Transport of self-field dominated dc low-energy electron beams through low-pressure gases

A high-intensity dc electron beam with energy on the order of 100 keV propagating through a rarefied gas may be space-charge neutralized and made self-focusing by means of the beam-generated plasma. The purpose of this research was to investigate the propagation of partially and fully neutralized electron beams and to identify the ranges of gas pressure and other parameters which would permit high-quality electron beam transport, with linear self-focusing forces. Experiments were performed to measure transverse current density distributions for an electron beam of energy 60 keV, current 270 mA, propagating through nitrogen at pressures from 10−6 to 10−5 Torr. Theories were developed to describe the radial distributions of neutralization in the two distinct cases of incomplete and complete neutralization. The neutralization distributions in each case were calculated numerically and using a beam sheath tracing program, theoretical values of the beam current density distributions were obtained. Good agreement was generally found between experimental data and the calculations. The results confirm that under conditions where complete and uniform neutralization was expected, the beam optics were, in fact, linear, while partial neutralization produced serious nonlinearities.