Glow plasma gas measurement signal processing

Abstract

To provide methods, apparatuses and systems for enhanced determination of the gas composition of a sample gas using glow discharge optical emission spectroscopy (GD-OES) for gas analysis.SOLUTION: A method comprises: generating one or more oscillating electromagnetic fields within a plasma cell so as to excite particles within the cell and produce glow discharge plasma in the plasma cell, and controlling operating conditions of the plasma cell while flowing a gas mixture through the plasma cell to maintain glow discharge optical emissions from the plasma within a desired operating range; monitoring one or more glow discharge optical emissions from the plasma in the plasma cell, where monitoring comprises measuring the optical emissions at twice the plasma excitation frequency, or measuring a signal that correlates with the optical emissions; and processing the signal during each excitation cycle of the electromagnetic excitation so as to determine the concentration of a gas within a gas mixture flowing through the plasma cell.SELECTED DRAWING: Figure 3

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This patent application claims priority from UK patent applications Nos. 2005140.5 and 2005079.5 (published as UK Patent No. 2583025), which are incorporated herein by reference. [Technical Field]

[0002] The present disclosure relates to methods, devices and systems for processing signals obtained from glow plasma emissions. [Background technology]

[0003] Plasma consists of ionized gas molecules in a mixture of free electrons, neutral molecules, and photons of light of various wavelengths. Plasma can take many forms, both natural, such as stars, nebulae, flames, and lightning, or artificial, such as arc discharges in high-voltage electric fields. Plasma can be generated at both high and low pressures. Low-pressure plasmas have advantages, such as lower strike and sustain voltages and a lower quench rate due to lower species density, but the cost and complexity of achieving these low pressures increases, and the total amount of ionized molecules may be lower than in high-pressure plasmas. Plasmas are used in material processing applications, such as surface cleaning to prepare substrates for thin-film deposition. Plasmas are also used in plasma lighting, ozone generation, computer chip etching, and solar cell fabrication.

[0004] In a glow plasma, electrons and other ionized species are not in thermal equilibrium, and the energy associated with their excitation in the electric field can be much higher than the average energy of the mixture. The electric field of the species and inelastic collisions between the accelerated electrons and gas molecules create excited and ionized species. Subsequent radiative decay to lower energy levels emits characteristic photons of radiation, which gives the "glow" discharge its name.

[0005] Glow discharges can occur in direct current (DC) or alternating current (AC) excitation fields. DC fields involve direct contact of the electrodes in a gaseous environment, which can be undesirable for the electrode's performance and lifespan. AC fields can be coupled to the gaseous sample through a dielectric barrier, thus protecting the electrodes from direct contact with the gas. Dielectric barrier discharge (DBD) plasmas are also used for industrial ozone generation.

[0006] Glow plasmas play an important role in gas analysis. Optical emission spectroscopy (OES) is a technique for species identification and quantification, in which the light emitted from excited species within the glow plasma is analyzed. The position of the emission line in the electromagnetic spectrum indicates the species' identity, and its intensity indicates the concentration of that gas species in a gas mixture (shown in Figure 1). Glow discharge optical emission spectroscopy (GD-OES) has been used to analyze the surfaces of conductive solid materials but has not been the preferred technique for gas analysis. Most conventional GD-OES systems use low-pressure glow discharge plasmas, although measurements above atmospheric pressure are still possible in some cases. In gas analysis, glow plasmas can be used to analyze a wide range of gases, including pollutants. This analysis can be used to control industrial processes to minimize light emission levels and gases of interest, optimize process efficiency, reduce power demand, and ultimately reduce greenhouse gas production, which is a component of most electricity generation and heat production. Summary of the Invention

[0007] A method, apparatus and system are provided for processing optical signals from a stabilized glow plasma in real time with improved signal-to-noise recovery.

[0008] According to one aspect of the present invention, a method for producing a glow discharge plasma includes generating one or more oscillating electromagnetic fields within a plasma cell to excite particles within the cell, generating a glow discharge plasma within the plasma cell, and controlling operating conditions of the plasma cell while flowing a gas mixture through the plasma cell to maintain a glow discharge emission from the plasma within a desired operating range; monitoring one or more glow discharge emissions from a plasma of the plasma cell; monitoring the emission, which may include measuring the emission at twice the plasma excitation frequency or measuring a signal correlated with the emission; processing the signal during each excitation period of the electromagnetic excitation to determine the concentration of gas in the gas mixture flowing through the plasma cell; A method for generating a stable plasma is provided, including:

[0009] The step of generating one or more oscillating electromagnetic fields may include generating an oscillating electric field, an oscillating magnetic field, or a combination of both an oscillating electric field and an oscillating magnetic field within the plasma cell to excite particles (atoms, molecules, or charged species) within the cell and generate a glow discharge from the plasma within the plasma cell. According to some of the methods, devices, and systems described herein, a relatively stable glow discharge emission can be maintained from the plasma within the plasma cell by monitoring one or more glow discharge emissions or one or more signals correlated with the emissions and providing dynamic resonant feedback control of operating conditions, such as the electric or magnetic field used to excite the particles within the plasma cell. The emission from the stabilized glow plasma can then be used for glow discharge optical emission spectroscopy (GD-OES) and non-optical analysis for gas analysis and other applications.

[0010] Monitoring one or more glow discharge emissions from the plasma of the plasma cell can be performed in real time during each excitation period, and the methods and apparatus described herein provide improved signal-to-noise recovery. The optical signal is monitored at twice the excitation frequency of the plasma. Monitoring the signal at twice the excitation frequency (2f) improves the signal-to-noise ratio by narrowing the signal's frequency band and isolating the frequency from the excitation frequency (f), for example, by using a notch filter. Signal detection techniques for examining and determining the 2f signal may include one or more of lock-in detection, synchronous detection, frequency domain analysis using, for example, a fast Fourier transform (FFT), and time-domain or frequency-domain matched filter techniques, shape filters, or other suitable detection means. The signal may be acquired as peak height, peak area, integration of the 2f signal, or any other suitable technique using appropriate filtering (e.g., median filter) and / or ensemble averaging and / or moving averages. Our recognition that it is desirable to measure the signal in real time throughout the monitoring process influenced the design and implementation of our optical detection system. For example, silicon detectors for ultraviolet or visible light are an economical and efficient means of measuring light, but the inventors determined that a fast response time is desirable and chose detectors with inherently low capacitance. Signal collection may occur after passing through the transmission band of an optical filter, or using a dispersive grating or other suitable wavelength-selective device.

[0011] Many conventional glow plasma discharges use a self-oscillating Royer transformer to maintain a stable plasma, which allows 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 inductive feedback techniques (European Patent Application Publication No. 1381257). One modern innovation is the use of secondary stabilizing electrodes for applying a transverse electric field and / or for electron injection (European Patent Application Publication No. 3265806).

[0012] Another more recent example method, described in GB 2583897, maintains a stable glow discharge plasma 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 the induced signal across the plasma cell, and comparing the induced signal with a reference signal to obtain a differential signal. This comparison is performed at plasma resonance. References to "resonance" and "resonance condition" herein should not be construed as a limitation to peak resonance unless this is explicitly stated, as explained below. Control signals are then applied to at least two electrodes in the plasma cell based on the resulting differential signal to achieve the desired voltage gradient for excitation required for a stable glow under resonance conditions. All of this is accomplished in real time during each excitation period. The present inventors have improved and extended the teachings of GB 2583897 and known GD-OES and other plasma emission systems by monitoring the output signal at twice the excitation frequency (2f instead of f). This results in an improved signal-to-noise ratio.

[0013] The drive frequency may be periodically or irregularly scanned or chirped over a defined frequency range, the plasma excitation frequency may be actively configured to coincide with a peak resonance associated with the species mixture of interest, or a phase-locked loop circuit may be used to indicate changes in the phase or resonant frequency of a self-oscillating plasma system. [Brief explanation of the drawings]

[0014] Various features of exemplary apparatus, systems and methods are now described, by way of example only, with reference to the accompanying drawings, in which: [Figure 1A] 1 is a schematic diagram of components of an optical emission spectroscopy system for gas analysis that can use a spectrometer and light emitted by a dielectric barrier discharge (DBD) in a plasma cell. [Figure 1B] FIG. 1 is a diagram of a glow plasma. [Figure 2] FIG. 1 is a diagram of the excitation waveform and the 2f optical waveform. [Figure 3]FIG. 1 is a schematic diagram of the components of a plasma cell resonant current feedback control system. [Figure 4] FIG. 1 is a diagram of 2f signal processing techniques. [Figure 5] FIG. 1 is a diagram illustrating the advantages of using lock-in detection. [Figure 6] FIG. 2f signal amplitude for hydrogen / nitrogen mixtures. [Figure 7] A comparison of 2f amplitude and spectrometer amplitude results. [Figure 8] FIG. 10 is a diagram of the signal waveform of an argon / nitrogen mixture. [Figure 9] FIG. 10 is a diagram of the lock-in amplitude for argon / nitrogen mixtures. [Figure 10] FIG. 1 is a diagram showing the relationship between the phase difference between 1f electrical pumping and 2f optical signal and the impurity concentration. [Figure 11A] FIG. 1 is a diagram of an opto-mechanical electrode arrangement. [Figure 11B] FIG. 1 is a diagram of an opto-mechanical electrode arrangement. [Figure 12] FIG. 11C illustrates the results obtained using the conventional self-oscillating drive circuit and optomechanical cell shown in FIG. 11B. DETAILED DESCRIPTION OF THE INVENTION

[0015] Plasma is an electrically conductive, ionized gas, so it can interact with external electric and magnetic fields. The main components of plasma are free neutral atoms or molecules, positively charged ions or metastable species, free electrons, and various high-energy photons. These species are in a constant state of collision. The degree of ionization of a plasma is the ratio of the number densities of charged and neutral species.

[0016] There are three main light-producing processes in plasma: i. Free-bound transition or recombination radiation Free electrons in a plasma can also be captured by ions, a process known as radiative recombination. If this capture or recombination occurs to the ground state, a photon with an energy greater than the ionization potential of the ion or atom is emitted, resulting in a continuous spectrum. Alternatively, if recombination occurs to an excited energy level, the electrons can decay through the excited state to the ground state by emitting a photon of a specific wavelength, producing an emission line characteristic of that ionic species. ii. bound-bound transition These transitions occur when the change in energy of an electron in an atom or molecule is such that the electron remains bound or attached to the atom or molecule both before and after the change. If the energy increases, a photon moving around in the plasma is typically absorbed. If the energy decreases, the photon is emitted. Bound-bound transitions in plasma can produce both emission and absorption lines that are characteristic of the atomic or molecular species. iii. Free-Free Transition: Bremsstrahlung In any plasma, there are many unbound electrons that are free to interact with other species. When a free electron in a plasma passes near an ionized atom or molecule, the free electron accelerates or decelerates, resulting in a net change in the electron's kinetic energy. Quantum mechanics dictates that this change in energy is quantified and mediated by the absorption or emission of photons by the electron. Because these photons can be of any wavelength, the radiation produced in this process has a continuous spectrum and is also known as thermal bremsstrahlung.

[0017] Within the plasma itself, several processes occur that enable the above transitions, the most important of which are collisions between species. An important set of collisions are those between electrons / charged species and neutral species, which result in ionization. For this to occur, the electrons or some of the charged species must have kinetic energy that exceeds the ionization potential of the gas in question. Conversely, collisions can also lead to recombination processes, where neutral species can be created by collisions between charged species of opposite polarity.

[0018] One method of generating a glow plasma is by high-voltage radio frequency (RF) excitation of a gas 100 flowing through a dielectric barrier vessel 110 (e.g., glass or quartz) surrounded by conductive electrodes 120, 130 to form a plasma cell 140, as shown schematically in FIGS. 1A and 1B. During each RF excitation cycle, charged species in the plasma experience two peak accelerations in both directions in the electric field. On the positive side of the sinusoidal excitation, this acceleration peaks near the crest of the waveform. Similarly, on the negative side of the sinusoidal excitation, the charged species experience a peak acceleration of opposite polarity near the trough of the waveform. Because all three light-generation processes described above also peak during these high-acceleration events, the instantaneous light generated from any glow plasma exhibits two distinct peaks and troughs during each individual cycle of RF excitation, and the emission spectrum 160 characteristic of the constituent gases can be measured using a spectrometer or other detector 150.

[0019] This was experimentally verified by the inventors by using a high-speed photodiode amplifier circuit to detect instantaneous optical signals passing through a narrow optical bandpass filter centered at 337 nm in a N2 glow plasma. This is clearly shown in Figure 2, where the excitation signal 200 is at frequency f and the detected signal 300 is at twice the excitation frequency (2f). Note that there is a phase shift between the excitation and drive waveforms due to instrumentation, the inertia of the plasma species, and other factors.

[0020] Until relatively recently, accurately acquiring and processing these signals in real time has been extremely difficult. Furthermore, specific design recommendations exist to ensure that detectors respond quickly enough at commonly used drive frequencies (tens of kHz or higher), which are generally not required or used in standard optical plasma measurements. This is 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 the non-intuitive approach we propose has not previously been explored.

[0021] Our realization that the light emitted from a plasma has a second-harmonic (2f) component correlated to the plasma's RF excitation waveform allows us to extract the light from a noisy background with extremely high noise rejection. One conventional method for achieving this is called lock-in detection. The greatest strength of a lock-in detector is its ability to extract the amplitude and phase of a signal even in extremely noisy environments. In effect, lock-in detection is like a Fourier transform using a single frequency (2f) component and setting all other coefficients to zero. Typically, lock-in detection uses a homodyne detection scheme followed by low-pass filtering to extract the amplitude and phase of the desired signal relative to a periodic reference (see Figure 4). The shape of the 2f periodic reference waveform can take many forms, including sine waves, square waves, or other shapes appropriate for optimal extraction of the desired process signal. This detection occurs in a well-defined, narrow frequency band around the reference frequency, effectively rejecting all contributions at other frequencies from other spurious light sources. Using this technique, a photodiode or other suitable detector (photomultiplier, bolometer, pyroelectric, or thermopile detector) can detect the plasma light in an extremely narrow band, significantly reducing the contributions of thermal and shot noise from the amplifier. The extent of the improvement achievable by using the lock-in technique can be seen in the simulated data in Figure 5. Graph (a) shows a clean 2f signal, while (b) shows the effect of adding large amounts of random noise to the signal. Even with this significant noise, lock-in detection can effectively recover the original signal, as seen in graph (c). In practice, silicon photodiodes are an economical and versatile solution, offering fast detection over a wide range of wavelengths, potentially in the ultraviolet, visible, and near-infrared. Another important consideration is the presence of ambient light. Most light detection from glow plasmas occurs near the near-ultraviolet and visible wavelength bands, which are also wavelength bands where many spurious ambient light sources exist. Detection of the second harmonic of the plasma light improves rejection of ambient light sources, which typically modulate at much lower frequencies, making the task of shielding the plasma detector much easier.Optimally, the plasma excitation drive, whether a drive system or a self-oscillating system, is free of 2f components and / or distortions that may include 2f components, as this can contribute to spurious 2f noise and / or offsets.

[0022] When using a glow plasma spectrometer for trace gas species measurements, two important signal processing objectives must be simultaneously achieved. First, a fairly high gain is required to enable detection of weak emissions. Second, a much higher-resolution digitizer is required to provide measurement resolution. Post-processing of the 2f signal is crucial for weak emission components. The second-harmonic signal can be processed in two ways, such as in conventional techniques with analog lock-in detection using multipliers and low-pass filters. However, these analog circuits are prone to drift and incur a significant penalty in the form of additional noise contributions. Alternatively, in modern electronic configurations, the second-harmonic signal is directly digitized with a high-speed, low-noise analog-to-digital converter (ADC). All further processing is then performed digitally to limit noise contributions. To cover a wide range of gas concentrations across the full range of transmission strengths likely to be encountered in industrial applications, an ADC with a resolution of 16 bits or greater is preferably used. The second-harmonic signal itself may be treated as a digital frame or scan. A high-speed, time-critical, real-time data acquisition algorithm using a microprocessor performs all digital lock-in operations, including shape reconstruction algorithms such as averaging front-end frames, multiplying by a reference 2f frame, and optionally using FFT techniques to optimize computational efficiency. In practice, weak 2f signals can be corrupted by a mixture of random and systematic distortions, so shape identification of weak profiles can play an effective role in signal reconstruction. Experiments during the work, which can be realized in various configurations, have shown that a unique combination of a high-gain, AC-coupled analog front-end followed by high-speed digital signal processing can be an excellent combination for successfully restoring weak signals. An example of a 2f signal processing technique is shown in Figure 4. The excitation waveform is typically sinusoidal, which can be particularly convenient when implementing lock-in or FFT techniques. However, other profiles may be used, and instrumentation and / or other factors may distort the excitation profile, which may be important for any post-processing, especially the use of shape filters or matched filters.Phase angles and / or signals at frequencies other than 2f also contain signal enhancement information or background composition data and can be used to extract extra signal processing information. For example, changes in phase angle may be related to the target gas concentration in the mixture and / or the background mixture composition in the case of non-binary gas mixtures.

[0023] In summary, photodetection at 2f is superior to conventional photodiode photodetection at DC due to the following advantages: 1. Improved signal-to-noise ratio 2. Extremely narrow bandwidth detection leading to less thermal and shot noise 3.Improved removal rate of spurious light sources such as ambient light.

[0024] However, real-time photodetection at 2f at high frequencies 600 has the following new design recommendations for optimal performance, as shown in Figure 4. 1. A photodiode gain stage 610 with DC cancellation, including: Low capacitance and high shunt resistance of the photodiode element Techniques to reduce junction capacitance, such as reverse biasing a photodiode. High-speed transimpedance amplifier. Active cancellation of the DC optical signal, as high-gain amplification would not be possible without it 2. Active suppression of the 1f signal (e.g., via a notch filter 620) (because without this high-gain amplification 630 would not be possible) 3. High-speed analog-to-digital conversion 640 (e.g., oversampling of 2f signals by more than 10 times) 4. Demodulation and lock-in detection followed by filtering in the digital domain 650 to avoid further noise contribution from the analog circuitry

[0025] Detailed Implementation and Results While the exemplary device detailed below uses glow discharge optical emission spectroscopy for gas analysis, the signal processing methods described herein also enhance processing for any other form of oscillator-driven glow plasma. This embodiment (according to GB 2583897) takes advantage of the improved stability of glow plasma at atmospheric pressure for in-line gas analysis. While this embodiment could be realized, for example, using a Royer transformer and / or secondary stabilized electrodes to apply a transverse electric field or perform electron injection, the former has a narrow operating range, and the latter involves additional construction and operational complexity. Furthermore, in the case of electron injection, the presence of such secondary electrodes in the gas flow exposes them to potential contamination and corrosion. When a gas of interest is transported into the plasma, it becomes excited, and the light emitted by radiative decay is detected by a spectrometer, revealing its unique wavelength signature. OES provides non-intrusive, highly specific information about not only the plasma's chemistry but also the relative concentrations of its species. Unlike conventional GD-OES systems, gas analysis applications often require that the gas stream itself not be in physical contact with the high voltage electrodes to avoid sputtering effects or chemical reactions at the electrodes. Also, many gas analysis applications involve a continuous flow of the gas of interest, requiring fast response for spectroscopic detection and species identification.

[0026] This embodiment overcomes the plasma stability shortcomings of conventional methods at fundamental plasma energy levels, enabling a stable glow plasma under a very wide range of conditions (e.g., composition, gas type, gas concentration, and flow rate). This is achieved through control of the plasma operating conditions. In one embodiment, this control is achieved by monitoring the plasma current on a periodic basis (i.e., during each excitation period) and using feedback control to maintain the plasma current at a specified value. This feedback can be achieved in several ways, for example, by controlling the voltage gradient across the plasma cell in real time.

[0027] References to "resonance" or "resonance condition" or "resonance frequency" in this patent specification are intended to refer to the functional excitation frequency range (resonance frequency band) of the glow plasma, which depends on the gas composition and the physical dimensions of the plasma cell, among other considerations such as ambient conditions. While a glow plasma can be actively maintained within this range, there is typically an optimal frequency or peak resonant frequency within the functional glow plasma excitation frequency range, where the greatest energy transfer efficiency (maximum or peak resonance) to the plasma occurs. For a fixed frequency, the voltage gradient across the plasma cell may be adapted via a feedback mechanism to maintain a stable glow plasma even as the gas composition and / or environmental conditions change. Alternatively, for fixed gas composition and / or ambient conditions, the frequency may be scanned to find the optimal (maximum) resonant peak, or a combination of the two methods may be implemented. The impedance between the electrical excitation source and the plasma cell should be optimized to achieve both stabilization and optimal energy transfer. Optimization parameters may be determined theoretically, empirically, or a combination of both.

[0028] There is a composition-dependent resonant voltage gradient across the plasma cell that sustains the glow plasma, which 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 actively adjusting the voltage applied to the electrodes in real time (cycle-by-cycle) using a feedback circuit or other suitable means, the glow plasma can be stabilized and maintained over a wide range of compositions and ambient conditions. The feedback circuit used is compatible with fast feedback implementation.

[0029] The shape of the input excitation waveform may be tailored to, for example, a sine wave, a square wave, a sawtooth, a smooth non-sinusoidal function, or any other suitable waveform or combination of waveforms, although in most practical RF electrical implementations the waveform delivered across the plasma will be in the form of a quasi-sinusoidal wave.

[0030] In one exemplary device, the plasma cell is driven by at least one pair of electrodes separated by a specified distance through a dielectric barrier, such as ceramic, glass, or quartz, on either side of the cell and a gap inside the plasma cell, forming a channel through which the target gas flows and through which plasma is formed. At least one inlet and at least one outlet are provided to allow the target gas to enter and exit. The electrodes are typically connected to the dielectric barrier by mechanical and / or adhesive means. Two or more pairs of electrodes may be desirable, for example, when an extended region of the glow plasma is required. The size and shape of the electrodes are important in some exemplary applications because they define the extension and shape of the plasma region. Extended electrodes or multiple electrodes can be useful for detecting multiple species, allowing for the placement of multiple wavelength-selective devices and multiple photodetectors. While a single pair of electrode geometries can be convenient for simplicity of design and manufacture, similar 2f implementations with two or more pairs of electrodes may be envisioned, e.g., with a second pair of electrodes lying orthogonal to the first pair and / or longitudinally in series and / or coaxially for simultaneous and / or sequential electromagnetic excitation.

[0031] Exemplary electrodes include defined discontinuities, such as a mesh or grid-like structure with round, square, or other defined gaps in the electrode structure. However, electrodes with continuous surface structures may also be used, which are easier to design and assemble and have higher capacitance for the same external dimensions. A grid-like electrode structure can reduce the high current densities associated with filament formation and further allow the use of a photodetector behind the electrode that measures light through holes in the electrode. Coplanar, continuous circular electrodes are advantageously used because of their symmetry, which does not impart an inherent bias that promotes localized plasma instabilities (breakdowns). However, other shapes are possible and may potentially be advantageous, particularly in mechanical and flow configurations.

[0032] Due to the improved stability achievable using the methods and apparatus described herein, a wider range of electrode designs can be practically implemented. Similarly, the shape of the electrodes can be selected to modify the profile of the plasma region that is formed, which can be useful for optimizing the plasma geometry for particular flow regimes and / or light emission or collection designs.

[0033] The electrode area also affects the cross-sectional area of ​​the plasma and therefore the amount of light emitted; a larger area results in more light being emitted, but at the expense of a larger input power. Additionally, a larger electrode area increases the system's capacitance, which allows for stronger current feedback and therefore improved performance. Ideally, the electrical and mechanical properties of the dielectric, and therefore the impedance and capacitance characteristics of the dielectric barrier, are stable over time for optimal operational stability. Additionally, the material and electrical properties of the dielectric are relevant factors when determining the optimal thickness of the dielectric barrier. A dielectric barrier that is too thin may have poor current-limiting properties, while a barrier that is too thick requires a high voltage to penetrate the barrier. The voltage gradient across the gap is a controlling factor in initiating and sustaining plasma. For a fixed voltage, the smaller the gap, the greater the voltage gradient. This means that a smaller gap can use a lower voltage to induce plasma compared to a larger gap that induces the same voltage gradient. This is a consideration when attempting to initiate a plasma in a high-ionization-energy gas such as nitrogen. Lower voltages have further advantages due to electrically safe design, easier transformer construction requirements (fewer turns), and less power usage. In addition, a smaller gap results in a larger capacitance, which allows for more sensitive current feedback and improves sensitivity, especially in non-optical detection modes. There is a practical limit to the size of the gap, as a smaller gap results in a larger pressure drop across the cell and can result in very low light output. Therefore, a reasonable gap size is used, taking into account the factors mentioned above as well as manufacturability and cost. In some embodiments, an insulator is provided around the electrodes (encapsulation) to avoid the occurrence of corona discharge.

[0034] To monitor the light output, at least one optically transmissive element must be present within the cell that is transparent to the wavelength band of light of interest (typically the ultraviolet and visible regions of the electromagnetic spectrum). The at least one optically transmissive element may include one or more of a window, lens, diffraction grating, optical filter, and spectrometer. These optical elements should be photostable to ultraviolet and visible light and not emit light in the wavelength band of interest as a result of photon absorption. Multiple optical fibers can be useful for transmitting the light output to non-line-of-sight destinations and / or from the high temperature region containing the plasma cell to a cooler region where the electronics can operate within their operational ambient temperature limits. Furthermore, optical fibers allow the detector(s) and / or signal processing electronics to be located away from the plasma cell and associated high electromagnetic fields.

[0035] For gas detection, the output light may be detected by one or more detectors 150, 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 that creates a spectral plot across the emission wavelength band. The variation in the intensity of the emission lines with gas composition can be used for speciation and quantification. Plasma byproducts are present in the exhaust from the cell 140. These can be useful for etching plasma surfaces, cleaning, chemical manufacturing, or other purposes. The byproducts may also contain harmful gas species, which may require appropriate treatment or consideration. The amount of plasma byproducts generated and present in the exhaust gas depends on the gas composition, pressure, flow rate, cell size, and electrode area, among other factors.

[0036] Any materials used to hold or encapsulate the plasma cell should be photostable to ultraviolet and visible light and should not emit light within the wavelength band of interest as a result of photon absorption. Additionally, electromagnetic shielding can be useful to protect or contain the plasma cell and / or associated electronics from internal or external sources of electromagnetic interference. While the device has been advantageously described as being capable of functioning at atmospheric pressure or above, in some circumstances it may be desirable to operate at subatmospheric pressures, for example, to reduce the required starting and sustaining voltages and / or operating power, or to reduce the density of harmful by-products.

[0037] In some embodiments, the plasma cell may be maintained at a defined, fixed temperature. This can prevent condensation and increase plasma stability. In addition, the gas sample entering the cell may be maintained at a defined, fixed temperature. This has the advantage of increasing the thermal stability of the gas entering the cell, thus stabilizing the power output, and also of lowering the voltage required to initiate and maintain the plasma through density reduction when thermal energy is applied to the gas sample by maintaining it at a temperature higher than ambient. However, both of these options involve increased power for heating. For optimal stability, the flow rate through the plasma cell should also be maintained at a defined, fixed flow rate through an appropriate flow control means, such as a flow controller.

[0038] Also, embodiments may be designed to add one or more dopants to the gas sample before the plasma begins to form, for example, trace amounts of water may be added for signal processing reasons, as described in U.S. Patent No. 8,239,171.

[0039] A dielectric barrier discharge (DBD) is a discharge configuration in which at least one pair of electrodes, both in contact with a dielectric, acts as a current limiter. Under certain conditions, a unique type of discharge mode for DBDs exists, in which the discharge appears as a diffuse glow uniformly covering the entire electrode surface. Preionization of the gas by electrons and metastable species from previous discharges, as well as the interaction between the plasma and the dielectric surface, play important roles in the formation of this glow mode. The shape, size, and spacing of the electrodes, as well as the properties and thickness of any dielectric barrier between the electrodes and the plasma, play an important role in determining the optimal plasma field for the glow discharge mode. While in principle, the electrodes can be in direct contact with the gas being measured, in practice, the first example has electrodes protected by a dielectric barrier (e.g., glass or ceramic, or any dielectric capable of withstanding high temperatures and high electromagnetic fields). Additionally, depending on the gas being measured, aggressive and corrosive components (e.g., free radicals, ionized molecules, and / or chemically corrosive gases / byproducts) may be present, so in such situations, the dielectric surface must be corrosion-resistant. However, the use of a dielectric barrier precludes the use of high voltage DC fields.

[0040] The use of AC fields means that waveform, frequency, and amplitude are important parameters for the glow plasma stability to be achieved. When the plasma is used for gas analysis, the signal generated by the plasma in the presence of analyte(s) can be measured, for example, by optical detection (e.g., passband filters (wavelength selection) and photo (silicon) detectors), which may be in the ultraviolet and / or visible light spectrum. The intensity of the light emitted in the individual passbands indicates the chemical species and concentration of the analyte. This requirement means that in this optical range, there must be at least one transparent window or other optical element in the plasma gas cell for this type of gas detection.

[0041] The typical electrical characteristics of a DBD plasma at atmospheric pressure can be described as follows: A discharge is initiated when the voltage across the gas gap rises above the breakdown voltage by applying an external AC high voltage to the electrodes. The decomposition of the gas in the gap generates a plasma, causing a rapid rise in the electrode current. In many conventional DBDs, this uncontrolled rise in plasma current can form a filamentary discharge at this stage. This is characterized by a rapid change in the filamentary channel resistance due to the rapidly growing space charge forming self-propagating streamers. Charged particles generated in the plasma accumulate on the dielectric surface adjacent to the electrodes, creating an electric field that opposes the applied electric field. This reduces the net electric field across the gap, causing a rapid decrease in the current. After the discharge ends, the remaining charge on the dielectric surface creates a residual electric field in preparation for the next electric field cycle.

[0042] In optical emission spectroscopy, the presence of filamentary discharges every half cycle leads to undesirable, noisy signals at the detector. Over time, filamentary discharges can also erode the surface of dielectric barriers, such as quartz, leading to measurement drift and eventual failure of the barrier. Therefore, it is desirable to operate DBD plasmas in a stable, diffuse glow mode to achieve low noise and drift.

[0043] This plasma current waveform provides a means of detecting the presence of filamentary discharges every half cycle. Therefore, by implementing control of the plasma current, it is possible to suppress or mitigate the formation of undesired filaments. Such control is performed cycle by cycle with a suitably high bandwidth. According to one embodiment, another important parameter for DBD plasma operation is its RF excitation frequency. If the excitation frequency is too low, electrons and charged species on the dielectric surface accumulate too quickly, and the opposing electric field excessively suppresses the rise of the plasma voltage. In addition, some species recombination occurs at the interface. These effects can result in the plasma not starting or being quenched prematurely. Conversely, if the excitation frequency is too high, electrons and charged species generated in the plasma bulk are trapped within the interelectrode gap and cannot reach the barrier surface to create the necessary opposing electric field. This can also lead to unstable atmospheric DBD plasmas. The solution is to control the excitation frequency so that the plasma remains within an optimal (relatively narrow) frequency range for stable glow operation. This is referred to herein as the resonant frequency band of the feedback system.

[0044] Resonant Current Feedback Control Circuit An exemplary resonant feedback control circuit is described below that can actively adjust the voltage applied to the electrodes on a cycle-by-cycle basis during each cycle of the AC excitation voltage, thereby controlling, for example, the plasma current and / or excitation frequency.

[0045] FIG. 3 shows an exemplary implementation of the basic elements of a feedback circuit for plasma current control.

[0046] The plasma cell 500 allows for the formation of plasma. The plasma cell is driven by at least one pair of electrodes 501 separated by a defined distance. The electrodes 501 are typically connected to a dielectric barrier (not shown) by mechanical and / or adhesive means. The distance provided between the electrodes 501 forms a gap 502 inside the plasma cell 500, which in combination with a gas inlet opening 503 and a gas outlet opening 504 forms a channel through which a flow of gas of interest can flow through the plasma cell 500. The plasma is formed within the gap 502 of the plasma cell 500.

[0047] A sense resistor 505 is provided as part of a feedback circuit to measure the signal across the plasma cell 500 when an AC excitation voltage is applied to the electrodes 501, creating a voltage gradient across the electrodes 501. The sense resistor 505 is configured to measure the signal induced across the plasma cell such that the instantaneous plasma current iT(t) flows through the sense resistor 505, producing a sense voltage V measured by a meter. s The sense resistor 505 is connected to the secondary winding or coil of the high voltage transformer 506 substantially halfway (center tap) of the secondary winding or coil of the high voltage transformer 506, allowing the sense resistor 505 to be referenced to ground. Although a planar transformer is shown, the transformer 506 is not limited thereto. The transformer 506 is preferably a low winding capacitance transformer, as described in more detail below.

[0048] The connection between amplifier 507 and sense resistor 505 is such that amplifier 507 receives a sense voltage V from sense resistor 505 through a first amplifier input. s A second amplifier input is provided to amplifier 507 so that amplifier 507 can receive a reference waveform 508 that can be used to set both the amplitude and frequency of the plasma current. The reference waveform may be provided by, for example, a function generator. Although a sinusoidal reference waveform is shown, other waveform profiles may be utilized, such as square wave, sawtooth, triangular, or other suitable waveforms or combinations of waveforms.

[0049] Amplifier 507 is preferably a high speed amplifier capable of generating an error signal 509 proportional to the difference between a reference waveform 508 and the actual plasma cell waveform detected by sense resistor 505 .

[0050] The amplifier 507 sends an error signal 509 to a power driver stage 510 or control unit that has a resonant LCR network that is used to drive the primary winding or coil of the transformer 506 .

[0051] The error signal drives the bases of complementary high-power NPN-PNP transistors in a push-pull configuration, which in turn drives the transformer primary through a fixed LCR network. The tight magnetic coupling between the planar transformer's secondary and primary windings transfers this RF energy cycle by cycle to the secondary, which is directly coupled to the plasma electrode. Operating the entire system at resonance ensures that energy is efficiently transferred to the plasma capacitance with each drive cycle.

[0052] The configuration of Figure 3 is just one implementation of a resonant current feedback control circuit; other implementations may be used. As explained, a planar transformer is not required, but may provide additional benefits, which are described in more detail below. High-voltage isolation circuitry is not required for plasma current detection, because it would adversely affect the bandwidth of such a comparison, making it too slow to detect the onset of filament formation. The feedback circuitry used is compatible with high-speed feedback implementations.

[0053] The feedback control circuit allows the plasma current to be directly compared to a known reference waveform that can be used to control or set the amplitude and / or frequency of the plasma current.

[0054] The AC excitation voltage applied to the electrode 501 is adjusted on a cycle-by-cycle basis with reference to a reference waveform, i.e., the adjustment is made by adjusting the detected voltage V measured from the plasma cell during each cycle of the excitation voltage. s and a reference waveform. Active periodic adjustment of the voltage applied to electrode 501 is achieved, allowing the glow plasma to be stable over a wide range of changing conditions, such as gas composition and / or ambient conditions. Adjustment of the voltage applied to electrode 501 can compensate for the changing conditions, allowing the plasma to be stabilized during each period of the AC excitation voltage, i.e., in real time.

[0055] For example, every cycle, the excitation frequency of the excitation voltage can be measured and increased to avoid plasma failure or premature quenching. Alternatively, the excitation frequency can be decreased to prevent electrons and charged species from becoming trapped in the electrode gap, thereby allowing them to reach the dielectric barrier surface and form an opposing electric field. In other words, the excitation frequency can be controlled to remain within an optimal (relatively narrow) frequency range in which the plasma is stably glowing.

[0056] As previously mentioned, to achieve the required fast time response of the detection system, silicon photodiodes (i.e., low active silicon area) with inherently low capacitance and high shunt resistance were used alone or in arrays, with wavelength selection achieved by optical bandpass filters, as shown in Figure 4. Additionally, the photodiodes were operated in reverse bias to further reduce capacitance. The signal was gain-up using a high-speed transimpedance amplifier, which also enabled active cancellation of the DC optical signal, since high-gain amplification would otherwise be impossible. Active suppression of the 1f signal was achieved by a notch filter. High-speed analog-to-digital conversion was achieved by a high-resolution ADC and at least 10x oversampling of the 2f signal. After demodulation and lock-in detection, filtering was performed in the digital domain to avoid additional noise contribution from the analog circuitry.

[0057] The 2f component of the plasma optical signal consists of amplitude and phase. In addition to using the 2f amplitude, the phase also reveals important plasma characteristics related to the collisional energy transfer that occurs within each half-cycle of excitation. At the plasma excitation peak, the acceleration of ion species is greatest, leading to a large number of collisional energy transfer events. As the concentration of impurities added to the plasma changes, the energy transfer collision events also change, causing a small variation in the plasma's bulk capacitance. This variation in plasma capacitance manifests as a "phase" shift in the 2f optical signal relative to the excitation 1f waveform as the impurity concentration changes. An experimental example of this phase shift in the 2f optical signal due to impurity concentration is described in the Experimental Results section.

[0058] Experimental results Note that a 2f optical signal can exist even in the absence of actively emitting species due to bremsstrahlung background emission. Also, depending on the gas mixture, the 2f optical signal may result from 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), enhancement of emission due to the presence of another species (e.g., nitrogen by helium), or a combination of two or more of these processes. For example, the enhancement of nitrogen emission at 337 nm by helium is due to helium's lower ionization energy, which therefore enhances nitrogen excitation and therefore emission (Penning ionization). Note that the relationship between the 2f signal (e.g., amplitude or integrated area) obtained by any appropriate technique and gas concentration may be linear or nonlinear, depending on the gas mixture and concentration range. In the case of a nonlinear signal, the output can be linearized from empirical or theoretical fitting, polynomials, other appropriate mathematical relationships, or a combination of two or more of these.

[0059] 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 337 nm. The results are plotted from the oscilloscope trace of the 2f peak height versus gas concentration. It can be seen that hydrogen nonlinearly quenches the nitrogen emission.

[0060] This correspondence between the 2f method and conventional DC-type measurements is clearly shown in Figure 7, where the reference amplitude is obtained from the amplitude reading of a spectrometer (using a diffraction grating and photodiode array) at 337 nm, and both methods show the same intensity vs. concentration relationship, but the 2f method offers improved signal-to-noise performance.

[0061] We also conducted experiments to demonstrate the 2f detection method using 2f amplitude and lock-in detection, as shown in Figures 8(a)-(c). This was performed using argon in a nitrogen background while monitoring the optical signal after an optical passband filter centered at 780 nm with a silicon detector. The asymmetry in the 2f signal is a result of asymmetry in the position and orientation of the electrodes and / or detector. The 2f optical output maximum is clearly visible in all three graphs (a)-(c), as it increases in amplitude with argon concentration. Figure 9 shows the relationship between amplitude and argon concentration after 2f lock-in detection, which is slightly nonlinear over this concentration range.

[0062] As described in the previous section, the phase difference between the 1f electrical pump and the 2f optical signal may be related to the impurity concentration, and one example is shown in Figure 10. Figure 10 shows the decrease in phase angle with increasing nitrogen concentration.

[0063] The improved signal processing capabilities of 2f signals are not limited to any particular opto-mechanical design, electronic drive, or signal processing format, but are generally applicable. In addition to the examples in FIGS. 1A and 1B, 3, and 4, two further examples are shown in FIGS. 11A and 11B for design diagrams for different opto-mechanical electrode configurations. In either of these diagrams, different appropriate electronic drive and signal processing formats can be used, as described above. These diagrams are shown with a single pair of electrodes and a single detector, which can be convenient for simplifying design and manufacturing. However, similar 2f implementations with two or more pairs of electrodes and / or two or more detectors may be envisioned, e.g., with a second pair of electrodes orthogonal to the first pair and / or longitudinally in series and / or coaxially, for simultaneous and / or sequential electromagnetic excitation. These secondary electrodes may further be coupled to a secondary optical detection system for multi-species detection.

[0064] In FIG. 11A, a 1f plasma excitation driver circuit 700 used to generate an electromagnetic field across the cell is electrically connected to an inner axial electrode 701 and an outer cylindrical electrode 702. The inner electrode 701 is protected from the aggressive plasma environment by a tight-fitting protective tubular sheath 703, which also functions as an inner dielectric barrier. In this illustration, the outer tube 704 and inner protective tube 703 may be made of quartz or other suitable dielectric material for reasons explained in the previous section. The sample gas is contained within the interior volume between the outer tube 704 and the inner tube 703. The electrodes 701 and 702 may be dielectrically coupled for electrical, mechanical, and chemical reasons. A gas inlet 705 and a gas outlet 706 allow the gas to be analyzed to pass in and out of the cell. A window 707 in the electrode 702 allows emission from excited species at 2f to be detected by an appropriate detector, such as a silicon detector 708, after passing through a wavelength-selective element 709, such as an optical bandpass filter. The location, size and shape of the formed plasma 710 are determined by the cell design and materials. The detected optical signal can then be appropriately processed, such as by means shown in FIG.

[0065] In FIG. 11B, a 1f plasma excitation driver circuit 800 used to generate an electromagnetic field across the cell is electrically connected to a circular top electrode 801 and a circular bottom electrode 802. Both electrodes are protected from the aggressive plasma environment by the dielectric barrier of the plasma cell 803 and are also adhesively bonded to the dielectric barrier. In this illustration, the dielectric material of the cell 803 may be made of quartz or other suitable dielectric material for reasons explained in the previous section. The sample gas is contained within the interior volume of the cell 803. A gas inlet 804 and a gas outlet 805 allow the gas to be analyzed to pass into and out of the cell 803. A window 806 in the cell 803 allows the emission from the excited species at 2f to be detected by an appropriate detector, such as a silicon detector 807, after passing through a wavelength-selective element, such as an optical bandpass filter 808. The location, size, and shape of the formed plasma 809 are determined by the cell design and materials. The detected optical signal can then be appropriately processed, such as by means shown in FIG. 4.

[0066] The graph in Figure 12 shows that similar results are obtained when a conventional self-oscillating drive circuit is used instead of the resonant current feedback control circuit shown in Figure 4, and the optomechanical cell shown schematically in Figure 11B is used instead of the cell shown in Figure 1A. The self-oscillating drive has a narrower range of function than the previous resonant feedback example, but nevertheless clearly shows the relationship between 2f signal amplitude and nitrogen concentration. [Explanation of symbols]

[0067] 100 gas 110 Dielectric barrier container 120 Conductive electrode 130 Conductive electrode 140 Plasma Cell 150 detectors 160 Emission Spectrum 200 excitation signals 300 detection signal 500 plasma cells 501 Electrode 502 Gap 503 Gas inlet opening 504 Gas outlet opening 505 Detector Resistor 506 High Voltage Transformer 507 Amplifier 508 Reference waveform 509 Error signal 510 Resonant LCR Network 600 High Frequency 610 Photodiode gain stage 620 Notch Filter 630 High Gain Amplification 640 Digital Transformation 650 Digital Domain 700 1f plasma excitation driver circuit 701 Electrode 702 Electrode 703 Inner Tube 704 outer tube 705 Gas inlet 706 Gas outlet 707 Window 708 Silicon Detector 709 Wavelength Selective Element 710 Plasma 800 1f plasma excitation driver circuit 801 Upper electrode 802 bottom electrode 803 Plasma Cell 804 Gas inlet 805 Gas outlet 806 Windows 807 Silicon Detector 808 Optical Bandpass Filter 809 Plasma

Claims

1. generating one or more oscillating electromagnetic fields within a plasma cell to excite particles within the cell, generating a glow discharge plasma within the plasma cell, and controlling operating conditions of the plasma cell while flowing a gas mixture through the plasma cell to maintain a glow discharge emission from the plasma within a desired operating range, wherein controlling the operating conditions includes adapting an excitation waveform, frequency, current, and / or voltage via a feedback mechanism to maintain a stable glow plasma despite changes in gas composition and / or environmental conditions; monitoring one or more glow discharge emissions from the plasma in the plasma cell; monitoring the optical emission, which may include measuring the optical emission at twice the plasma excitation frequency or measuring a signal correlated with the optical emission; processing the measured light emission or the signal correlated with the light emission during each excitation period of electromagnetic excitation to determine concentrations of gases in the gas mixture flowing through the plasma cell; A method comprising:

2. The method of claim 1, wherein the generated one or more oscillating electromagnetic fields excite particles in the cell at an excitation frequency to generate plasma emission of a peak intensity at twice the excitation frequency, and wherein monitoring the emission at twice the plasma excitation frequency includes monitoring the plasma emission of the peak intensity.

3. 3. The method of claim 1 or 2, wherein one or more oscillating electromagnetic fields excite particles in excitation periods, and controlling the operating conditions of the plasma cell comprises adjusting one or more operating conditions in each excitation period of the one or more oscillating electromagnetic fields.

4. The method of any one of claims 1 to 3, wherein said monitoring step comprises measuring said emission or signal in real time using at least one photodetector.

5. The method of any one of claims 1 to 4, wherein the optical signal is collected after passing the emitted light through at least one wavelength selective device.

6. The method of any one of claims 1 to 5, wherein the step of processing the signal is completed in real time using digital signal processing.

7. 7. The method of any one of claims 1 to 6, wherein a notch filter is used to narrow the frequency band of the emission or signal and separate the frequencies from the excitation frequency.

8. 8. The method of any one of claims 1 to 7, wherein processing the measured luminescence or signal comprises one or more of lock-in detection, synchronous detection, frequency domain analysis, and time domain or frequency domain matched filter techniques, or shape filters.

9. 9. The method of any one of claims 1 to 8, wherein measuring the emission or signal comprises measuring a peak height, a peak area or an integral of an optical signal, or measuring a phase difference between an excitation signal having a first excitation frequency (1f) and an optical signal measured at twice the excitation frequency (1f).

10. The method described in claim 9, wherein the phase difference between an excitation signal having the first excitation frequency (1f) and an optical signal measured at twice the excitation frequency (1f) is used to determine one or more characteristics of the plasma related to changes in the transfer of collision energy that occur within each half period of the excitation signal.

11. The method according to any one of claims 1 to 10, wherein signal post-processing is applied, said signal post-processing comprising a median filter and / or an ensemble average and / or a moving average.

12. The method of any one of claims 1 to 11, wherein the drive current for generating the one or more oscillating electromagnetic fields has a sinusoidal drive waveform.

13. 12. The method of any one of claims 1 to 11, wherein the drive current for generating the one or more oscillating electromagnetic fields has a drive waveform that is a square wave, a sawtooth, a smooth non-sinusoidal function, or a combination of such waveforms.

14. 14. The method of any one of claims 1 to 13, comprising controlling the phase angle of the 2f optical signal between a 1f excitation waveform and a 2f optical signal and / or controlling characteristics of frequency signal components other than 2f to enhance determination of gas concentration and / or background gas composition.

15. The method of claim 14 , wherein the step of controlling a characteristic of a frequency signal component comprises controlling an amplitude, width, or area of ​​the frequency signal component.

16. The method of claim 1 , wherein the electromagnetic field across the plasma cell is an electric field generated by an AC excitation voltage, and the controlling occurs on an excitation cycle basis.

17. The method of claim 1 , wherein the electromagnetic field is a magnetic field generated by an alternating excitation current of an electromagnet, and the controlling occurs on an excitation cycle basis.

18. 18. The method of claim 16 or 17, generating a combination of electromagnetic fields, including one or more electric fields generated according to claim 16 and / or one or more magnetic fields generated according to claim 17.

19. 19. The method of any one of claims 1 to 18, wherein said measuring said signal comprises measuring the amplitude of an optical signal at twice the excitation frequency (2f signal amplitude), said measuring being used to determine the concentration of a gas in a gas mixture.

20. 20. The method of claim 19, wherein the change in 2f signal amplitude with gas concentration is linearized from a mathematical relationship.

21. a plasma cell having an inlet and an outlet for flowing a gas mixture into the plasma cell; an electromagnetic field generator that generates one or more oscillating electromagnetic fields within the plasma cell to excite particles within the cell and generate a glow discharge plasma within the plasma cell; a controller that controls operating conditions of the plasma cell while flowing the gas mixture through the plasma cell to maintain a glow discharge emission from the plasma within a desired operating range, wherein controlling the operating conditions includes adapting an excitation waveform, frequency, current, and / or voltage via a feedback mechanism to maintain a stable glow plasma despite changes in gas composition and / or environmental conditions; one or more photodetectors coupled to one or more measurement circuits configured to monitor glow discharge emissions from the plasma of the plasma cell, wherein the monitoring of the emissions includes measuring the emissions at twice a plasma excitation frequency or measuring a signal correlated with the emissions; A system comprising: The system includes a signal processor configured to process the measured emission or measured signal during each excitation period of electromagnetic excitation to determine a concentration of gas in the gas mixture flowing through the plasma cell.

22. The system described in claim 21, wherein the generated one or more oscillating electromagnetic fields excite particles in the cell at an excitation frequency to generate plasma emission of a peak intensity at twice the excitation frequency, and wherein monitoring the emission at twice the plasma excitation frequency includes monitoring the plasma emission of the peak intensity.

23. 23. The system of claim 21 or 22, wherein a photodiode is used to monitor the light emission, the photodiode having an inherent low capacitance and high shunt resistance, and used to monitor the light emission in the ultraviolet, visible or near infrared range.

24. 24. The system of claim 23, wherein the photodiode is reverse biased to reduce the intrinsic capacitance.

25. The system of any one of claims 21 to 24, wherein signal collection occurs after passing the emitted light through a wavelength selective device.

26. 26. The system according to any one of claims 21 to 25, wherein the glow plasma is controlled in a self-oscillating manner by using a Royer transformer, and a stable plasma can be maintained.

27. A system according to any one of claims 21 to 25, wherein secondary stabilising electrodes are used for applying a transverse electric field and / or for electron injection.

28. 26. The system of claim 21, wherein the electromagnetic field generator is a voltage generator, and a stable glow discharge plasma is maintained by applying an input signal from the voltage generator to at least two 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 comparing the induced signal with a reference signal using a comparator to obtain a difference signal, and the controller determines a control signal, which is applied to the at least two electrodes in the plasma cell based on the obtained difference signal to achieve a desired voltage gradient for excitation required for a stable glow under resonance conditions.

29. 30. The system of claim 28, wherein the induced signal is a plasma current and the reference signal is a drive current waveform.

30. A system as described in claim 28 or 29, wherein the electromagnetic field is generated by an alternating excitation voltage and the control is performed on a cycle-by-cycle basis for each excitation period.

31. 31. The system of claim 30, wherein the AC excitation voltage is controlled to have a frequency within a determined resonant frequency band.

32. A system described in any one of claims 28 to 31, wherein an electric field is generated between two or more electrodes in the plasma cell, and controlling the operating conditions includes controlling the voltage gradient between the electrodes to obtain a desired current between the electrodes.

33. The system of any one of claims 28 to 32, wherein said determining comprises comparing a measured voltage proportional to the plasma current to a reference voltage.

34. The system of any one of claims 28 to 33, wherein each of a plurality of different operating conditions of the plasma cell is controlled in response to changes in the operating conditions of the plasma cell.

35. 35. The system of any one of claims 28 to 34, wherein the control of a plurality of operating conditions comprises frequent adjustment of one or more electrical input parameters and / or infrequent adjustment of one or more physical configuration parameters.

36. 36. The system of any one of claims 21 to 35, further comprising means for transferring energy to the gas molecules before they enter the plasma cell, for example for controlling the temperature, pressure, excitation or ionization of the gas molecules.

37. 37. The system of any one of claims 28 to 36, wherein the drive frequency is periodically or irregularly scanned or chirped over a defined frequency range, wherein the plasma excitation frequency is actively configured to coincide with a peak resonance associated with the species mixture being analyzed, or wherein a phase locked loop circuit is used to indicate changes in the phase or resonant frequency of the self-oscillating plasma system.

38. A system according to any one of claims 21 to 37, wherein a sample gas is maintained at a predetermined controlled temperature before entering the plasma cell.

39. The system of any one of claims 21 to 38, wherein the plasma cell is maintained at a predetermined, controlled temperature.

40. 40. The system of any one of claims 21 to 39, wherein the flow rate of gas through the plasma cell is maintained at a predetermined controlled rate.

41. 41. The system of claim 40, wherein the flow rate is adapted by a feedback system to maintain a plasma current of the plasma at a determined value.

42. The system of any one of claims 21 to 41, wherein one or more dopants are added to the gas before it enters the plasma cell.

43. 43. The system of claim 42, wherein the dopant is water.

44. 44. The system of any one of claims 21 to 43, wherein the plasma cell is at or maintained at atmospheric or superatmospheric pressure.

45. A system according to any one of claims 21 to 44, wherein the pressure within the plasma cell is adapted by a feedback system to maintain the plasma current of the plasma at a determined value.

46. 22. The system of claim 21, wherein generating an electromagnetic field comprises generating a magnetic field within the plasma cell using an electromagnet.

47. The system of any one of claims 21 to 46, wherein the plasma cell comprises at least one optically transmissive element that is transparent to a wavelength band of light of interest.

48. 48. The system of claim 47, wherein the optically transmissive element is photostable and does not emit light and may include one or more of a window, a lens, a diffraction grating, an optical filter, or a spectrometer.

49. 49. The system of any one of claims 21 to 48, wherein a plurality of optical fibers are used to transmit optical power to non-line-of-sight destinations and / or from a high temperature region containing the plasma cell to a cooler region where electronics can operate within their operational ambient temperature limits, and / or allow the detector and / or signal processing electronics to be located away from the plasma cell and the one or more electromagnetic fields of the electromagnetic field generator.

50. 50. The system of any one of claims 21 to 49, comprising a plurality of electrode pairs, one or more secondary electrode pairs being orthogonal to and / or in series longitudinally and / or coaxially with the first electrode pair for simultaneous and / or sequential electromagnetic excitation.

51. A system described in any one of claims 21 to 50, wherein measuring the luminescence or signal includes measuring the peak height, peak area or integral value of the optical signal, or measuring the phase difference between an excitation signal having a first excitation frequency (1f) and an optical signal measured at twice the excitation frequency (1f).

52. The system described in claim 51, wherein the phase difference between an excitation signal having the first excitation frequency (1f) and an optical signal measured at twice the excitation frequency (1f) is used to determine one or more characteristics of the plasma related to changes in the transfer of collision energy that occur within each half period of the excitation signal.

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