Propagation Velocity of the Ionization Front Driven by a High-Current Relativistic E-Beam through the Active Laser Media of Increased Pressure

Abstract

Relativistic electron beams (REB) are widely used for the creation of an active laser medium. They provide homogeneous excitation of large volumes in gas-flow and chemical lasers at low and intermediate pressures. On transition to the increased pressures of gaseous mixtures, the dynamics of excitation starts to depend on the propagation velocity of the ionization wave driven by REB. The fact that the REB-front velocity can evidently differ from the initial electron velocity and be determined by the motion of the ionization front has been discovered by measurements at low and intermediate pressures in electrically neutral gases [1–3]. With a rise in pressure the front velocity seems to approach the initial velocity of the beam electrons, V_bo ≅ C, since the time of electrical neutralization is inversely proportional to the gas concentration. Indeed, such an increase in velocity has been observed in the pressure region from P ≅ 0.1 torr up to P ≅ 1 torr [4]. With a further pressure rise to P > 100 torr, and especially at the transition to electrically-negative gases, the velocity of the beam current does not remain constant but drops with pressure, as the experimental results given below show. Therefore, the lower limit of the radiation pulse duration of the chemical laser initiated by REB rises with a rise in the pressure of the gas mixture, and this fact should be taken into account in extrapolation of the experimental data to the range of higher gas pressures and REB currents [5].