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.