Optical Emission Spectroscopy Diagnostics Research Articles

Influence of electrode transparency on the plasma properties of a cylindrical IEC plasma source

This study investigates the influence of cathode and anode grid transparency on the plasma properties of a Cylindrical Inertial Electrostatic Confinement plasma source. The primary focus is to elucidate how variations in the transparency of the electrodes impact plasma characteristics and the operational performance of the source, particularly during spray jet mode. Employing optical emission spectroscopy (OES) and Langmuir probe diagnostics, the plasma behavior is analyzed comprehensively. The results demonstrate that cathode transparency has a pronounced effect on discharge current and plasma density within the cathode. Conversely, the transparency of the anode has only a little influence the ignition characteristics and operational stability of the plasma source. The findings highlight the importance of utilizing cathode transparency to adjust both the pressure and voltage operating ranges of the IEC source in spray jet mode, optimizing its performance for various applications.

Plasma parameter measurement on a RIT-10 using empirical correlations between non-invasive optical emission spectroscopy and Langmuir diagnostics

Characterising and understanding the plasma properties of a rf-coupled electric propulsion device is crucial during testing, qualification and development. Therefore, the optimization of existing diagnostic systems as well as the development of new ones is an important area of electric propulsion research. Here, we present an approach to non-invasively determine the plasma parameters of an operating radio-frequency ion-thruster. For this purpose, a correlation between non-invasive optical emission (OE) spectroscopy and intrusive Langmuir probe diagnostics measurements is established for a reference system. Both types of measurements are performed simultaneously for a wide range of operation points yielding a large reference data set. Based on a principal component analysis (PCA), a correlation between plasma parameters and corresponding OE spectra at different operational points is established. This correlation can then be applied to OE spectra of the plasma of an operating thruster to obtain non-invasively the corresponding plasma parameters, i.e., without having to employ intrusive Langmuir probes. This approach for evaluating optical spectroscopic data in terms of plasma parameters has no need for a theoretical microscopic modeling of the plasma. This makes this approach very versatile and easily transferable to cases where other propellants are used, since no knowledge of excitation cross sections or transition matrix elements and other microscopic parameters of the species of the plasma is required. Such an approach enables continuous monitoring of a thruster’s behavior during the qualification process.

Impact of self-sputtering in high power impulse magnetron sputtering (HiPIMS) with helium

Conventional magnetron discharge is a widely used technology for many applications. In the last decade, the high current density sputtering regime has been an interesting alternative for tailoring thin film properties. In this paper, we focused on the electrical characterization of the helium magnetron plasma operated at average gas pressure (5 Pa) with a molybdenum target. Optical emission spectroscopy diagnostics also supports this study by providing information on electron density evolution. The analysis of the plasma–surface interaction zone on the target unveiled the physical mechanisms associated with the high current density range (6 A cm−2), corresponding to different discharge regimes. The self-sputtering yield plays a key role in high-power impulse magnetron sputtering discharge operated with helium. The electron density is highly dependent on the presence of a metal.

Non-thermal plasma multi-jet platform based on a flexible matrix.

A new plasma source design that merges the main characteristics of capacitive dielectric barrier discharge (DBD) and cold atmospheric plasma jet (CAPJ) is discussed. The DBD system contains a flexible, porous matrix consisting of silica aerogel, which is comprised between two biased electrodes. The helium flow supply subjected to a sinusoidal voltage of around 5 kV in amplitude and 15 kHz in frequency provides a set of plasma jets that propagates more than 1cm beyond the active DBD region. The studied plasma multi-jet system consists of an array of three aligned jets that flow in the laminar regime, and it is intended for treating the surfaces of 3D objects and large areas. CAPJ performance is discussed as a hypothetical morphing source in flat and bent configurations. Electrical characterization and optical emission spectroscopy diagnostics have provided current-voltage waveforms and the composition of the CAPJ through the aerogel layer, respectively. This novel source is promising for biomedical applications that require full adaptation of plasma parameters to delicate samples, such as wound healing and treatment of surgical margins in plasma-based cancer surgery.

Arc plasma ablation of quartz crystals

Spherical quartz stones of around 1 cm in diameter have been exposed to anodic arc discharges in a helium atmosphere at 300 Torr. The arc current flowing between the graphite electrodes was set either in continuous DC mode (30–150 A) or in pulsed mode at 2 Hz (220 A peak). The ablation rate in each sample was systematically measured after several seconds of arc plasma treatment. Optical emission spectroscopy (OES) diagnostics and 2D fluid simulations of the arc discharge have shed light on the heat flux transport and the heating mechanisms of the quartz crystals. A linear correlation is found between the absorbed power density and the resulting rate of penetration, which yields a maximal value of 15 cm h−1 for approximately 150 W cm−2. The linear fit on the slope provides a specific energy of 40 kJ cm−3. The incident energy flux onto the sample surface promoted a phase transition from crystalline to glassy silica, as characterized via Raman spectroscopy. This study points out the strong potential of arc plasma technology for geothermal drilling applications.

Optical diagnostics of the characteristics of a square unipolar nanosecond pulse-driven atmospheric pressure helium plasma jet

Compared with the traditional sinusoidal voltage source, a short rising nanosecond voltage source can generate a high electron current for a short rising time. This paper investigates how the nanopulse parameters such as the voltage amplitude, pulse duration, and repetition frequency affect the radical generation and the plasma bullet propagation in an atmospheric pressure helium plasma jet. An intensified charge-coupled device was used to observe the bullet propagation in the nanosecond gate mode. The plasma bullet’s propagation speed is mainly affected by the applied voltage and externally biased electrodes rather than the pulse duration or the driving frequency. In contrast, optical emission spectroscopy diagnostics estimate that the radical density inside the atmospheric pressure plasma jet mainly increases with the repetition frequency. At the same time, the population of high-energy electrons can be controlled with the unipolar voltage amplitude. Thus, unipolar nanosecond pulses make it possible to control the emitting charges and the generated radicals independently.

Improvement of catalytic activity of graphene oxide by plasma treatment

A considerable enhancement of the catalytic activity of graphene oxide (GO) was achieved as a result of plasma treatment in a hydrogen glow discharge. The results obtained for the hydroisomerization of 1-octene, together with the catalyst characterization showed that plasma exposure leads to the formation of defects in the material and of surface acidic species. Optical emission spectroscopy diagnostics of the plasma revealed that the electron temperature and the dissociation of molecular hydrogen are important parameters that can be tuned for optimization of the treatment process.

(Invited) Improving Nanosynthesis By Means of Pulsed Anodic Arc Discharge

Anodic arc discharge operated with a periodic voltage signal shows better performance in nanomaterial synthesis compared to steady DC arc discharge. The main advantages of pulsed arcs consist of improved process control, high arc stability, tailoring of material properties, and growth of high-quality materials thanks to reduced production of macroparticles. This study shows the relevance of pulse frequency (1-5 Hz), duty cycle (10-50%), peak arc voltage (~50 V) and peak arc current (~200 A) on determining arc discharge properties. All arc processes are performed in a helium atmosphere at around 300 Torr. Pulsed arc discharges show ablation rates and average power consumption values of the order of 1 mg/s and 1 kW, respectively, which are competitive with typical values of standard DC anodic arcs. Pulsed arc syntheses of low-dimensional carbon (graphene, nanotubes) and molybdenum disulfide nanomaterials are treated as examples for this new deposition method. Plasma parameters have been estimated by optical emission spectroscopy and fast Langmuir probe diagnostics. Characterization techniques such as Raman spectroscopy, scanning electron microscopy and x-ray diffraction have provided information on structural and morphological properties of the cathode deposit. The basic physical processes during arc discharge are described in terms of a global model that considers the total pressure evolution of the system. Such model provides a scenario for the observed erosion dynamics and optical emission patterns associated to the different arc discharge phases. The electrical properties of pulsed arc discharges have been characterized and related to basic plasma structure. In conclusion, pulsed anodic arc plasmas are promising for the synthesis of nanomaterials with special physical and surface properties suitable for electrical, mechanical and optical applications.

Optical emission spectroscopy diagnostics of DBD plasma with particles in a two-dimensional spouted bed

To gain an insight into the effect of fluidization behavior of particles on the characteristic of plasma in a plasma-enhanced spouted bed, we investigated the changes of plasma optical emission spectroscopy (OES) with fluidized particles in this paper. Fluidization experiments under the irradiation of a dielectric barrier discharge (DBD) plasma generator were performed in a two-dimensional spouted bed. Essential operation parameters, such as particle type, applied voltage, gas velocity, and particle number were changed. The pressure drop of particles, emission intensity (EI) of plasma, and high-speed camera images of plasma pattern were measured. As the results, the fluidization behavior of particles had been changed by the plasma irradiation, especially for polypropylene (PP) particles. The emission spectra of argon (Ar) plasma had been changed significantly with particles. The emission intensities of the Ar I transition lines were increased, and Ar II lines were decreased with PP particles compared to the case of no particle. The emission intensity of Ar I lines was enhanced with the applied voltage regardless of the addition of particles. The emission intensity changed with a gas velocity versus particle numbers was identified with the fluidization behavior, that emission intensity was increased at first and then decreased. The gas velocities corresponded to the highest value of emission intensity with various particle number were similar to the trend of the changes of minimum spouted velocity. The electrical conductivity and color of particles may affect the difference of pressure drop and detected emission intensity for two kinds of particles. Our research extends the knowledge of the relationship between fluidization behavior and plasma optical characteristic.

Study of the influence of magnetic field profile on plasma parameters in a simple mirror trap

This work presents the multiple diagnostics characterization of the plasma in an axis-symmetric simple mirror trap as a function of magnetic field profile (mirror ratios and magnetic field gradient), especially in the quasi-flat B field configuration that is typical of Microwave Discharge Ion Sources, and also of neutral gas pressure and microwave power. The simultaneous use of Optical Emission Spectroscopy, Langmuir Probe and X-ray diagnostics allows the characterization of the whole electron energy distribution function (EEDF), from a few eV to hundreds of keV . Results show non-linear behaviour under small variations of even one source parameter and strong influence on EEDF of the Bmin/BECR ratio. Benefit and next developments will be highlighted.

Self-stabilized discharge filament in plane-parallel barrier discharge configuration: formation, breakdown mechanism, and memory effects

Single self-stabilized discharge filaments were investigated in the plane-parallel electrode configuration. The barrier discharge was operated inside a gap of 3 mm shielded by glass plates to both electrodes, using helium-nitrogen mixtures and a square-wave feeding voltage at a frequency of 2 kHz. The combined application of electrical measurements, ICCD camera imaging, optical emission spectroscopy and surface charge diagnostics via the electro-optic Pockels effect allowed the correlation of the discharge development in the volume and on the dielectric surfaces. The formation criteria and existence regimes were found by systematic variation of the nitrogen admixture to helium, the total pressure and the feeding voltage amplitude. Single self-stabilized discharge filaments can be operated over a wide parameter range, foremost, by significant reduction of the voltage amplitude after the operation in the microdischarge regime. Here, the outstanding importance of the surface charge memory effect on the long-term stability was pointed out by the recalculated spatio-temporally resolved gap voltage. The optical emission revealed discharge characteristics that are partially reminiscent of both the glow-like barrier discharge and the microdischarge regime, such as a Townsend pre-phase, a fast cathode-directed ionization front during the breakdown and radially propagating surface discharges during the afterglow.

Characterization of microwave plasma in a multicusp using 2D emission based tomography: Bessel modes and wave absorption

A tomographic method based on the Fourier transform is used for characterizing a microwave plasma in a multicusp (MC), in order to obtain 2D distribution of plasma emissions, plasma (electron) density (Ne) and temperature (Te). The microwave plasma in the MC is characterized as a function of microwave power, gas pressure, and axial distance. The experimentally obtained 2D emission profiles show that the plasma emissions are generated in a circular ring shape. There are usually two bright rings, one at the plasma core and another near the boundary. The experimental results are validated using a numerical code that solves Maxwell's equations inside a waveguide filled with a plasma in a magnetic field, with collisions included. It is inferred that the dark and bright circular ring patterns are a result of superposition of Bessel modes (TE11 and TE21) of the wave electric field inside the plasma filled MC, which are in reasonable agreement with the plasma emission profiles. The tomographically obtained Ne and Te profiles indicate higher densities in the plasma core (∼1010 cm−3) and enhanced electron temperature in the ECR region (∼13 eV), which are in agreement with earlier results using a Langmuir probe and optical emission spectroscopy (OES) diagnostics.

Analysis of Hα spectral line profile in a DC hydrogen discharge

This contribution reports an experimental study of characteristics of the neutrals in a DC hydrogen discharge, designed for production of NiH. The investigations are carried out at gas pressure in the range p = (200–1600) mTorr and current I = 200 mA. Optical emission spectroscopy diagnostics are based on analysis of the Hα line profile. The profile is formed from the emission of three energetically distinct groups of atoms in the n = 3 excited state. The first one is the group of thermalized atoms, with a mean energy of about 0.1 eV, the second—fast atoms, with mean energies about 10 eV and the third—superfast atoms with a mean energy of about 90 eV. The experimental technique leads to accurate values of the relative densities of these atoms and their mean energies for various working conditions.

Fast, hot electron production and ion acceleration in a helicon inductive plasma

A large, time-averaged, double layer-like plasma potential drop of 80 V over several hundred Debye lengths has been observed in the magnetic expansion region on the Madison Helicon eXperiment. It is operated in an inductive mode at 900 W and low argon operating pressures (0.12–0.20 mTorr) in the collisionless regime. The plasma space potential drop is due to the formation of a double layer-like structure in the magnetic expansion region and is much higher than the potential drop caused by a Boltzmann expansion. With the plasma potential drop, a locally negative potential ion hole region at lower pressures with a higher electron density than ion density has been observed just the downstream of the potential drop region. Two-temperature Maxwellian electron distributions with a warm (Te≈15 eV) and bulk (Te≈5 eV) components are observed just upstream of the double layer validated through a RF compensated Langmuir probe and an optical emission spectroscopy (OES) diagnostics. In the expansion chamber downstream of the double layer-like potential drop, a single warm (Te≈15 eV) Maxwellian electron distribution is observed via both the Langmuir probe and OES diagnostics. Ion beam energies of 65 eV are also observed downstream of the potential drop.

Steam reforming of toluene as biomass tar model compound in a gliding arc discharge reactor

Non-thermal plasma is considered a promising and attractive approach for the removal of tars from biomass gasification to deliver a clean and high quality syngas (a mixture of H2 and CO). In this study, an AC gliding arc discharge (GAD) reactor has been developed for the conversion of toluene as a tar model compound using nitrogen as a carrier gas. The presence of steam in the plasma reaction produces OH radicals which open a new reaction route for the conversion of toluene through a stepwise oxidation of toluene and intermediates, resulting in a significant enhancement in both the conversion of toluene and the energy efficiency of the plasma process. The effects of steam-to-carbon (S/C) molar ratio, toluene feed rate and specific energy input (SEI) on the performance of the plasma steam reforming of toluene have been investigated. The optimal S/C molar ratio was found to be between 2 and 3 for high toluene conversion and energy efficiency. The maximum toluene conversion of 51.8% was achieved at an optimal S/C molar ratio of 2, a toluene feed flow rate of 4.8ml/h and a SEI of 0.3kWh/m3, while the energy efficiency of the plasma process reached a maximum (∼46.3g/kWh) at a toluene feed flow rate of 9.6ml/h and a SEI of 0.19kWh/m3. H2, CO and C2H2 were identified as the major gas products with a maximum syngas yield of 73.9% (34.9% for H2 and 39% for CO). Optical emission spectroscopy (OES) has been used to understand the role of steam on the formation of reactive species in the plasma conversion of toluene. The possible reaction pathways in the plasma conversion of toluene have also been proposed by combined means of the analysis of gas and liquid samples and OES diagnostics.

Electron scattering cross sections for the modelling of oxygen-containing plasmas*

This work proposes a set of electron scattering cross sections for molecular and atomic oxygen, with interest for the modelling of oxygen-containing plasmas. These cross sections, compiled for kinetic energies up to 1 keV, are part of the IST-LISBON database with LXCat, being used as input data to the LoKI (LisbOn KInetics) numerical code. The cross sections for ground-state molecular oxygen describe elastic and inelastic collision mechanisms, the latter including rotational excitations/de-excitations (treated using either a discrete or a continuous approach), vibrational and electronic excitations (including dissociation), dissociative attachment and ionisation. This set yields calculated swarm parameters that reproduce measurements within 5–20% (transport parameters) and within a factor of 2 difference (Townsend coefficients), for reduced electric fields in the range 10-3–103 Td. The cross sections describing the kinetics of atomic oxygen by electron-impact comprise elastic mechanisms, electronic excitation and ionisation from O(3P) ground-state, dissociation of O2(X,a,b) (including dissociative ionisation and attachment) and of O3, and detachment. These cross sections are indirectly validated, together with other elementary data for oxygen, by comparing the densities of O((4S0)3p 5P) obtained from the self-consistent modelling and from calibrated optical emission spectroscopy diagnostics of microwave-sustained micro-plasmas in dry air (80% N2: 20% O2), produced using a surface-wave excitation (2.45 GHz frequency) within a small radius capillary (R = 345 μm) at low pressure (p = 300 Pa). The calculated densities are in good qualitative agreement with measurements, overestimating them by a factor ∼1.5.

Spectroscopic study of neutral species in a planar-coil inductive discharge in hydrogen

This contribution reports on an experimental study of characteristics of the neutrals in a single element of a matrix source of negative hydrogen ions. The investigations are carried out in hydrogen discharges maintained at a frequency of 27 MHz, applied power varied in the limits P = (50–150) W and pressure in the range p = (90–160) mTorr. An optical emission spectroscopy diagnostics, based on analysis of the spectral line profile and the intensity distribution of the Fulcher-α molecular band, is used. The profile of the atomic line reveals the existence of thermal as well as of non-thermal (fast) hydrogen atoms in the discharge. For processing the experimental data a line shape model, accounting for details of the plasma kinetics and the fine structure of the line, is used. The temperature of the atoms in the excited n = 3 state, the mean energy of the fast atoms, the ratio between the densities of these two groups of atoms and the relative population of the fine structure components of the n = 3 hydrogen state are determined. The atomic temperature is estimated to be equal to the temperature of the excited (n = 3) atoms. The molecular temperature is determined from the analysis of the intensity distribution of the molecular Fulcher-α band and is found to be about two times lower than the atomic temperature.

A collisional radiative model of hydrogen plasmas developed for diagnostic purposes of negative ion sources.

A collisional radiative model of low-pressure hydrogen plasmas is elaborated and applied in optical emission spectroscopy diagnostics of a single element of a matrix source of negative hydrogen ions. The model accounts for the main processes determining both the population densities of the first ten states of the hydrogen atom and the densities of the positive hydrogen ions H(+), H2 (+), and H3 (+). In the calculations, the electron density and electron temperature are varied whereas the atomic and molecular temperatures are included as experimentally obtained external parameters. The ratio of the Hα to Hβ line intensities is calculated from the numerical results for the excited state population densities, obtained as a solution of the set of the steady-state rate balance equations. The comparison of measured and theoretically obtained ratios of line intensities yields the values of the electron density and temperature as well as of the degree of dissociation, i.e., of the parameters which have a crucial role for the volume production of the negative ions.

Investigation of helium barrier discharges with small admixtures of oxygen

Barrier discharges in helium and in helium with small admixtures of oxygen were investigated by electrical measurements, the spatiotemporally resolved optical emission spectroscopy and surface charge diagnostics via the electro-optic Pockels effect. As already known, in pure helium a diffuse discharge is typically formed because of the significant role of the metastable species. However, even a very small oxygen admixture (0.025 vol.%) causes the transition to a filamentary mode as a result of the effective quenching of helium metastables by oxygen molecules. This effect was indicated by a significant decrease of N the first negative system emission. The transition region was characterized by several Townsend-like discharge breakdowns becoming more and more unstable with an increasing O2 admixture. The formation of the atmospheric pressure Townsend-like discharge was confirmed by the spatiotemporally resolved emission. The development of the surface charges agrees qualitatively and quantitatively well with the transported charge during the discharge breakdown calculated from the discharge current.

Diagnostics of Air-Breathing Electric Thrusters

Air-Breathing Electric Thrusters (ABET) diagnostics is addressed, with on-ground prototypes in mind. It is based on detailed volume averaged Global Models and focuses on emission spectroscopy. Notably, the obtained optical emission spectroscopy diagnostics tools give important information about the thruster propellant constitution and also the ionization degree of each constituent. This allows for trade-off between various prototypes on the basis of their characterization and optimization.