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Background
This programme is designed to develop fundamental understanding, embodied in models and simulations, and measurement techniques. These activities will contribute to the fundamental science of the field, and will be directly supportive of activities in the more applied parts of the programme that are discussed below. We note that the background programmes referred to above are covering several areas of central interest, which will be drawn on as needed, but these will not be discussed in detail here.
Objectives
The modelling elements of this work aim to develop fundamental understanding, and formulate that understanding in models and simulations that will add value to the experimental programmes. We note that modelling of low-pressure plasmas is covered by a background programme and will not be discussed here. That project will also be demonstrating control strategies in a simulation context, which will inform the development of control schemes discussed below. The modelling work proposed here focuses on atmospheric pressure plasmas, and especially their interaction with liquid drops. The remaining elements of the fundamentals programme are concerned with developing diagnostics for use in other areas of the project, and for validating models.
Modelling
A background project will develop models for atmospheric pressure plasma discharges, and has demonstrated a two-dimensional simulation appropriate for the Labline and the discharge region of the PlasmaStream. This project will extend that work in two directions. First, the PlasmaStream system has a complex geometry that has not yet been fully addressed, especially in the region between the discharge tube and the sub-strate. This task will extend the existing models to address this area, by adopting a finite element approach. Second, an essential feature of the experimental approach involves the injection of liquid precursors into the discharge. It is certain that there is a crucial synergistic interaction between the plasma and the droplets, but the nature of that interaction is unknown. We will investigate the interaction between the plasma and the liquid drops, starting from previous work concerned with other contexts, from which there will be important differences. These works show that the droplet size distribution is the product of a dynamic equilibrium. The aim will be to study firstly, non-thermal interactions between the discharge and the droplets, and secondly, transport of droplets through the discharge region and onto the substrate. This, of course, will be pursued in close collaboration with the experimental work discussed below. We note that plasma-liquid interactions is a topic of emerging interest in plasma science, in fields ranging from fusion to medical physics. There are also obvious connections with the field of dusty plasmas.
Plasma Diagnostics
Any process control strategy must be based on robust sensors that can be deployed in an industrial environment. It is not clear that such sensors yet exist for plasma processing, and especially not for atmospheric pressure plasmas. This activity seeks to explore routes to developing such sensors, at the same time enhancing our ability to examine fundamental physics issues. We will be examining electrical and optical measurement techniques.
Electric Diagnostics
It is well-known that a plasma discharge excited by an oscillating current exhibits a rich harmonic spectrum. Such spectra are readily shown to be sensitive to many plasma and process parameters, and techniques such as principal component analysis can be used to establish a relationship between such parameters and certain spectral characteristics. This procedure is attractive, because of the uninvasive character of the measurement, but is not completely satisfactory, because the spectra are sensitive to various extraneous factors, so that a complex calibration is needed for each chamber, which likely needs to be repeated often, for instance after maintenance. Consequently, this method has not seen wide adoption, in spite of its merits.
An important aim is to investigate the physical origin of the harmonic spectrum of a plasma discharge, with a view to understanding which plasma parameters affect the spectrum. For example, sheath nonlinearity is almost certainly important, bu are other phenomena, such as ion acoustic waves, also significant? This understanding will be used to formulate a physics-based model for the plasma aspects of the frequency spectrum, with a view to separating plasma effects from circuit effects. If this can be done, then one can seek to develop an interpretation procedure with much less need for calibration, and where such calibration is limited to dealing with circuit tolerances. This experimental aspect of this work will be carried out using extant equipment, and the experiments will be combined with a mixture of analytic and numerical modelling to develop a quantitative understanding of the frequency response. An important aim will be to investigate electronegative discharges.
We will also be considering electrical diagnostics with potential for application to atmospheric pressure plasmas, such as pulsed Langmuir probes, and asymmetric pulsed energy analysis to measure ion fluxes. The latter technique has the potential to be a powerful diagnostic for chemically complex atmospheric pressure discharges.
Langmuir probes and less established resonance probes will also be employed. The basic concept behind a plasma resonance or hairpin probe is not new, but the idea has attracted interest recently because it has the possibility to measure electron density more reliably than a angmuir probe, especially in the presence of interference from radio-frequency oscillations in the plasma potential. Moreover, the hairpin probe may be more suitable for development as process sensor than a Langmuir probe, and extension to a more powerful diagnostic yielding information about the ionic composition may also be possible by investigating the time response of a hairpin probe to which a pulsed bias is applied. The aim of this activity is to develop hairpin probes that can be integrated with low-pressure plasma processing tools, and to use these probes to characterize aspects of the processing plasma.
Optical Diagnostics
Optical emission spectroscopy is a well-known plasma diagnostic, especially useful because of its non-invasive character. In recent years it has become possible to carry out such spectroscopy with time resolution within the cycle of oscillating current discharges. These measurements can be post-processed to obtain information about electron-impact excitation processes with temporal and spatial resolution, and this provides an extremely interesting and sensitive test for models and simulations. Moreover, such emission can be shown to be sensitive to changes in process conditions, which can, for example, be an indicator of a process end-point. Phase resolved optical emission spectroscopy (PROES) is therefore not just an important tool for developing fundamental understanding of both high- and low-pressure plasmas, but it also has potential for development as a process sensor.
We intend to exploit both these avenues, adopting PROES as a diagnostic for model validation, and as a sensor for process management.
Acoustic Diagnostics
In recent unpublished work, Law has shown that acoustic diagnostics have great potential application to atmospheric pressure discharges. This background work will be exploited where appropriate in the present programme.
Droplet characterisation
Characterisation of the density and size distribution of droplets introduced into at-mospheric pressure discharges is of obvious importance. Laser scattering will be used to determine the droplet size distribution, but we also propose to investigate
a potentially more powerful approach based on photoacoustic methods. Such tech-niques have in the past been used to characterize soot and particulate sizes in engine emissions. For example, Keller et al. fabricated open cell photoacoustic etectors based on laser stimulation of the gas under test, and an array of speakers and phase-locked loop (PLL) systems to measure deviations from acoustic resonances within a windowless measurement chamber. However the reliance on PLL techniques limits the application of this technique to a single harmonic, thus throwing away much useful measurement data.
Our new Open Cell Photoacoustic Detection (OCPD) technology will enable the si- multaneous measurement of time (photoacoustic spectroscopic signal) and frequency information (photoacoustic Fourier signal) which represents a new diagnostic/metrology for atmospheric plasmas. Briefly, the atmospheric plasma under test is surrounded by a cylindrical open-ended chamber, whose dimensions are chosen to produce acoustic resonances in the kHz range. A pulsed laser beam excites the azimuthal nodes which propagate along the cylinder axis. If the laser beam pulse width is significantly shorter than the acoustic response time (i.e. nano- or microseconds vs. milliseconds) then the short heat pulse generated by the laser acts as a broadband acoustic source, exciting all eigenmodes of the resonator simultaneously. Thus, in the time domain, one observes a decaying photoacoustic envelope function, and in the frequency domain a series of peaks each corresponding to the acoustic harmonics in the chamber. The time domain Photoacoustic Spectroscopy (PAS) signal can be used to determine properties of the plasma under test such as mass specific scattering cross-sections of any particulates/droplets, their fractal dimensions, concentrations, dimensions and density. Furthermore, in the frequency domain, any changes in the plasma envi- ronment (density, temperature, contamination) will manifest themselves as changes in the resonant frequencies including frequency shifts and relative intensities. This form of Photoacoustic Impedance Fourier Spectroscopy (PIFS) is little understood and we will seek to correlate this data to atmospheric plasma conditions in order to develop this new diagnostic/metrology tool. Early tests on PAS/PIFS have been undertaken as part of a DCU MEng project and these confirm the feasibility of the technique.
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