GB2594050A

GB2594050A - Glow plasma gas measurement signal processing

Info

Publication number: GB2594050A
Application number: GB2005140.5A
Authority: GB
Inventors: Alizadeh Barhram; Lopez Martin
Original assignee: Servomex Group Ltd
Current assignee: Servomex Group Ltd
Priority date: 2020-04-06
Filing date: 2020-04-07
Publication date: 2021-10-20
Also published as: TW202204858A; GB202005140D0

Abstract

Methods and systems are disclosed for the enhanced determination of the composition of a sample gas using glow discharge optical emission spectroscopy (GD-OES) for gas analysis. A first method comprises: generating an oscillating electric and/or magnetic field within a plasma cell to excite particles within the cell to produce a glow discharge from a plasma in the plasma cell; monitoring one or more glow discharge optical emissions from the plasma in the plasma cell at twice the excitation frequency; and processing this signal in real time during each excitation cycle to obtain a signal to noise enhancement and to determine the concentration of a gas within a gas mixture. The operating conditions of the plasma cell are also controlled to maintain the glow discharge optical emissions from the plasma within a desired operating range.

Description

Glow plasma gas measurement signal processing

Background

Plasma is composed of ionised gas molecules in a mixture of free electrons, neutral molecules and photons of light of various wavelengths. Plasma can take many forms, both naturally occurring, such as in stars, nebulae, flames and lightning, or man-made such as arc discharges in high strength electric fields. Plasmas may occur both at high and low pressures. Reduced pressure plasmas have the advantages of requiring a lower strike voltage (ignition of plasma) and maintenance voltage (voltage to sustain a plasma), as well as decreased incidence of quenching due to lower species density, but there are increased costs and complexity associated with obtaining this lower pressure and the total amount of ionised molecules may be reduced compared to a higher pressure plasma. Plasmas are used in material processing applications such as surface cleaning for the preparation of substrates for thin film depositing. They are also used in plasma lighting, ozone production, etching of computer chips, and manufacture of solar cells.

Within a glow plasma, electrons and other ionised species are not in thermal equilibrium, and the energy associated with the excited within the electric field may be well above that of the average energy for the mixture. The species electric field and inelastic collisions between the accelerated electrons and gas molecules lead to the creation of excited and ionised species. The subsequent radiative decay to lower energy levels results in the emission of characteristic photons of radiation that gives the name of "glow" discharge.

Glow discharges may take place in either direct current (DC) or alternating current (AC) excitation fields. DC fields involve direct electrode contact within the gaseous environment, which may be undesirable for the properties and lifetime of the electrode. AC fields can be coupled to the gaseous sample via a dielectric barrier, thus the electrodes are shielded from direct contact with the gas. Dielectric barrier discharge (DBD) plasmas have also been used in industrial ozone production.

Glow plasmas have an important application in gas analysis. Optical Emission Spectroscopy (OES) is a technique for species identification and quantification, where light emission from excited state species within a glow plasma is analysed. The location of the emission lines in the electromagnetic spectrum indicates the identity of the species and the intensity denotes the concentration of that gas species in the gas mixture (as shown in Figure 1). Although glow discharge optical emission spectroscopy (GD-OES) has been used in the analysis of surfaces of solid, conducting materials, it has not been the preferred technology for gas analysis. Most conventional GD-OES systems use a low-pressure glow discharge plasma, but measurements at atmospheric pressure or higher may still be possible on some instances. In gas analysis, glow plasmas may be used to analyse a wide variety of gases including pollutants, and the analysis may be used to control an industrial process to minimise emission levels and gases of interest to optimise process efficiency, reducing power demand and ultimately reducing production of greenhouse gases, which is part of most power production and heat production.

Summary

Provided are methods, apparatuses and systems for processing optical signals from stabilised glow plasmas in real time with enhanced signal to noise recovery.

In one aspect of the invention, there is provided a method for generating a stable plasma comprising: generating an oscillating electric field and or an oscillating magnetic field (or a combination of both oscillating electric and magnetic fields) within a plasma cell to excite particles (atoms, molecules or charged species) within the cell, to produce a glow discharge from a plasma in the plasma cell; wherein monitoring one or more glow discharge optical emissions from the plasma in the plasma cell is performed in real time during each excitation cycle and results in improved signal to noise recovery. The monitoring of the optical signal is at twice the excitation frequency of the plasma. Monitoring the signal at twice the excitation frequency (2f) results in signal to noise ratio improvements, due to the narrowing of the frequency bandwidth of the signal and separation in frequency from the excitation frequency (f) (e.g. by using a notch filter). Signal detection techniques to examine and determine the 2f signal may include one or more of the following: Lock-in detection, synchronous detection, frequency domain analysis such as by using Fast Fourier Transforms (FFTs) and time or frequency domain matched filter techniques, shape filters or other appropriate detection means. The signal may be taken as the peak heights, the peak areas, as integrals of the 2f signal or any other appropriate technique with suitable filtering (e.g. median filter) and/or ensemble averaging and/or moving averaging. This requirement to measure the signal in real time throughout has implications for the design and implementation of the optical detection system. For example, silicon detectors for ultraviolet or visible light are an economical and efficient means of measuring the light, but the requirement for a rapid response time means that the detectors must have intrinsically low capacitance. The signal collection could be achieved after passing through the transmission band of an optical filter or by using a dispersive grating or other appropriate wavelength selection device. Optical emissions from glow plasmas can be used in glow discharge optical emission spectroscopy (GD-OES) for gas analysis and in other applications.

Many conventional glow plasma discharges use a Royer transformer in a self-oscillating scheme to maintain a stable plasma. This enables a controlled glow plasma to be maintained over a narrow range of conditions. Various techniques have been used to improve the stability and flexibility of conventional glow plasmas, including using inductive feedback techniques (EP1381257A2).0ne contemporary innovation is the use of secondary stabilisation electrodes (EP3265806A1) to apply a transverse electric field and/or to provide electron-injection.

Another more recent example method (UK Patent Application No. 1904896.6) is where a stable glow discharge plasma is maintained in a plasma cell by applying an input signal to two or more electrodes in the plasma cell to generate a voltage gradient between the electrodes, measuring an induced signal across the plasma cell, and comparing the induced signal with a reference signal to obtain a difference signal. This comparison is performed at plasma resonance. References to 'resonance' and 'resonant conditions' herein are, as explained below, not to be interpreted as a limitation to peak resonance unless this is stated explicitly. A control signal is then applied to the at least two electrodes in the plasma cell based on the obtained difference signal to achieve a desired voltage gradient for the excitation that is needed for a stable glow under resonance conditions. This method is all achieved in real time during each cycle.

Brief Description of drawings

Various features of exemplary apparatus, systems and methods are described below, by way of example only, with reference to the accompanying drawings in which: Figure 1 is a schematic representation of components of an optical emission spectroscopy system, which can use a spectrometer and light emitted by dielectric barrier discharge (DBD) in a plasma cell, for gas analysis; Figure 2 is an illustration of a glow plasma; Figure 3 is an illustration of the excitation waveform and 2f optical waveform; Figure 4 is an illustration of the Lock-in detection technique; Figure 5 is an illustration of the benefits of using lock-in detection; Figure 6 is an illustration of the 2f signal amplitudes for hydrogen/nitrogen mixtures; Figure 7 is an illustration of the comparison between 2f amplitude and spectrometer amplitude results; Figure 8 is an illustration of the signal waveforms for argon/nitrogen mixtures; and Figure 9 is an illustration of the Lock-in amplitude for argon/nitrogen mixtures. Detailed Description Since a plasma is an ionised gas that is electrically conductive, it is able to interact with external electric and magnetic fields. The major constituents of a plasma are free neutral atoms or molecules, positively charged ions or metastable species, free electrons and a variety of energetic photons. These species are in a state of constant collisions. The degree of ionisation in a plasma is the ratio of number density of charged species to the neutral species.

There are three main light production processes in a plasma described as follows: i. free-bound transitions or recombination radiation: A free electron in a plasma can also be captured by an ion; also known as radiative recombination. If this capture or recombination is to the ground state, a photon with an energy greater than the ionisation potential of the ion or atom is emitted, producing a continuous spectrum. Alternatively, if the recombination is to an excited energy level, the electron cascades down through the excited states to the ground state by releasing photons of unique wavelengths, thus producing the emission lines characteristic of that ion species.

ii. bound-bound transitions These transitions happen when a change to the energy of an electron within an atom or a molecule is such that the electron remains attached or bound to the atom or molecule both before and after the change. In the case where energy is increased, typically a photon roaming the plasma is absorbed. When the energy is reduced, a photon is emitted. Bound-bound transitions in a plasma can produce both emission and absorption lines unique to the atomic or molecular species.

iii. free-free transitions: Bremsstrahlung In any plasma there are many unbound electrons which can freely interact with other species. When a free electron in a plasma passes close to an ionised atom or molecule, it is either accelerated or decelerated. This results in a net change of the kinetic energy of that electron.

Quantum mechanics dictates that this change of energy is quantised and mediated by either absorbing or emitting a photon by the electron. Since these photons can be of any wavelength, radiation produced in this process has a continuous spectrum and is also known as thermal bremsstrahlung.

Within a plasma itself several processes occur that enable the above transitions, with the most important process being collisions among species. An important set of collisions is those between the electrons/charged species and neutrals which leads to ionisation. For this to happen, a fraction of the electrons or charged species need to have kinetic energies exceeding the ionisation potential of the gas of interest. Conversely, collisions can also lead to the recombination process, where the impact between the charged species of opposite polarity can lead to the production of neutral species.

One of the most common methods of producing a glow plasma is by means of high voltage radio frequency (RF) excitation of a gas flowing in a dielectric barrier vessel (commonly glass or quartz) surrounded by conductive electrodes (see Figure 2). In each typical RF excitation cycle, charged species in a plasma experience peak acceleration in opposite directions in the electric field twice. On the positive side of the sinewave excitation, this acceleration peaks near the top of the waveform. Similarly, on the negative side of the sinewave excitation, the charged species experience peak acceleration in the opposite polarity near the trough of the waveform. Since all three light production processes described above will also peak during these high acceleration events, the instantaneous light produced from any glow plasma will exhibit two distinct peaks and troughs during each individual typical cycle of RF excitation.

This has been experimentally verified by the inventors using a high-speed photodiode amplifier circuit detecting the instantaneous light signal through a narrow optical bandpass filter centred around 337nm in a N2 glow plasma. This is clearly illustrated in Figure 3, where the excitation signal at frequency f and the detected signal is at twice the excitation frequency (2f). Note that there is a phase shift between the excitation and drive waveforms due to instrumentation, plasma species inertia and other factors.

Until relatively recently, the accurate acquisition and processing of these signals would have been extremely difficult to accomplish in real time. There are also specific design recommendations to make the detectors sufficiently fast to respond at the drive frequencies which are typically used (10s of kHz or higher), which are not generally required or used in standard optical plasma measurements. This is the reason why many plasma gas detectors use an integrated or DC signal as the processed detection signal. It is also one of the main reasons why this non-intuitive approach proposed by the inventors has not been considered previously.

The knowledge that the emitted light from a plasma has a 2' harmonic (2f) component correlated to the plasma RF excitation waveform allows it to be extracted from noisy backgrounds with extremely high noise rejection. One conventional methodology to achieve this is called Lock-in detection. The key strength of a Lock-in detector is its ability to extract a signal amplitude and phase in very noisy environments. In effect, Lock-in detection is like a Fourier Transform with a single frequency (2f) component, with all other coefficients being set to zero. Typically, it uses a homodyne detection scheme followed by low-pass filtering to extract a desired signal amplitude and phase relative to a periodic reference (see Figure 4). The shape of the 2f periodic reference waveform could take many forms including sinusoidal, square wave or other appropriate shape to optimally extract the desired process signal. This detection occurs in a well-defined narrow frequency band around the reference frequency, efficiently rejecting all other frequency contributions from other spurious sources. Using this technique allows the photodiode or other appropriate detection means (e.g. photomultiplier, bolometer, pyroelectric or thermopile detector) detection of plasma light at an extremely narrow bandwidth leading to significantly lower thermal and shot noise contribution from the amplifiers. The magnitude of improvement that can be achieved by using the Lock-in technique can be seen in Figure 5 for simulated data. Graph (a) shows the clean 2f signal, whilst (b) shows the effect of adding large scale random noise to the signal. Even with such large noise, Lock-in detection is capable of recovering the original signal effectively as can be seen in graph (c). In practice, a silicon photodiode is both an economical and versatile solution, with potentially fast detection across a broad range of ultraviolet, visible and near infrared wavelengths. Another important consideration is the presence of ambient light. Most light detection from a glow plasma occurs in the near ultraviolet and visible band of wavelengths. This is also the band of wavelengths where significant spurious ambient light sources exist. The 2nd harmonic detection of plasma light gives an enhanced rejection of ambient sources, which will be generally modulating at a much lower frequency, making the task of light shielding of a plasma detector significantly easier. Whether in a driven or self-oscillating system, the plasma excitation drive is optimally free from 2f components and/or distortions which might contain 2f components, since this could contribute to spurious 2f noise and/or offsets.

If a glow plasma spectroscopic instrument is to be used for measuring trace levels of a gas species, it must be capable of achieving two important signal processing objectives at the same time. Firstly, a significantly higher gain is required to make the weak emission detectable. Secondly, a much higher resolution digitiser is needed to provide measurement resolution. Post processing of the 2f signal is extremely important in the case of a weakly emitting component. The 2nd harmonic signal may be processed in two ways such as in a conventional design by means of analogue Lock-in detection using multipliers and low-pass filters. These analogue circuits are prone to drift however and come with the hefty penalty of additional noise contribution. Alternatively, in a modern electronic architecture, the 2nd harmonic signal will be digitised directly in a fast low-noise Analogue to Digital Converter (ADC). All processing is then performed digitally from here onwards to limit noise contribution.

In order to cover a wide range of gas concentrations over all the possible range of intensity transmissions encountered in industrial applications, preferably a 16-bit or higher resolution ADC will be required. The 2nd harmonic signal itself may treated as a digital frame or a scan. A high-speed time critical real-time data acquisition algorithm using a micro-processor performs all the digital Lock-in tasks including the front-end frame averaging, multiplication by a reference 2f frame, potentially followed by a proprietary shape recovery algorithm possibly using FFT techniques to optimise computational efficiency. In practice, a weak 2f signal may be corrupted by a mix of random and systematic distortions, therefore, shape discrimination of the weak profiles may play a vital part of the signal recovery. Experiments during the feasibility work with various configurations showed that a unique blend of high-gain AC-coupled analogue front end followed by propriety high-speed digital signal processing may give the best combination to successfully recover weak signals. The excitation waveform is typically sinusoidal in shape, which may be convenient, especially if Lock-in or FFT techniques are performed, however, other profiles may be used and instrumentation and/or other factors may distort the excitation profile and this may be significant in any post processing, especially with regard to any shape or matched filter usage. The phase angle and/or signal at frequencies other than 2f also contain signal enhancement information or background compositional data and be used to extract extra signal processing information. For example, a change in the phase angle may be related to the target gas concentration within a mixture and/or related to background mixture composition for non-binary gas mixtures.

In summary, light detection at 2f is superior to conventional photodiode light detection at DC due to the following advantages: 1. Improved signal to noise ratio.

2. Detection at an extremely narrow bandwidth leading to lower thermal and Shot noise.

3. Enhanced rejection of spurious sources of light such as ambient light.

However, detection of light at 2f in real time at high frequencies has the following new design recommendations for optimal performance: 1. Low capacitance and high shunt resistance of the photodiode element.

2. Techniques to lower junction capacitance such as reverse biasing of photodiode.

3 High speed trans-impedance amplifier.

4 Active cancellation of DC light signal, since without this high gain amplification is not possible.

Active suppression of if signal (e.g. via a notch filter), since without this high gain amplification is not possible.

6 Fast analogue to digital conversion (e.g. >10 times oversampling of the 2f signal).

7 Demodulation and Lock-in detection followed by filtering performed in digital domain to avoid additional noise contribution from analogue circuits.

Detailed embodiments and results An example apparatus, described in detail below, uses glow discharge optical emission spectroscopy in gas analysis, although this method herein described for signal processing will also have processing enhancements for any other oscillating drive glow plasma formats. This example (as per UK Patent Application No. 1904896.6) uses improved glow plasma stability at atmospheric pressure, for on-line gas analysis. This example could also be accomplished, for example, using a Royer transformer and/or secondary stabilising electrodes to apply a transverse electric field or to provide electron-injection, but the former would have a narrower operating range and the latter would involve extra build and operational complexity. Also, in the case of electron injection, the presence of such secondary electrodes within the gas stream would expose them to potential contamination and corrosion. As the gas of interest is carried into the plasma, it is excited, and the light emitted by the radiative decay is detected by a spectrometer for its unique wavelength signature. OES offers a non-intrusive and very specific information not only on plasma chemistry, but also on the relative concentration of the species. Unlike conventional GD-OES systems, gas analysis applications often require that the gas stream itself does not come into physical contact with the high voltage electrodes to avoid any sputtering effects or chemical reactions on the electrodes. In many gas analysis applications, the gas of interest is in a continuous flow regime which requires a fast response for spectroscopic detection and species identification.

The present example addresses the plasma stability shortcomings of previous methods at the fundamental plasma energy level, to enable stable glow plasma under a very wide range of conditions (e.g. composition, types of gases, concentrations of gases and flow rates). This is achieved via control of the plasma working conditions. In an example, this control is achieved through cycle-by-cycle monitoring of the plasma current (i.e. during each excitation cycle) and using feedback control to maintain the plasma current at a defined value. This feedback may be achieved by several methods such as by control of the voltage gradient across the plasma cell in real time.

References in this patent specification to "resonance" or "resonant conditions" or "resonant frequency" are intended to refer to the functional excitation frequency range (resonant frequency band) for a glow plasma which will be dependent on the gas composition and physical dimensions of the plasma cell, amongst other considerations, such as ambient conditions. Within this range, a glow plasma can be actively maintained, but there will typically be an optimum frequency or peak resonance frequency within the functional glow plasma excitation frequency range, where maximum energy transfer efficiency to the plasma occurs (maximum or peak resonance). For a fixed frequency, the voltage gradient across the plasma cell may be adapted through a feedback mechanism to maintain a stable glow plasma with changing gas composition and/or ambient conditions. Alternatively, for a fixed gas composition and/or ambient conditions, the frequency may be scanned to find the optimum (maximum) resonance peak, or a combination of the two methods may be implemented. The impedance between the electrical excitation source and the plasma cell should be optimised to achieve both stabilisation and optimum energy transfer. The optimisation parameters may be determined theoretically, empirically or a combination of both.

There is a compositionally dependent, resonant voltage gradient across the plasma cell that will maintain a glow plasma, and this can be achieved electrically by adjusting the voltage applied to the electrodes. For example, if a defined and/or fixed plasma current profile is maintained by using a feedback circuit or other appropriate means to actively adjust the voltage applied to the electrodes in real time (cycle to cycle), the glow plasma may be stabilised and maintained over a wide range of compositional and ambient conditions. The feedback circuit used is one that is able to cope with high speed feedback implementation.

The input excitation waveform shape may be adjusted, for example, to a sine wave, square wave, saw tooth, a smooth, non-sinusoidal function or other appropriate waveform or combinations of waveforms. However, in most practical, high frequency, electrical implementations, the waveform delivered across the plasma may become pseudo-sinusoidal in form.

In one example apparatus, the plasma cell is driven by at least one pair of electrodes, which are separated by a defined distance, through dielectric barriers such as ceramic, glass or quartz on opposite sides of the cell and the gap of the interior of the plasma cell, which forms a channel though which the gas of interest flows and within which the plasma is formed. At least one inlet and at least one outlet are provided to allow entrance and exit of the gas of interest. The electrodes are typically connected to the dielectric barrier by mechanical and/or adhesive means. More than one pair of electrodes may be desirable, for example, if an extended region of glow plasma is required. The size and shape of the electrodes are important in some example applications, since they define the plasma region extension and shape.

Example electrodes are provided with defined discontinuities in the electrode construction, such as in a mesh or lattice-like construction with round, square or other defined shape gaps.

Nevertheless, electrodes with continuous surface construction may be used and are easier to design and assemble and will have higher capacitance for the same external size. A lattice-like electrode construction can lower the high current densities associated with filamentary formation and may also allow the use of optical detectors behind the electrodes measuring light through the holes in the electrodes. Identical, planar, continuous, circular electrodes may advantageously be used, due to the symmetry giving no intrinsic bias to encourage localisation of any plasma instability (breakdown). However, other shapes are possible and potentially advantageous, in particular mechanical and flow configurations.

Thanks to the enhanced stability that is achievable using methods and apparatus described herein, a wider range of electrode designs can be practically implemented. Likewise, the electrodes' shapes can be chosen to modify the profile of the plasma region formed and this may be useful to optimise the plasma geometry for particular flow regimes and/or optical emission or collection designs.

The area of the electrodes will affect the plasma cross sectional area and hence the amount of light emitted, with a larger area increasing the emitted light accordingly, although this will be at the expense of higher input power. Additionally, larger surface area electrodes will increase the capacitance of the system and this will enable enhanced current feedback and hence increased performance. Ideally, the electrical and mechanical properties of the dielectric and hence also the impedance and capacitive properties of the dielectric barriers are stable with time for optimal operational stability. In addition, the material and electrical properties of the dielectric are relevant factors when deciding the optimal thickness of the dielectric barriers. If the dielectric barriers are too thin, the current limiting properties may be insufficient, and if the barriers are too thick, an increased voltage will be required to penetrate the barrier. The voltage gradient across the gap is a control factor when inducing the plasma initiation and maintenance. For a fixed voltage, the smaller the gap, the larger will be the voltage gradient. This means that a small gap will enable the use of a lower voltage to induce a plasma in comparison with a large gap to induce the same voltage gradient. This is a consideration when attempting to initiate plasma in high ionisation energy gases such as nitrogen. A lower voltage can also have advantages for electrical safety design, easier transformer build requirement (fewer turns) and lower power usage. Additionally, a small gap will create a larger capacitance, which will enable more sensitive current feedback and enhance sensitivity, especially in non-optical detection mode. There is a practical limit to the size of the gap used because, as the gap becomes smaller, there will be a larger pressure drop across the cell and the optical output may become very low. A compromise gap size is therefore used, which takes account of the above-mentioned factors, as well as manufacturability and cost. In some examples, insulation is provided around the electrodes (encapsulation) to avoid corona discharge formation.

In order to monitor the optical output, at least one optically transmissive element must be present in the cell, transparent to the wavelength range of the light of interest (typically in the ultraviolet and visible parts of the electromagnetic spectrum). The at least one optically transmissive element may include one or more of the following: windows, lenses, diffraction gratings, optical filters or spectrometers. These optical elements should be photostable to ultraviolet and visible light and also not luminesce in the wavelength range of interest as a consequence of photon absorption. Optical fibres may be useful in transferring the optical output to a non-line-of-sight destination and/or from a hot region containing the plasma cell to a cooler region where the electronics can operate within their operational ambient temperature limits. Additionally, fibre optics allow the siting of the detector and/or signal processing electronics at a distance away from the plasma cell and the high associated electromagnetic fields.

For gas detection, the output light may be detected by detectors such as photodetectors (e.g. silicon or InGaAs photodiodes), or thermal based detectors (e.g. pyroelectric detectors, bolometers or thermopiles) or, alternatively, the output light may be collected by a spectrometer, which creates a spectral plot across an emission wavelength range. The changes of intensifies of emission lines with gas composition may be used for speciation and quantification. Plasma by-products are present in the exhaust from the cell. These may be useful for plasma surface etching, cleaning, chemical production or other purposes. The byproducts may also contain hazardous gaseous species, which may require appropriate treatment or consideration. The amount of plasma by-products produced and present in the exiting gas will be dependent on the gas composition, pressure, flow rate, cell size and electrode area amongst other factors.

All materials used to hold or encapsulate the plasma cell should be photostable to ultraviolet and visible light and also not luminescence within the wavelength range of interest as a consequence of photon absorption. Additionally, electromagnetic shielding may be useful to shield or contain the plasma cell and/or associated electronics from internal or external sources of electromagnetic interferences. Although the device has been described advantageously as being able to function at atmospheric pressure or higher, it may be desirable, in some circumstances, to operate at sub-atmospheric pressures, for example, to lower the required initiation and maintenance voltage and/or operational power or to lower the density of harmful by-products.

In some embodiments, the plasma cell may be maintained at a defined, fixed temperature.

This may prevent condensation and enhance plasma stability. In addition, the gas sample entering the cell may be maintained at a defined, fixed temperature. This has the advantages of increasing the thermal stability of the gas entering the cell, hence stabilising the output and lowering the voltage required to strike and maintain the plasma through the decrease in density, when held at higher than ambient temperature and with the addition of thermal energy to the gas sample. However, both of these options involve increased power for the heating. For optimal stability, the flow rate through the plasma cell should also be maintained at a defined, fixed flow rate through suitable flow control means, such as a flow controller.

Embodiments may also be designed to add one or more dopants to the gas sample prior to plasma entry. For example, trace amounts of water may be added for signal processing reasons, as described in US8239171.

A dielectric barrier discharge (DBD) is a form of discharge in which both electrodes of the at least one pair are in contact with a dielectric material. This dielectric layer acts as a current limiter. Under certain conditions, a unique type of discharge mode in DBDs is present, where the discharge appears as a diffuse glow, covering the entire electrode surface uniformly. Gas pre-ionisation by electrons and metastable species from previous discharges and the interaction between the plasma and the dielectric surfaces play important roles in the formation of this glow mode. The shape, size and separation of the electrodes, as well as the properties and thickness of any dielectric barrier between the electrodes and the plasma will be crucial to determining the optimal plasma field in glow discharge mode. Although, in principle, the electrodes could be in direct contact with the gas to be measured, in practice, a first example has electrodes protected by a dielectric barrier (e.g. glass or ceramic or any dielectric that can withstand high temperatures and high electromagnetic fields). Additionally, depending on the gases to be measured, there may be aggressive, corrosive components present (e.g. free radicals, ionised molecules and/or chemically corrosive gases/by-products) and, therefore, the dielectric surface must be corrosion resistant in these circumstances. The use of a dielectric barrier, however, precludes the use of a high voltage DC field.

The use of an AC field means that the waveform, frequency and amplitude are important parameters for the stability of any glow plasma to be achieved. When the plasma is used for gas analysis, the signal generated by the plasma when the measurand(s) is (are) present may be determined by, for example, optical detection (for example with a passband filter (wavelength selection) and optical (silicon) detector), which may be in the ultraviolet and/or visible light spectrum. The intensity of emitted light at an individual passband is indicative of the measurand speciation and concentration. This requirement means that there must be, in this optical range, at least one transparent window or other optical element within the plasma gas cell for this type of gas detection.

A typical electrical characteristic of a DBD plasma at atmospheric pressure can be described as follows. Applying an external AC high voltage to the electrodes causes the discharge to initiate when the voltage across the gas gap rises above the breakdown voltage. The breakdown of the gas in the gap causes a plasma to be formed and the electrode current rises rapidly. In many conventional DBDs, this uncontrolled rise in plasma current can lead to the formation of filamentary discharges in this phase. This is characterised by a fast change in filamentary channel resistance, as a rapidly growing space charge forms a self-propagating streamer. Charged particles produced in the plasma accumulate on the dielectric surfaces adjacent to the electrodes, creating an electric field opposing the applied field. This causes a decrease in the net electric field across the gap and, therefore, the current diminishes rapidly. The charges remaining on the dielectric surfaces after the discharge ends, produce a residual electric field ready for the next cycle of the field.

In optical emission spectroscopy, the presence of filamentary discharges during each half cycle leads to undesirably noisy signals at the detector. The filamentary discharges will also, over long time periods, erode the surface of the dielectric barrier, such as quartz, leading to measurement drift and ultimate failure of the barrier. It is therefore desirable to operate the DBD plasma in a stable diffuse glow mode to achieve low noise and low drift.

The plasma current waveform provides a means of detecting the presence of the filamentary discharges during each half cycle. Therefore, by implementing control of the plasma current, it is possible to control or mitigate the formation of undesirable filaments. Such a control is on a cycle-by-cycle basis with a reasonably high bandwidth.

Another important parameter of the DBD plasma operation, according to one example, is the frequency of its RF excitation. When the excitation frequency is too low, electrons and charged species on the dielectric surfaces accumulate too quickly and the opposing electric field overly suppresses the rise of the plasma voltage. Additionally, some recombination of species on the boundary surface takes place. These effects, together, will result in either non-initiation or premature quenching of the plasma. Conversely, with too high excitation frequency, the electrons and charged species generated in the plasma bulk become confined within the inter-electrode gap and cannot reach the barrier surface to form the necessary opposing electric field. This will also lead to an unstable atmospheric DBD plasma. The solution is to control the excitation frequency to remain within the optimum (relatively narrow) frequency range over which the plasma is in a stable glow operation. This is referred to herein as the resonant frequency band of the feedback system.

As was mentioned earlier, in order to achieve the required fast time response of the detection system, intrinsically low capacitance, high shunt resistance silicon photodiodes (i.e. low active silicon area) were used either singly or in an array with wavelength selection achieved via optical bandpass filters and, additionally, the photodiodes were operated in reverse biase to lower the capacitance even more. The signal was gained up using a high-speed trans-impedance amplifier, which also enabled active cancellation of DC light signal, since without this high gain amplification is not possible. Active suppression of if signal was accomplished via a notch filter. Fast analogue to digital conversion was performed by a high ADC and oversampling by a factor of at least 10 times of the 2f signal. Demodulation and Lock-in detection was followed by filtering performed in digital domain to avoid additional noise contribution from analogue circuits.

Experimental Results: Note that due to bremsstrahlung background radiation, an optical 2f signal may be present, even in the absence of any actively emitting species. Also, dependent on the gas mixture, the 2f optical signal may be due to direct photon emission by an excited gas species (e.g. argon in nitrogen), quenching or reduction of the emission of another species (e.g. nitrogen by oxygen or hydrogen) or enhancement of emission by the presence of another species (e.g. nitrogen by helium) or a combination of two or more of these processes. Enhancement of nitrogen emission at 337nm by helium, for example, is due to the fact that the helium has a lower ionisation energy and hence can enhance the nitrogen excitation and therefore optical emission (Penning ionisation). Note that the relationship of 2f signal (such as the amplitude or integrated area) derived via whatever appropriate technique with gas concentration may be linear or non-linear, dependent on the gas mixture and concentration range. In the case of a non-linear signal, the output may be linearised from an empirical or theoretical fit, polynomial, other appropriate mathematical relationship or combination of two or more of these.

The performance of the 2f detection method was verified using new signal electronics and software. Figure 6 shows the effect of hydrogen on nitrogen emission at 337nm. The results are plotted of the 2f peak heights derived from oscilloscope traces vs the gas concentration.

It can be seen that the hydrogen is quenching the nitrogen emission in a non-linear way.

The correspondence of this 2f method to the conventional DC type measurement is clearly shown in Figure 7, where the reference amplitude is derived from a spectrometer at 337 nm (using a diffraction grating and photodiode array) amplitude reading and shows an identical relationship of intensity with concentration by whichever method is employed, but with the 2f method providing performance improvements in the signal to noise.

Experiments were also performed to illustrate the 2f detection method using 2f amplitude and Lock-in detection which are shown in Figures 8 (a) to (c). This was performed for argon in a background of nitrogen, with optical signal monitoring after an optical pass band filter centred at 780nm with a silicon detector. Asymmetry of the 2f signal is a result of asymmetry in the electrodes and/or the detector position and orientation. The optical output maxima at 2f are clearly seen in all three graphs (a) to (c), as are the increasing amplitudes with argon concentration. Figure 9 shows the relationship of the amplitude after 2f Lock-in detection with argon concentration and can be seen to be slightly non-linear over this concentration range.

Claims

CLAIMS1. A method comprising: generating an oscillating electric and/or magnetic field within a plasma cell to excite particles within the cell, to produce a glow discharge from a plasma in the plasma cell, and controlling the operating conditions for the plasma cell to maintain the glow discharge optical emissions from the plasma within the desired operating range; and monitoring one or more glow discharge optical emissions from the plasma in the plasma cell; wherein the monitoring of the optical signal is at twice the excitation frequency and comprises measuring and processing the signal or signals in real time during each excitation cycle and wherein the monitoring is used to determine the concentration of a gas within a gas mixture.
2. A method according to claim 1, wherein the monitoring comprises measuring the optical emissions in real time using an optical detector.
3. A method according to claims 1 or 2, wherein the optical signal is collected after passing through the transmission band of an optical filter or by using a dispersive grating or other appropriate wavelength selection device.
4. A method according to any of claims 1 to 3, wherein processing the signal is completed in real time using digital signal processing.
5. A method according to any of claims 1 to 4, wherein a notch filter is used to narrow the frequency bandwidth of the signal and obtain separation in frequency from the excitation frequency.
6. A method according to any of the preceding claims, wherein processing the signal comprises one or more of the following: Lock-in detection, synchronous detection, frequency domain analysis such as by using Fast Fourier Transforms (FFTs) and time or frequency domain matched filter techniques or shape filters.
7. A method according to any of the preceding claims, wherein measuring of the signal comprises measuring peak heights, peak areas or integrals of the signal.
8. A method according to any of the preceding claims, wherein post signal filtering is applied, the post signal filtering comprising a median filter and/or ensemble averaging and/or moving averaging.
9. A method according to any preceding claim, wherein the drive waveform is sinusoidal.
10. A method according to any preceding claim, wherein the drive waveform is a square wave, saw tooth, a smooth, non-sinusoidal function or a combination of such waveforms.
11. A method according to any preceding claim, wherein the phase angle of the 2f signal between the excitation waveform and optical signal and/or the amplitude, width, area or other feature of frequency signal components other than at 2f is used to enhance determination of gas concentration and/or background gas composition.
12. A method according to claim 1, wherein the electric field across the plasma cell is generated by an alternating excitation voltage and said controlling is undertaken on an excitation cycle-by-execution cycle basis.
13. A method according to claim 1, wherein the magnetic field is generated by an alternating excitation current in an electromagnet and said controlling is undertaken on a cycle-by-cycle basis.
14. A method according to claim 12 or claim 13 which uses a combination of the electric and magnetic fields of either one or both of claims 12 and 13.
15. A method according to any of the preceding claims, wherein the monitoring of the optical signal comprises measuring the amplitude of the 2f signal and where said monitoring is used to determine the concentration of a gas within a gas mixture.
16. A method according to claim 15, wherein the change in 2f signal amplitude with gas concentration is linearised from an empirical or theoretical fit, polynomial, other appropriate mathematical relationship or combination of two or more of these.
17. A system comprising: a voltage controller to generate an oscillating electric field within a plasma cell to excite particles within the cell, to produce a glow discharge from a plasma in the plasma cell and using a controller to control the operating conditions for the plasma cell to maintain the glow discharge optical emissions from the plasma within the desired operating range; and/or a magnetic field generator to generate an oscillating magnetic field within a plasma cell to excite particles within the cell, to produce a glow discharge from a plasma in the plasma cell and using a controller to control the operating conditions for the plasma cell to maintain the glow discharge optical emissions from the plasma within the desired operating range; and monitoring one or more glow discharge optical emissions from the plasma in the plasma cell using one or more optical detectors linked to measurement circuits; wherein said monitoring of the optical signal is at twice the excitation frequency and comprises measuring and processing the signal in real time during each excitation cycle using a signal processor; and wherein said monitoring is used to determine the concentration of a gas within a gas mixture using a signal processor.
18. A system according to claim 17, wherein photodiodes, such as silicon photodiodes, are used to monitor the optical emissions, said photodiodes having intrinsically low capacitance and high shunt resistance and being used to monitor the optical emissions in the ultraviolet, visible or near infrared light range.
19. A system according to claim 18, wherein the photodiodes are reverse biased to reduce the intrinsic capacitance.
20. A system according to any of the claims 17 to 19, wherein a signal collection is achieved after passing through the transmission band of an optical filter or by using a dispersive grating or other appropriate wavelength selection device.
21. A system according to any of claims 17-20, wherein the glow plasma is controlled by using a Royer transformer in a self-oscillating scheme to maintain a stable plasma, to enable a controlled glow plasma to be maintained over a narrow range of operating conditions.
22. A system according to any of claims 17 to 20, wherein secondary stabilisation electrodes are used to apply a transverse electric field and/or to provide electron-injection.
23. A system according to any of claims 17 to 20, wherein a stable glow discharge plasma is maintained in a plasma cell by applying an input signal from a voltage generator to two or more electrodes in the plasma cell to generate a voltage gradient between the electrodes, measuring an induced signal across the plasma cell using a meter, and using a comparator to compare the induced signal with a reference signal to obtain a difference signal; and wherein a controller determines a control signal which is then applied to the at least two electrodes in the plasma cell based on the obtained difference signal to achieve a desired voltage gradient for the excitation that is needed for a stable glow under resonance conditions.
24. A system according to claim 23, wherein the induced signal is the plasma current and the reference signal is the drive current waveform.
25. A system according to claim 23 or 24, wherein the electric field is generated by an alternating excitation voltage and said controlling is undertaken on a cycle-by-cycle basis.
26. A system according to claims 23 to 25, wherein the alternating excitation voltage is controlled to have a frequency within a determined resonant frequency band.
27. A system according to any of claims 23 to 26, wherein the electric field is generated between two or more electrodes within the plasma cell, and controlling the operating conditions comprises controlling the voltage gradient between the electrodes, to achieve a desired electrical current between the electrodes.
28. A system according to any of the preceding claims 23 to 27, wherein the controlling comprises adapting an excitation waveform, frequency, current and/or voltage.
29. A system according to any of the preceding claims 23 to 28, wherein the determining comprises comparing a measured voltage proportional to the plasma current with a reference 40 voltage
30. A system according to any of the preceding claims 23 to 29, which is responsive to changing operating conditions for the plasma cell to control each of a plurality of different operating conditions for the plasma cell.
31. A system according to claim 23 to 30, wherein the control of a plurality of operating conditions comprises high frequency adjustments to one or more electrical input parameters and/or low frequency adjustments to one or more physical configuration parameters.
32. A system according to any one of the preceding claims 11 to 27, further comprising: transferring energy to gas molecules before the molecules enter the plasma cell, such as controlling temperature, pressure, excitation or ionisation of the gas molecules.
33. A system according to any of claims 23 to 32, wherein a drive frequency is scanned or chirped across a defined frequency range either on a regular or variable basis and a plasma excitation frequency is actively adapted to coincide with the peak resonance related to a species mixture to be analysed.
34. A system according to any of claims 17 to 33, wherein a sample gas is maintained at a determined, controlled temperature prior to entry to the plasma cell.
35. A system according to any of claims 17 to 34, wherein the plasma cell is maintained at a determined, controlled temperature.
36. A system according to any of claims 17 to 35, wherein a flow rate of gas through the plasma cell is maintained at a determined, controlled flow rate.
37. A system according to any of claims 23 to 35, wherein the flow rate is adapted with a feedback system to maintain the plasma current at a determined value.
38. A system according to any of claims 17 to 37, wherein one or more dopants are added to the sample gas prior to entry to the plasma cell.
39. A system according to claim 38, wherein a dopant is water.
40. A system according to any of claims 17 to 39, wherein the plasma cell is at or maintained at atmospheric pressure or higher than atmospheric pressure.
41. A system according to any one of claims 17 to 40, wherein pressure in the plasma cell is adapted with a feedback system to maintain the plasma current at a determined value.
42. A system according to claim 17, wherein generating a magnetic field within a plasma cell is accomplished using an electromagnet.
43. A system according to any of the claims 17 to 42, wherein the plasma cell has at least one optically transmissive element, transparent to the wavelength range of the light of interest.
44. A system according to claim 43, wherein the optically transmissive element is photostable and non-luminescing and may include one or more of the following: windows, lenses, diffraction gratings, optical filters or spectrometers.
45. A system according to any of the claims 17 to 44, wherein optical fibres are used to transfer the optical output to a non-line-of-sight destination and/or from a hot region containing the plasma cell to a cooler region where the electronics can operate within their operational ambient temperature limits and/or allowing the siting of the detector and/or signal processing electronics at a distance away from the plasma cell and the high associatedelectromagnetic fields.