Observed current enhancement with propagation of the eta electron beam in air

It is observed with the ETA electron beam propagating in air that the net current exceeds the beam current for pressures between 50 and 500 Torr. Microwave measurements indicate that this current enhancement is distinctly different than that associated with the two-stream instability at low pressure. Detailed measurements of its dependence on pressure, current, beam size, and propagation distance are given. A theory is proposed that attributes the additional forward current to energetic secondary electrons pushed forward by the beam self-magnetic field. Detailed comparison of measurements with the results of a non-local Boltzmann code are made.