Charge and current neutralization phenomena in low-impedance diode structures

In this paper is described an experiment to examine low energy electron beam transport (β ∼ 0.33) in a low impedance (one ohm) diode structure. Both charge and current neutralization as a function of pressure from 1 mTorr to 2 Torr were determined for the first time with an electron beam facility modified to permit rise times of 2 ns and pulse widths of 5 ns with a peak intensity of 25 A/cm². The results of the experiment indicate that for electron beams with this short time duration, charge neutralization will occur for pressures in the range 200–300 mTorr. Furthermore, Langmuir oscillations were observed in the 200 to 300 mTorr pressure regime and late-time plasma “tails”, explained in terms of the magnetic diffusion time, were observed for pressures ≥ 400 mTorr. Current neutralization of the beam was observed at 900 mTorr. Qualitative agreement is obtained between theory and experiment.