Session FR4: Gas Phase Plasma Chemistry

Control of reactive species formation in atmospheric pressure plasmas using pulsed power deposition

The success of atmospheric pressure plasmas in a wide range of applications relies on the production of a diverse array of reactive species. As a result, control of reactive species densities is fundamentally important for the development and optimisation of plasma-based processes. In addition, the rate of delivery of reactive species to targets and the energy efficiency of production are often of particular importance. Due to the non-linear nature of plasma-chemical processes, these parameters are often challenging to optimise on an empirical basis. Further, many application-relevant plasma systems operate using pulsed power deposition and utilise a wide variety of temporal power deposition schemes where the peak and average power deposition, temporal pulse width and pulse repetition frequency can vary over orders of magnitude. In order to effectively understand and optimise the operating conditions of such sources, experimentally validated simulations using plasma-chemical reaction schemes with sufficient detail as to capture the main reaction pathways both during the active plasma and afterglow phases are essential. In this contribution, the state of development and experimental validation of such reaction schemes for N₂, O₂ and H₂O-containing plasma sources is presented and discussed. These schemes are subsequently used together with zero-dimensional plasma-chemical kinetics simulations to systematically study the production of application-relevant reactive species such as NO and H₂O₂ as a function of the power deposition profile. As application-relevant model systems, dielectric barrier discharges and discharges formed in bubbles within liquids are chosen. Special attention is paid to the timescales for the formation and consumption of reactive species and their relationship to the temporal duration of the pulsed power deposition and the pulse repetition frequency.

    

The Promise of Data-Driven Methods for Characterization, Diagnostics and Control of Plasma Processing of Complex Surfaces

Data-driven methods can create unprecedented opportunities for characterization, diagnostics and process control of non-equilibrium plasmas (NEPs), which are increasingly used for treatment of heat and pressure sensitive materials in surface etching/functionalization, environmental, and biomedical applications.  Some of the main challenges in modeling and control of NEP applications arise from their inherent complexity and variability. Firstly, the behavior of NEPs are highly nonlinear and spatio-temporally distributed, which are hard to model due to their mechanistic complexity. Secondly, the NEP effects on complex surfaces are generally poorly understood. And thirdly, NEPs exhibit run-to-run variations and time-varying dynamics, whereby the same NEP treatment may be carried out under similar conditions, but yield different results. In this talk, we will discuss how advances in machine learning and data-driven optimization methods can be leveraged for: 1. modeling and simulation of NEPs to enhance understanding of plasma-surface mechanisms; 2. real-time diagnostics to “soft sense” plasma and surface characteristics; 3. plasma process control to realize reproducible and effective control of NEP processes; and 4. active learning-guided design of experiments to explore parameter space of NEP processes systematically.

    

Controlling O3 production in low-temperature He+O2 atmospheric-pressure plasmas using tailored voltage waveforms

Ozone (O₃) plays a key role in many medical applications of atmospheric-pressure plasmas. However, at high concentrations it is toxic, which means its production must be carefully controlled. Electrical control mechanisms are favoured over mechanical ones because of the flexibility and control speed that can be achieved. In this work, tailored voltage waveforms (TVWs) are employed to drive a radio-frequency He+O₂ plasma in a COST-like source. The TVWs consist of up to 5 harmonics (N), and form peaks, valleys and sawtooth waveforms. The density of O₃ is measured in the far effluent using Fourier Transform Infrared Spectroscopy.

When different waveforms of constant peak-to-peak voltage are applied, including extra harmonics in the driving waveform considerably enhances O₃ production until an upper limit of N=4. At this limit, an increase in power deposition likely increases the background gas temperature and drives the decomposition of O₃ into O₂. When a constant power is supplied to the plasma rather than a constant voltage, O₃ production can also be enhanced but the effect is not as strong as with constant voltage. The increased O₃ production at constant power is linked to a power deposition that is more localised in time, due to increased potential gradients in the plasma.   

Author not Attending

In parts of the world where water treatment infrastructure is lacking, the WHO estimates that 2 billion people are drinking water from a contaminated source and that nearly half a million people die annually from waterborne illnesses. To address this need, we are developing a solar powered point-of-use plasma disinfection applicator. Preliminary results show that over short treatment periods 6 log reduction in E. Coli is realizable in modest sized treatment volumes. An important consideration is that the final product is safe to drink. This apparatus allows for internal plasma chemistry to be tuned, thereby allowing for the reduction in undesired products, NO2 and NO3, and maximization of the high-disinfection products, O3, H2O2, and OH radicals. Here we study the species selectivity of the plasma system; absorption spectroscopy is used to monitor relative production rates for a range of reactive species generated by the device over a broad parameter space, including gas flow and excitation frequency. We also present operation data featuring a low cost power supply. This work aims to lay the foundation for modeling the gas phase chemistry of this device and the resulting activation of the liquid water.   

Author not Attending

A plasma microchannel is used to investigate the effect of a uniform nanosecond dielectric barrier discharge (ns-DBD) plasma on deflagration to detonation transition (DDT) for dimethyl ether (DME) in DME:O₂:Ar mixtures at atmospheric pressure and room temperature. Different quantities of discharges are applied across the length of the microchannel ahead of ignition. As the flamefront travels through the plasma region it is imaged using a high speed camera to trace flamefront position and velocity over time as well as to identify DDT. It is shown that a small number of plasma discharge pulses prior to ignition result in reduced DDT onset time and distance by 60% and 40%, respectively, when compared to the results without pre-excitation by ns discharges. The results also show that an increase of plasma discharge pulses results in an extended DDT onset time and distance of 224% and 94%, respectively. The present experiments provide insights to control DDT for applications in advanced propulsion engines.