Transition from interpulse to afterglow plasmas driven by repetitive short-pulse microwaves in a multicusp magnetic field

Abstract

In the power-off phase, plasmas generated by repetitive short-pulse microwaves in a multicusp magnetic field show a transitive nature from interpulse to afterglow as a function of pulse duration tw = 20–200 μs. The ionized medium can be driven from a highly non equilibrium to an equilibrium state inside the pulses, thereby dictating the behavior of the plasma in the power-off phase. Compared to afterglows, interpulse plasmas observed for tw < 50 μs are characterized by a quasi-steady-state in electron density that persists for ∼ 20–40 μs even after the end of the pulse and has a relatively slower decay rate (∼ 4.3 × 104 s−1) of the electron temperature, as corroborated by optical measurements. The associated electron energy probability function indicates depletion in low energy electrons which appear at higher energies just after the end of the pulse. The transition occurs at tw ∼ 50 μs as confirmed by time evolution of integrated electron numbers densities obtained from the distribution function.