Measurement Of Erosion Rates Of A Relativistic Electron Beam

Erosion rates have been measured for a 2.5 MeV, 0.5 μsec electron beam propagating on a laser-formed plasma channel in the ion-focused regime. Various values of transverse magnetic field were also applied. The erosion rate is defined as the speed at which the beam front recedes from the beam head. The ejection of plasma electrons by the space-charge of the electron beam as it propagates induces a longitudinal electric field that in turn slows the beam electrons. When the beam electrons lose sufficient energy, they are lost to the wall. This loss process is termed inductive erosion. If there is an external transverse magnetic field present, electrons in the unneutralized head of the beam are swept away. This process is termed magnetic erosion. The plasma channel was formed using an electron beam pumped KrF laser in low density fill (3 x 10
^-5
Torr) trimethylamine (TMA). The electron beam propagated 91 meters on this channel and the time-resolved total current was monitored at B-dot stations every 14 meters. Beam currents ranged from 0.3 kA to 0.75 kA The channel plasma densities ranged from 2 x 10
⁹
cm
^-3
to 1.5 x 10
¹⁰
cm
^-3
and transverse magnetic fields of Oto 4 G. The total erosion rate was determined by measuring the change in the pulse width as the beam propagated along the channel. Good agreement was obtained between the data and erosion formulae obtained from numeric fit to numerous simulations.