Table of contents

We report an abrupt Nb ion formation in a direct current hot magnetron sputtering discharge as a result of target temperature increase to a certain point (1900 K in our case). The effect is explained by a significant thermionic emission from the target surface, leading to an enhanced electron impact ionization in plasma volume. The phenomenon is especially pronounced for Nb (refractory metal), for which higher target temperatures can be reached. The volume density mapping is undertaken for Nb neutrals and ions (by laser-based spectroscopy), emphasizing the found effects.

The present study investigates the physics of a nanosecond-pulsed microplasma operated at a pressure of 200 mbar with the help of a particle-in-cell simulation with Monte Carlo treatment of collision (PIC/MCC) and (semi-)analytical models. The discharge is ignited in a 1 mm gap between two parallel molybdenum electrodes by applying a voltage in the kV-range for several tens to hundreds of nanoseconds. A PIC/MCC simulation code is developed in order to describe the experiment performed under identical conditions. The simulation includes the external electrical circuit to perform ab-initio calculations of the complete experimental setup. Notable features of the PIC/MCC are (i) the adjustment of super-particle weights to reduce the computational load during drastic changes of the plasma density, which reaches values up to a few $10^{19}\,\mathrm{m}^{-3}$, and (ii) the inclusion of dissociative recombination of the $\mathrm{N_4}^+$ ions, which is the key loss process for the charge carriers in the plasma bulk regions. The current and voltage waveforms obtained from the simulation are compared to the experimentally measured ones and good agreement is found. After ignition, the discharge establishes a quasi-steady state exhibiting spatial features similar to a conventional DC glow discharge. Using the PIC/MCC results, reasonable approximations are identified, which allow the development of various analytical fluid models for the individual plasma regions. These models are able to reproduce the key features of the discharge in agreement with the PIC/MCC results. The simplified models for the different discharge regions can be combined to describe the global behaviour of the discharge and—in a next step—might be used to develop computationally efficient global chemistry models that account for the different power dissipation mechanisms along the discharge gap.

A nanosecond atmospheric pressure plasma jet operated in pure nitrogen is studied by spatially and temporally resolved optical emission spectroscopy complementing the companion paper (Kuhfeld et al 2023 Plasma Sources Sci. Technol.32 084001), where the discharge is investigated by means of Particle-in-Cell/Monte Carlo collisions (PIC/MCC) simulations and fluid models. Two temporal phases of the evolution of the discharge are identified: a fast breakdown and a quasi-DC phase. It is shown that during the breakdown phase several ionization waves develop, while after the breakdown the discharge has a structure similar to DC glow discharges, in agreement with the modeling predictions. The results of the measurements of the spatial-temporal dynamics of the light emission are compared with the distribution of densities of the ${\mathrm N}^+_2({\mathrm B}^2\Sigma^+_u)$ and ${\mathrm N}_2({\mathrm C}^3\Pi_u)$ states reconstructed from the PIC/MCC simulations. A good agreement is demonstrated.

This work presents a reaction mechanism for oxygen plasmas, i.e. a set of reactions and corresponding rate coefficients that are validated against benchmark experiments. The kinetic scheme is validated in a DC glow discharge for gas pressures of 0.2–10 Torr and currents of 10–40 mA, using the 0D LisbOn KInetics simulation tool and available experimental data. The comparison comprises not only the densities of the main species in the discharge—$\mathrm{O_2}(\mathrm{X}^3\Delta_\mathrm{g}^-)$, $\mathrm{O_2}(\mathrm{a^1}\Delta_\mathrm{g})$, $\mathrm{O_2}(\mathrm{b^1}\Sigma_\mathrm{g^+})$ and $\mathrm{O(^3P)}$—but also the self-consistent calculation of the reduced electric field and the gas temperature. The main processes involved in the creation and destruction of these species are identified. Moreover, the results show that the oxygen atoms play a dominant role in gas heating, via recombination at the wall and quenching of $\mathrm{O_2}(\mathrm{X}^3\Sigma_\mathrm{g^-,v})$ vibrations and $\mathrm{O_2}$ electronically-excited states. It is argued that the development and validation of kinetic schemes for plasma chemistry should adopt a paradigm based on the comparison against standard validation tests, as it is done in electron swarm validation of cross sections.

Ozone, O₃, is a strong oxidizing agent often used for water purification. O₃ is typically produced in dielectric barrier discharges (DBDs) by electron-impact dissociation of O₂, followed by three-body association reactions between O and O₂. Previous studies on O₃ formation in low-temperature plasma DBDs have shown that O₃ concentrations can drop to nearly zero after continued operation, termed the ozone-zero phenomenon (OZP). Including small (<4%) admixtures of N₂ can suppress this phenomenon and increase the O₃ production relative to using pure O₂ in spite of power deposition being diverted from O₂ to N₂ and the production of nitrogen oxides, N_xO_y. The OZP is hypothesized to occur because O₃ is destroyed on the surfaces in contact with the plasma. Including N₂ in the gas mixture enables N atoms to occupy surface sites that would otherwise participate in O₃ destruction. The effect of N₂ in ozone-producing DBDs was computationally investigated using a global plasma chemistry model. A general surface reaction mechanism is proposed to explain the increase in O₃ production with N₂ admixtures. The mechanism includes O₃ formation and destruction on the surfaces, adsorption and recombination of O and N, desorption of O₂ and N₂, and NO_x reactions. Without these reactions on the surface, the density of O₃ monotonically decreases with increasing N₂ admixture due to power absorption by N₂ leading to the formation of nitrogen oxides. With N-based surface chemistry, the concentrations of O₃ are maximum with a few tenths of percent of N₂ depending on the O₃ destruction probability on the surface. The consequences of the surface chemistry on ozone production are less than the effect of gas temperature without surface processes. An increase in the O₃ density with N-based surface chemistry occurs when the surface destruction probability of O₃ or the surface roughness was decreased.

The problem of assessing apparent secondary emission coefficient based on electrical properties of direct-current (DC) glow discharges using numerical modeling is revisited. An analysis of potential sources of uncertainties resulting from errors in experimental data as well as from model assumptions and approximations is presented. An estimation method based on a previously developed analytical model of a DC glow discharge is suggested. Application of the method is demonstrated in the example available in the literature of current–voltage characteristics of DC glow discharges in argon with copper electrodes. Values obtained for the considered data fall into two distinguishable groups, closely corresponding to those for clean and dirty copper cathode surfaces. The obtained preliminary results suggest the feasibility of estimating apparent secondary emission coefficients in DC discharges using numerical modeling.

Organometallic positive ions were identified in inductively coupled plasmas by means of mass spectrometry during the etching of Ge, Sb, Se materials. A preliminary study was focused on identifying M_xH_y + (M = Ge, Sb, Se) positive ion clusters during a H₂/Ar etching process. The methane addition to the H₂/Ar mixture generates CH_x reactive neutral species. The latter react with the metalloids within gas phase to form M_xC_yH_z⁺ organometallic ions. In addition, the etching of Sb₂Se₃ and Ge_19.5Sb_17.8Se_62.7 bulk targets forms mixed products via ion-molecule reactions as evidenced by the presence of SeSbC_xH_y⁺ ion clusters. Changes in surface composition induced by the newly formed organometallic structures were investigated using in situ x-ray photoelectron spectroscopy. In the case of the Ge and Sb surfaces, (M)–M–C_x environments broadened the Ge 2p_3/2, Ge 3d, Sb 3d and Sb 4d spectra to higher values of binding energy. For the Se surface, only the hydrogen and methyl bonding could explain the important broadening of the Se 3d core level. It was found that the Ge₃₉Se₆₁ thin film presents an induced (Ge)–Ge–Se entity on the Ge 2p_3/2 and Ge 3d core levels.

The behaviour of nitrogen plasma mixed with varying proportions of argon (10%–80%) is investigated under different RF discharge conditions. It is observed that at a relatively low RF power of 200 W (E-mode) the dissociation fraction (DF) of nitrogen increases with the growing concentration of argon, whereas the opposite happens for a higher RF power of 1000 W (H-mode), when the DF rapidly falls from a high value as the argon percentage starts to increase. This rising trend of DF closely follows the argon metastable fraction (MF) in the E-mode, and for the H-mode it is not followed until the argon percentage crosses the 20% mark. The electron density, temperature and electron energy probability function (EEPF) are obtained using a RF compensated Langmuir probe and to evaluate the vibrational and rotational temperatures, DF, MF etc, a separate optical emission spectroscopy technique is incorporated. At 5 × 10⁻³ mbar of working pressure and 10% argon content the EEPF profile reveals that the plasma changes from non-Maxwellian to Maxwellian as the RF power jumps from 200 W to 1000 W, and for a fixed RF power the high energy tail tends to move upwards with the gradual increment of argon. These observations are reverified theoretically by considering electron–electron collision frequency and electron bounce frequency as a function of electron temperature. Overall, all the major experimental phenomena in this study are explained in terms of EEPF profile, electron–electron collision effect, electron and gas temperature, electron density and argon metastable population.

Micro direct current (DC) ion thrusters have broad application prospects as propulsion systems for micro spacecrafts due to their advantages of high discharge reliability and efficiency. Experiments in the literature show that the plasma discharge under the axial ring cusp hybrid (ARCH) magnetic field has higher discharge efficiency in the discharge chamber of micro DC ion thruster. In this paper, a 2d-3v axisymmetric particle-in-cell/Monte Carlo collision numerical model is developed for the ARCH discharge. This model takes the thermal electron emission including the Schottky effect, various collision processes including the Bohm-type anomalous conductivity and the uneven background gas density distribution in the cathode-anode gap into account. The spatial distributions of plasma characteristics are presented and the advantages of ARCH discharge compared with traditional 3-ring discharge in the discharge chamber of the micro DC ion thruster are analyzed. The longer electron path length and the change of ionization region improve the discharge efficiency in small-scale discharges. Two primary methods for the discharge confinement in the miniature ion thrusters, that is, the magneto-static 'cusp confinement' through magnetic cusps and the electrostatic 'sheath refection confinement' through the backplate with the lower potential are investigated. The sensitivity of macroscopic current characteristics and microscopic plasma characteristics in the ARCH discharge to the magnetic field strength and backplate biased potential are explored. It is found that there is an optimal magnetic field to maximize the utilization of propellant and minimize the discharge loss. The electrostatic 'sheath refection confinement' is conducive to the reduction of discharge loss, however, it is also accompanied by the decline of propellant utilization. The above results provide further support for the design optimization of the micro DC ion thruster discharge chamber.

Aiming at the quantitative diagnostics of boron monohydryde, BH, in fusion plasmas, we present elastic, electronic excitation, and ionization cross sections of BH for the first time. The calculations were performed by the R-matrix and Born Encounter Bethe methods utilized by Quantemol-EC software. To examine the uncertainty due to the calculation conditions, we compared the results by different basis sets and internuclear distances of the target model. We found that the uncertainties are typically within ∼10%. Rate coefficients were derived and fitted to an Arrhenius function. The derived decay rate per photon, $S/XB$, agreed with the value presented by Lieder et al (the ASDEX-Upgrade Team) (1994 Eur. Phys. Soc. Conf. Plasma Phys.2 722).

Streamer discharges are the primary mode of electric breakdown of air in lightning and high voltage technology. Streamer channels branch many times, which determines the developing tree-like discharge structure. Understanding these branched structures is for example important to describe streamer coronas in lightning research. We simulate branching of positive streamers in air using a 3D fluid model where photoionization is included as a discrete and stochastic process. The probability and morphology of branching are in good agreement with dedicated experiments. This demonstrates that photoionization indeed provides the noise that triggers branching, and we show that branching is remarkably sensitive to the amount of photoionization. Our comparison is therefore one of the first sensitive tests for Zheleznyak's photoionization model, confirming its validity.

In capacitively coupled radio frequency discharges, the interaction of the plasma and the surface boundaries is linked to a variety of highly relevant phenomena for technological processes. One possible plasma-surface interaction is the generation of secondary electrons (SEs), which significantly influence the discharge when accelerated in the sheath electric field. However, SEs, in particular electron-induced SEs (δ-electrons), are frequently neglected in theory and simulations. Due to the relatively high threshold energy for the effective generation of δ-electrons at surfaces, their dynamics are closely connected and entangled with the dynamics of the ion-induced SEs (γ-electrons). Thus, a fundamental understanding of the electron dynamics has to be achieved on a nanosecond timescale, and the effects of the different electron groups have to be segregated. This work utilizes $1d3v$ particle-in-cell/Monte Carlo collisions simulations of a symmetric discharge in the low-pressure regime (p = 1 Pa) with the inclusion of realistic electron-surface interactions for silicon dioxide. A diagnostic framework is introduced that segregates the electrons into three groups ('bulk-electrons', 'γ-electrons', and 'δ-electrons') in order to analyze and discuss their dynamics. A variation of the electrode gap size $L_\mathrm{gap}$ is then presented as a control tool to alter the dynamics of the discharge significantly. It is demonstrated that this control results in two different regimes of low and high plasma density, respectively. The fundamental electron dynamics of both regimes are explained, which requires a complete analysis starting at global parameters (e.g. densities) down to single electron trajectories.

The Electrical Asymmetry Effect (EAE) provides control of the mean ion energy at the electrodes of multi-frequency capacitively coupled radio frequency plasmas (CCP) by tuning the DC self-bias via adjusting the relative phase(s) between the consecutive driving harmonics. Depending on the electron power absorption mode, this phase control affects the ion flux in different ways. While it provides separate control of the mean ion energy and flux in the α-mode, limitations were found in the γ- and Drift-Ambipolar modes. In this work, based on experiments as well as kinetic simulations, the EAE is investigated in the striation-mode, which is present in electronegative CCPs driven by low frequencies. The discharge is operated in CF₄ and is driven by two consecutive harmonics (4/8 MHz). The simulation results are validated against measurements of the DC self-bias and the spatio-temporally resolved dynamics of energetic electrons. To include heavy particle induced secondary electron emission realistically, a new computationally assisted diagnostic is developed to determine the corresponding secondary electron emission coefficient from a comparison of the DC self-bias obtained experimentally and from the simulations. Based on the validated simulation results, the EAE is found to provide separate control of the mean ion energy and flux in the striation mode, while the axial charged particle density profiles and the number of striations change as a function of the relative phase. This is understood based on an analysis of the ionization dynamics.

We present a computational study of positive streamers in air propagating over dielectric plates with square channels running orthogonal to the propagation direction. The study uses a newly developed non-kinetic Particle-In-Cell model based on Îto diffusion and kinetic Monte Carlo, which does not introduce artificial smoothing of the plasma density or photo-electron distributions. These capabilities permit the computational study to use high-resolution grids with large time steps, and also incorporates geometric shielding for particle and photon transport processes. We perform Cartesian 2D simulations for channel dimensions ranging from 250 µm to 2 mm, and track streamers over a distance of 4 cm and times ranging up to 300 ns, for various voltages ranging from 15 kV to 30 kV. These baseline simulations are supplemented by additional studies on the effects of transparency to ionizing radiation, streamer reignition, dielectric permittivity, and 3D effects. The computer simulations show: 1) Larger channels restrict streamer propagation more efficiently than narrow channels, and can lead to arrested surface streamers. 2) Geometric shielding of ionizing radiation substantially reduces the number of starting electrons in neighboring channels, and thus also reduces the onset point of streamer reignition. 3) Decreasing the streamer channel separation leads to slower streamers. 4) Increasing the dielectric permittivity increases the discharge velocity. The results are of generic value to fields of research involving streamer-dielectric interactions, and in particular for high-voltage technology where streamer termination is desirable.

The work is devoted to the development of laser absorption spectroscopy (LAS) of plasma-assisted processes for application under industrial conditions. The interpretation of the LAS measurements was revised by taking into consideration the temperature gradient along the absorption path, which is unavoidable in a reactor for thermochemical treatment. The revision is based on the measurement of $\mathrm{HCN}$, $\mathrm{NH_3}$, $\mathrm{H_2O}$ and $\mathrm{CO}$ molecular lines in an industrial-scale, active screen plasma nitrocarburizing reactor with a steel active screen. It shows that an effective temperature determined from Doppler broadening could be assigned to each measured spectral line. The effective temperature does not only reflect the temperature gradients along the line-of-sight but also the line strength dependence on temperature for the specific spectroscopic transition. Lower limit estimates of the molecular densities are proposed based on the determined effective temperatures under the assumption of a Boltzmann distribution of the population density over the molecular levels at any local volume of the reactor. For a more accurate interpretation of LAS data of plasma-assisted processes, the spatial distribution of the temperature along line-of-sight has to be known and needs to be taken into account to obtain the molecular densities.

The density evolution of $\mathrm{HCN}$, $\mathrm{NH_3}$, $\mathrm{H_2O}$ and $\mathrm{CO}$ molecules over time was monitored by laser absorption spectroscopy in a low pressure DC pulsed discharge in N₂–H₂ gas mixtures with addition of CH₄ or O₂. The discharge was maintained in an industrial-scale, active screen (AS) plasma nitrocarburizing (ASPNC) reactor with a steel AS. The measured species densities were analysed using a simplified kinetic model that includes three characteristic times for chemical processes in the ASPNC reactor. The shortest time (1–4 min) was associated with the gas residence time in the reactor, the middle one (about 20 min) was assigned to surface reactions on the AS and workload, whereas the largest one (about 3–5 h) was assigned to surface reactions on the cold reactor walls. The work highlights the importance of monitoring the gas composition during plasma nitrocarburizing processes in order to maintain defined treatment conditions and compensate for continuously changing chemical kinetics at the internal reactor surfaces.

Runaway electrons (RAEs) are believed to affect the dynamics of ultra-fast gas breakdown significantly. In this work, considering the field enhancement effect near the micro-protrusion on the cathode surface, the formation of RAEs and diffuse discharge in atmospheric pressure air is investigated by two-dimensional particle-in-cell/Monte Carlo collision simulation. It is found that the beam amplitude of RAEs is dictated by the field enhancement factor and the initial energy of electrons obtained near the micro-protrusion is decisive for their converting to RAEs, which precede the low energy electrons and guide the discharge propagation by improving pre-ionization. As a result, the discharge transfers from the filamentary mode without RAEs to the diffuse mode under the high pre-ionization degree due to RAEs and a wide streamer with a diameter comparable with the gap distance is formed, which transfers from spherical to conical shape. The results of this study illustrate the fundamental process of RAE formation and how RAEs influence streamer dynamics during ultra-fast gas breakdown process.

When a phosphate-buffered saline (PBS) solution is exposed to atmospheric-pressure plasmas generated in air, hydrogen peroxide $\mathrm{H}_{2}\mathrm{O}_{2}$ in the solution is known to be decomposed by hypochlorite $\mathrm{OCl}^{-}$, which is formed in the solution from reactions between chlorine ions $\mathrm{Cl}^{-}$ present in the PBS solution and plasma-generated reactive oxygen species. Global numerical simulations of liquid-phase chemical reactions were performed to identify the reaction mechanisms of $\mathrm{H}_{2}\mathrm{O}_{2}$ decomposition by solving known liquid-phase chemical reactions self-consistently. It has been confirmed that the decomposition of $\mathrm{H}_{2}\mathrm{O}_{2}$ is indeed mostly due to the presence of $\mathrm{OCl}^{-}$ in the solution. However, this study has also found that, in the presence of abundant hydroxyl ($\mathrm{OH}$) radicals, the most efficient $\mathrm{H}_{2}\mathrm{O}_{2}$ decomposition pathway can be a series of reactions that we call a chlorine monoxide cycle, where $\mathrm{OCl}^{-}$ first reacts with $\mathrm{OH}$ to generate chlorine monoxide $\mathrm{ClO}$, which then decomposes $\mathrm{HOCl}$, rather than $\mathrm{OCl}^{-}$ directly decomposing $\mathrm{H}_{2}\mathrm{O}_{2}$. The chlorine monoxide cycle generates $\mathrm{OH}$ as one of its byproducts, so once this cycle is initiated, it continues until either $\mathrm{ClO}^{-}$ or $\mathrm{H}_{2}\mathrm{O}_{2}$ runs out, as long as none of the intermediates are scavenged by other reactions.

LiDB is a newly developed database of molecular vibrational and vibronic state radiative lifetimes. It has been created with the aim of enabling radiative effects to be properly captured in low-temperature plasma models. Datasets have been generated for 36 molecules using comprehensive and highly accurate molecular line lists from the ExoMol spectroscopic database. The main data output of LiDB is radiative lifetimes at vibrational state resolution. Partial lifetimes, which give information on the dominant decay channels in a molecule, are also provided. LiDB is freely available to the scientific community and is hosted at www.exomol.com/lidb. Users can dynamically view molecular datasets or use a specially-designed application programming interface to make data requests. LiDB will continue to expand in the future by adding more molecules, important isotopologues, and neutral and singly-charged atomic species.

Many applications involving atmospheric pressure plasma-substrate interactions are enabled by the large fluxes of short-lived reactive species such as OH radicals to the substrate, nonetheless, the accurate measurement of radical densities and fluxes at substrates at atmospheric pressure has received little attention to date, particularly for surface ionization waves. We report the measurement of the OH density distribution in a surface discharge on a fused silica (quartz) substrate generated by an impinging atmospheric pressure plasma jet in dry and humid helium. The OH density is measured by microscopic laser induced fluorescence with a spatial resolution of 10 µm in the direction perpendicular to the quartz substrate. The measured OH diffusive flux varied for the investigated experimental conditions by almost three orders of magnitude and had a maximum value of 1.7 × 10¹⁵ cm⁻² s⁻¹. The corresponding surface loss probability of OH on the quartz surface was determined to be ∼0.01. The high spatial resolution was required to accurately resolve the near surface gradient of OH radicals.

We have performed hybrid kinetic-fluid simulations of a positive column in alternating current (AC) argon discharges over a range of driving frequencies f and gas pressure p for the conditions when the spatial nonlocality of the electron energy distribution function (EEDF) is substantial. Our simulations confirmed that the most efficient conditions of plasma maintenance are observed in the dynamic regime when time modulations of mean electron energy (temperature) are substantial. The minimal values of the root mean square electric field and the electron temperature have been observed at f/p values of about 3 kHz Torr⁻¹ in a tube of radius R = 1 cm. The ionization rate and plasma density reached maximal values under these conditions. The numerical solution of a kinetic equation allowed accounting for the kinetic effects associated with spatial and temporal nonlocality of the EEDF. Using the kinetic energy of electrons as an independent variable, we solved an anisotropic tensor diffusion equation in phase space. We clarified the role of different flux components during electron diffusion in phase space over surfaces of constant total energy. We have shown that the kinetic theory uncovers a more exciting and rich physics than the classical ambipolar diffusion (Schottky) model. Non-monotonic radial distributions of excitation rates, metastable densities, and plasma density have been observed in our simulations at pR > 6 Torr cm. The predicted off-axis plasma density peak in the dynamic regime has never been observed in experiments so far. We hope our results stimulate further experimental studies of the AC positive column. The kinetic analysis could help uncover new physics even for such a well-known plasma object as a positive column in noble gases.

Corona discharge is a widely-used phenomenon that requires a sharp electrode to generate a strong electric field (10⁶ V m⁻¹) at high voltages (typically in the tens of kV). The advent of nanoelectrodes has overcome the technical limitations of traditional electrodes, dramatically improving the density of discharge points and enabling low voltage (several kV) corona discharges with nanometer-sized tips. Consequently, nanoelectrode discharge technology has the potential to revolutionize the miniaturization of plasma equipment in the future. However, research on the discharge characteristics of nanoelectrodes is still relatively sparse. This paper focuses on an array of carbon nanotubes (ACNTs) and proposes a numerical simulation model based on the hybrid hydrodynamics model and ion migration model. The accuracy and efficiency of this model are demonstrated by a high degree of agreement between the results from numerical simulations and experiments. In addition, the corona discharge characteristics of ACNTs are studied and discussed, particularly the spatiotemporal evolution of charged particles near the tip. This paper may provide a method of analysis for optimizing and broadly applying nanoelectrodes.

Recent results on the efficiency of CO₂ conversion in discharges in CO₂–N₂ mixtures are discussed. Conditions are considered when the dominating conversion mechanism is dissociation of CO₂ molecules in collisions with nitrogen molecules in several electronically excited states. Its efficiency is determined by the values of dissociation yields in these collisions. Knowledge of dissociation yields for various N₂ excited states is rather poor. In this paper, the effects of variation of these yields on the conversion efficiency are evaluated. Comparison of the obtained estimates with available experimental data allows ascertaining the yield values.

Sterilization of skin prior to surgery is challenged by the reservoir of bacteria that resides in hair follicles. Atmospheric pressure plasma jets (APPJs) have been proposed as a method to treat and deactivate these bacteria as atmospheric plasmas are able to penetrate into structures and crevices with dimensions similar to those found in hair follicles. In this paper, we discuss results from a computational investigation of an APPJ sustained in helium flowing into ambient air, and incident onto a layered dielectric similar to human skin in which there are idealized hair follicles. We found that, depending on the location of the follicle, the bulk ionization wave (IW) incident onto the skin, or the surface IW on the skin, are able to launch IWs into the follicle. The uniformity of treatment of the follicle depends on the location of the first entry of the plasma into the follicle on the top of the skin. Typically, only one side of the follicle is treated on for a given plasma pulse, with uniform treatment resulting from rastering the plasma jet across the follicle over many pulses. Plasma treatment of the follicle is sensitive to the angle of the follicle with respect to the skin, width of the follicle pocket, conductivity of the dermis and thickness of the underlying subcutaneous fat layer, the latter due to the change in capacitance of the tissue.

In multi-component vacuum arc discharge, light ions and heavy ions usually have different spatial distributions of density and velocity. Previous research has suggested that the difference in spatial distribution of light and heavy ions is due to the mixing effect of cathode spot jet. However, in this work, the ion collision is found to be an important factor leading to the separation of light and heavy ions. In this paper, multi-fluid model is used to study the effect of ion collisions on separation mechanism in multi-component vacuum arc. The simulation results show that, during the jet mixing process, the collisions between different ions will reduce the velocity of light ions, and greatly increase the density and temperature. As a result, the pressure expansion of light ions is significantly enhanced. In addition, the collision between different ions will also increase the size of jet mixing region for light ions, which makes the plasma jet of light ions mixing more fully. These effects make their isotropic expansion dominant, and the ion density at the center is not much different from that at the edge. However for heavy ions, the collision between different ions has little influence on their movement. The pressure is far less than the inertia force, so the density of heavy ion mainly distributes along the convection direction, and the center is greater than the edge. This is the main separation mechanism of ion angular flux. It is also found there are three main factors leading to the separation of light and heavy ions: ion mass, ion density and ion temperature. The separation effect can be enhanced by increasing ion temperature, decreasing ion density and selecting electrode components with significant differences in elemental mass. This paper provides an insight into the mechanism of ion separation in multi-component vacuum arc.