Investigating the formation and kinetics of O and OH in a He–H2O plasma jet using absorption spectroscopy and 0D modelling.

Atmospheric pressure plasmas are effective sources for reactive species, making them applicable for industrial and biomedical applications. We quantify ground-state densities of key species, atomic oxygen (O) and hydrogen (H), produced from admixtures of water vapour (up to 0.5%) to the helium feed gas in a radio-frequency-driven plasma at atmospheric pressure. Absolute density measurements, using two-photon absorption laser induced fluorescence, require accurate effective excited state lifetimes. For atmospheric pressure plasmas, picosecond resolution is needed due to the rapid collisional de-excitation of excited states. These absolute O and H density measurements, at the nozzle of the plasma jet, are used to benchmark a plug-flow, 0D chemical kinetics model, for varying humidity content, to further investigate the main formation pathways of O and H. It is found that impurities can play a crucial role for the production of O at small molecular admixtures. Hence, for controllable reactive species production, purposely admixed molecules to the feed gas is recommended, as opposed to relying on ambient molecules. The controlled humidity content was also identified as an effective tailoring mechanism for the O/H ratio.

Single frequency, geometrically symmetric Radio-Frequency (rf) driven atmospheric pressure plasmas exhibit temporally and spatially symmetric patterns of electron heating, and consequently, charged particle densities and fluxes. Using a combination of phase-resolved optical emission spectroscopy and kinetic plasma simulations, we demonstrate that tailored voltage waveforms consisting of multiple rf harmonics induce targeted disruption of these symmetries. This confines the electron heating to small regions of time and space and enables the electron energy distribution function to be tailored.

Low power micro-propulsion sources are currently being developed for a variety of space missions. Electrothermal plasma thrusters are of specific interest as they enable spatial control of the power deposition to the propellant gas. Understanding the mechanisms whereby electrical power is coupled to the propellant will allow for optimisation of the heating and fuel efficiencies of electrothermal sources. Previous studies of radio-frequency (rf) plasmas have shown a dependence of the gas and electron heating mechanisms on the local collisionality. This is of particular importance to thrusters due to the large pressure gradients that exist between the inlet and outlet when expanding into vacuum. In this work, phase-resolved optical emission spectroscopy and numerical simulations were employed to study plasma heating in an asymmetric rf (13.56 MHz) electrothermal microthruster operating in argon between 186 -226 Pa (1.4 -1.7 Torr) plenum pressure, and between 130 -450 V (0.2 -5 W). Three distinct peaks in the phase-resolved Ar(2p 1 ) electron impact excitation rate were observed, arising from: sheath collapse heating, sheath expansion heating and heating via secondary electron collisions. These experimental findings were corroborated with the results of 2D fluid/Monte-Carlo simulations performed using the Hybrid Plasma Equipment Model (HPEM). The influence of each mechanism with respect to position within the plasma source during 1 an α-γ mode transition, where plasma heating is driven via bulk and sheath heating, respectively, was investigated. Sheath dynamics were found to dictate the electron heating at the inlet and outlet, as distinct from the centre of the thruster where interactions of secondary electrons were found to be the dominant electron heating mechanism. Optimisation of the heating mechanisms that contribute to the effective exhaust temperature will directly benefit electrothermal thrusters used on miniaturized satellite platforms.

Radio-frequency driven atmospheric pressure plasmas are efficient sources for the production of reactive species at ambient pressure and close to room temperature. Pulsing the radio-frequency power input provides additional control over species production and gas temperature. Here, we demonstrate the controlled production of highly reactive atomic oxygen and nitrogen in a pulsed radio-frequency ( MHz) atmospheric-pressure plasma, operated with a small % air-like admixture (/ at ) through variations in the duty cycle. Absolute densities of atomic oxygen and nitrogen are determined through vacuum-ultraviolet absorption spectroscopy using the DESIRS beamline at the SOLEIL synchrotron coupled with a high resolution Fourier-transform spectrometer. The neutral-gas temperature is measured using nitrogen molecular optical emission spectroscopy. For a fixed applied-voltage amplitude (234 V), varying the pulse duty cycle from 10% to 100% at a fixed 10 kHz pulse frequency enables us to regulate the densities of atomic oxygen and nitrogen over the ranges of – and – , respectively. The corresponding 11 K increase in the neutral-gas temperature with increased duty cycle, up to a maximum of K, is relatively small. This additional degree of control, achieved through regulation of the pulse duty cycle and time-averaged power, could be of particular interest for prospective biomedical applications.