STRUCTURES OF AN ELECTRON-BEAM DRIFTING IN VACUUM AND IN LOW-PRESSURE GASES

We performed a series of tests on the propagation of intense electron beams with ν/γ≃0.40 and 0.85 in a vacuum (∼10-4 Torr) and in a few gases (nitrogen, helium, and carbon dioxide) at pressures ranging from 50 mTorr to 1 Torr. Beam currents were recorded step by step by a Faraday cup movable along the drift-tube axis up to about (1/2) m from the anode of the field emission diode. Current neutralization and almost complete transport of the electron beam along the drift tube occur at different pressures in different gases. Structures in the transmitted currents appear as a function of the drift distance, markedly at pressures slightly lower than that of complete neutralization of the beam. They are in good agreement with Lawson's predictions.