Plasma Sources Science and Technology

The effect of additives of Ar, Kr and Xe on the probability of О(³P) surface recombination, ${\gamma _o}$, on Pyrex in O₂ DC discharge has been studied in order to investigate the impact of vacuum ultraviolet (VUV) radiation on the atom loss. It is shown that within the measurement error, there is no effect of the sort of noble gas, as well as its amount on ${\gamma _o}$. The fluxes of VUV photons onto the tube surface under the considered conditions have been experimentally estimated. The phenomenological analysis of the results has revealed that the lifetime of reversible surface defects generated by VUV radiation should be small enough to have no effect on the kinetics of surface recombination of oxygen atoms. As estimated, this lifetime cannot exceed several seconds, which corresponds to the activation energy of the defect relaxation process of no more than ∼0.9 eV.

A method for measuring individual ion density and for measuring radial distribution is developed in an argon/helium mixture inductively coupled plasma using the floating harmonic method (FHM) and the electron energy probability function (EEPF) measurement. In order to measure the ion density, the ion saturation current is measured using the FHM, and the electron temperature and electron density are obtained from the EEPF measurement. In addition, plasma quasi-neutrality, ion acoustic wave dispersion relation, and ion saturation current formula are used to obtain the ion density. Ion density is compared for the case of using the common Bohm velocity and for the case of using the individual Bohm velocity. Under various conditions, increasing power results in higher density of both argon and helium ion, while argon ion density increase and helium ion density decreases with pressure. With increasing argon ratio and pressure, electron density increases, but electron temperature deceases. Consequently, fewer electrons are available to ionize He, leading to a reduction in helium ion density. The measured radial distributions of argon and helium ion at 5 mTorr reveal that the argon ion distribution exhibits a center-high distribution, whereas the helium ion distribution is relatively uniform.

Plasmas interacting with liquid surfaces produce a complex interfacial layer where the local chemistry in the liquid is driven by fluxes from the gas phase of electrons, ions, photons, and neutral radicals. Typically, the liquid surface has at best mild curvature with the fluxes of impinging plasma species and applied electric field being nominally normal to the surface. With liquids such as water having a high dielectric constant, structuring of the liquid surface by producing a wavy surface enables local electric field enhancement due to polarization of the liquid, as well as producing regions of higher and lower advective gas flow across the surface. This structuring (or waviness) can naturally occur or can be achieved by mechanical agitation such as with acoustic transducers. Electric field enhancement at the peaks of the waves of the liquid produces local increases in sources of reactive species and incident plasma fluxes which may be advantageous for plasma driven solution electrochemistry (PDSE) applications. In this paper, results are discussed from a computational investigation of pulsed atmospheric pressure plasma jets onto structured water solutions containing AgNO₃ as may be used in PDSE for silver nanoparticle (NP) formation. The solution surface consists of standing wave patterns having wavelength and wave depth of hundreds of microns to 1 mm. The potential for structured liquid surfaces to facilitate spatially differentiated chemical selectivity and enhance NP synthesis in the context of PDSE is discussed.

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.

Three-body attachment plays an important role in dry and humid air, in which O₂ and H₂O are considered as a third body. This work reports quantitative validation of Monte Carlo simulations against recent experiments (de Urquijo et al 2023 J. Phys. D: Appl. Phys.57 125205) for attachment processes due to slow electrons in humid air for a wide range of H₂O mole fractions from 2% to 50%. The simulation results are in excellent agreement with measurements of density-normalized attachment coefficients in pulsed Townsend experiments, when Taniguchi's cross sections are used for three-body electron attachment. It is found from the Monte Carlo simulations that the magnitude of cross sections for electron attachment with H₂O as a third body is a factor 7 higher in magnitude than the one for O₂. This validation supports the use of Taniguchi's cross section for three-body electron attachment with O₂ and H₂O, within the Biagi's cross sections for air and H₂O, for quantitative calculations of electron attachment in humid air.

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.

Neutral beam injection for fusion requires a negative hydrogen ion source (NHIS) capable of delivering high-quality H^- beams, and pulsed modulated discharges are one potential method of increasing H^- density volume generation. A two-dimensional fluid model is developed to investigate a NHIS operating under pulsed modulated discharge. The vibrationally excited states of hydrogen molecules are grouped to save computational time and improve model stability. The results reveal that with the increase of pulse duty cycle and pulse frequency, the cycle-averaged H^- density exhibits a non-monotonic variation. The optimal H^- ions enhancement occurs at pulse frequency of 5 kHz ~ 8 kHz and duty cycle of 30% ~ 50%. The enhancement is also pressure-dependent and is more pronounced at low pressures. Notably, the cycle-averaged H^- density from pulsed modulated discharge at 0.4 Pa even surpasses that from CW discharge at 0.8 Pa. This demonstrates the feasibility of operating a NHIS at lower pressures, potentially reducing co-extracted electrons and minimizing H^- ions destruction during extraction. Additionally, an alternative dual-pulse ion source is designed to address the temporal inhomogeneity of the plasma parameters in pulsed modulated discharges, and promising simulation results are achieved. Finally, the model is compared with experimental measurements to verify the accuracy.

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-induced fluorescence (LIF) is a widely used non-invasive method for characterizing NOx emission, mostly in combustion applications, but also in many plasma facilities. Under the carbon-free prerequisite, non-thermal plasma-assisted combustion (PAC) 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.

We study how the choice of input data affects simulations of positive streamers in humid air, focusing on H₂O cross sections, photoionization models, and chemistry sets. Simulations are performed in air with a mole fraction of 0%, 3% or 10% H₂O using an axisymmetric fluid model. Five H₂O cross section sets are considered, which lead to significant differences in the resulting electron attachment coefficient. As a result, the streamer velocity can vary by up to about 50% with 10% H₂O. We compare results with three photoionization models: the Naidis model for humid air, the Aints model for humid air, and the standard Zheleznyak model for dry air. With the Naidis and in particular the Aints model, there is a significant reduction in photoionization with higher humidities. This results in higher streamer velocities and maximal electric fields, and it can also cause streamer branching in our axisymmetric simulations. Three humid air chemistry sets are considered. Differences between these sets, particularly in the formation of water clusters around positive ions, cause the streamer velocity to vary by up to about 50% with 10% H₂O. A sensitivity analysis is performed to identify the most important chemical reactions in these chemistries.

Reaction rate constants of rotational transitions in the N2+ ion (j -> j', j ≦ 36) induced by collisions with helium atoms have been calculated using recently reported cross-sections [ChemPhysChem, 25:e202300469, 2024] obtained via classical trajectories run on the ground-state potential energy surface of the N2+/He collision complex obtained at a MCSCF/aug-cc-pVQZ level. Weak to medium electric fields (E/N = 1-100 Td) have been considered. In addition, the role of vibrational excitations in N2+ (v=0 -> v=1) is also briefly discussed and shown negligible under the conditions considered. The calculated rate constants have been used to model the relaxation dynamics of rotationally excited N2+ ions and to estimate its characteristic time scales as well as effective temperatures of relaxed N2+ ions.

Plasmas interacting with liquid surfaces produce a complex interfacial layer where the local chemistry in the liquid is driven by fluxes from the gas phase of electrons, ions, photons, and neutral radicals. Typically, the liquid surface has at best mild curvature with the fluxes of impinging plasma species and applied electric field being nominally normal to the surface. With liquids such as water having a high dielectric constant, structuring of the liquid surface by producing a wavy surface enables local electric field enhancement due to polarization of the liquid, as well as producing regions of higher and lower advective gas flow across the surface. This structuring (or waviness) can naturally occur or can be achieved by mechanical agitation such as with acoustic transducers. Electric field enhancement at the peaks of the waves of the liquid produces local increases in sources of reactive species and incident plasma fluxes which may be advantageous for plasma driven solution electrochemistry (PDSE) applications. In this paper, results are discussed from a computational investigation of pulsed atmospheric pressure plasma jets onto structured water solutions containing AgNO₃ as may be used in PDSE for silver nanoparticle (NP) formation. The solution surface consists of standing wave patterns having wavelength and wave depth of hundreds of microns to 1 mm. The potential for structured liquid surfaces to facilitate spatially differentiated chemical selectivity and enhance NP synthesis in the context of PDSE is discussed.

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-induced fluorescence (LIF) is a widely used non-invasive method for characterizing NOx emission, mostly in combustion applications, but also in many plasma facilities. Under the carbon-free prerequisite, non-thermal plasma-assisted combustion (PAC) 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.

Accurate measurement of magnetic field strengths is critical in many plasma environments, ranging from astrophysical systems to fusion energy research. In this work, a non-perturbative laser-based optical diagnostic known as quantum beat spectroscopy is demonstrated to be good alternative for measuring the magnetic field strength in low-pressure laboratory plasmas. The technique is investigated using both ns and fs pulsed lasers in an argon plasma. Preliminary results for a helium plasma are also given. Zeeman-split J = 1 electron states with transitions from metastable states were identified and tested for neutral argon $(^2\textit{P}^{\textrm{o}}_{1/2})4p$$^2[1/2]$ and neutral helium $1s3p$$^1P^{\textrm{o}}_1$. Magnetic fields are measured with sub-Gauss precision at near single laser pulse acquisition rates.

Nanoparticles (NPs) 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.

Numerical analysis of an atmospheric pressure glow discharge (APGD) in helium is carried out. Numerical models are spatially one- and two-dimensional and based on drift-diffusion theory of gas discharges. On the basis of the current–voltage and current density–voltage characteristic curves, the effects of the temperature regime on the cathode surface (cooled vs uncooled), the value of the secondary electron emission coefficient, and the thermal diffusion on the discharge parameters are studied. The possible transition of the discharge to an obstructed mode with gas heating is investigated. An analysis of the formation of normal APGD was carried out, which revealed good agreement with experimental data. The spontaneous emergence of cathode spots is illustrated and discussed.

Surface dielectric barrier discharge (SDBD) ignited directly from the liquid electrodes at the 3-phase gas/liquid/solid interface represents a novel approach in both water and polymer surface treatment methods. This study investigates the gaseous and liquid-phase reactive oxygen and nitrogen species (RONS) generated by this discharge. The impact of the discharge power and treatment duration on the concentration of these species in both gas and liquid is explored. The spatial development of ozone, the prevailing molecule produced by air dielectric barrier discharge, is studied. The production yields of plasma-generated species in the gas are described. Additionally, the electrical measurements of the SBDB with liquid electrode are presented and its characteristics are discussed. The combined investigation of RONS production yields, electrical discharge characterization, and in-situ ozone evolution provides important information regarding the presence of the reactive species in the vicinity of the plasma discharge, supporting further development and targeted applications of this technology.

To achieve a uniform argon plasma in an atmospheric pressure jet, we explored the possibility of using dual-frequency excitation in a coaxial dielectric barrier discharge device. Two separate ring-shaped electrodes outside an alumina tube generate the two frequencies. The upstream electrode is powered at low frequency (LF, $17 \, \mathrm{kHz}$), while the downstream electrode is powered at radio frequency (RF, $27 \, \mathrm{MHz}$). To assess the interaction with the substrate, a grounded electrode, covered by a glass substrate, is placed $5 \, \mathrm{mm}$ from the outlet of the alumina tube. We analyze the device with a fluid model and compare the results with experimental electrical and optical characterization. As the plasma is ignited by the LF, positive streamers develop from the LF electrode and reach the substrate in a few hundred nanoseconds. At this stage, the substrate is charged and a surface discharge propagates on the glass; however, no additional ionization occurs in the jet. As RF is added, the plasma is sustained by modulation in the Ω regime. At the same time, the LF polarization propagates through the plasma, influencing the sheaths on the facing dielectric walls. When the voltage drop in the sheaths exceeds a threshold level, an additional γ mode originates due to the secondary electrons emitted by the surfaces, leading to an increase in power dissipation. This coexistence of the two regimes is observed in the simulation, and it is validated experimentally by time-resolved photoemission measurements. As a result, the dual-frequency plasma exhibits a filamentary structure similar to that of an LF-driven jet. However, RF excitation caused diffused pre-ionization of the gas, which reduced the charge density gradients, resulting in wider microdischarge channels and a lower average electric field. Streamers propagation is therefore limited, while an ion flow to the substrate is maintained and controlled by the LF polarization.

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, γ^∗. 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 - 200 Pa and voltage amplitudes ranging from 150 V - 350 V at a driving frequency of 13.56 MHz. A mode 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 collision simulations employing a range of γ^∗ 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.