Plasma Sources Science and Technology

Magnetron sputtering deposition has become the most widely used technique for deposition of both metallic and compound thin films and is utilized in numerous industrial applications. There has been a continuous development of the magnetron sputtering technology to improve target utilization, increase ionization of the sputtered species, increase deposition rates, and to minimize electrical instabilities such as arcs, as well as to reduce operating cost. The development from the direct current (dc) diode sputter tool to the magnetron sputtering discharge is discussed as well as the various magnetron sputtering discharge configurations. The magnetron sputtering discharge is either operated as a dc or radio frequency discharge, or it is driven by some other periodic waveforms depending on the application. This includes reactive magnetron sputtering which exhibits hysteresis and is often operated with an asymmetric bipolar mid-frequency pulsed waveform. Due to target poisoning the reactive sputter process is inherently unstable and exhibits a strongly non-linear response to variations in operating parameters. Ionized physical vapor deposition was initially achieved by adding a secondary discharge between the cathode target and the substrate and later by applying high power pulses to the cathode target. An overview is given of the operating parameters, the discharge properties and the plasma parameters including particle densities, discharge current composition, electron and ion energy distributions, deposition rate, and ionized flux fraction. The discharge maintenance is discussed including the electron heating processes, the creation and role of secondary electrons and Ohmic heating, and the sputter processes. Furthermore, the role and appearance of instabilities in the discharge operation is discussed.

In this review we describe a transient type of gas discharge which is commonly called a streamer discharge, as well as a few related phenomena in pulsed discharges. Streamers are propagating ionization fronts with self-organized field enhancement at their tips that can appear in atmospheric air, or more generally in gases over distances larger than order 1 cm times N₀/N, where N is gas density and N₀ is gas density under ambient conditions. Streamers are the precursors of other discharges like sparks and lightning, but they also occur in for example corona reactors or plasma jets which are used for a variety of plasma chemical purposes. When enough space is available, streamers can also form at much lower pressures, like in the case of sprite discharges high up in the atmosphere. We explain the structure and basic underlying physics of streamer discharges, and how they scale with gas density. We discuss the chemistry and applications of streamers, and describe their two main stages in detail: inception and propagation. We also look at some other topics, like interaction with flow and heat, related pulsed discharges, and electron runaway and high energy radiation. Finally, we discuss streamer simulations and diagnostics in quite some detail. This review is written with two purposes in mind: first, we describe recent results on the physics of streamer discharges, with a focus on the work performed in our groups. We also describe recent developments in diagnostics and simulations of streamers. Second, we provide background information on the above-mentioned aspects of streamers. This review can therefore be used as a tutorial by researchers starting to work in the field of streamer physics.

Since decades, the PECVD ('plasma enhanced chemical vapor deposition') processes have emerged as one of the most convenient and versatile approaches to synthesize either organic or inorganic thin films on many types of substrates, including complex shapes. As a consequence, PECVD is today utilized in many fields of application ranging from microelectronic circuit fabrication to optics/photonics, biotechnology, energy, smart textiles, and many others. Nevertheless, owing to the complexity of the process including numerous gas phase and surface reactions, the fabrication of tailor-made materials for a given application is still a major challenge in the field making it obvious that mastery of the technique can only be achieved through the fundamental understanding of the chemical and physical phenomena involved in the film formation. In this context, the aim of this foundation paper is to share with the readers our perception and understanding of the basic principles behind the formation of PECVD layers considering the co-existence of different reaction pathways that can be tailored by controlling the energy dissipated in the gas phase and/or at the growing surface. We demonstrate that the key parameters controlling the functional properties of the PECVD films are similar whether they are inorganic- or organic-like (plasma polymers) in nature, thus supporting a unified description of the PECVD process. Several concrete examples of the gas phase processes and the film behavior illustrate our vision. To complete the document, we also discuss the present and future trends in the development of the PECVD processes and provide examples of important industrial applications using this powerful and versatile technology.

Processes at the plasma boundaries, including the electrodes, can significantly influence plasma properties, among them the plasma density, the flux-energy distribution of various particle species, etc. The emission of secondary electrons, in particular, can lead to ionization avalanches, which strongly increase the plasma density and change the discharge operation mode as a function of the operating conditions. Using reliable values to characterize the efficiency of such processes is indispensable for accurate numerical modeling. There is, however, a lack of such data for surface coefficients for arbitrary combinations of the plasma species and electrode materials and surface conditions. In this work, we investigate the α- to γ-mode mode transition induced by changes of the operating conditions (voltage, pressure) in capacitively coupled argon plasmas for different electrode surface materials (copper, nickel, gold, aluminum, and stainless steel) and target the determination of the effective in-situ secondary electron emission coefficient, $\gamma^\ast$. The first is accomplished by phase-resolved optical emission spectroscopy applied to measure the spatio-temporal distribution of the electron-impact excitation rate from the ground state into a high-threshold-energy level of a tracer gas (neon). The studies are conducted for pressures between 50 Pa and 200 Pa and voltage amplitudes ranging from 150 V to 350 V at a driving frequency of 13.56 MHz. A transition from the α- to the γ-mode is shown to take place at different pressures for different materials. The combination of these measurements with particle-in-cell / Monte Carlo collisions simulations employing a range of $\gamma^\ast$ values allows the determination of the effective 'in-situ' electron yield for the given set of operating conditions. The simulations also shed light on the contributions of the various species, argon ions, metastable atoms, and vacuum-ultraviolet photons to electron emission from the electrodes. The findings suggest that for precise modeling individual secondary electron yields specific to different electrode surface materials should be used and multiple species should be included in the models that describe secondary electron emission at the electrodes.

This work proposes an updated set of electron-impact cross sections (CSs) for carbon dioxide (CO₂) by quantitatively identifying CO₂ dissociation within the two electronic excitation channels proposed by Phelps. In particular, the CS with energy threshold at 7 eV is considered with a 15% dissociation branching ratio and is associated with dissociation into O(¹D) + CO(X), while the one with threshold at 10.5 eV is used entirely for dissociation into O(³P) + CO(a³Π_r). Experimental data on CO₂ dissociation rate coefficients at moderate reduced electric field (E/N), CO₂ conversion efficiencies at high E/N, and electron transport coefficients for E/N∈[10⁻², 10³] Td are used to validate the updated set and demonstrate its completeness and consistency over a wide range of E/N. Notably, the updated CS set enables the full coupling between the electron and chemical kinetics, a feature lacking in most existing CS sets. The updated set is applied to study electron kinetics in CO₂–Ar and CO₂–N₂ mixtures, revealing significant modifications in the electron energy distribution function and CO₂ dissociation rate coefficient due to mixture composition. The updated CS set is made available at the IST-Lisbon database within LXCat.

High-power impulse magnetron sputtering (HiPIMS) delivers a high target power in short pulses, enhancing the ionization and energy of sputtered atoms and thus providing more possibilities to control the film properties. This study explores the effect of various pulse configurations (unipolar HiPIMS, bipolar HiPIMS, chopped unipolar, and chopped bipolar HiPIMS) to increase energy flux to an insulated surface (e.g. substrate or growing film). The chopped bipolar HiPIMS configuration, featuring several short positive pulses replacing a single long positive pulse, is introduced, and the total energy fluxes are subsequently measured using a passive thermal probe. Moreover, the effect of the probe's capacitance with respect to the ground is systematically investigated by connecting an external capacitor. Results show that for an insulated surface with low capacitance, bipolar pulse configurations do not significantly increase energy flux to the surface due to its rapid charging by plasma ions. Conversely, high surface capacitance facilitates an increase in energy flux, as a large potential difference between the plasma and the surface remains even for a long positive pulse. For medium surface capacitance (tens of nF), chopping the positive pulse in bipolar HiPIMS effectively increases the energy delivered to the film by discharging the surface in the off-times. The thermal probe measurements also confirm that energy to the film can be increased for unipolar HiPIMS configurations by splitting the negative pulse into several shorter pulses.

Laser-produced transient tin plasmas are the sources of extreme ultraviolet (EUV) light at 13.5 nm wavelength for next-generation nanolithography, enabling the continued miniaturization of the features on chips. Generating the required EUV light at sufficient power, reliability, and stability presents a formidable multi-faceted task, combining industrial innovations with attractive scientific questions. This topical review presents a contemporary overview of the status of the field, discussing the key processes that govern the dynamics in each step in the process of generating EUV light. Relevant physical processes span over a challenging six orders of magnitude in time scale, ranging from the (sub-)ps and ns time scales of laser-driven atomic plasma processes to the several μs required for the fluid dynamic tin target deformation that is set in motion by them.

Nanoparticles formed in nonthermal plasmas (NTPs) can have unique properties and applications. However, modeling their growth in these environments presents significant challenges due to the non-equilibrium nature of NTPs, making them computationally expensive to describe. In this work, we address the challenges associated with accelerating the estimation of parameters needed for these models. Specifically, we explore how different machine learning models can be tailored to improve prediction outcomes. We apply these methods to reactive classical molecular dynamics data, which capture the processes associated with colliding silane fragments in NTPs. These reactions exemplify processes where qualitative trends are clear, but their quantification is challenging, hard to generalize, and requires time-consuming simulations. Our results demonstrate that good prediction performance can be achieved when appropriate loss functions are implemented and correct invariances are imposed. While the diversity of molecules used in the training set is critical for accurate prediction, our findings indicate that only a fraction (15%–25%) of the energy and temperature sampling is required to achieve high levels of accuracy. This suggests a substantial reduction in computational effort is possible for similar systems.

Laser-induced fluorescence is a widely used non-invasive method for characterizing NO_x emission, mostly in combustion applications, but also in many plasma facilities. Under the carbon-free prerequisite, non-thermal plasma-assisted combustion is a promising technology to address the low flammability issues of ammonia (NH₃) flames, but nitric oxide (NO) emission remains unknown. NO quantification in such plasma-flame environments is a daunting task due to largely unknown fluorescence quenching, which urgently drives this study. In this work, we map the NO fluorescence lifetime (τ) in an NH₃/air flame sustained in a nanosecond pulsed discharge (NPD) at various time delays. Firstly, in both burnt and unburnt zones, τ increases slightly in the first 2 μs after the discharge, and then almost remains constant. Secondly, the impact of NPD on τ differs between the burnt and unburnt zones. In the burnt zone, τ of NO exhibits a modest increase (less than 10%) compared to that without NPD pulses, which can be theoretically explained by the temperature rise (i.e. decreased number density) due to the NPD pulse. Besides, a shock front originates from the anode in the burnt zone and exhibits a dip in τ. This further supports the decisive role of number density in quenching of laser-excited NO(A). However, in the unburnt zone, where plasma-induced NO is primarily generated, within the measured 1–100 μs delay after the discharge, τ is unexpectedly long, twice that of the theoretical calculation. It might be attributed to the plasma-induced NH₃ decomposition and other excited radicals at low temperatures. These experimental findings clarify, for the first time, the impact of non-thermal NPD on NO(A) quenching, providing a foundation for quantitative analysis of NO in plasma applications.

Physical vapor deposition (PVD) refers to the removal of atoms from a solid or a liquid by physical means, followed by deposition of those atoms on a nearby surface to form a thin film or coating. Various approaches and techniques are applied to release the atoms including thermal evaporation, electron beam evaporation, ion-driven sputtering, laser ablation, and cathodic arc-based emission. Some of the approaches are based on a plasma discharge, while in other cases the atoms composing the vapor are ionized either due to the release of the film-forming species or they are ionized intentionally afterward. Here, a brief overview of the various PVD techniques is given, while the emphasis is on sputtering, which is dominated by magnetron sputtering, the most widely used technique for deposition of both metallic and compound thin films. The advantages and drawbacks of the various techniques are discussed and compared.

Intermediate pressure (0.2 to 6 Torr) capacitive discharges with feedstock mixtures containing N₂ are commonly used for thin film processing. Using previous one-dimensional (1D) particle-in-cell (PIC) simulations of an intermediate pressure capacitive N₂ discharge, (Kawamura et al 2021 Plasma Sources Sci. Technol.30 035001) we develop a 2D electromagnetic (EM) fluid model including secondary emission to study the effect of the α to γ transition on plasma uniformity in a large area 1.6 Torr N₂ reactor with radius R = 1.8 m and gap size l = 2.5 cm. In the α-mode, the hot γ electrons due to ion-induced secondary emission from the electrodes play a minimal role in the discharge while, in the γ-mode, they are essential for its maintenance. The 1D PIC simulations of a 2.5 cm gap, 1.6 Torr N₂ capacitive discharge showed an α to γ transition, characterized by a rise in density and a collapse of the sheath widths when the rf sheath voltage amplitude V₁ exceeded a breakdown voltage $V_\textrm{B}$. Smaller sheath widths enhance EM effects which may negatively affect plasma uniformity and stability. In the 2D EM fluid simulations at lower powers, the discharge is fully in the α-mode with $V_1 \lt V_\textrm{B}$ for $0\unicode{x2A7D} r\unicode{x2A7D} R$. As power is raised, the enhanced finite wavelength effect causes $V_1 \gt V_\textrm{B}$ (γ-mode) toward the radial center, and $V_1 \lt V_\textrm{B}$ (α-mode) toward the radial edge with the transition point $r = r_\textrm{B}$ approaching R. A fully γ-mode discharge where $V_1 \gt V_\textrm{B}$ for all r is obtainable at 6.78 MHz but not at 13.56 MHz due to the enhanced EM effect at the higher frequency.

The populations of the 2s$^2{\mathrm{S}}_{1/2}$, 2p$^2{\mathrm{P}}_{1/2}$, and 2p$^2{\mathrm{P}}_{3/2}$ states of atomic hydrogen were examined in a low-density inductively coupled hydrogen plasma using diode laser absorption spectroscopy at the Balmer-α line. The Doppler broadened absorption spectra were not fitted with the theoretical spectra under the assumption that the population distribution of the three states followed the statistical weights. The fitting accuracy improved when the population ratio was allowed to deviate from the statistical weights; however, the improvement remained insufficient, especially for spectra observed at low electron densities and low hydrogen pressures. A reasonable fit was achieved only when a high-temperature component was introduced into the velocity distribution function of the 2s$^2{\mathrm{S}}_{1/2}$ state. It was considered that the high-temperature component was produced via the electron-impact dissociative excitation of molecular hydrogen, because the population of the high-temperature component was proportional to the electron density. The experimental results clearly indicated a decrease in the population of high-temperature 2s$^2{\mathrm{S}}_{1/2}$ with increasing electron density and hydrogen pressure. This trend is qualitatively reasonable considering the collisional transfer from the 2s$^2{\mathrm{S}}_{1/2}$ to the 2p$^2{\mathrm{P}}_{1/2}$ and 2p$^2{\mathrm{P}}_{3/2}$ states. However, the cross-sections of the collisional transfer were insufficiently small to explain the experimental observations quantitatively. In addition, the experimental results suggest lower metastability of low-temperature 2s$^2{\mathrm{S}}_{1/2}$ than that of the high-temperature component, which cannot be explained by collision cross-sections. Thus, a more sophisticated model should be investigated in future studies to obtain a deeper understanding of the kinetics governing the 2s$^2{\mathrm{S}}_{1/2}$, 2p$^2{\mathrm{P}}_{1/2}$, and 2p$^2{\mathrm{P}}_{3/2}$ states.

Self organized striation structures have been observed in electronegative capacitive discharges under certain operating conditions, which include high electronegativity and an ion plasma frequency comparable to the driving frequency. In this study, striations in capacitive chlorine discharges were explored using one-dimensional particle-in-cell/Monte Carlo collisional simulations with a 2.54 cm gap driven by a sinusoidal rf voltage of 13.56 MHz. The properties of the discharges are explored focusing on the striations, as the gas pressure, driving voltage amplitude, and secondary electron emission processes are varied. The most realistic secondary electron emission model includes contribution from ions, electrons, and neutrals bombarding the electrodes. The striations start to appear at pressure around 15 Pa and increase in amplitude with increased pressure. We find that the amplitude and the number of striations increase with the addition of secondary electron emission processes to the discharge model. Furthermore, the most realistic model for secondary electron emission is used to explore the striation structures as driving voltage amplitude, driving frequency, and gas pressure is varied. As the pressure is increased, the striation amplitude increases but the number of striations remains unchanged. Higher driving voltage and higher driving frequency increase the ion critical density, resulting in the formation of striation patterns, even when the pressure is low. Increasing the driving frequency further leads to a denser arrangement of striations, with tighter striation gaps, while higher voltage results in a smaller bulk width.

A novel method of uniformity control in low pressure capacitively coupled radio-frequency plasmas by an individually driven sidewall electrode is proposed and verified by two-dimensional kinetic simulations. At the sidewall electrode, energetic electron beams are generated by sheath expansion heating and propagate radially towards the reactor center. Upon reaching the edge of the planar powered electrode, where the wafer is located in plasma processing applications, they interact with the time-modulated RF sheath at this electrode. Depending on the amplitudes of and the phase shift between the voltage waveforms applied to the sidewall and the wafer electrode at the same frequency, they reach the wafer electrode edge at different times within one period of the local sheath oscillation. If the radially propagating beam electrons arrive at the wafer electrode edge, when the local sheath is thick, they will be accelerated upwards and cause significant local ionization peaks at the wafer electrode edge. This effect is demonstrated to improve the radial plasma uniformity at low pressures drastically, if a center high plasma density profile is present in the absence of an RF driven sidewall electrode. If the beam electrons arrive at the wafer electrode edge, when the local sheath is collapsed, they will continue propagating radially towards the center and the effect on the uniformity will be reduced. Thus, the voltage amplitude applied to the sidewall electrode and its phase relative to the RF voltage applied to the planar powered electrode are found to be effective parameters to electrically control the radial plasma uniformity above the wafer.

In order to assess the importance of collisional processes in the neutralization of a gridded ion thruster plume this study presents a series of simulations carried out with a planar, full-PIC code provided of a library of electron, ion and neutral collisions. In particular, we find that the inclusion of electron inelastic collisions, such as ionization and excitation, results fundamental in the generation of a population of electrons trapped in the plume's potential well, which play a major role in the neutralization of the plume and in the formation of the electric potential map. A further investigation is therefore carried out on the properties of the electron populations present in the plume, suggesting that the trapped population results mostly insensitive to the emission properties of the cathode, but displays a strong dependence on the inclusion of inelastic collisions and a slow approach to stationary conditions dictated by the timescales of these collisions. Given the symmetry of the plume bulk even in presence of an externally mounted cathode, we further extend the study to an axisymmetric simulation case with an annular cathode, that allows the evaluation of a three-dimensional expansion process. The build up of the trapped electron population in this case is even slower, because of the smaller neutral density observed when expanding the plume in three dimensions, and an acceleration strategy that speeds-up the approach to steady state without altering it is therefore proposed. Nevertheless, the main governing physical processes observed in the planar case remain prominent also in this latter case.

Theoretical study of the electron kinetics (i.e. the velocity distribution and the transport parameter) in gases is generally conducted using the electron Boltzmann equation. The year 2022 marked 150 years since the formulation of the Boltzmann equation. Even in the last several decades, the historical progress has been made synchronously with the development of innovative technologies in gaseous electronics and in combination with the appearance of computers with sufficient speed and memory. Electron kinetic theory based on the Boltzmann equation has mostly been developed as the swarm physics in the hydrodynamic regime in the dc and radio frequency electric fields. In particular, the temporal characteristics are understood in terms of the collisional relaxation times between electron and gas molecule. There are two main theoretical approaches based on the Boltzmann equation for finding the velocity distribution. One is the traditional description of the electron kinetics, starting from the Boltzmann statistics in velocity space under a uniform density or a small density gradient of electrons. The other most recent approach is based on the phase-space tracking of the velocity distribution where the electron transport parameter is given by the moment of the electron density distribution in position space. In the present paper, we will explore the historical development of the electron Boltzmann equation with respect to three key items: collision term, solution method, and intrinsic electron transport in a hydrodynamic regime involved as the key elements in the low-temperature collisional plasma. The important topics listed in a table are briefly noted and discussed.

Over the past decade, extensive modeling practices on low-temperature plasmas have revealed that input data such as microscopic scattering cross-sections are crucial to output macroscopic phenomena. In Monte Carlo collision (MCC) modeling of natural and laboratory plasma, the angular scattering model is a non-trivial topic. Conforming to the pedagogical purpose of this overview, the classical and quantum theories of binary scattering, such as the commonly used Born–Bethe approximation, are first introduced. Adequate angular scattering models, which MCC simulation can handle as input, are derived based on the above theories for electron–neutral, ion–neutral, neutral–neutral, and Coulomb collisions. This tutorial does not aim to provide accurate cross-sectional data by modern approaches in quantum theory, but rather to introduce analytical angular scattering models from classical, semi-empirical, and first-order perturbation theory. The reviewed models are expected to be readily incorporated into the MCC codes, in which the scattering angle is randomly sampled through analytical inversion instead of the numerical accept–reject method. These simplified approaches are very attractive, and demonstrate in many cases the ability to achieve a striking agreement with experiments. Energy partition models on electron–neutral ionization are also discussed with insight from the binary-encounter Bethe theory. This overview is written in a tutorial style in order to serve as a guide for novices in this field, and at the same time as a comprehensive reference for practitioners of MCC modeling on plasma.

As long-distance space travel requires propulsion systems with greater operational flexibility and lifetimes, there is a growing interest in electrodeless plasma thrusters that offer the opportunity for improved scalability, larger throttleability, running on different propellants and limited device erosion. The majority of electrodeless designs rely on a magnetic nozzle (MN) for the acceleration of the plasma, which has the advantage of utilizing the expanding electrons to neutralize the ion beam without the additional installation of a cathode. The plasma expansion in the MN is nearly collisionless, and a fluid description of electrons requires a non-trivial closure relation. Kinetic electron effects and in particular electron cooling play a crucial role in various physical phenomena, such as energy balance, ion acceleration, and particle detachment. Based on experimental and theoretical studies conducted in recognition of this importance, the fundamental physics of the electron-cooling mechanism revealed in MNs and magnetically expanding plasmas is reviewed. In particular, recent approaches from the kinetic point of view are discussed, and our perspective on the future challenges of electron cooling and the relevant physical subject of MN is presented.

The field of low-temperature plasmas (LTPs) excels by virtue of its broad intellectual diversity, interdisciplinarity and range of applications. This great diversity also challenges researchers in communicating the outcomes of their investigations, as common practices and expectations for reporting vary widely in the many disciplines that either fall under the LTP umbrella or interact closely with LTP topics. These challenges encompass comparing measurements made in different laboratories, exchanging and sharing computer models, enabling reproducibility in experiments and computations using traceable and transparent methods and data, establishing metrics for reliability, and in translating fundamental findings to practice. In this paper, we address these challenges from the perspective of LTP standards for measurements, diagnostics, computations, reporting and plasma sources. This discussion on standards, or recommended best practices, and in some cases suggestions for standards or best practices, has the goal of improving communication, reproducibility and transparency within the LTP field and fields allied with LTPs. This discussion also acknowledges that standards and best practices, either recommended or at some point enforced, are ultimately a matter of judgment. These standards and recommended practices should not limit innovation nor prevent research breakthroughs from having real-time impact. Ultimately, the goal of our research community is to advance the entire LTP field and the many applications it touches through a shared set of expectations.

The enduring contributions of low temperature plasmas to both technology and science are largely a result of the atomic, molecular, and electromagnetic (EM) products they generate efficiently such as electrons, ions, excited species, and photons. Among these, the production of light has arguably had the greatest commercial impact for more than a century, and plasma sources emitting photons over the portion of the EM spectrum extending from the microwave to soft x-ray regions are currently the workhorses of general lighting (outdoor and indoor), photolithography for micro- and nano-fabrication of electronic devices, disinfection, frequency standards (atomic clocks), lasers, and a host of other photonic applications. In several regions of the EM spectrum, plasma sources have no peer, and this article is devoted to an overview of the physics of several selected plasma light sources, with emphasis on thermal arc and fluorescent lamps and the more recently-developed microcavity plasma lamps in the visible and ultraviolet/vacuum ultraviolet regions. We also briefly review the physics of plasma-based metamaterials and plasma photonic crystals in which low temperature plasma tunes the EM properties of filters, resonators, mirrors, and other components in the microwave, mm, and sub-mm wavelength regions.

The characteristics of nitric oxide (NO) production in a kHz pulsed atmospheric pressure Ar plasma jet, operated in ambient air, are explored by measuring the absolute density of the ground state NO(X²Π) using laser-induced fluorescence (LIF), and the emission of the excited state NO(A²Σ⁺) using optical emission spectroscopy (OES). The quenching rates of the NO(A²Σ⁺) are determined and the rate constants by air and Ar are evaluated as k_q,air = 4.7(±0.2) × 10^-17 m³s^-1 and k_q,Ar = 1.4 × 10^-19 m³s^-1. The time-resolved NO measurement is carried out within one discharge cycle. It is found that NO generation shows fast rise accompanied by the propagation of the plasma ionization wave and a constant n_gnd remains in the afterglow time. n_gnd is in the order of magnitude of 10¹⁹ m^-3 in our plasma. Spatially resolved n_gnd is mapped for the plasma plume and the influences of voltage, flow rate, gas admixture and pulse frequency on NO production are discussed. NO generation is revealed to benefit from increased injected power and a flow rate that leads to an appropriate amount of mixing with ambient air (e.g., 4 slm in this work). The addition of N₂ is optimal for a higher n_gnd and air continuously contributes to the NO(X²Π) formation below 3%. The NO excitation proportion per voltage pulse is found to be almost the same. Through analysis, it is confirmed that NO is produced mostly from the reactions involving atomic N and metastable N₂(A).

A low-pressure double-inductively coupled plasma device is used to study the fundamental plasma parameters, plasma chemistry, and UV photon emission from the first excited state of nitric oxide, NO(A), in gas mixtures of nitrogen and oxygen. In addition to the gas mixture, rf power and gas pressure are varied, and the E-H mode transition of the inductively coupled plasma is studied specifically. The gas temperature and UV photon emission are measured by optical emission spectroscopy (OES), the absolute density of the nitric oxide electronic ground state by laser-induced fluorescence (LIF), as well as electron density and electron temperature by a multipole resonance probe (MRP). A simple collisional-radiative model for UV emission from NO(A) is developed, which takes the measured densities of ground state nitric oxide and electrons, as well as the electron temperature and neutral gas temperature, as input parameters. The results reveal the links between the absolute densities of ground state nitric oxide, the excitation of this species driven by electron impact and collisions with nitrogen metastables, quenching of the nitrogen metastables, and the resulting UV photon emission rate. The density of ground state nitric oxide is shown to increase with power, while the discharge remains in E-mode, and to decrease significantly with the transition into H-mode, when sufficient rf power is deposited in the discharge. Despite the lower densities of ground state nitric oxide in H-mode, the UV photon emission intensity increases continuously with higher rf powers and over the E-H transition. This effect is shown to be caused by increased excitation of NO(A) by nitrogen metastables in H-mode, which is sufficient to overcompensate the decrease in ground state nitric oxide density.

In this work, we present a revised set of electron collision cross sections for 1,1,1,2 tetrafluoroethane (C2H2F4). The cross-section set is the result of a standard swarm analysis of measured drift velocities (W) and effective ionization coefficients ((α-η)/N under pulsed-Townsend conditions, where α is the ionization coefficient, and η is the attachment coefficient. Calculations and numerical analysis are performed for pure C2H2F4 and its mixtures with argon over a wide range of reduced electric fields E/N (where E is the electric field and N is the gas number density). A special attention was given to the region of low and moderate electron energies, where electron attachment and vibration excitation are dominant inelastic channels of scattering. The cross-section set we derive provides a good balance of energy, momentum, and the number of charged particles. Using a Monte Carlo simulation technique and the derived cross-section set as input, we calculate other transport coefficients and properties, such as characteristic energy, diffusion coefficients, and rate coefficients for individual collisional processes. We expect this cross-section set and electron swarm transport coefficients to be useful in modelling non-equilibrium plasmas as well as in studying resistive plate chambers in high-energy physics.

In this study, we investigate a constant-current mode DC powered argon gliding arc discharge (GAD) operating at atmospheric pressure. The evolution of the discharge is comprehensively illustrated using multi-resolution electrical signals. Moreover, the impact of discharge current (ID) and gas injection flowrate (Φ) on the discharge characteristics is explored through electrical signals, fast Fourier transform and fast camera images. We observed that, within one discharge period, the GAD initiates with a spark breakdown and rapidly transits to an abnormal glow discharge within approximately 1 µs. Subsequently, it evolves into a normal glow discharge over approximately 40 µs until the discharge extinguishes. Two distinct discharge operation modes are identified in the limits of our experimental window, periodic and quasi-continuous, as a function of ID, each exhibiting different responses to Φ. Analysis of current waveforms and fast camera images reveals that discharge extinction occurs due to the fast deionization process in the periodic discharge operation mode, while it results from the complex interplay between the discharge and gas flow in the quasi-continuous discharge operation mode. In the latter mode, the discharge power and the duty cycle are much higher than the one observed for the periodic mode, which suggests potentially better performances for the quasi-continous mode. Finaly, several discharge parameters and plasma parameters are estimated for the normal glow discharge mode (ID = 50 mA and Φ = 10 slm). In this mode, the evaluated average electric field in the plasma column and discharge power (PD) are 4.5 kV/m and 39.85 W while the spatially-averaged gas temperature, electron temperature and electron density are approximately 521±33 K, 1.35 eV (15 660 K), and 2.8-7.1 × 1014 cm-3, respectively.

Gas temperature is an essential parameter in terms of thermodynamics and kinetics of a plasma chemical reaction, but the gas temperature in dielectric barrier discharge (DBD) plasma is difficult to be measured and often misunderstood as near room temperature. In this work, the axial distributions of center (Tc) and wall (Tw) temperatures in a bare, water-cooled, heat-insulated and furnace-heated DBD reactors are measured. It is found that Tc is significantly higher than Tw and hasasymmetric bow-likeprofile of axial distribution, which indicates axial and radial nonuniformity of gas temperature in DBD plasma. Based upon radial distribution of the gas temperature in the DBD plasma by simulation, a radial-averaged gas temperature (TgRA) is obtained, which confirms that the Tc is a reliable representative of gas temperature in DBD plasma. Finally, the effects of flow rate, discharge power,specific energy input, alternating-current (AC) frequency and pulse modulation on the gas temperature are investigated.

The characteristics of nitric oxide (NO) production in a kHz pulsed atmospheric pressure Ar plasma jet, operated in ambient air, are explored by measuring the absolute density of the ground state NO(X²Π) using laser-induced fluorescence (LIF), and the emission of the excited state NO(A²Σ⁺) using optical emission spectroscopy (OES). The quenching rates of the NO(A²Σ⁺) are determined and the rate constants by air and Ar are evaluated as k_q,air = 4.7(±0.2) × 10^-17 m³s^-1 and k_q,Ar = 1.4 × 10^-19 m³s^-1. The time-resolved NO measurement is carried out within one discharge cycle. It is found that NO generation shows fast rise accompanied by the propagation of the plasma ionization wave and a constant n_gnd remains in the afterglow time. n_gnd is in the order of magnitude of 10¹⁹ m^-3 in our plasma. Spatially resolved n_gnd is mapped for the plasma plume and the influences of voltage, flow rate, gas admixture and pulse frequency on NO production are discussed. NO generation is revealed to benefit from increased injected power and a flow rate that leads to an appropriate amount of mixing with ambient air (e.g., 4 slm in this work). The addition of N₂ is optimal for a higher n_gnd and air continuously contributes to the NO(X²Π) formation below 3%. The NO excitation proportion per voltage pulse is found to be almost the same. Through analysis, it is confirmed that NO is produced mostly from the reactions involving atomic N and metastable N₂(A).

A low-pressure double-inductively coupled plasma device is used to study the fundamental plasma parameters, plasma chemistry, and UV photon emission from the first excited state of nitric oxide, NO(A), in gas mixtures of nitrogen and oxygen. In addition to the gas mixture, rf power and gas pressure are varied, and the E-H mode transition of the inductively coupled plasma is studied specifically. The gas temperature and UV photon emission are measured by optical emission spectroscopy (OES), the absolute density of the nitric oxide electronic ground state by laser-induced fluorescence (LIF), as well as electron density and electron temperature by a multipole resonance probe (MRP). A simple collisional-radiative model for UV emission from NO(A) is developed, which takes the measured densities of ground state nitric oxide and electrons, as well as the electron temperature and neutral gas temperature, as input parameters. The results reveal the links between the absolute densities of ground state nitric oxide, the excitation of this species driven by electron impact and collisions with nitrogen metastables, quenching of the nitrogen metastables, and the resulting UV photon emission rate. The density of ground state nitric oxide is shown to increase with power, while the discharge remains in E-mode, and to decrease significantly with the transition into H-mode, when sufficient rf power is deposited in the discharge. Despite the lower densities of ground state nitric oxide in H-mode, the UV photon emission intensity increases continuously with higher rf powers and over the E-H transition. This effect is shown to be caused by increased excitation of NO(A) by nitrogen metastables in H-mode, which is sufficient to overcompensate the decrease in ground state nitric oxide density.

This study examines the emergence of an ionization-attachment plasma instability in a geometrically symmetric capacitively coupled plasma (CCP) reactor with stainless steel electrodes operated in oxygen. Periodic fluctuations in optical emission intensity were observed under varying conditions of pressure and voltage. To interpret the experimental data, a kinetic (PIC/MCC) simulation approach was employed, with a particular focus on the influence of external parameters, including the voltage amplitude and the O₂⁺ ion induced secondary electron emission coefficient (SEEC, γ). By varying the γ coefficient at fixed pressure and voltage amplitude, the aim was to achieve the best possible agreement between the measured and computed oscillation frequencies. The experimentally observed oscillation frequency of f_in = 0.25 kHz at φ₀ = 655 V voltage amplitude (with 13.56 MHz RF excitation) was reproduced in the simulation with γ = 0.0042 at φ₀ = 600V. The results highlight the critical role of these parameters in the onset and characteristics of plasma instability and correspond to a computationally assisted diagnostic to determine ion induced secondary electron emission coefficients.

Self organized striation structures have been observed in electronegative capacitive discharges under certain operating conditions, which include high electronegativity and an ion plasma frequency comparable to the driving frequency. In this study, striations in capacitive chlorine discharges were explored using one-dimensional particle-in-cell/Monte Carlo collisional simulations with a 2.54 cm gap driven by a sinusoidal rf voltage of 13.56 MHz. The properties of the discharges are explored focusing on the striations, as the gas pressure, driving voltage amplitude, and secondary electron emission processes are varied. The most realistic secondary electron emission model includes contribution from ions, electrons, and neutrals bombarding the electrodes. The striations start to appear at pressure around 15 Pa and increase in amplitude with increased pressure. We find that the amplitude and the number of striations increase with the addition of secondary electron emission processes to the discharge model. Furthermore, the most realistic model for secondary electron emission is used to explore the striation structures as driving voltage amplitude, driving frequency, and gas pressure is varied. As the pressure is increased, the striation amplitude increases but the number of striations remains unchanged. Higher driving voltage and higher driving frequency increase the ion critical density, resulting in the formation of striation patterns, even when the pressure is low. Increasing the driving frequency further leads to a denser arrangement of striations, with tighter striation gaps, while higher voltage results in a smaller bulk width.

A novel method of uniformity control in low pressure capacitively coupled radio-frequency plasmas by an individually driven sidewall electrode is proposed and verified by two-dimensional kinetic simulations. At the sidewall electrode, energetic electron beams are generated by sheath expansion heating and propagate radially towards the reactor center. Upon reaching the edge of the planar powered electrode, where the wafer is located in plasma processing applications, they interact with the time-modulated RF sheath at this electrode. Depending on the amplitudes of and the phase shift between the voltage waveforms applied to the sidewall and the wafer electrode at the same frequency, they reach the wafer electrode edge at different times within one period of the local sheath oscillation. If the radially propagating beam electrons arrive at the wafer electrode edge, when the local sheath is thick, they will be accelerated upwards and cause significant local ionization peaks at the wafer electrode edge. This effect is demonstrated to improve the radial plasma uniformity at low pressures drastically, if a center high plasma density profile is present in the absence of an RF driven sidewall electrode. If the beam electrons arrive at the wafer electrode edge, when the local sheath is collapsed, they will continue propagating radially towards the center and the effect on the uniformity will be reduced. Thus, the voltage amplitude applied to the sidewall electrode and its phase relative to the RF voltage applied to the planar powered electrode are found to be effective parameters to electrically control the radial plasma uniformity above the wafer.

This study investigates electron dynamics in three distinct discharge modes of a cross-field atmospheric pressure plasma jet: the non-neutral, quasi-neutral, and constricted modes. Using a hybrid Particle-In-Cell/Monte Carlo Collisions (PIC/MCC) simulation, we systematically vary the applied voltage and driving frequency to explore these modes and their transitions. At low power, the discharge operates in a non-neutral mode, characterized by near-extinction behavior, analogous to the chaotic mode in other plasma devices. As power increases, the plasma transitions to a quasi-neutral mode, exhibiting the Ω- and Penning-mode heating mechanisms,
similar to the bullet mode in parallel-field jets. At high power, the discharge enters a constricted mode, where plasma density increases significantly, and the discharge contracts toward the electrodes along the entire channel. Experimental validation using phase-resolved optical emission spectroscopy (PROES) confirms the existence of the constricted mode as a distinct operational regime. These findings provide deeper insights into discharge mode transitions, contributing to the optimization of atmospheric pressure plasmas for various applications.

Oscillations within the plume of a Helicon Plasma Thruster are experimentally characterized. An array of electrically-floating probes is used to resolve the power spectral density, the coherence, and the wavevector components along both the azimuthal and parallel directions relative to the magnetic field, at various locations and for two mass flow rates. Various features are identified: a very low-frequency (≃ 4 kHz), global oscillation mode, attributed to an ionization instability in the source; and a broadband, low-coherence, essentially-azimuthal, ion-acoustic-like spectrum at < 200 kHz. Within this range, a mild peak of larger coherence is found at ≃ 50 kHz, followed by a weak secondary peak at ≃ 100 kHz at some locations. A bi-coherence analysis discards any nonlinear interactions within the < 1 MHz band. However, significant coupling is found between the broadband fluctuations and the driving frequency of 13.56 MHz, which suggests the former are likely the result of a parametric decay instability.

Nanoparticles formed in nonthermal plasmas (NTPs) can have unique properties and applications. However, modeling their growth in these environments presents significant challenges due to the non-equilibrium nature of NTPs, making them computationally expensive to describe. In this work, we address the challenges associated with accelerating the estimation of parameters needed for these models. Specifically, we explore how different machine learning models can be tailored to improve prediction outcomes. We apply these methods to reactive classical molecular dynamics data, which capture the processes associated with colliding silane fragments in NTPs. These reactions exemplify processes where qualitative trends are clear, but their quantification is challenging, hard to generalize, and requires time-consuming simulations. Our results demonstrate that good prediction performance can be achieved when appropriate loss functions are implemented and correct invariances are imposed. While the diversity of molecules used in the training set is critical for accurate prediction, our findings indicate that only a fraction (15%–25%) of the energy and temperature sampling is required to achieve high levels of accuracy. This suggests a substantial reduction in computational effort is possible for similar systems.

Laser-induced fluorescence is a widely used non-invasive method for characterizing NO_x emission, mostly in combustion applications, but also in many plasma facilities. Under the carbon-free prerequisite, non-thermal plasma-assisted combustion is a promising technology to address the low flammability issues of ammonia (NH₃) flames, but nitric oxide (NO) emission remains unknown. NO quantification in such plasma-flame environments is a daunting task due to largely unknown fluorescence quenching, which urgently drives this study. In this work, we map the NO fluorescence lifetime (τ) in an NH₃/air flame sustained in a nanosecond pulsed discharge (NPD) at various time delays. Firstly, in both burnt and unburnt zones, τ increases slightly in the first 2 μs after the discharge, and then almost remains constant. Secondly, the impact of NPD on τ differs between the burnt and unburnt zones. In the burnt zone, τ of NO exhibits a modest increase (less than 10%) compared to that without NPD pulses, which can be theoretically explained by the temperature rise (i.e. decreased number density) due to the NPD pulse. Besides, a shock front originates from the anode in the burnt zone and exhibits a dip in τ. This further supports the decisive role of number density in quenching of laser-excited NO(A). However, in the unburnt zone, where plasma-induced NO is primarily generated, within the measured 1–100 μs delay after the discharge, τ is unexpectedly long, twice that of the theoretical calculation. It might be attributed to the plasma-induced NH₃ decomposition and other excited radicals at low temperatures. These experimental findings clarify, for the first time, the impact of non-thermal NPD on NO(A) quenching, providing a foundation for quantitative analysis of NO in plasma applications.

High-power impulse magnetron sputtering (HiPIMS) delivers a high target power in short pulses, enhancing the ionization and energy of sputtered atoms and thus providing more possibilities to control the film properties. This study explores the effect of various pulse configurations (unipolar HiPIMS, bipolar HiPIMS, chopped unipolar, and chopped bipolar HiPIMS) to increase energy flux to an insulated surface (e.g. substrate or growing film). The chopped bipolar HiPIMS configuration, featuring several short positive pulses replacing a single long positive pulse, is introduced, and the total energy fluxes are subsequently measured using a passive thermal probe. Moreover, the effect of the probe's capacitance with respect to the ground is systematically investigated by connecting an external capacitor. Results show that for an insulated surface with low capacitance, bipolar pulse configurations do not significantly increase energy flux to the surface due to its rapid charging by plasma ions. Conversely, high surface capacitance facilitates an increase in energy flux, as a large potential difference between the plasma and the surface remains even for a long positive pulse. For medium surface capacitance (tens of nF), chopping the positive pulse in bipolar HiPIMS effectively increases the energy delivered to the film by discharging the surface in the off-times. The thermal probe measurements also confirm that energy to the film can be increased for unipolar HiPIMS configurations by splitting the negative pulse into several shorter pulses.