Current measurements in a gas filled drift tube

The authors report systematic measurement of currents in a low-pressure, gas-filled drift tube into which an intense relativistic electron beam injected. Typically, a 0.5-MeV, 80-kA, 100-ns electron beam of 2.54-cm diameter is injected through a foil anode into a gas-filled drift tube of 15-cm diameter. It is observed that, as gas pressure in the drift tube increases, the magnitude of the net current increases up to a peak value and then decreases. The pressure at which this peak value is achieved is interpreted as the critical pressure required for full-charge neutralization of the beam. Above this critical gas pressure, the waveform of the current decays nearly exponentially with a slow time constant. The decay time constant is attributed to L/R of the drift tube and plasma column, and the resistivity of the plasma can be conveniently inferred from the time constant