Plasma Sources Science and Technology

Mode transition phenomenon in an external discharge Hall thruster (XHT) has been reported under different anode voltage and magnetic flux density. Experimental data reveal a transition point indicating an optimal magnetic field for the thruster. Mode transition with varying anode voltages is more pronounced, showing performance trends of rise, saturation, and decline, with transition thresholds at 180 V and 220 V. Moreover, benefiting from the channel-less discharge of the XHT, the emission spectral image of the entire discharge region was acquired for the first time. The thickness of the luminous region from 140 to 180 V changed from 4 mm to 2 mm (50.0% decrease), and from 180 to 240 V changed from 2.0 mm to 1.8 mm (10.0% decrease). The finding provides new insights and a valuable dataset for the investigation of related thrusters and the understanding of discharge mechanisms. It showed that a strong correlation between the mode transition process and the spatial variation of the ionization region. A combined dynamical and statistical model was developed to predict the steady ionization region boundaries, focusing on the dynamical behavior of electrons and the magnetized electrons region in E × B field. The ionization region formation requires magnetized electrons, gas concentration, and sufficient electron energy, making the varying definitions of its boundary the primary cause of mode transition. Furthermore, a mode locus plot method was developed to exactly reveal the plume structure and parameters change, which was verified by the experimental observation. It is worth noting that although the model is based on the of an XHT, the analytical methods therein can be extended to explain the widespread mode transition phenomenon in conventional Hall thrusters, as well as to E × B field discharges at vacuum or low atmospheric pressure.

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 (I_D) 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 I_D, 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 those observed for the periodic mode, which suggests potentially better performances for the quasi-continuous mode. Finally, several discharge parameters and plasma parameters are estimated for the normal glow discharge mode (I_D = 50 mA and Φ = 10 slm). In this mode, the evaluated average electric field in the plasma column and discharge power (P_D) are 4.5 kV m⁻¹ and 39.9 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 measure and often misunderstood as near room temperature. In this work, the axial distributions of center (T_c) and wall (T_w) temperatures in bare, water-cooled, heat-insulated and furnace-heated DBD reactors are measured. It is found that T_c is significantly higher than T_w and has an asymmetric bow-like profile of axial distribution, which indicates axial and radial nonuniformity of gas temperature in DBD plasma. Based upon the radial distribution of the gas temperature in the DBD plasma by simulation, a radial-averaged gas temperature (T_g^RA) is obtained, which confirms that the T_c is a reliable representative of gas temperature in DBD plasma. Finally, the effects of flow rate, discharge power, specific energy input, alternating-current frequency and pulse modulation on the gas temperature are investigated.

This study examines the emergence of an ionization-attachment plasma instability in a geometrically symmetric capacitively coupled plasma 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 O$_2^+$ 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 $f_\textrm{in}$ = 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 = 600 V. 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.

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 $\textit{n}_\mathrm{gnd}$ of the ground state $\mathrm{NO}(X^{2}\Pi)$ using laser-induced fluorescence, and the emission of the excited state $\mathrm{NO}(A^{2}\Sigma^{+})$ using optical emission spectroscopy. The quenching rates of $\mathrm{NO}(A^{2}\Sigma^{+})$ are determined and the rate constants by air and Ar are evaluated as $k_{q, \mathrm{air}} =\,4.7 (\pm 0.2)$$\times 10^{-17}\, \mathrm{m^{3}\,s^{-1}}$ and $k_{q, \mathrm{Ar}}\,=\,1.4\times\,10^{-19}\, \mathrm{m^{3}\,s^{-1}}$. 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 $\textit{n}_\mathrm{gnd}$ remains in the afterglow time. $\textit{n}_\mathrm{gnd}$ is in the order of magnitude of 10¹⁹ m⁻³ in our plasma. Spatially resolved $\textit{n}_\mathrm{gnd}$ 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 1% $\mathrm{N_{2}}$ is optimal for a higher $\textit{n}_\mathrm{gnd}$ and air continuously contributes to the $\mathrm{NO}(X^{2}\Pi)$ 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 $\mathrm{N_{2}}(A)$.

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 exhaust of a small ablative pulsed plasma thruster (PPT), fed with polytetrafluoroethylene and operated at 1000 V of
discharge voltage and 6 μF of capacitance, is characterized by means of a novel diagnostic system. The technique time-recon-
structs its plume cross-sectional expansion, and consists of an array of electrostatic wire probes biased at the ion saturation
regime. The two-dimensional and time-dependent ion current distribution is reconstructed from the probe data using a variable
separation algorithm. The new method is valid for plumes of both unsteady and steady-operation electrical plasma thrusters.
The PPT plume contains at least three distinct ion groups with different mean velocities, with the second one carrying the
major part of the ion current. Spatially, the plume exhibits a single-peaked profile in the direction perpendicular to the
PPT electrodes, while in the direction parallel to them it features two peaks and a greater divergence angle. A small spatial
asymmetry involving a deviation of the current towards the cathode and to one of the sides of the channel is also present.

Laser-driven tin droplets represent the primary technique for generating extreme ultraviolet (EUV) radiation in lithography light sources. The implementation of a vertical external magnetic field is a critical strategy for reducing the ion debris produced by the tin plasma. Nonetheless, the detailed mechanisms through which the external magnetic field affects ion debris dynamics have not yet been fully elucidated. In our study, numerical simulations were conducted using the radiation hydrodynamics code FLASH to investigate the magnetohydrodynamic properties of tin plasma under an external magnetic field with a strength of 1 T. Our results reveal, for the first time, the intermediate-timescale (tens to hundreds of nanoseconds) evolution of tin plasma under an external magnetic field, providing new insights into the mechanisms of ion debris mitigation in EUV lithography systems. The magnetic field guides the plasma from the front and rear surfaces of the target to flow to both sides, forming a vortex street phenomenon, accompanied by backflow opposing the vortex street flow. Beyond the vortex region, the plasma transitions to laminar flow, where hydrodynamic instabilities between layers of varying velocities are significantly suppressed. These phenomena are recognized as significant factors to the effective trapping of tin plasma by an externally applied magnetic field in a vertical configuration.

The barrier discharges in CO2 are investigated using optical emission spectroscopy and electrical measurements, with support of spatially one-dimensional fluid modelling. The study is focused on a sinusoidal driven atmospheric pressure Townsend discharge (APTD) and a single- and multi-filament barrier discharge at atmospheric pressure. The spectra of weak CO2 barrier discharge emission in UV- VIS-NIR range are recorded. The optical emission of selected vibrational spectral bands is analysed and the E/N in barrier discharges is determined using developed intensity ratio method and kinetic data available in the literature. The technique of time-correlated single photon counting is used to obtain necessary sub-nanosecond temporal resolution and high signal sensitivity for the single-filament experiment. After necessary calibration, the E/N of 330Td is measured in the barrier discharge streamer head. It is coherent with results of numerical modelling. The E/N determined in APTD has a value of 110Td which agrees within 10% with the results of electrical equivalent circuit model. The uncertainties coming from kinetic data and the low sensitivity of some rate coefficient ratios complicate easy and broad utilization of the method. Future steps are therefore proposed, including uncertainty quantification and sensitivity analysis of the simplified model and necessary synthetic spectra fitting.

This study examines the emergence of an ionization-attachment plasma instability in a geometrically symmetric capacitively coupled plasma 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 O$_2^+$ 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 $f_\textrm{in}$ = 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 = 600 V. 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.

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 $\textit{n}_\mathrm{gnd}$ of the ground state $\mathrm{NO}(X^{2}\Pi)$ using laser-induced fluorescence, and the emission of the excited state $\mathrm{NO}(A^{2}\Sigma^{+})$ using optical emission spectroscopy. The quenching rates of $\mathrm{NO}(A^{2}\Sigma^{+})$ are determined and the rate constants by air and Ar are evaluated as $k_{q, \mathrm{air}} =\,4.7 (\pm 0.2)$$\times 10^{-17}\, \mathrm{m^{3}\,s^{-1}}$ and $k_{q, \mathrm{Ar}}\,=\,1.4\times\,10^{-19}\, \mathrm{m^{3}\,s^{-1}}$. 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 $\textit{n}_\mathrm{gnd}$ remains in the afterglow time. $\textit{n}_\mathrm{gnd}$ is in the order of magnitude of 10¹⁹ m⁻³ in our plasma. Spatially resolved $\textit{n}_\mathrm{gnd}$ 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 1% $\mathrm{N_{2}}$ is optimal for a higher $\textit{n}_\mathrm{gnd}$ and air continuously contributes to the $\mathrm{NO}(X^{2}\Pi)$ 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 $\mathrm{N_{2}}(A)$.

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 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 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.

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, the absolute density of the nitric oxide electronic ground state by laser-induced fluorescence, as well as electron density and electron temperature by a multipole resonance probe. 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.

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. Two main features are identified: a very low-frequency (${\simeq}4$ kHz), global oscillation mode, attributed to an ionization instability in the source; and a broadband, low-coherence, essentially-azimuthal, ion-acoustic-like spectrum at ${\lt}200$ kHz. Within this range, a mild peak of larger coherence is found at ${\simeq}50$ kHz, followed by a weak secondary peak at ${\simeq}100$ kHz at some locations. A bi-coherence analysis discards any nonlinear interaction within the ${\lt}1$ MHz band. However, significant coupling is found between the broadband fluctuations and the Helicon driving frequency of 13.56 MHz, which suggests that the former are likely the result of a parametric decay instability.

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.

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.