Ionization evolution in low-pressure air irradiated by a ~70-kv electron-beam

Summary form only given. We are investigating electron-beam-driven gas ionization over a range of beam current density and air pressure, with emphasis on the weakly-collisional regime. A roughly 70-kV, 2-4 kA, 80-ns electron beam is injected into a 10-cm-deep gas cell. The peak beam current density is varied between 6-300 A/cm2. The beam is injected into dry air at pressures from 0.020 to 100 torr. This pressure range covers the transition from collisionless to collisional behavior. A highly-sensitive interferometer at 1.06 mum measures the time-dependent electron density integrated across the beam. The laser location is varied axially and radially to get the spatial profile of the electron density. This is compared with the time-integrated beam profile obtained from contact radiography. We also record the net cell current measured outside the beam channel. We see that at low beam current density, the measured electron density is consistent with simple collisional ionization, and the net current is equal to the injected current. As the current density increases, we measure an enhanced electron density that is attributed to the effect of inductively-driven plasma current. A strong influence of air pressure is seen, with the density enhancement being most pronounced in the neighborhood of 0.1 torr. As the beam current density increases still further, a pronounced increase in plasma density occurs on axis. The plasma conductivity becomes high enough that inductive currents drive increased ionization for almost a microsecond after the diode current has ceased, leading to a local ionization fraction on the order of unity. Examination of the net current signals indicates that only a fraction of the plasma current connects to the cell walls. This fraction can be strongly affected by the wall material. Results will be compared with predictions of a Monte Carlo model developed in tandem with these experiments.