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.
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J T Gudmundsson 2020 Plasma Sources Sci. Technol. 29 113001
Sander Nijdam et al 2020 Plasma Sources Sci. Technol. 29 103001
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.
Ronny Brandenburg 2017 Plasma Sources Sci. Technol. 26 053001
Dielectric barrier discharges (DBDs) are plasmas generated in configurations with an insulating (dielectric) material between the electrodes which is responsible for a self-pulsing operation. DBDs are a typical example of nonthermal atmospheric or normal pressure gas discharges. Initially used for the generation of ozone, they have opened up many other fields of application. Therefore DBDs are a relevant tool in current plasma technology as well as an object for fundamental studies. Another motivation for further research is the fact that so-called partial discharges in insulated high voltage systems are special types of DBDs. The breakdown processes, the formation of structures, and the role of surface processes are currently under investigation. This review is intended to give an update to the already existing literature on DBDs considering the research and development within the last two decades. The main principles and different modes of discharge generation are summarized. A collection of known as well as special electrode configurations and reactor designs will be presented. This shall demonstrate the different and broad possibilities, but also the similarities and common aspects of devices for different fields of applications explored within the last years. The main part is devoted to the progress on the investigation of different aspects of breakdown and plasma formation with the focus on single filaments or microdischarges. This includes a summary of the current knowledge on the electrical characterization of filamentary DBDs. In particular, the recent new insights on the elementary volume and surface memory mechanisms in these discharges will be discussed. An outlook for the forthcoming challenges on research and development will be given.
R Snyders et al 2023 Plasma Sources Sci. Technol. 32 074001
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.
Oscar O Versolato 2019 Plasma Sources Sci. Technol. 28 083001
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.
T von Woedtke et al 2022 Plasma Sources Sci. Technol. 31 054002
Plasma medicine refers to the application of nonequilibrium plasmas at approximately body temperature, for therapeutic purposes. Nonequilibrium plasmas are weakly ionized gases which contain charged and neutral species and electric fields, and emit radiation, particularly in the visible and ultraviolet range. Medically-relevant cold atmospheric pressure plasma (CAP) sources and devices are usually dielectric barrier discharges and nonequilibrium atmospheric pressure plasma jets. Plasma diagnostic methods and modelling approaches are used to characterize the densities and fluxes of active plasma species and their interaction with surrounding matter. In addition to the direct application of plasma onto living tissue, the treatment of liquids like water or physiological saline by a CAP source is performed in order to study specific biological activities. A basic understanding of the interaction between plasma and liquids and bio-interfaces is essential to follow biological plasma effects. Charged species, metastable species, and other atomic and molecular reactive species first produced in the main plasma ignition are transported to the discharge afterglow to finally be exposed to the biological targets. Contact with these liquid-dominated bio-interfaces generates other secondary reactive oxygen and nitrogen species (ROS, RNS). Both ROS and RNS possess strong oxidative properties and can trigger redox-related signalling pathways in cells and tissue, leading to various impacts of therapeutic relevance. Dependent on the intensity of plasma exposure, redox balance in cells can be influenced in a way that oxidative eustress leads to stimulation of cellular processes or oxidative distress leads to cell death. Currently, clinical CAP application is realized mainly in wound healing. The use of plasma in cancer treatment (i.e. plasma oncology) is a currently emerging field of research. Future perspectives and challenges in plasma medicine are mainly directed towards the control and optimization of CAP devices, to broaden and establish its medical applications, and to open up new plasma-based therapies in medicine.
Jon Tomas Gudmundsson et al 2022 Plasma Sources Sci. Technol. 31 083001
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.
P J Bruggeman et al 2016 Plasma Sources Sci. Technol. 25 053002
Plasma–liquid interactions represent a growing interdisciplinary area of research involving plasma science, fluid dynamics, heat and mass transfer, photolysis, multiphase chemistry and aerosol science. This review provides an assessment of the state-of-the-art of this multidisciplinary area and identifies the key research challenges. The developments in diagnostics, modeling and further extensions of cross section and reaction rate databases that are necessary to address these challenges are discussed. The review focusses on non-equilibrium plasmas.
Stephan Reuter et al 2015 Plasma Sources Sci. Technol. 24 054001
Absorption spectroscopy (AS) represents a reliable method for the characterization of cold atmospheric pressure plasma jets. The method's simplicity stands out in comparison to competing diagnostic techniques. AS is an in situ, non-invasive technique giving absolute densities, free of calibration procedures, which other diagnostics, such as laser-induced fluorescence or optical emission spectroscopy, have to rely on. Ground state densities can be determined without the knowledge of the influence of collisional quenching. Therefore, absolute densities determined by absorption spectroscopy can be taken as calibration for other methods. In this paper, fundamentals of absorption spectroscopy are presented as an entrance to the topic. In the second part of the manuscript, a review of AS performed on cold atmospheric pressure plasma jets, as they are used e.g. in the field of plasma medicine, is presented. The focus is set on special techniques overcoming not only the drawback of spectrally overlapping absorbing species, but also the line-of-sight densities that AS usually provides or the necessity of sufficiently long absorption lengths. Where references are not available for measurements on cold atmospheric pressure plasma jets, other plasma sources including low-pressure plasmas are taken as an example to give suggestions for possible approaches. The final part is a table summarizing examples of absorption spectroscopic measurements on cold atmospheric pressure plasma jets. With this, the paper provides a 'best practice' guideline and gives a compendium of works by groups performing absorption spectroscopy on cold atmospheric pressure plasma jets.
Pedro Viegas et al 2022 Plasma Sources Sci. Technol. 31 053001
Plasma jets are sources of repetitive and stable ionization waves, meant for applications where they interact with surfaces of different characteristics. As such, plasma jets provide an ideal testbed for the study of transient reproducible streamer discharge dynamics, particularly in inhomogeneous gaseous mixtures, and of plasma–surface interactions. This topical review addresses the physics of plasma jets and their interactions with surfaces through a pedagogical approach. The state-of-the-art of numerical models and diagnostic techniques to describe helium jets is presented, along with the benchmarking of different experimental measurements in literature and recent efforts for direct comparisons between simulations and measurements. This exposure is focussed on the most fundamental physical quantities determining discharge dynamics, such as the electric field, the mean electron energy and the electron number density, as well as the charging of targets. The physics of plasma jets is described for jet systems of increasing complexity, showing the effect of the different components (tube, electrodes, gas mixing in the plume, target) of the jet system on discharge dynamics. Focussing on coaxial helium kHz plasma jets powered by rectangular pulses of applied voltage, physical phenomena imposed by different targets on the discharge, such as discharge acceleration, surface spreading, the return stroke and the charge relaxation event, are explained and reviewed. Finally, open questions and perspectives for the physics of plasma jets and interactions with surfaces are outlined.
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Matjaž Panjan 2024 Plasma Sources Sci. Technol. 33 055015
A high-frame-rate camera with microsecond-level time resolution was used to make systematic investigations of plasma self-organization and spoke dynamics during individual HiPIMS pulses. The plasma was imaged for a range of argon pressures (0.25–2 Pa) and peak discharge currents (10–400 A) using an Al target. The experiments revealed that plasma evolves through three characteristic stages as the discharge current increases. In stage I, which is present from the current onset and up to ∼25 A, spokes are azimuthally long and rotate in the −Ez× B direction. The spoke behavior is similar to the one observed in DCMS discharges. The number of spokes depends on pressure and the current growth rate. At the lowest pressure (0.25 Pa) a single spoke is present in discharge, while at higher pressures (1–2 Pa) two spokes are most often observed. The spoke velocity depends on the number of spokes, current growth rate and pressure. A single spoke rotates with velocities in the 4–15 km s−1 range, while two spokes rotate in the 1–9 km s−1 range depending on the pressure and growth rate. Following stage I, the plasma undergoes a complex reorganization that is characterized by aperiodic spoke patterns and irregular dynamics. In stage II spokes are less localized, they merge, split and propagate either in the retrograde or prograde direction. After chaotic plasma reorganization, more ordered spoke patterns begin to form. Spokes in stage III are azimuthally shorter, typically exhibit a triangular shape and rotate in the Ez× B direction. In general, the spoke dynamics is less complicated and is only influenced by the pressure. Spokes rotate faster at higher pressures than at lower ones; velocities range from 9 km s−1 at 0.25 Pa to 6 km s−1 at 2 Pa. The spoke velocity in stage III is largely unaffected by the discharge current or number of spokes. Stage III can be further divided into sub-stages, which are characterized by different current growth rates, spoke sizes and shapes. In general, the spoke evolution is highly reproducible for pulses with similar discharge current waveforms.
Zheng Zhao et al 2024 Plasma Sources Sci. Technol. 33 055014
Positive streamer behaviors under repetitive pulses are predominantly dependent on the availability of free electrons. If surface residual electrons stored from previous discharges could be intentionally released and involved into the next discharge, an alternative control freedom is provided apart from voltage waveform tailoring methods that mainly attract or repel gaseous residual charges. Evolutions of repetitively pulsed surface streamers in compressed (0.2 MPa) air were investigated after low-photon-energy pulsed visible (532 nm) and infrared (1064 nm) laser irradiations. Pulse-sequence and temporally resolved diagnostics were implemented to investigate effects of laser parameters (irradiation moment, wavelength, energy) and gas composition. A 2D surface streamer fluid simulation was performed to qualitatively unveil impacts of localized plasma patches. The surface streamer morphology and emission light are significantly and repeatably affected by the laser irradiation before the streamer inception, while, variations totally disappear without the solid surface. The secondary streamer is prolonged accompanied by a higher flashover probability after the pulsed laser irradiation in compressed air. Intriguingly, influences of the infrared laser persist for tens of microseconds before the next voltage pulse. Residual charge dynamics under the laser irradiation are analyzed, where the additional increase of of low electron bound energy is emphasized. The laser induced surface trapped electron desorption is achieved through the direct or the step-wise process, dependent on the laser energy and the surface trap state distribution.
N Yu Babaeva and G V Naidis 2024 Plasma Sources Sci. Technol. 33 055013
Characteristics of low-current stationary axially symmetric discharges in longitudinal laminar flows of atmospheric-pressure air calculated in the framework of a two-dimensional model are presented. Non-equilibrium discharge regimes, in the current range from 1 to 100 mA, are considered for gas flow velocities up to 50 m s−1. It is shown that variation of the flow velocity substantially affects the discharge characteristics, such as the width of discharge column, the electric field inside the gap, the current density etc. Validity of the obtained results is confirmed by their comparison with available experimental data.
Dante Filice and Sylvain Coulombe 2024 Plasma Sources Sci. Technol. 33 055011
Sub-breakdown radiofrequency (RF) discharges enabled by a nanosecond (ns) pulse ignition source are studied at atmospheric pressure in a range of gas mixtures from completely inert (in Ar) to completely reactive (in CO2). An electrical characterisation of the continuous wave (CW) RF discharge (13.56 MHz) is performed to determine plasma impedance and plasma power dissipation. Two different measurement methods to electrically characterize the system are described and compared. One method uses in-situ measurements of discharge parameters (voltage, current and the phase angle), and the other method performs ex-situ measurements of the load circuit using a vector network analyser. It was found that RF plasma power deposition depended on the applied RF power as well as the gas mixture composition. Using the in-situ voltage, current and phase angle measurements, plasma power deposition was calculated to be as much as 85% and 76% of the applied RF power for the pure Ar and pure CO2 cases, respectively. A preliminary qualitative assessment of the plasma composition was performed by optical emission spectroscopy, and CO2 conversion by mass spectrometry. CO2 to CO conversions of 11.2% and 5.5% in a 20:80 (CO2:Ar) mixture and in 100% CO2, respectively, were observed. This study demonstrates a RF plasma source for gas conversion applications at atmospheric pressure in a completely reactive gas.
Pawandeep Singh et al 2024 Plasma Sources Sci. Technol. 33 055012
The sheath-edge electric field () is an important parameter to patch the quasi-neutral pre-sheath and non-neutral sheath regions. The choice of significantly influences the theoretically estimated values of the sheath width, potential, and ion density distribution inside the sheath, as determined by the Poisson equation. The precise nature of has been a persistent subject of investigation, giving rise to the question of whether it should be zero or possess a finite value, as proposed by various authors. In this study, we determine the values of by solving Poisson's equation as a boundary-value problem, utilizing experimentally determined values of sheath radius from a DC-biased hairpin probe. The obtained values of are found to be finite and closely align with the analytical expressions presented by Riemann (1991 J. Phys. D: Appl. Phys.24 493) and Kaganovich (2002 Phys. Plasmas9 4788). Additionally, the impact of electron-penetrating sheaths and interacting sheaths on the applicability of the hairpin probe in low-pressure plasmas is briefly discussed.
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Wei Yang 2024 Plasma Sources Sci. Technol. 33 023001
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.
June Young Kim et al 2023 Plasma Sources Sci. Technol. 32 073001
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.
Luís L Alves et al 2023 Plasma Sources Sci. Technol. 32 023001
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.
Sander Nijdam et al 2022 Plasma Sources Sci. Technol. 31 123001
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.
Karsten Arts et al 2022 Plasma Sources Sci. Technol. 31 103002
This article discusses key elementary surface-reaction processes in state-of-the-art plasma etching and deposition relevant to nanoelectronic device fabrication and presents a concise guide to the forefront of research on plasma-enhanced atomic layer etching (PE-ALE) and plasma-enhanced atomic layer deposition (PE-ALD). As the critical dimensions of semiconductor devices approach the atomic scale, atomic-level precision is required in plasma processing. The development of advanced plasma processes with such accuracy necessitates an in-depth understanding of the surface reaction mechanisms. With this in mind, we first review the basics of reactive ion etching (RIE) and high-aspect-ratio (HAR) etching and we elaborate on the methods of PE-ALE and PE-ALD as surface-controlled processing, as opposed to the conventional flux-controlled processing such as RIE and chemical vapor deposition (CVD). Second, we discuss the surface reaction mechanisms of PE-ALE and PE-ALD and the roles played by incident ions and radicals in their reactions. More specifically, we discuss the role of transport of ions and radicals, including their surface reaction probabilities and ion-energy-dependent threshold effects in processing over HAR features such as deep holes and trenches.
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Lee et al
The generation of low-energy electrons is essential for the plasma source of the charge neutralizer system within the ion implanter process of semiconductors and displays, owing to their exceptional capability of being effectively transported along their ion beams. In this study, we propose a method to produce non-Maxwellian electron energy probability functions (eepfs) characterized by low-energy-abundant electrons, specifically below 5 eV, across an electron extraction system. In the electron transport region with an axial magnetic field under conditions of high discharge voltage and gas flow rate, we observed a significant increase in low-energy electrons in eepfs. The simple global model proposed to analyze these results demonstrated that the wall loss of electrons can be reduced by an elevated plasma potential, which is influenced by the ionization rate in the transport region. These results are consistent with the experimentally measured plasma potential and electron density. Additionally, the reduction in wall losses and increased ionization rate within the transport region resulted in the relaxation of the plasma potential gradient. This phenomenon effectively inhibited the cutting of low-energy electrons within the eepfs, thereby facilitating their consequential transport to the target. This study emphasizes the significance of increasing the ionization rate and minimizing the potential gradient for the dual purposes of generating low-energy electrons and directing them towards the target.
Zhou et al
In the study of electronegative CF4 capacitively coupled plasmas (CCP), plasma modulation is typically achieved by varying parameters such as pressure and voltage, et al. In this work, the particle-in-cell/Monte Carlo (PIC/MC) method is used to simulate modulation of CF4 CCP with injection of anions (F-) ion beam (FB). The results demonstrate that FB injection effectively enhances the dissociation collision process between F- ions and neutral molecules, thus altering the densities of electrons and ions. An effective modulation of the characteristic parameters of the plasma of CF4 can be achieved by controlling the current and energy of FB. Particularly noteworthy is the transition of the heating mode from the DA mode to the dissociation mode as the FB current increases to 0.038 A (energy fixed at 10 keV) or when the FB energy exceeds 10 keV (current fixed on 0.038 A). This transition is attributed to the generation of a substantial number of electrons through dissociative collisions. This approach provides insight into the controlled modulation of plasma characteristics in CF4 CCP, offering potential applications in various plasma-based technologies.
Xue et al
Moving the laser focus to the vicinity of the gas-liquid interface is the key point for many new enhanced and new methods to improve the quality of spectral signals in water LIBS detection. Understanding the generation and evolution characteristics of the plasma induced by pulsed laser near the gas-liquid interface is of great significance for the establishment of evolution models and improvement of these new LIBS methods. In this paper, a set of slow horizontal flow auxiliary system is established to provide an ideal flat gas-liquid two-phase interface experimental condition. Experimental research on vertical incidence plane system was conducted using techniques such as time-resolved imaging, plasma characterization diagnosis, and spectral analysis. And the detection capabilities of the system were also tested. The characteristics and mechanisms of LIBS near the gas-liquid two-phase interface were investigated with the laser incident on the sample along the vertical direction. Simulation of the laser beam focusing process and observation of laser beam spot images show that the shift of plasma generation position relative to the focal point results from the refraction of the laser beam entering the solution from the air and the 'interface effect' of propagation on the vertical direction. Moreover, the plasma forms only the optical power density surpasses the breakdown threshold. In this work, plasma with smaller size, rounder shape, stronger radiation, higher temperature, and higher density can be produced when the focus position is in the liquid column 0.3 mm away from the upper interface. Simultaneously, for example, the Mg ion line at 285.213 nm, the obtained spectral intensity to signal-to-background ratio reaches the maximum value, and a better spectral signal can be obtained, which is 2-4 times of other positions, and the detection limits of the elements Na, Mg, and Ca also reach the lowest level, with 1.6-2.4 times of the detection limit of other focusing positions for Mg and 1.4-1.7 times for Ca, respectively.
Huang et al
This work numerically studies densities and mechanisms of OH species generated in atmospheric-pressure air dielectric barrier discharges with the model validated by experiments. The power consumption is measured, and the number of microdischarges (MDs) generated within a half period is captured by an intensified CCD camera. The OH densities of cases with various H2O concentrations are measured using ultraviolet absorption spectroscopy. The numerical model integrating the 1.5D discharge fluid model and 3D background gas model (BGM) is adopted to predict the MD behavior and the generation of species related to OH generation. The simulated OH densities cover the range of 1.1×10^19 and 1.6×10^19 m-3 in the cases studied, agreeing with those measured. The simulated results show that most OH radicals are generated in MDs, while the reactive section contributes around 2% of the total OH generation. The detailed analysis shows that atomic oxygen (O(1D) and O) and O3 contribute most of the OH generation in the MDs. In contrast, the self-association reactions (i.e., 2OH + M → H2O2 + M and 2OH → O + H2O) and NOx species consume more than 64% of OH radicals generated in MDs. In the BGM, it is interesting to find that reactive species NOx play significant roles in both the OH generation and depletion in the reactive section. The distributions of species related to the OH species obtained by the BGM are presented to elucidate the detailed chemistry of OH species in the reactive section.
Kotov et al
The CO2→CO+½O2 conversion experiment [F~A~D'Isa et al. 2020 Plasma Sources Sci. Technol.29 105009] has been compared with thermo-chemical calculations. The experiment is a 2.45 GHz plasma torch with straight channel in the effluent. The 1.5D model of the CO2/CO/O2/O/C mixture without turbulent transport has been applied with plasma acting only as prescribed heat source. The parameter range covered is specific energy input (SEI) 0.3-5 eV/molecule at pressure p=0.9 bar, and SEI=0.6-2 eV/molecule at p=0.5, 0.2 bar. The calculated conversion χ is always close to experimental values. At the same time, the calculated temperatures T deviate significantly from the experiment, especially for p=0.2 bar. The calculated T were also found to be sensitive with respect to the uncertain model parameters, but χ is not sensitive. According to the model the net conversion is driven mainly by the radial diffusion of CO and O from the hot core toward the wall and steep radial temperature gradients. The main factor which reduces the energy efficiency is re-oxidation of CO at the edge of the hot plasma region and downstream. The net conversion in the model is driven to large extent by the radial diffusion of CO and O 
from the hot core toward the wall and steep radial temperature gradients. The main factor which reduces the energy efficiency is re-oxidation of CO at the edge of the hot plasma region and downstream. The 1.5D approximation applied has the principle limitation that the impact of the realistic bulk flow field on the chemical process could not be studied. Hence the results must be considered as 
preliminary and have to be confirmed with a more elaborate and accurate model of the vortex stabilized flows inside the reactor.
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Matjaž Panjan 2024 Plasma Sources Sci. Technol. 33 055015
A high-frame-rate camera with microsecond-level time resolution was used to make systematic investigations of plasma self-organization and spoke dynamics during individual HiPIMS pulses. The plasma was imaged for a range of argon pressures (0.25–2 Pa) and peak discharge currents (10–400 A) using an Al target. The experiments revealed that plasma evolves through three characteristic stages as the discharge current increases. In stage I, which is present from the current onset and up to ∼25 A, spokes are azimuthally long and rotate in the −Ez× B direction. The spoke behavior is similar to the one observed in DCMS discharges. The number of spokes depends on pressure and the current growth rate. At the lowest pressure (0.25 Pa) a single spoke is present in discharge, while at higher pressures (1–2 Pa) two spokes are most often observed. The spoke velocity depends on the number of spokes, current growth rate and pressure. A single spoke rotates with velocities in the 4–15 km s−1 range, while two spokes rotate in the 1–9 km s−1 range depending on the pressure and growth rate. Following stage I, the plasma undergoes a complex reorganization that is characterized by aperiodic spoke patterns and irregular dynamics. In stage II spokes are less localized, they merge, split and propagate either in the retrograde or prograde direction. After chaotic plasma reorganization, more ordered spoke patterns begin to form. Spokes in stage III are azimuthally shorter, typically exhibit a triangular shape and rotate in the Ez× B direction. In general, the spoke dynamics is less complicated and is only influenced by the pressure. Spokes rotate faster at higher pressures than at lower ones; velocities range from 9 km s−1 at 0.25 Pa to 6 km s−1 at 2 Pa. The spoke velocity in stage III is largely unaffected by the discharge current or number of spokes. Stage III can be further divided into sub-stages, which are characterized by different current growth rates, spoke sizes and shapes. In general, the spoke evolution is highly reproducible for pulses with similar discharge current waveforms.
Cheng-Liang Huang et al 2024 Plasma Sources Sci. Technol.
This work numerically studies densities and mechanisms of OH species generated in atmospheric-pressure air dielectric barrier discharges with the model validated by experiments. The power consumption is measured, and the number of microdischarges (MDs) generated within a half period is captured by an intensified CCD camera. The OH densities of cases with various H2O concentrations are measured using ultraviolet absorption spectroscopy. The numerical model integrating the 1.5D discharge fluid model and 3D background gas model (BGM) is adopted to predict the MD behavior and the generation of species related to OH generation. The simulated OH densities cover the range of 1.1×10^19 and 1.6×10^19 m-3 in the cases studied, agreeing with those measured. The simulated results show that most OH radicals are generated in MDs, while the reactive section contributes around 2% of the total OH generation. The detailed analysis shows that atomic oxygen (O(1D) and O) and O3 contribute most of the OH generation in the MDs. In contrast, the self-association reactions (i.e., 2OH + M → H2O2 + M and 2OH → O + H2O) and NOx species consume more than 64% of OH radicals generated in MDs. In the BGM, it is interesting to find that reactive species NOx play significant roles in both the OH generation and depletion in the reactive section. The distributions of species related to the OH species obtained by the BGM are presented to elucidate the detailed chemistry of OH species in the reactive section.
Vladislav Kotov et al 2024 Plasma Sources Sci. Technol.
The CO2→CO+½O2 conversion experiment [F~A~D'Isa et al. 2020 Plasma Sources Sci. Technol.29 105009] has been compared with thermo-chemical calculations. The experiment is a 2.45 GHz plasma torch with straight channel in the effluent. The 1.5D model of the CO2/CO/O2/O/C mixture without turbulent transport has been applied with plasma acting only as prescribed heat source. The parameter range covered is specific energy input (SEI) 0.3-5 eV/molecule at pressure p=0.9 bar, and SEI=0.6-2 eV/molecule at p=0.5, 0.2 bar. The calculated conversion χ is always close to experimental values. At the same time, the calculated temperatures T deviate significantly from the experiment, especially for p=0.2 bar. The calculated T were also found to be sensitive with respect to the uncertain model parameters, but χ is not sensitive. According to the model the net conversion is driven mainly by the radial diffusion of CO and O from the hot core toward the wall and steep radial temperature gradients. The main factor which reduces the energy efficiency is re-oxidation of CO at the edge of the hot plasma region and downstream. The net conversion in the model is driven to large extent by the radial diffusion of CO and O 
from the hot core toward the wall and steep radial temperature gradients. The main factor which reduces the energy efficiency is re-oxidation of CO at the edge of the hot plasma region and downstream. The 1.5D approximation applied has the principle limitation that the impact of the realistic bulk flow field on the chemical process could not be studied. Hence the results must be considered as 
preliminary and have to be confirmed with a more elaborate and accurate model of the vortex stabilized flows inside the reactor.
Dante Filice and Sylvain Coulombe 2024 Plasma Sources Sci. Technol. 33 055011
Sub-breakdown radiofrequency (RF) discharges enabled by a nanosecond (ns) pulse ignition source are studied at atmospheric pressure in a range of gas mixtures from completely inert (in Ar) to completely reactive (in CO2). An electrical characterisation of the continuous wave (CW) RF discharge (13.56 MHz) is performed to determine plasma impedance and plasma power dissipation. Two different measurement methods to electrically characterize the system are described and compared. One method uses in-situ measurements of discharge parameters (voltage, current and the phase angle), and the other method performs ex-situ measurements of the load circuit using a vector network analyser. It was found that RF plasma power deposition depended on the applied RF power as well as the gas mixture composition. Using the in-situ voltage, current and phase angle measurements, plasma power deposition was calculated to be as much as 85% and 76% of the applied RF power for the pure Ar and pure CO2 cases, respectively. A preliminary qualitative assessment of the plasma composition was performed by optical emission spectroscopy, and CO2 conversion by mass spectrometry. CO2 to CO conversions of 11.2% and 5.5% in a 20:80 (CO2:Ar) mixture and in 100% CO2, respectively, were observed. This study demonstrates a RF plasma source for gas conversion applications at atmospheric pressure in a completely reactive gas.
A D Pajdarová et al 2024 Plasma Sources Sci. Technol. 33 055007
Time-resolved Langmuir probe diagnostics at the discharge centerline and at three distances from the target (, , and ) was carried out during long positive voltage pulses (a duration of and a preset positive voltage of ) in bipolar high-power impulse magnetron sputtering of a Ti target (a diameter of ) using an unbalanced magnetron. Fast-camera spectroscopy imaging recorded light emission from Ar and Ti atoms and singly charged ions during positive voltage pulses. It was found that during the long positive voltage pulse, the floating and the plasma potentials suddenly decrease, which is accompanied by the presence of anode light located on the discharge centerline between the target center and the magnetic null of the magnetron's magnetic field. These light patterns are related to the ignition of a reverse discharge, which leads to the subsequent rise in the plasma and the floating potentials. The reversed discharge is burning up to the end of the positive voltage pulse, but the plasma and floating potentials have lower values than the values from the initial part of the positive voltage pulse. Secondary electron emission induced by the impinging Ar+ ions to the grounded surfaces in the vicinity of the discharge plasma together with the mirror configuration of the magnetron magnetic field are identified as the probable causes of the charge double-layer structure formation in front of the target and the ignition of the reverse discharge.
Katerina Polaskova et al 2024 Plasma Sources Sci. Technol.
The cold atmospheric plasma jets change their character when interacting with the different surfaces. Since such interaction is the primary area of plasma jet applications, it is essential to monitor the process. The non-linearity of the RF plasma slit jet (PSJ) was analyzed using the VI probes and a novel method, the non- intrusive antenna measurements. Regardless of the experimental setup and gas mixture (Ar, Ar/O2 , Ar/N2), the PSJ frequency spectrum consisted of the following main features: dominant fundamental frequency peak, relatively strong odd harmonics, and significantly weaker even harmonics. The lowest degree of non-linearity was recorded for the Ar PSJ ignited against a grounded target. Admixing a molecular gas increased the discharge non-linearity as observed by even harmonics intensities. It was attributed to the enhancement of secondary electron emission from the dielectric surfaces. In addition to the non-linearity analysis, the antenna spectra were for the first time used to determine the semi-quantative values of the PSJ-radiated electric field. The electric fields decreased by a factor of 2 after the admixing of nitrogen and oxygen molecular gases. Out of the studied targets, the highest electric fields were observed when plasma impinged on the grounded targets, followed by the floating target (2x lower) and the PSJ ignited in the open space configuration (4x lower than in the grounded target configuration).
M S Benilov 2024 Plasma Sources Sci. Technol. 33 055002
When a hot arc spot has just formed on the cathode surface, e.g. in the course of arc ignition on a cold cathode, a significant part of the current still flows in the glow-discharge mode to the cold surface outside the spot. The near-cathode voltage continues to be high at all points of the cathode surface. The mean free path for collisions between the atoms and the ions within the plasma ball near the spot is comparable to, or exceeds, the thickness of the ionization layer, which is a part of the near-cathode non-equilibrium layer where the ion current to the cathode is generated. The evaluation of the ion current to the cathode surface under such conditions is revisited. A fluid description of the ion motion in the ionization layer is combined with a kinetic description of the atom motion. The resulting problem admits a simple analytical solution. Formulas for the evaluation of the ion current to the cathode for a wide range of conditions are derived and the possibilities of using these formulas to improve the accuracy of existing methods for modeling high-pressure arc discharges in relation to glow-to-arc transitions are discussed.
Pedro Viegas et al 2024 Plasma Sources Sci. Technol. 33 055003
Surface recombination in an oxygen DC glow discharge in a Pyrex (borosilicate glass) tube is studied via mesoscopic modelling and comparison with measurements of recombination probability. A total of 106 experimental conditions are assessed, with discharge current varying between 10 and 40 mA, pressure values ranging between 0.75 and 10 Torr, and fixed outer wall temperatures () of −20, 5, 25 and . The model includes O+O and O+O2 surface recombination reactions and a dependent desorption frequency. The model is validated for all the 106 studied conditions and intends to have predictive capabilities. The analysis of the simulation results highlights that for and the dominant recombination mechanisms involve physisorbed oxygen atoms () in Langmuir–Hinshelwood (L-H) recombination and in Eley–Rideal (E-R) recombination , while for and processes involving chemisorbed oxygen atoms () in E-R and L-H also play a relevant role. A discussion is taken on the relevant recombination mechanisms and on ozone wall production, with relevance for higher pressure regimes.
Ranna Masheyeva et al 2024 Plasma Sources Sci. Technol. 33 045019
The electron power absorption mechanisms in electronegative capacitively coupled plasmas in CF4 are investigated using particle-in-cell/Monte Carlo collisions simulations at a pressure of 60 Pa, a driving frequency of 13.56 MHz for voltage amplitudes in the interval of 100300 V, where pronounced self-organized density variations, i.e. striations, develop. The calculations are based on the Boltzmann term analysis, a computational diagnostic method capable of providing a complete spatio-temporal description of electron power absorption. The discharge undergoes an electron power absorption mode transition from the drift-ambipolar- to the striation-mode at φ0 = 180 V. Although Ohmic power absorption is found to be the dominant electron power absorption mechanism in the parameter range considered, the electron power absorption mode transition can be inferred from the behaviour of the spatio-temporally averaged ambipolar power absorption as a function of the voltage amplitude. Furthermore, it is shown, that as a consequence of the presence of striations, the temporal modulation of the electron density leads to a temporal modulation of the ambipolar electric field, which is responsible for the striated structures of various physical quantities related to electrons, such as the electron temperature and the ionization source function.
Duarte Gonçalves et al 2024 Plasma Sources Sci. Technol. 33 045020
Atmospheric-pressure microplasma jets (µAPPJs) are versatile sources of reactive species with diverse applications. However, understanding the plasma chemistry in these jets is challenging due to plasma-flow interactions in heterogeneous gas mixtures. Spatial metastable density profiles help to understand these physical and chemical mechanisms. This work focuses on controlling the shielding gas around a µAPPJ. We use a dielectric barrier discharge co-axial reactor where a co-flow shields the pure argon jet with different N2–O2 gas mixtures. A voltage pulse (4 kV, 1 µs, 20 kHz) generates a first discharge at the pulse's rising edge and a second discharge at the falling edge. Tunable diode laser absorption spectroscopy measures the local Ar(1s5) density. A pure N2 (100%N2–0%O2) co-flow leads to less reproducible and lower peak Ar(1s5) density (). Increasing the O2 admixture in the co-flow yields narrower Ar(1s5) absorbance profiles and increases the Ar(1s5) density ( to ). The position of the peak density is closer to the reactor for higher O2 fractions. Absence of N2 results in comparable Ar(1s5) densities between the first and second discharges (maxima of and , respectively). Local Ar(1s5) density profiles from pure N2 to pure O2 shielding provide insights into physical and chemical processes. The spatially-resolved data may contribute to optimising argon µAPPJ reactors across the various applications and to validate numerical models.