is found and compared with the experimentally measured quantity