Current multiplication during relativistic electron-beam propagation in a subTorr-pressure gas

The current multiplication observed when a relativistic electron beam is propagated in an initially neutral gas at subTorr pressures (0.1–0.3 Torr) is generally attributed to the two-stream instability. A general expression for this current gain is derived for a scattered beam propagating in a neutral gas, which takes into account a quasihydrodynamic expression for the beam energy loss. Moreover, the beam-to-plasma density ratio is obtained as a function of time for a given beam pulse by numerically solving the electron energy and continuity equations. It is found that at a given beam-to-plasma density ratio, the current gain increases with increasing the beam energy, with the increase being most rapid for the small scattering angles. A finite average scattering angle is seen to reduce the current multiplication. The experimentally observed peak current gains are in good agreement with the predicted peak gains obtained by taking into account the time-evolved beam-to-plasma density ratio.