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.

Particle energy distributions and metastable atoms in transient low pressure interpulse microwave plasma

“…The basic experimental set-up consists of a pulsed microwave source (2.45 GHz/9.45 GHz/or variable frequency TWT amplifier based source) whose output is launched into a cylindrical vacuum chamber (VC), as shown in figure 1. Plasma confinement is brought about by using a multicusp (MC, 12 cm in diameter and 30 cm in length) [19][20][21]. The mode of the axially launched waves belongs predominantly to k ⊥ B where k is the wave vector and B is the static magnetic field.…”

“…The pulse energy used to ignite the plasma is kept fixed at 60 mJ in the experiments. The details of the experimental system have been described in [19][20][21]. The transient plasmas, confined inside MC, have been created with: (i) short pulse (~1 μs) microwaves of peak power 60 kW at 9.45 GHz and (ii) medium-pulse (~20 μs), microwaves of moderate peak power (~3 kW) at 2.45 GHz.…”

“…In pulsed microwave plasma experiments [19], it has been shown that by varying the pulse duration (t w ), one can attain (i) an afterglow plasma (t w > 50 μs) where a steady state plasma is produced inside the pulses, and after the end of the pulse a decaying plasma is obtained or, (ii) an interpulse plasma (t w < 50 μs) where non-equilibrium, transient plasmas are observed to exhibit quite different properties in the power-off phase. The plasma grows beyond the end of the pulse followed by a quasi-steady-state in the power-off phase of the discharge, that is maintained from a few to tens of microseconds during the time between the pulses [20,21].…”

“…The plasma grows beyond the end of the pulse followed by a quasi-steady-state in the power-off phase of the discharge, that is maintained from a few to tens of microseconds during the time between the pulses [20,21]. The transition time from one state to the other is a function of the microwave pulse duration and can be determined from the EEPF profile in the power-off phase [19]. The EEPF in the non-equilibrium plasma therefore plays an important role in dictating the behavior and properties of the temporally evolving plasma after the end of the pulse.…”

Observation of ion heating during stimulated Buneman instability in a temporally growing plasma

“…The discriminator bias (V g ) is swept from 0 to −200 V to obtain the collector current (I c ). f (υ) is then proportional to the first derivative of I c with V g [25]. A gridded ion energy analyzer is employed for measuring the ion energy distribution function (IEDF) as well in a similar way by inverting the biasing scheme, where the potentials to the different grids are maintained as in ref.…”