High-voltage nanosecond pulses in a low-pressure radiofrequency discharge

TL;DR

This study investigates how high-voltage nanosecond pulses affect low-pressure RF argon plasma, revealing a two-phase response with a bright flash and a subsequent dark phase, supported by experimental and simulation analysis.

Contribution

It demonstrates the impact of nanosecond pulses on plasma dynamics and elucidates the mechanisms behind the flash and dark phases through combined experiments and simulations.

Findings

High-voltage pulses cause a bright flash with increased emission.

Residual electrons and secondary emissions drive plasma excitation.

High-density plasma screens RF fields, leading to the dark phase.

Abstract

An influence of a high-voltage (3-17 kV) 20 ns pulse on a weakly-ionized low-pressure (0.1-10 Pa) capacitively-coupled radiofrequency (RF) argon plasma is studied experimentally. The plasma evolution after pulse exhibits two characteristic regimes: a bright flash, occurring within 100 ns after the pulse (when the discharge emission increases by 2-3 orders of magnitude over the steady-state level), and a dark phase, lasting a few hundreds \mu s (when the intensity of the discharge emission drops significantly below the steady-state level). The electron density increases during the flash and remains very large at the dark phase. 1D3V particle-in-cell simulations qualitatively reproduce both regimes and allow for detailed analysis of the underlying mechanisms. It is found that the high-voltage nanosecond pulse is capable of removing a significant fraction of plasma electrons out of the discharge gap, and that the flash is the result of the excitation of gas atoms, triggered by residual electrons accelerated in the electric field of immobile bulk ions. The secondary emission from the electrodes due to vacuum UV radiation plays an important role at this stage. High-density plasma generated during the flash provides efficient screening of the RF field (which sustains the steady-state plasma). This leads to the electron cooling and, hence, onset of the dark phase.