Plasma Diagnostics Research Articles

Spectral and electric diagnostics of low-current arc plasmas in CO2 with N2 and H2O admixtures

Plasma diagnostics is a key tool to support the further development of plasma-induced chemical conversion of greenhouse gases (such as CO2) into high-value chemicals. For this reason, spectroscopic and electric measurements of low current (below 1.7 A), stationary arc plasmas in CO2 at atmospheric pressure with addition of N2 or H2O are reported. High-speed photography, imaging emission spectroscopy and time-resolved electrical measurements are used to obtain time-space resolved gas temperatures as well as the electric-field current characteristics of the discharge. It is found that the lowest average electric field in a CO2 arc plasma at atmospheric pressure is ∼20 kV mm−1 at a current between 0.8 and 1 A. If the current decreases below this level, the arc remains in vibrational–translational (VT) equilibrium by increasing the electric field. However, VT equilibrium conditions can be only maintained until a threshold minimum current of 0.33 ± 0.05 A, at which the arc transitions into a non-equilibrium condition with further increasing electric fields (reaching 68 ± 15 V mm−1 at 0.03 A). The addition of N2 or H2O did not influence the electrical characteristics of the CO2 arc within to the tested mixtures. However, there is only a significant decrease in the electric field of the formed transition arcs and the threshold minimum current in the presence of N2. The spectra of the low-current CO2 arc is found to be dominated by emission from the C2 Swan band system and the O I 777 nm triplet peak. However, the CN band dominates the spectra even when small amounts (0.5 wt%) of N2 is present in the plasma. The gas temperature at the axis of the CO2 arc plasma decreased slightly with decreasing current, from an estimated 7000 K at 1 A down to 6300 K at 0.4 A. The thermal radius of the arc is estimated to be larger than 1.2 mm, more than two times larger than the optical radius obtained from the emitted radiation. The addition of N2 and H2O (up to 7 and 9 wt% respectively) lead to only to a 500 K decrease in the axial arc temperature.

The latest developments of microwave diagnostics for high temperature plasma in ELVA-1 company

For nearly 30 years, we have been designing and supplying instruments for microwave diagnostics of high temperature plasma. This report provides a description of the mm-wave components we utilize to make diagnostics within the frequency range of 26–330 GHz. While most of these components are standard and readily available on the market, we have also developed a few specific devices that simplify the architecture of our instruments. The article includes descriptions of these devices: Backward Wave Oscillators (BWO), Impact Ionization Avalanche Transit-Time diode (IMPATT) sources, IMPATT Active Frequency Multipliers (AFM), Noise Sources, and Electronically Controlled Attenuators. Furthermore, we offer an overview of the microwave plasma diagnostics we have supplied, including ECE radiometers operating at 50–220 GHz, as well as heterodyne interferometers operating at fixed frequency 94 GHz, 140 GHz, or 300 GHz. We also discuss methods employed to ensure measurement stability and present the achieved results. The advent of the new era of modern Monolithic Microwave Integrated Circuit (MMIC) based devices has brought forth exciting possibilities. As an example, we discuss the upgrade of the low noise receiver for the Collective Thomson Scattering (CTS) diagnostic at Wendelstein 7-X, which enables ion temperature measurements in the plasma core [1]. Lastly, we provide a list of MMIC-based devices that are currently available and have garnered the attention of the plasma diagnostics community.

Magnetic Field and Plasma Diagnostics for Solar Coronal Mass Ejections: A Case Study Using the Forward Modeling Approach

Magnetic Field and Plasma Diagnostics for Solar Coronal Mass Ejections: A Case Study Using the Forward Modeling Approach

Two-Dimensional Distribution of Plasma Electric Potential in the T-10 Tokamak

Heavy ion beam probe (HIBP) is a unique plasma diagnostics that makes it possible to measure the electric potential φ of high-temperature plasma and its fluctuations y, as well as the density ne and poloidal magnetic field Bpol fluctuations. Position of the point of performing measurements in the plasma vertical cross-section depends on the beam energy and angle of its entrance into the plasma. The variation of these two parameters makes it possible to construct a two-dimensional (2D) detector grid, which covers the domain of possible measurements. The measurement results obtained in the detector grid points provide for constructing 2D distributions of plasma parameters. For the OH and ECRH stages of the T-10 tokamak shots, 2D distributions of the plasma electric potential are presented for the regime with the on-axis magnetic field of Bt = 2.2 T, plasma current of Ipl = 230 kA, line-average density of ne ≈ 1.1 × 1019 m–3 and off-axis ECRH power of PECRH = 1.7 MW.

Shielded ionisation discharge (SID) probe for spatiotemporal profiling of pulsed molecular beam.

Intense pulsed supersonic molecular beams are used in many applications such as tokamak fueling, edge plasma diagnostics, ion beam profile monitors, laser cluster experiments, chemical kinetics, etc. Measurement of absolute density is required to optimize beam sources used in these experiments. Absolute density measurement of a continuous molecular beam is challenging due to its small size and rarefied flow, which makes it even more difficult for a pulsed molecular beam due to its transient nature. In this work, we demonstrate a novel probe to measure the spatiotemporal evolution of the absolute number density of a pulsed supersonic molecular beam. The probe is named the Shielded Ionization Discharge probe. It measures density using localized discharge within the molecular beam created by the thermionic emission of electrons from a hot filament. We describe the design, calibration, and characterization of the developed probe. The performance of the probe is demonstrated by measuring the spatial and temporal profiles of a pulsed supersonic molecular beam of 1.5ms duration.

Recent development of LIF diagnostics in ASIPP

In this article, recent research progress of laser induced fluorescence (LIF) diagnostics in ASIPP is reviewed. The fundamental goal of ASIPP's effort on the LIF diagnostics is to develop a working velocity distribution function (VDF) diagnostic to monitor helium ash removal in burning plasmas. Real-time monitoring of helium ash VDF requires consideration in radiation safety, measurement response time, and compatibility of tokamak operation, as well as ion/neutral VDFs with a temperature at possibly >20 eV. These considerations, almost unique to large-scale instruments or specifically to tokamak plasmas, require renewed research and development (R&D) efforts in optical design, automation, and expansion of laser functionality for the LIF diagnostics that were often not considered in conventional applications of the LIF diagnostics. To this end, ASIPP invested a series of research effort into the development of LIF techniques. Signal evaluation has been performed on a diagnostics-test device (LTS), and the obtained signal strength has been extrapolated to tokamak edge and divertor relevant environments with promising results. The extrapolations compared the available solid angle and plasma density between the test device and the design scenario where the LIF diagnostics will potentially be implemented in a tokamak edge and divertor plasma. The estimation neglects the higher density of energetic electrons in the edge and divertor regions, making it a conservative assessment. Automated post-DAQ processing of LIF signals using both “conventional” methods and an AI-augmented method were also explored. Both approaches yielded usable results, but the AI-trained method showed a faster response, which is advantageous for real-time plasma diagnostics. Basic mechanisms of LIF diagnostics were also explored. Specifically, the limitations of lock-in modulation was demonstrated to determine the limitation of time-resolved measurements using such methods. De-modulation and degradation of the LIF signal occurred as the modulation frequency exceeded 1/10th of the fluorescence frequency. This finding sets a practical limit of the modulation frequency up to which we can benefit from lock-in amplification. These published works along with other unpublished progress will be thoroughly discussed in this article to illustrate ASIPP's effort towards the implementation of LIF diagnostics in future tokamak devices and to promote the application of LIF diagnostics in other fields of research.

Annihilation-gamma-based diagnostic techniques for magnetically confined electron–positron pair plasma

Efforts are underway to magnetically confine electron–positron pair plasmas to study their unique behaviour, which is characterized by significant changes in plasma time and length scales, supported waves and unstable modes. However, use of conventional plasma diagnostics presents challenges with these low-density and annihilating matter–antimatter plasmas. To address this problem, we propose to develop techniques based on the distinct emission provided by annihilation. This emission exhibits two spatial correlations: the distance attenuation of isotropic sources and the back-to-back propagation of momentum-preserving 2 $\gamma$ annihilation. We present the results of our analysis of the $\gamma$ emission rate and the spatial profile of the annihilation in a magnetized pair plasma from direct pair collisions, from the formation and decay of positronium as well as from transport processes. In order to demonstrate the effectiveness of annihilation-based techniques, we tested them on annular $\gamma$ emission profiles produced by a $\beta ^+$ radioisotope on a rotating turntable. Direct and positronium-mediated annihilation result in overlapping volumetric $\gamma$ sources, and the 2 $\gamma$ emission from these volumetric sources can be tomographically reconstructed from coincident counts in multiple detectors. Transport processes result in localized annihilation where field lines intersect walls, limiters or internal magnets. These localized sources can be identified by the fractional $\gamma$ counts on spatially distributed detectors.

Assessment of the spatial distribution of low-temperature plasma parameters on plasma-beam installation

At present continue, studies of the interaction of plasma with structural materials of the first wall and divertor of the ITER international experimental fusion reactor on operating tokamaks and simulated plasma facilities. In the research of the interaction of plasma with the surface of candidate materials TNR an important role plasma parameters play. For determination of local values of the main plasma parameters there is a separate field of research – plasma diagnostics. Plasma diagnostics is the determination of local and instantaneous values of main its parameters: concentration and temperature of charged plasma particles, as well as the distribution functions of various plasma components by energies.This works presents the results of a research to the spatial distribution of local parameters (temperature electrons (Te) and plasma concentration (ne)) of a low-temperature plasma on a plasma-beam installation (PBI) with the help a movable Langmuir probe with the following geometrical parameters of the electrode: length – 4 mm; diameter 0.5 mm. The probe sweep was with the help a Keithley 6487 picoammeter. The processing of volt-ampere characteristics (VAC) was carried out by standard methods.To assess the spatial distribution of the above-mentioned parameters of low-temperature plasma, experiments were carried out with the location of the probe at a distance of 5 mm, 9 mm and 11 mm from the center of the plasma beam at different energies of the primary electron beam in the range from 0.5 keV to 5 keV.The experiments results showed a smooth decrease in plasma concentration in the radial direction.

Four-framed X-ray imaging crystal spectrometer for time-resolved laser plasma diagnostics

X-ray imaging crystal spectrometers with high spectral and spatial resolutions are the important diagnostic tools for acquiring high-quality plasma information in the study of laser–matter interactions. In this paper we propose a new multi-channel imaging crystal spectrometer with coupling X-ray framing camera to record the dynamic evolution of a laser-induced plasma: a four-channel imaging mica crystal spectrometer with a spatial resolution of 40–60 μm for diagnosing the aluminum (Al) plasma targets was developed. Calibration experiments revealed that the spectral resolution is higher than 1700 at Al Kα3 characteristic lines (1496.4eV). Further, we established an aiming method based on the positioning ball and two monitoring CCDs. The optical design, mechanical design, and experimental measurements of the four-channel imaging mica crystal spectrometer are discussed.

De-noising of microwave reflectometry signal using maximal overlap discrete wavelet packet transform for plasma density measurement

For microwave reflectometry-based fusion plasma diagnostics, the acquired data is contaminated by the surrounding noise environment, making it difficult to measure the beat frequency accurately. Discrete wavelet and wavelet packet transforms are unsuitable as the former cannot decompose detailed coefficients, and the latter is a time-variant transform. This paper presents a correlation-based Maximal Overlap Discrete Wavelet Packet Transform (MODWPT), which decomposes the signal using different mother wavelet families. The suitable mother wavelet is chosen based on the metrics like signal-to-noise ratio (SNR), peak signal-to-noise ratio (PSNR), mean squared error (MSE), and mean absolute error (MAE). The data used for analysis is the stationary tokamak wall, for which the beat frequency and distance are measured. After rigorous analysis, it is found that sym12 outperforms with the following values 0.0493 (MSE), 0.1736 (MAE), 4.3334(SNR(dB)), 13.0634(PSNR(dB)) respectively. This technique can be extended to plasma scenarios for accurately measuring beat frequency and density profile.

Advances and Challenges in Combustion Control of Radio Frequency Oscillating Plasma Discharge

Article Advances and Challenges in Combustion Control of Radio Frequency Oscillating Plasma Discharge Linyan Wang , Xiao Yu , Graham Reader , and Ming Zheng * Department of Mechanical, Automotive and Materials Engineering, University of Windsor, 401 Sunset Avenue, Windsor, N9B 3P4 Ontario, Canada * Correspondence: mzheng@uwindsor.ca Received: 18 July 2023 Accepted: 13 September 2023 Published: 20 September 2023 Abstract: Oscillating plasma (corona) ignition is a promising technique for producing a larger initial ignition volume for achieving cleaner combustion. The oscillating plasma system provides a quick response to the ignition command compared with the conventional transistor-coil-ignition systems (TCI). Subsequently, the shorter combustion duration contributes to further improvement of engine efficiency under lean/diluted conditions. The challenges of oscillating plasma ignition appear under elevated background pressures, mostly owing to fewer plasma streamers and even void discharge, thus the advantage of oscillating plasma discharge over spark discharge is compromised. To suffice the industrial applications, the challenges of plasma generation and control platforms are investigated and discussed in this work. The challenges include ignition control, background condition impacts, and other external disturbances. A flexible modulation for oscillating plasma generation is established, with the measurements of discharge voltage and current. High-speed imaging, simultaneous with electrical waveform measurements is applied to record the plasma formation and flame propagation. This research provides advances in the ignition control for the oscillating plasma discharge, adding a foundation and reference for the oscillating plasma diagnostics under engine-like conditions with variations of pressure, temperature, gas composition, and flow pattern.

Procedure qualification and integration of the continuous external Rogowski diagnostics to the superconducting magnets of ITER tokamak

Continuous External Rogowski (CER) are the primary plasma current diagnostics for the ITER tokamak, as well as the only diagnostics built into the ITER Toroidal Field Coils (TF coils). The integration of CER diagnostics to the TF coils faced numerous technical challenges, such as: the integrated components have to withstand the maximum pressure foreseen during ITER TF coil resin impregnation (350 kPa), comply with the high vacuum requirements that the helium leak rate shall be <2.7 × 10−9 Pa-m3/s, and survive at cryogenic temperature. The CER diagnostics integration campaign turned out to be a marked success, thanks to a comprehensive pre-qualification program and well-coordinated collaborations among the participating parties. All four sets of CER diagnostics have so far been integrated to the respective TF coils. As a pioneering attempt of building plasma diagnostics into reactor-scale superconducting magnets, the potential implications of this venture to the diagnostics development for DEMO and future fusion reactors are discussed. The gained experiences and lessons learned from this venture are summarized at last.

Solution-cathode glow discharge under high NaCl concentration: Impact on excitation conditions and analyte transfer processes

The Solution-Cathode Glow Discharge (SCGD) is a useful plasma source for elemental analysis, gathering atomization/excitation/ionization capabilities for aqueous solutions with simplicity and low consumption. When combined with Optical Emission Spectrometry (OES), ng/mL limits of detection can be achieved for multiple elements. Additionally, this plasma source presents certain robustness against the presence of dissolved non-analyte ions. Nevertheless, the mechanisms by which the SCGD can transfer the dissolved analytes from the solution into the plasma, and the mechanisms of how concomitant ions in the sample solution affect the performance of the analysis, are still not fully understood. Therefore, fundamental plasma diagnostics of the SCGD are performed in this work, including electrical measurements, electron densities and rotational and excitation temperatures. In particular, these measurements are performed introducing solutions with NaCl concentrations up to 3.5 g/L in order to understand how they affect the atomization/excitation efficiency and the analyte solution-to-plasma transfer processes of the source. The information is correlated with the signal produced by three analytes (Cd, Zn and Cu).

Role of Oxygen in Amorphous Carbon Hard Mask Plasma Etching.

In the current and next-generation Si-based semiconductor manufacturing processes, amorphous carbon layer (ACL) hard masks are garnering considerable attention for high-aspect-ratio (HAR) etching due to their outstanding physical properties. However, a current limitation is the lack of research on the etching characteristics of ACL hard masks under plasma etching conditions. Given the significant impact of hard mask etching on device quality and performance, a deeper understanding of the etching characteristics of ACL is necessary. This study aims to investigate the role of oxygen in the etching characteristics of an ACL hard mask in a complex gas mixture plasma etching process. Our results show that a small change of oxygen concentration (3.5-6.5%) can significantly alter the etch rate and profile of the ACL hard mask. Through our comprehensive plasma diagnostics and wafer-processing results, we have also proven a detailed mechanism for the role of the oxygen gas. This research provides a solution for achieving an outstanding etch profile in ACL hard masks with sub-micron scale and emphasizes the importance of controlling the oxygen concentration to optimize the plasma conditions for the desired etching characteristics.

INVESTIGATION OF SPATIAL DISTRIBUTION OF METAL VAPOURS ADMIXTURES IN THE PLASMA OF AN ELECTRIC ARC DISCHARGE

This work focuses on diagnosing the plasma in an electric arc discharge in an argon flow using optical emission spectroscopy. The method employed for determining the population of energy levels and the concentration of metal atoms based on the absolute values of emission intensity is described and validated. The experimental setup includes a spectrograph and an RGB CMOS matrix as the emission registration device. By obtaining the absolute values of the spectral radiances of Cu I lines and considering the axial symmetry of the electric arc discharge, the local radiation intensity of these lines is determined. Radial distributions of copper atom concentrations are then calculated using the absolute values of emission intensities and the radial distribution of the excitation temperature, which is determined using the Boltzmann plots technique. Two methods are employed for calculating the atom concentrations. The first method involves Boltzmann plots based on four spectral lines of Cu I and the corresponding excitation temperature. The second method determines the concentrations directly from the population of copper's energy levels, which are derived from the absolute values of emission intensity of the Cu I spectral lines. The results obtained from these two methods exhibit a coincidence of within 20%, supporting the recommendation of this technique for plasma diagnostics in electric arc discharges.

Benchmarking calculations of excitation energies and transition properties with spectroscopic accuracy of highly charged ions used for the fusion plasma and astrophysical plasma

Atomic radiative data such as excitation energies, transition wavelengths, radiative rates, and level lifetimes with high precision are the essential parameters for the abundance analysis, simulation, and diagnostics in fusion and astrophysical plasmas. In this work, we mainly focus on reviewing our two projects performed in the past decade. One is about the ions with Z ≲ 30 that are generally of astrophysical interest, and the other one is about the highly charged krypton (Z = 36) and tungsten (Z = 74) ions that are relevant in research of magnetic confinement fusion. Two different and independent methods, namely, multiconfiguration Dirac–Hartree–Fock (MCDHF) and the relativistic many-body perturbation theory (RMBPT) are usually used in our studies. As a complement/extension to our previous works for highly charged tungsten ions with open M-shell and open N-shell, we also mainly focus on presenting and discussing our complete RMBPT and MCDHF calculations for the excitation energies, wavelengths, electric dipole (E1), magnetic dipole (M1), electric quadrupole (E2), and magnetic quadrupole (M2) transition properties, and level lifetimes for the lowest 148 levels belonging to the 3l 3 configurations in Al-like W61+. We also summarize the uncertainties of our systematical theoretical calculations, by cross-checking/validating our datasets from our RMBPT and MCDHF calculations, and by detailed comparisons with available accurate observations and other theoretical calculations. The data are openly available in Science Data Bank at https://doi.org/10.57760/sciencedb.10569.

30-min decayless kink oscillations in a very long bundle of solar coronal plasma loops

The energy balance in the corona of the Sun is the key to the long-standing coronal heating dilemma, which could be potentially revealed by observational studies of decayless kink oscillations of coronal plasma loops. A bundle of very long off-limb coronal loops with the length of 736pm 80 Mm and a lifetime of about 2 days are found to exhibit decayless kink oscillations. The oscillations are observed for several hours. The oscillation amplitude is measured at 0.3–0.5 Mm, and the period at 28–33 min. The existence of 30-min periodicity of decayless kink oscillations indicates that the mechanism compensating the wave damping is still valid in such a massive plasma structure. It provides important evidence for the non-resonant origin of decayless kink oscillations with 2–6 min periods, i.e., the lack of their link with the leakage of photospheric and chromospheric oscillations into the corona and the likely role of the broadband energy sources. Magnetohydrodynamic seismology based on the reported detection of the kink oscillation, with the assistance of the differential emission measure analysis and a background coronal model provides us with a comprehensive set of plasma and magnetic field diagnostics, which is of interest as input parameters of space weather models.

Effects of spark dielectric barrier discharge plasma on water sterilization and seed germination

This study investigated the effects of spark dielectric barrier discharge (SDBD) plasma on water sterilization and seed germination using an in-situ treatment method. The plasma diagnostics used I–V characteristics and optical emission spectroscopy. The electron, rotational, and vibrational temperature, as well as electron density were measured. The findings revealed that SDBD plasma treatment caused significant changes in the physicochemical properties of the liquids. In addition, the concentrations of H2O2, NO2− and NO3− were increased after 8 min of plasma treatment to 30, 40, and 100 mgL−1, respectively. The effectiveness of the SDBD plasma treatment was evaluated based on bacterial inactivation and seed germination. After 8 min of plasma treatment, an almost complete removal of E. coli and S. aureus was achieved. Furthermore, the germination of coriander seeds was enhanced when they were treated with SDBD plasma for 5 min. SDBD plasma shows promise for water sterilization, seed germination, and sustainability.

A neural network approach to kinetic Mie polarimetry for particle size diagnostics in nanodusty plasmas

The analysis of the size of nanoparticles is an essential task in plasma technology and dusty plasmas. Light scattering techniques, based on Mie theory, can be used as a non-invasive and in-situ diagnostic tool for this purpose. However, the standard back-calculation methods require expertise from the user. To address this, we introduce a neural network that performs the same task. We discuss how we set up and trained the network to analyze the size of plasma-grown amorphous carbon nanoparticles (a:C–H) with a refractive index n in the range of real(n) = and imag(n) = and a radius of up to several hundred nanometers, depending on the used wavelength. The diagnostic approach is kinetic, which means that the particles need to change in size due to growth or etching. An uncertainty analysis as well as a test with experimental data are presented. Our neural network achieves results that agree with those of prior fitting algorithms while offering higher methodical stability. The model also holds a major advantage in terms of computing speed and automation.

Electron impact fine-structure cross-section calculations of Cu and their applications in the diagnostics of laser-produced Cu-plasma through the collisional radiative model

We present the calculations of electron impact excitation cross-sections for the transitions from the ground state of Cu 3d104sS1/22 to the excited fine structure energy levels of the configurations 3d94s2,3d94s4p,3d104l,(l=p,d,f),3d105l,(l=s,p,d),3d10nl,(n=6,7;l=s,p) using the fully relativistic distorted wave theory. In addition, cross-sections for the transitions from the metastable states 3d94s2D5/2,3/22 to the upper excited levels are also presented. For calculating these cross-sections, we have obtained the atomic wave functions of copper using the multi-configuration Dirac–Fock approximation theory. To ensure the accuracy of these wave functions, we compared our calculated oscillator strengths of some dipole-allowed transitions of Cu with the available earlier reported values. We have utilized all the calculated electron impact fine-structure level excitation cross-sections from the ground as well as the metastable states of Cu to develop a fine structure-resolved collisional-radiative (CR) model for the diagnostics of laser-produced Cu plasma at atmospheric pressure. The model has been applied for diagnostics by coupling it with the spatially resolved optical emission spectroscopy (OES) measurements of Singh and Sharma [Phys. Plasmas 23 122104 (2016)] in the presence and absence of a static magnetic field. For diagnostics, the intensities of three OES observed strong atomic emission lines of Cu viz, 510.5, 515.3, and 521.8 nm have been utilized in the absence and presence of a magnetic field at different axial lengths (z = 0.5–6.5 mm) from the Cu target. Also, these intensities are corrected using the internal reference self-absorption method to overcome the effect of radiation trapping. The normalized intensities of the considered lines obtained from the CR model have been compared with their corresponding OES-measured values to extract the plasma parameters, i.e., electron temperature and electron density. Further, the obtained electron temperatures from the CR model are compared with the corresponding values reported in the experimental paper of Singh and Sharma [Phys. Plasmas 23 122104 (2016)] obtained using the Boltzmann plot method.