Atomic Hydrogen Dynamics and Neutral Depletion in a High Power Linear Plasma Device

Abstract

Neutral particles are an important constituent of low temperature plasma. Neutral particles are involved in collisions in the plasma, where they can form new species and are an energy and momentum sink. However, there are few methods that make direct measurements of neutral species in the plasma.

A two-photon absorption laser-induced fluorescence (TALIF) diagnostic is established to investigate the properties of neutral atomic hydrogen in a high density pulsed plasma. A Nd:YAG-pumped dye laser at 205.1nm is used to directly probe the ground state of atomic hydrogen, by monitoring the fluorescence at 656nm. The TALIF diagnostic is capable of making both spatially and temporally resolved measurements of the atomic hydrogen.

The TALIF diagnostic is used to examine the atomic hydrogen densities and temperatures in high power (1-20kW) discharges in the MAGnetised Plasma Interaction Experiment (MAGPIE) helicon device. Neutral atomic hydrogen is observed to decrease in density above 5kW applied RF power, where the plasma pressure becomes comparable to the gas pressure. This decrease in density is explained to be due to neutral depletion of the atomic hydrogen in the centre of the chamber, which to the author's knowledge are the first experimental measurements of neutral depletion of hydrogen in a linear plasma device.

Temporal measurements in the afterglow of the higher power (>5kW) plasma discharge show a replenishment of atomic hydrogen in the early afterglow. This replenishment is observed to be consistent with depletion of atomic hydrogen during the discharge and is due to the neutrals flowing back in to the depleted regions after the plasma has decayed. Radial measurements show a slight increase in atomic hydrogen further from the plasma core.

Atomic hydrogen is shown to display increasing temperature with higher applied powers with a constant radial temperature profile. There is no observed evidence of a secondary hotter population of atomic hydrogen.

A two-state decay of atomic hydrogen after the plasma discharge is observed at lower applied power (1kW). It is determined that different processes account for the two different decay rates with the second slower decay rate being due to atomic losses at the chamber walls. A single decay rate is observed at the higher powers which is also explained to be due to the same process. Show more