Session HT6: Poster Session I

Numerical simulation of low-temperature helium plasma source for biomedical applications

Numerical simulation of low-temperature helium plasma for biomedical applications was conducted. The plasma source is presented as a rod electrode located above the grounded plate. Helium acts as a working gas, which is supplied to the discharge through a quartz tube surrounding the rod electrode. An AC voltage with a frequency of 13 kHz and amplitude of up to 3 kV is applied to the electrode. Distance between rod tip and plate varies from 1 to 8 centimeters. Helium blow rate is considered in the range from 1 to 10 m / s. For a description of the discharge, in this paper, two-dimensional extended fluid model was presented. It consists of the continuity equations for calculating the concentration of particles, the energy balance equation for finding the electron temperature and the Poisson equation for electric fields. To calculate the velocity of neutral particles Navier-Stokes equations was solved, and thermal conductivity equation was solved for calculating the heating of the neutral gas.   

Surface discharges generated at metal-semiconductor-gas triple junctions.

Discharges in air at atmospheric pressure as well as high-pressure CO$_{\mathrm{2}}$ up to 15 atm are generated on silicon surfaces using reactor geometries typical of surface dielectric barrier discharges (DBDs), in order to investigate plasma generation and properties at metal-semiconductor-gas triple junctions. Short (10 ns) or long (200 ns) high-voltage pulses are applied at pulse repetition frequencies of 1 -- 1000 Hz. Both p- and n-type silicon are investigated at different doping levels. Discharge generation can be achieved at applied voltages of about 1 kV or less, despite using silicon layers of 0.5 -- 1 mm thickness. The discharge current differs in character from that of other types of nanosecond discharges, such as glows, sparks, and DBDs. The experimental characterization of plasma and surface properties is also presented.   

Generation of anomalously energetic suprathermal electrons by an electron beam interacting with a nonuniform plasma

Electrons emitted from electrodes are accelerated by the sheath electric field and become the electron beams penetrating the plasma. The electron beam can interact with the plasma in collisionless manner via two-stream instability and produce suprathermal electrons. In order to understand the mechanism of suprathermal electrons acceleration, a beam-plasma system was simulated using a 1D3V particle-in-cell code EDIPIC. These simulation results show that the acceleration may be caused by the effects related to the plasma nonuniformity. The electron beam excites plasma waves whose wavelength and phase speed gradually decrease towards anode. The short waves near the anode accelerate plasma bulk electrons to suprathermal energies. Rich complexity of beam- plasma interaction phenomena was also observed: intermittency and multiple regimes of two-stream instability in a dc discharge, band structure of the growth rate of the two-stream instability of an electron beam propagating in a bounded plasma, multi-stage acceleration of electrons in a finite system.   

Influence of pressure on ion energy distribution functions in EUV-induced hydrogen plasmas

Next-generation lithography tools currently use Extreme Ultraviolet (EUV) radiation to create even smaller features on computer chips. The high energy photons (92 eV) induce a plasma in the low pressure background gas by photoionization. Industries have realized that these plasmas are of significant importance with respect to machine lifetime because impacting ions affect exposed surfaces. The mass resolved ion energy distribution function (IEDF) is therefore one of the main plasma parameters of interest. In this research an ion mass spectrometer is used to investigate IEDFs of ions impacting on surfaces in EUV-induced plasmas. EUV radiation is focused into a vessel with a low pressure hydrogen environment. Here, photoionization creates free electrons with energies up to 76 eV, which further ionize the background gas. The influence of the pressure on plasma composition and IEDFs has been investigated in the range 0.1-10 Pa. In general the ion fluxes towards the surface increase with pressure. However, above 5 Pa the flux of $H_2^+$ is not affected by the increase in pressure due to the balance between the creation of $H_2^+$ and the conversion of $H_2^+$ to $H_3^+$. These results will be used to benchmark plasma scaling models and verify numerical simulations.   

Plasma forces on deposited particles

A plasma can have many effects on a substrate. In this contribution we focus on its effects on micrometer sized particles on the substrate. We are especially interested in forces acting on these particles. These have been suggested to be responsible for the lunar glow observed by the Apollo mission astronauts. They have recently also attracted interest as a possible cleaning mechanism for the high-tech industry. We will present experimental measurements of the forces acting on a particle on a substrate under influence of a plasma. To this extend we have developed two specialised experimental setups. They use extreme accelerations (up to one million times the earth gravitational acceleration) to balance forces on the particle. We will show quantitative measurements of the plasma force effects, and show what underlying physical effects cause them.   

Hydrodynamic ion sound instability in systems of a finite length

Plasmas permeated by an energetic ion beam is prone to the kinetic ion-sound instability that occurs as a result of the inverse Landau damping for ion velocity. It is shown here that in a finite length system there exists another type of the ion sound instability which occurs for $% v_{0}^{2} [Preview Abstract]

What is the size of a floating sheath? An answer

The formation of a non-neutral boundary sheath in front of material surfaces is universal plasma phenomenon. Despite several decades of research, however, not all related issues are fully clarified. In a recent paper, Chabert pointed out that this lack of clarity applies even to the seemingly innocuous question ``What the size of a floating sheath?'' [Plasma Sources Sci. Technol. 23 (2014) 065042] This contribution attempts to provide an answer that is not arbitrary: The size of a floating sheath is defined as the plate separation of an equivalent parallel plate capacitor. The consequences of the definition are explored with the help of a self-consistent sheath model, and a comparison is made with other sheath size definitions.   

Experimental Study of a Pulsating Anode Spot in Helium

Anode spots occur when a sufficiently small electrode is biased well above the plasma potential. Under these conditions, electrons are accelerated toward the electrode obtaining adequate energy to ionize the background gas near the face of the electrode. With a large enough bias, a threshold is exceeded causing the ionization region to rapidly expand into a high potential plasma encompassed by a double layer. While this secondary plasma can be stable, it is often observed to possess interesting dynamics. In this work, we examine a pulsating anode spot formed in helium above a solid electrode. Said spot exhibits no stable condition, but instead repeatedly forms and collapses with a frequency on the order of 10 kHz, varying with pressure and electron density. Higher frequency phenomena, on the order of 1 MHz, are also observed during the collapse of the spot. We consider several measurements of the spot properties in order to better understand the physics of its formation and collapse as well as the associated timescales.   

Observation of ExB effect on contact angle of incident deuterium ion at the graphite target of the weakly magnetized plasmas

Many fusion researches have been considered that the ion incident angle at the first wall or the divertor surface is that of B-field line. Ahedo predicted that the ion motion should be influenced by the E-field near the plasma boundary, that is the E x B drift. To verify his prediction, the discrepancy between the ion incident angle and the angle of B-field line to the surface was investigated in this study. A weakly magnetized D$_{\mathrm{2}}$ ECR plasma was used to investigate the ion incident angle. The ion incident angle was measured from a morphological change of a graphite target during ion irradiation, changing the B-field angle to the target surface. The B-field strength near the target was 700 gauss. Result reveals that the ion incident angle becomes 16\textordmasculine when the field angle is 85\textordmasculine , for example. The result is comparable with the estimation of the Ahedo's magnetic sheath model. With the model, it can be understood that the ion trajectory starts to deviate from the B-field line inside the presheath where the E-field start to increase. For the B-field strength of our device, however, the strong E-field inside the sheath accelerates ions only to the surface-normal direction, and no E x B drift occurs there. Details with a consideration of the B-field strength will be discussed.   

Mechanism of plasma-assisted ignition for H$_{\mathrm{2}}$ and C1-C5 hydrocarbons

Nonequilibrium plasma demonstrates ability to control ultra-lean, ultra-fast, low-temperature flames and appears to be an extremely promising technology for a wide range of applications, including aviation GTEs, piston engines, ramjets, scramjets and detonation initiation for pulsed detonation engines. To use nonequilibrium plasma for ignition and combustion in real energetic systems, one must understand the mechanisms of plasma-assisted ignition and combustion and be able to numerically simulate the discharge and combustion processes under various conditions. A new, validated mechanism for high-temperature hydrocarbon plasma assisted combustion was built and allows to qualitatively describe plasma-assisted combustion close and above the self-ignition threshold. The principal mechanisms of plasma-assisted ignition and combustion have been established and validated for a wide range of plasma and gas parameters. These results provide a basis for improving various energy-conversion combustion systems, from automobile to aircraft engines, using nonequilibrium plasma methods.   

Effects of ROS and RNS in non-equilibrium plasma enhanced oxidizing and nitriding

Plasma enhanced oxidizing and nitriding processes are of great interest for physics and applications [1]. However, despite all advances in plasma technology, mechanisms of non-equilibrium plasma chemistry are not quite clear, particularly concerning reactive oxygen and nitrogen species (ROS/RNS) in metastable states. We tried to study this matter more detail. Experiments were done in a low temperature magnetron with a non-self-sustained glow discharge in oxygen/nitrogen/argon mixtures, employing electrical and optical diagnostics. Measurements showed that plasma processing is accompanied by the formation of electronically excited particles ROS/RNS. Computer modeling by using 0D-kinetic and 1D-fluid models including ionization, excitation, dissociation-recombination, vibrational relaxation, collisional quenching and radiation revealed the most probable mechanisms of plasma-chemical transformations. Effects of metastables of singlet oxygen O$_{\mathrm{2}}^{\mathrm{\ast }}$(a,b) and nitrogen N$_{\mathrm{2}}^{\mathrm{\ast }}$(A) as well as small but important radicals O$^{\mathrm{\ast }}(^{\mathrm{1}}$D), N$^{\mathrm{\ast }}(^{\mathrm{2}}$D) were also examined. Our study confirms the role of ROS/RNS in plasma kinetics and indicates the way toward more efficient oxygen and nitrogen plasma processing. [1] M.A. Lieberman, A.J. Lichtenberg, Principles of Plasma Discharges and Materials Processing, John Wiley {\&} Sons, 2005.   

Aluminum Surface Morphology Evolution under High-Flux Helium Ions Bombardment

Aluminum samples with purity 99.99{\%} wt. were irradiated with He ion beam under ITER-like conditions using FALCON ion source. Aluminum has been used as the surrogate for plasma-material interaction studies. Typical parameters during steady-state expose were the following: He ion flux was 2-4x10$^{\mathrm{22}}$ m$^{\mathrm{-2}}$s$^{\mathrm{-1}}$, heat flux was above 1 MW m$^{\mathrm{-2}}$, average ion energy was 2 keV. The exposure fluence was well above 10$^{\mathrm{26}}$ m$^{\mathrm{-2}}$. Surface morphology evolution was investigated with SEM. Cone-like surface structures of a different size and shape were found on the surface. These structures are similar to cones observed after experiments on PISCES-B, Magnum-PSI, Pilot-PSI. Cone-like structures are arranged separately from each other at lower fluence. At higher fluence, their number is increased and they tend to form mountain-like clusters. Column-like structures growth with fluence and become higher. Grass-type structures and flakes were not readily observable on the surface at lower fluence, but could be found anywhere at higher fluence. Increasing the fluence, one can also observe formation of the crack network and its propagation. The observed structures, if present on the beryllium first wall, may cause exfoliation and local melting of the material with consequent excessive erosion of the first wall and contamination of the edge plasma.   

Separated effects of ions, metastables and photons on the properties of barrier layers on polymers

Analyses of a-C:H /a-Si:H multilayers on polymer substrates indicated that prolonged ion bombardment influences negatively the properties of the barrier layer, while a short plasma pretreatment can improve the barrier effect. This work is motivated by these results and investigates the influence of different reactive plasma components, namely ions, metastables and VUV-photons, on the properties of the grown barrier layer. To separate the different species and their influence on plasma pretreatment and film growth, we build a grid system, which repels the ions from the substrate, so that only metastables and VUV-photons have an effect on the layer. An integral part of this investigation is, to measure the photon fluxes to the substrate by an intensity calibrated VUV monochromator. For that, a differentially pumped monochromator with a spectral range 30 -- 300 nm is used, where the two most prominent argon lines at 104.9 and 106.8 nm can be measured. In this approach we are able to study the different effects of the plasma species and also possible synergy effects, to improve the properties of the barrier layer.   

Composite layers for barrier coatings on polymers

Amorphous hydrogenated carbon (a-C:H), amorphous hydrogenated silicon (a-Si:H), and SiO2 thin films are of high interest because they can serve as a gas barrier on polymers. To understand how the coating changes the overall barrier properties of the thin film-polymer system, optical, mechanical, and barrier properties have to be studied. One of the important characteristic of such coatings is their compressive stress, which has beneficial as well as unwanted effects. The stress can cause deformation of the bulk material or de-lamination of the film. The mechanical stability can be improved and it is possible to reduce cracking due to elongation, as the compressive stress can compensate externally applied tensile strain. Stress and mechanical properties of composite layers can be manipulated directly by embedding nanoparticles in an amorphous matrix film. Therefore nanoparticles and amorphous layers are investigated before they can be assembled in a composite layer. Growth rates as well as optical and mechanical properties are explored in this work. An inductively coupled plasma source was used for all amorphous layers and the silicon nanoparticles with diameter around 5 nm were produced in a capacitively coupled plasma reactor.   

Runaway Electron Preionized Diffuse Discharge and Its Impact on Plane Anode.

The spatial structure of a runaway electrons preionized diffuse discharge (REP DD) in nonuniform electric field and the influence of its plasma on the surface of a plane anode have been studied. In our experiments, we used a NPG-18/3500N high-voltage generator. The incident voltage had negative polarity, amplitude of \textasciitilde 20 kV, and FWHM of 6 ns; the discharge current was up to 200 A. The discharge plasma was formed in nitrogen by applying high voltage pulses to the interelectrode gap which was varied between 2 and 9 mm. Under such conditions, the specific input power reached up to 10 MW/cm$^{\mathrm{3}}$. It is established that diffuse channel is the initial stage of the discharge radiation; then anode spot, channel with high glow intensity based on the anode spot and spark channel are consecutively formed. Spark formation finished within 10--15 ns after the onset of the discharge. Microstructure of spark and diffuse channels with anode spot autograph have been detected. The traces of such discharge represents itself an aggregation of up to 100 microcraters with dimeters of 5--100 micrometers. It was also shown that diffuse discharge does not leave erosive action on an anode surface or on its carbon cover.   

Electric field strength determination in filamentary DBDs by CARS-based four-wave mixing

The electric field strength is a basic parameter of non-thermal plasmas. Therefore, a profound knowledge of the electric field distribution is crucial. In this contribution a four wave mixing technique based on Coherent Anti-Stokes Raman spectroscopy (CARS) is used to measure electric field strengths in filamentary dielectric barrier discharges (DBDs). The discharges are operated with a pulsed voltage in nitrogen at atmospheric pressure. Small amounts hydrogen (10 vol{\%}) are admixed as tracer gas to evaluate the electric field strength in the 1 mm discharge gap. Absolute values of the electric field strength are determined by calibration of the CARS setup with high voltage amplitudes below the ignition threshold of the arrangement. Alteration of the electric field strength has been observed during the internal polarity reversal and the breakdown process. In this case the major advantage over emission based methods is that this technique can be used independently from emission, e.g. in the pre-phase and in between two consecutive, opposite discharge pulses where no emission occurs at all. This work was supported by the Deutsche Forschungsgemeinschaft, Forschergruppe FOR 1123 and Sonderforschungsbereich TRR 24 ``Fundamentals of complex plasmas''.   

Measurement of Electron Density Using the Multipole Resonance Probe, Langmuir Probe and Optical Emission Spectroscopy in Low Pressure Plasmas with Different Electron Energy Distribution Functions

In recently publication$^{\mathrm{[1]}}$, the young diagnostic tool Multipole Resonance Probe (MRP) for electron density measurements was introduced. It is based on active plasma resonance spectroscopy (APRS). The probe was simulated und evaluated for different devices. The geometrical and electrical symmetry simplifies the APRS model, so that the electron density can be easily calculated from the measured resonance. In this work, low pressure nitrogen mixture plasmas with different electron energy distribution functions (EEDF) are investigated. The results of the MRP measurement are compared with measurements of a Langmuir Probe (LP) and Optical Emission Spectroscopy (OES). Probes and OES measure in different regimes of kinetic electron energy. Both probes measure electrons with low kinetic energy (\textless 10 eV), whereas the OES is influenced by electrons with high kinetic energy which are needed for transitions of molecule bands. By the determination of the absolute intensity of N$_{\mathrm{2}}$(C-B) and N$_{\mathrm{2}}^{\mathrm{+}}$(B-X) electron temperature and density can be calculated. In a non-maxwellian plasma, all plasma diagnostics need to be combined. [1] C. Schulz \textit{et al.}, IEEE Sensors Journal, \textbf{14}, No. 10, 2014.   

Probe measurements of electron energy spectrum in Helium/air micro-plasma at atmospheric pressure.

It is experimentally demonstrated that a wall probe may be a useful instrument for interpretation of electron energy spectrum in a micro-plasma with a nonlocal electron distribution function at atmospheric pressure. Two micro-plasma devices were fabricated with three layers of molybdenum metal foils with thickness of 0.1 mm separated by two sheets of mica insulation with thickness of 0.11 mm. In one device a hole with the diameter of 0.2 mm formed a cylindrical discharge cavity that passed through the entire five layers. In the second device the hole has the diameter of 0.065 mm. In both devices the inner molybdenum layer formed a wall probe, while the outer layers of molybdenum served as the hollow cathode and anode. The discharge was open into air with flow of helium gas. It is found that the wall probe I-V trace is sensitive to the presence of helium metastable atoms. The first derivative of the probe current with respect to the probe potential shows peaks revealing fast electrons at specific energies arising due to plasma chemical reactions. The devices may be applicable for developing analytical sensors for extreme environments, including high radiation and vibration levels and high temperatures.   

Time Resolution of Electron Density Measurement using Mach Zehnder Interferometer in Arc Discharge Plasma

Sample return mission from Jupiter Trojans is proposed for future mission in JAXA. Reentry velocity in this mission is estimated at 14 kilometers per second. Although an accurate estimation of radiation heating is required when reentry velocity is very high, it is reported that there is discrepancy between predicts and experimental results. Precursor photoionization is considered as the causation of it, and measurement of electron density over ahead of strong shock waves to behind of is acquired for figuring out this discrepancy. In this study, the goal is construction of Mach-Zehnder interferometer which is applicable to hyper velocity shock waves and is acquirable to electron density distribution in them. First, a plasma source has been developed for a measuring object. In addition, the interferometer will be applied to measure electron density.   

Measuring atomic oxygen densities and electron properties in an Inductively Coupled Plasma for thin film deposition.

Plasma Enhanced Pulsed Laser Deposition (PE-PLD) is an advanced way of depositing thin films of oxide materials by using a laser to ablate a target, and passing the resulting plasma plume through a background Inductively-Coupled Plasma (ICP), instead of a background gas as is done in traditional PLD. The main advantage of PE-PLD is the control of film stoichiometry via the direct control of the reactive oxygen species in the ICP instead of relying on a neutral gas background. The aim is to deposit zinc oxide films from a zinc metal target and an oxygen ICP. In this work, we characterise the range of compositions of the reactive oxygen species achievable in ICPs; in particular the atomic oxygen density. The density of atomic oxygen has been determined within two ICPs of two different geometries over a range of plasma powers and pressures with the use of Energy Resolved Actinometry (ERA). ERA is a robust diagnostic technique with determines both the dissociation degree and average electron energy by comparing the excitation ratios of two oxygen and one argon transition. Alongside this the electron densities have been determined with the use of a hairpin probe.   

Quantitative measurement of VUV radiation related to polymer pre-treatment in a microwave driven low pressure plasma

Plasma pre-treatment of polymers is used for a wide range of applications, e.g. prior to deposition of thin SiO$_{x}$ barrier films. At this, plasma generated particles and vacuum ultraviolet (VUV) radiation can reach the polymer surface. Both have a severe impact on the polymer interface, resulting in the production of e.g. dangling bonds. These modifications govern subsequent thin film growth. For understanding of pre-treatment processes, VUV radiation has to be quantified. Absolute VUV photon fluences are determined in situ, at the substrate holder, applying sodium salicylate (NaSal) as a scintillator. Therefore, VUV photons are quantified from 50 nm to 325 nm, due to constant quantum efficiency of NaSal, as integrals over defined wavelength ranges (50-110, 110-170, 170-200 and 200-325 nm). The set up allows for measurement with three scintillators. Each is equipped with optical filters. Observation of the fluorescence band is performed by means of optical fibers and a photomultiplier. Quantification is achieved by simultaneous measurement with an absolutely calibrated echelle spectrometer in the spectral range from 200 nm to 325 nm, taking into account observed plasma volumes. VUV photons are quantified for argon and oxygen plasmas as well as mixtures of both.   

Developing a diagnostic tool for measuring maximum effective temperature within high pressure electrodeless discharges.

Here we present an investigation into the feasibility of creating a diagnostic tool for obtaining maximum arc temperature measurements within a high pressure electrodeless discharge; utilizing integrating sphere measurements of optically thin lines emitted from mercury atoms within commercially available high pressure mercury lamp arc tubes. The optically thin lines chosen were 577 nm and 1014 nm from a 250 W high pressure mercury lamp operated at various powers. The effective temperature could be calculated by considering the relative intensities of the two optically thin lines and comparison with the theoretical ratio of the temperature dependent power emitted from the lines derived from the atomic spectral data provided by NIST. The calculations gave effective arc temperatures of 5755, 5804 and 5820 K at 200, 225, 250 W respectively. This method was subsequently used as a basis for determining maximum effective arc temperature within microwave-driven electrodeless discharge capsules, with varying mercury content of 6.07, 9.4 and 12.95 mg within $1\times$ $10-^6$ $m^3$ giving maximum effective temperatures of 5163, 4768 and 4715 K respectively at $\sim$240 W.   

Damage development of gallium nitride under plasma exposure

Plasma damage has been focused on since 1990s. In this era, this issue was manly targeted onto silicon-based semiconductors. However, since the gallium nitride (GaN) was paid attentions to after blue LEDs, they start to consider the damages given to GaN as well. We have so far utilized photoluminescence (PL) emission from the surface of GaN film to monitor the evolution of damage given by plasma exposure. This measurement gives us clues how plasma exposure changed intermediate electronical states in the film without taking the film out of the chamber. First of all, we analyzed the development of damage given by argon plasma, which is one of the most fundamental plasma to analyze. Argon plasma is responsible to give only physical damages over a GaN film. Our PL measurements showed a significant decrease within approximately 10 seconds after the plasma exposure started. This means that ions and radiations created from the plasma gives significant damages to the GaN film even short period of time. Chlorine-related gas is normally utilized for chemical etching. Chlorine species realize continuous damage layer removals, but some reports already mentioned that the processed device has difference electrical properties after the plasma exposure. In this presentation, we will show what happens to GaN film after the plasma exposure in terms of crystal structure and impurities of GaN, by connecting PL emission and ex-situ measurements.   

Time-resolved probe measurement in pulsed plasma using advanced boxcar technique

A novel plasma diagnostic method based on boxcar technique is developed for time-resolved measurement of electron energy distribution function (EEDF) in pulsed plasma. Pulsed plasma have been used for many applications including etching and deposition in semiconductor manufacturing because the pulsed plasma reduces the plasma induced damage (PID). In order to understand underlying physics of the pulsed plasma, probe measurement based on boxcar theory have been performed. However, in conventional method, measurement time is very long (over an hour for measurement of 1 kHz pulsed plasma), and displacement current, generated due to use of impulse voltage, can result in significant inaccuracy of EEDF especially in low density plasma. In this work, a novel method using sequential switching of dc voltage and reconstruction of measured current is proposed; the detail procedures were mentioned by Godyak and Alexandrovich (Proceeding of XXVIIth ICPIG, 2005). This method reduces the time required for the measurement to a few minutes, and there is not influence of the displacement current, resulting in reliable measurement of EEDF. Using this method, well-known characteristics in active and after-glow of the pulsed plasma are well observed.   

High time-resolution spatial distribution measurement of the ion flux and the electron temperature in an inductively coupled pulse plasma

The time-resolved spatial distribution of the plasma parameters are measured in the pulse-modulated inductively coupled argon discharge. During the initial active-glow period, the ion flux and the electron temperature beneath the antenna are higher than the center of the reactor. While the plasma is approaching a steady state active-glow, the spatial distributions of the ion flux and the electron temperature evolve a center-high profile. After the pulse is off, both the ion flux and the electron temperature are decreased maintaining their center-high profile. At the bottom surface, on the other hand, the center-high distribution profile is observed from the beginning of the active-glow, and maintained during the after-glow period. Compared to the continuous wave discharge, the spatial uniformity of the plasma parameters is improved during the active-glow period, and it is increased with the increasing pulse frequency. These spatial distribution characteristics can be explained by the discharge mechanism and the diffusion of charged particles, and it should be considered to achieve desirable process results in the pulse-modulated plasma material processing.   

Helium temperature measurements in a hot filament magnetic mirror plasma using high resolution Doppler spectroscopy.

Langmuir probe and spectroscopic diagnostics are used to routinely measure electron temperature and density over a wide operating range in a reconfigured Double Plasma device at University College Cork, Ireland\footnote{PJ Mc Carthy et al., 30$^{th}$ ICPIG, Belfast, 2011}. The helium plasma, generated through thermionic emission from a negatively biased tungsten filament, is confined by an axisymmetric magnetic mirror configuration using two stacks of NdFeB permanent magnets, each of length 20 cm and diameter 3 cm placed just outside the 15 mm water cooling jacket enclosing a cylindrical vacuum vessel of internal diameter 25 cm. Plasma light is analysed using a Fourier Transform-type Bruker spectrometer with a highest achievable resolution of 0.08 $cm^{-1}$. In the present work, the conventional assumption of room temperature ions in the analysis of Langmuir probe data from low temperature plasmas is examined critically using Doppler spectroscopy of the 468.6 nm He II line. Results for ion temperatures obtained from spectroscopic data for a variety of engineering parameters (discharge voltage, gas pressure and plasma current) will be presented.   

Optical emission spectroscopy of 'spokes' in a high power impulse magnetron discharge

Localized regions of intense light emission can be observed in front of the target of a high power impulse magnetron sputtering (HiPIMS) discharge. These regions are often referred to as 'spokes' and have been observed to rotate in E x B direction with frequencies in the order of 100 kHz. The spokes are located close to the target inside the zone of magnetic confinement where the magnetic field lines are closed. Outside this zone, the HiPIMS discharge has already been investigated thoroughly by the community using Langmuir probes. However, inside this zone a probe would change the magnetic field and disturbe the discharge. In this work, the spokes are therefore investigated using optical emission spectroscopy. A high resolution plane grating spectrograph combined with a fast, gated, intensified CCD camera is employed to analyse the discharge. Line broadening of the Balmer series of atomic hydrogen is studied by adding a small admixture of hydrogen to the argon used as the working gas. Additionally, the influence of reactive admixtures on the discharge, such as nitrogen or oxygen, is investigated.   

A computational study of the plasma-flow interplay in a reverse vortex microwave discharge for CO$_{\mathrm{2}}$ conversion

The problem of global warming due to greenhouse gas emission is one of the most prominent and urgent problems of the 21$^{\mathrm{st}}$ century. Recently, surface wave produced plasmas, created by a microwave discharge, have shown to be very efficient in the conversion of the main emitted greenhouse gas, namely CO$_{\mathrm{2}}$. This is the result of a high thermodynamic inequilibrium in which the CO$_{\mathrm{2}}$ is efficiently dissociated through vibrational excitation. Very promising results have been obtained in experiments using a reverse vortex gas flow (W.A. Bongers et al., ISPC 2015). Although it is known that reverse vortex gas flows tend to create a pressure and temperature drop in the center, it is unclear which effect the flow and the plasma have on each other. In this study we model this interplay between the reverse vortex gas flow and the plasma, to get a deeper understanding of the underlying processes. As a first step, Argon gas is used due to its simpler chemistry, limiting the computational costs. In a next step, a reduced chemistry set of CO$_{\mathrm{2}}$ will be implemented.   

Higher order moment models of electron transport in gases and liquids

This study seeks to extend an existing higher order (four) moment model to consider electron transport in gases and liquids. The impact of coherent scattering and other liquid effects are included into the moment model. By reconciling existing closure approximations into a new closure assumption, the subsequent moment model will be studied to understand the accuracies and sensitivities of various closure assumptions used in practice. A particular focus is the ability of the higher order moment model to treat spatially varying electric fields including interfaces, through comparison of the results with a space-time dependent solution of Boltzmann's equations.   

On judgement of electron transfer between two regions divided by the separatrix of confronting divergent magnetic fields applied to an inductively coupled plasma

In order to quantitatively evaluate the electron confinement effect of the confronting divergent magnetic fields (CDMFs) applied to an inductively coupled plasma,\footnote{T. Tsankov and U. Czarnetzki 2011 IEEE Trans. Plasma Sci. {\bf 39}, 2538.} we analyzed the electron transfer between two regions divided by the separatrix of the CDMFs in Ar at 0.67\,Pa at 300\,K using a Monte Carlo method. A conventional transfer judgement was simply based on the electron passage across the separatrix from the upstream source region to the downstream diffusion region. An issue was an overestimation of the transfer due to temporary stay of electrons in the downstream region. Electrons may pass the downstream region during their gyration even in case they are effectively bound to the upstream region, where their guiding magnetic flux lines run. More than half of the transfers were temporary ones and such seeming transfers were relevantly excluded from the statistics by introducing a newly chosen criterion based on the passage of electron gyrocenters across the separatrix and collisional events in the downstream region. Simulation results showed a tendency that the ratio of the temporary transfers excluded was higher under stronger magnetic fields because of higher cyclotron frequency.   

The Generalized Onsager Model and DSMC Simulations of High-Speed Rotating Flow with Swirling Feed

The generalized Onsager model for the radial boundary layer and of the generalized Carrier-Maslen model for the axial boundary layer at the end-caps in a high-speed rotating cylinder ((S. Pradhan {\&} V. Kumaran, J. Fluid Mech., 2011, vol. 686, pp. 109-159); (V. Kumaran {\&} S. Pradhan, J. Fluid Mech., 2014, vol. 753, pp. 307-359)), are extended to incorporate the angular momentum of the feed gas for a swirling feed for single component gas and binary gas mixture. For a single component gas, the analytical solutions are obtained for the sixth-order generalized Onsager equations for the master potential, and for the fourth-order generalized Carrier-Maslen equation for the velocity potential. In both cases, the equations are linearized in the perturbation to the base flow, which is a solid-body rotation. The equations are restricted to the limit of high Reynolds number and (length/radius) ratio, but there is no limitation on the stratification parameter. The linear operators in the generalized Onsager and generalized Carrier-Maslen equations with swirling feed are still self-adjoint, and so the eigenfunctions form a complete orthogonal basis set. The analytical solutions are compared with direct simulation Monte Carlo (DSMC) simulations. The comparison reveals that the boundary conditions in the simulations and analysis have to be matched with care. When these precautions are taken, there is excellent agreement between analysis and simulations, to within 15{\%}.   

The generalized Onsager model and DSMC simulations of high-speed rotating flows with product and waste baffles

The generalized Onsager model for the radial boundary layer and of the generalized Carrier-Maslen model for the axial boundary layer in a high-speed rotating cylinder ((S. Pradhan {\&} V. Kumaran, J. Fluid Mech., 2011, vol. 686, pp. 109-159); (V. Kumaran {\&} S. Pradhan, J. Fluid Mech., 2014, vol. 753, pp. 307-359)), are extended to a multiply connected domain, created by the product and waste baffles. For a single component gas, the analytical solutions are obtained for the sixth-order generalized Onsager equations for the master potential, and for the fourth-order generalized Carrier-Maslen equation for the velocity potential. In both cases, the equations are linearized in the perturbation to the base flow, which is a solid-body rotation. An explicit expression for the baffle stream function is obtained using the boundary layer solutions. These solutions are compared with direct simulation Monte Carlo (DSMC) simulations and found excellent agreement between the analysis and simulations, to within 15{\%}, provided the wall-slip in both the flow velocity and temperature are incorporated in the analytical solutions.   

Simulation for spatio-temporal variation of chemically active species in an atmospheric pressure streamer discharge..

Spatiotemporal variation of radical density in an atmospheric pressure plasma discharge has been investigated by two-dimensional numerical simulation. Behaviors of radicals are characterized by four areas as ”Hot anode region”, “Secondary streamer region”, “Primary streamer region”, and “Near-cathode region”. Although the reduced electric field in ”Hot anode region” is relatively high, the gas temperature also increases and the ozone destruction process proceed. On the other hand, in “Near-cathode region”, the high-energy radicals such as N(4S) is effectively produced because the instantaneous value of reduced electric field is high. Behaiviour of OH is also investigated. The results show that OH is effectively produced in “Secondary streamer region” and is not effective in “Hot anode region”. This is because the reduced electric filed in “Secondary streamer region” is sufficiently high for the dissociation of H2O by O(D) and N2(a) and the gas temperature in “Hot anode region ” is too high for the production of OH.   

Escape factors for Paschen 2p-1s lines in Ar, Kr, and Xe plasmas.

Radiation trapping is often observed when investigating low-temperature plasmas. Photons emitted from an upper state may be reabsorbed by a lower state before they leave the plasmas. To account for this effect, the ``escape factor'' as a function of optical depth is often adopted. In previous works several simple expressions of the escape factor were proposed for uniform plasmas with emission line profiles dominated by Doppler broadening and without line splitting due to hyperfine structure. These assumptions are valid for atoms e.g. Ar in uniform discharges. However, the excited state density in many low-temperature plasmas is non-uniform and the emission line profile can be influenced by collisional broadening. In this work, we study the escape factors of Paschen 2p-1s lines of Ar, Kr, and Xe in non-uniform plasmas. The collisional broadening and the hyperfine structure for Kr and Xe lines are both included. The calculated escape factor expression is verified particularly by an experiment in a low-pressure discharge. The escape factor equation for high- to atmospheric-pressure discharges is also provided.   

Planar Multipol-Resonance-Probe: A Spectral Kinetic Approach

Measuring plasma parameters, e.g. electron density and electron temperature, is an important procedure to verify the stability and behavior of a plasma process. For this purpose the multipole resonance probe (MRP) represents a satisfying solution to measure the electron density. However the influence of the probe on the plasma through its physical presence makes it unattractive for some processes in industrial application. A solution to combine the benefits of the spherical MRP with the ability to integrate the probe into the plasma reactor is introduced by the planar model of the MRP (pMRP). Introducing the spectral kinetic formalism leads to a reduced simulation-circle compared to particle-in-cell simulations. The model of the pMRP is implemented and first simulation results are presented.   

A progress report on the LXCat project

LXCat is an open-access, web-based platform (www.lxcat.net) for storing, exchanging and manipulating data for modeling the electron and ion components of low-temperature, non-equilibrium plasmas. The data types supported by LXCat are electron and ion scattering cross sections and rate coefficients, electron and ion swarm/transport parameters, ion-neutral interaction potentials, and optical oscillator strengths. On-line tools allow for searching, graphical display, and downloading of data, and an on-line Boltzmann solver allows users to calculate electron transport and rate coefficients in arbitrary gas mixtures if ``complete'' sets of cross sections for the individual components are available in the databases. At present, 24 public databases contributed by different groups around the world can be accessed on LXCat. The database contributors retain ownership and are responsible for the contents and maintenance of the individual databases. New contributors are welcome and can request an account and receive instructions for setting up a password-protected database on LXCat. This presentation will summarize the LXCat project objectives, its structure, the available databases, and the current status of the project.   

Sputtering, Plasma Chemistry, and RF Sheath Effects in Low-Temperature and Fusion Plasma Modeling

A new sheath boundary condition [Jenkins and Smithe, PSST {\bf 24}, 015020 (2015)] has been implemented in VSim, a plasma modeling code which makes use of both PIC/MCC and fluid FDTD representations. It enables physics effects associated with DC and RF sheath formation - local sheath potential evolution, heat/particle fluxes, and sputtering effects on complex plasma-facing components - to be included in macroscopic-scale plasma simulations that need not resolve sheath scale lengths. We model these effects in typical ICRF antenna operation scenarios on the Alcator C-Mod fusion device, and present comparisons of our simulation results with experimental data together with detailed 3D animations of antenna operation. Complex low-temperature plasma chemistry modeling in VSim is facilitated by MUNCHKIN, a standalone python/C++/SQL code that identifies possible reaction paths for a given set of input species, solves 1D rate equations for the ensuing system's chemical evolution, and generates VSim input blocks with appropriate cross-sections/reaction rates. These features, as well as principal path analysis (to reduce the number of simulated chemical reactions while retaining accuracy) and reaction rate calculations from user-specified distribution functions, will also be demonstrated.   

Comparison of initial seed electron generation mechanisms in kinetic simulations of positive streamers

Positive streamer simulations typically resort to initiation by artificially seeding a small region with an initial plasma. However, in order to simulate observed variations in breakdown voltages and times in pulsed voltage experiments [1], a more physical model for the generation of the initial plasma/electrons is necessary. This work will investigate several models of generating the initial seed plasma in an air-filled gap with a dielectric present: a ``typical'' artificial initial plasma, ionization of the background air due to cosmic rays, field emission from the dielectric, and simulation of radiation incident on surfaces prior to applying the voltage resulting in diffuse e$^{\mathrm{-}}$ and O$_{\mathrm{2}}^{\mathrm{-}}$ densities. 2D axisymmetric PIC-DSMC simulations using a detailed e$^{\mathrm{-}}$-air collision model including field-dependent detachment and photon transport [2] will be compared to experiments of an air gap with a dielectric cylinder and a 10 GV/s applied potential [1]. [1] L.B. Biedermann \textit{et al., Dielectric-Directed Surface Flashover under Atmospheric Conditions}, PPC-O-2-6, 2015. [2] C.H. Moore, \textit{et al.}, \textit{Development of PIC-DSMC Air Breakdown Model in the Presence of a Dielectric}, 43$^{\mathrm{rd}}$ IEEE ICOPS, June 19-23, 2016. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.   

A new approach to fluid modeling of Resistive Plate Chambers

We present a 1.5-dimensional model of Resistive Plate Chambers (RPCs) which are used for timing and triggering purposes in many high energy physics experiments. The model is based solely on the hydrodynamic approximation and assumes that the electron collisional source term in the continuity equation can be expanded in terms of gradients of the electron number density. Transport data used in this model are calculated using Monte Carlo simulations and a multi term solution of the Boltzmann equation. The model is employed to study the avalanche to streamer transition in RPCs under the influence of space charge effects and photoionization. In addition, this model is also used to calculate the average induced signals for different RPC configurations and applied electric field strengths. The results are compared with those obtained by classical fluid model with flux or bulk transport data as input parameters. Depending on the specific RPC configuration and applied electric field, the results for the induced charges calculated using these fluid models can differ as much as several hundred percents.   

Complementary approaches to model an RF plasma jet at atmospheric pressure

A nonthermal plasma jet has been investigated by three complementary model approaches. The argon jet consists of two concentric capillaries and two cylindrical electrodes driven by an RF voltage at 27.12 MHz. Investigations of a single filament in the active zone between both capillaries by means of a two-dimensional phase-resolved fluid model yields spatial profiles The heating profile deduced from this approach is used for a comprehensive description of the jet including gas flow and reactions of precursor molecules as well as their transport in the effluent. The obtained radial profiles of particle fluxes of precursor fragments onto the substrate qualitatively agree with measured The third model is devoted to the phenomena of self-organization observed e.g. in the regular azimuthal rotation of the filaments. Using the heating profile from the first approach, a three-dimensional hydrodynamic model of gas flow and heating is used to reveal the relation between the inclination of the filaments and the azimuthal gas velocity component.   

Comparative Study in Stabilization Methods for Capacitively Coupled Plasma Simulation Using Finite Element Method.

Many cases of hydrodynamic plasma analysis solve continuity equation for charged particles and energy balance equation for electron temperature adopting drift-diffusion approximation. In the transient convection-diffusion equation, finite element (FE) and finite difference schemes are unstable when convective term dominates diffusive term. In capacitively coupled plasma (CCP) cases, numerical instability is unavoidable due to enormous convection induced from the high electric field near the electrode. Several numerical stabilization methods have been developed to overcome this kind of instability problem in finite element scheme. Each of discontinuous Galerkin method (DGM), Petrov-Galerkin method (PGM) and characteristic Galerkin method (CGM) which are the developed stabilization methods, are applied to two-dimensional FE fluid code and suitability for CCP model is investigated.   

Simulation of protons energy relaxation in electron gas by molecular dynamics method

Our work is concerned with simulation of heavy charged particles energy relaxation in electron gas. The research was stimulated by antihydrogen experiments that are held in conditions far from conditions of well studied nuclear fusion or gas discharge experiments. We used numerical simulation as a tool to test existing theoretical approaches to classical Coulomb system kinetics. By means of molecular dynamics method we calculated dynamics of energy relaxation of protons in ultracold electron gas. We considered non neutral plasma when number of electrons is much greater than the number of protons. We have shown that boundary conditions have significant influence on simulation results. Two types of boundary conditions were considered – periodic boundary conditions and reflecting walls. The influence of number of particles in the simulation cell was studied. The problem of Coulomb potential modification on small distances was also considered. Simulations were performed for electron densities 10$^8$ cm$^{-3}$, initial temperatures for electrons is equal 10 K and for protons 100 K.   

A collisional-radiative model for low-pressure weakly magnetized Ar plasmas

Collisional-radiative (CR) models are widely investigated in plasma physics for describing the kinetics of reactive species and for optical emission spectroscopy. This work reports a new Ar CR model used in low-pressure (0.01--10 Pa) weakly magnetized (\textless 0.1 Tesla) plasmas, including ECR, helicon, and NLD discharges. In this model 108 realistic levels are individually studied, i.e. 51 lowest levels of the Ar atom and 57 lowest levels of the Ar ion. We abandon the concept of an ``effective level'' usually adopted in previous models for glow discharges. Only in this way the model can correctly predict the non-equilibrium population distribution of close energy levels. In addition to studying atomic metastable and radiative levels, this model describes the kinetic processes of ionic metastable and radiative levels in detail for the first time. This is important for investigation of plasma-surface interaction and for optical diagnostics using atomic and ionic line-ratios. This model could also be used for studying Ar impurities in tokamaks and astrophysical plasmas.   

Streamer propagation in air near and on curved dielectrics.

We simulate propagation of a positive streamer in air around a curved dielectric with a pronounced shading effect. In our setup a positive streamer is launched at the tip of a pin anode and propagates towards a grounded plate cathode. On the way of the streamer propagation path we place a curved dielectric body (e.g., a dielectric ball) of a diameter larger than the streamer diameter. This obstacle makes a streamer move around it. At the corner of the dielectric a surface streamer has a choice of moving along the surface or moving away from it. We explore physical mechanisms that can force a surface streamer to move all the way around a curved dielectric in air and nitrogen-oxygen mixtures. The potential candidates are secondary electron emission such as photoemission or field emission, higher dielectric permittivity, surface charge, lack of photoionization (in pure nitrogen). The problem is relevant for high-voltage technology, where surface streamers are often to be avoided.   

Development of hybrid computer plasma models for different pressure regimes

With increased performance of contemporary computers during last decades numerical simulations became a very powerful tool applicable also in plasma physics research. Plasma is generally an ensemble of mutually interacting particles that is out of the thermodynamic equilibrium and for this reason fluid computer plasma models give results with only limited accuracy. On the other hand, much more precise particle models are often limited only on 2D problems because of their huge demands on the computer resources. Our contribution is devoted to hybrid modelling techniques that combine advantages of both modelling techniques mentioned above, particularly to their so-called iterative version. The study is focused on mutual relations between fluid and particle models that are demonstrated on the calculations of sheath structures of low temperature argon plasma near a cylindrical Langmuir probe for medium and higher pressures. Results of a simple iterative hybrid plasma computer model are also given.   

Current gain of a pulsed dc discharge in low-pressure gases

Current and voltage waveforms of a pulsed gas discharge have been measured in a wide frequency range (20 to 300 kHz) for the two pressure values of 0.1 and 1 Torr using four different technology-relevant gases: nitrogen, oxygen, carbon tetrafluoride and sulfur hexafluoride. It is shown that the current can be substantially increased in the pulsed dc discharge, especially with electronegative gases, as compared with the discharge current relating to the same but constant voltage. The maximum 9-fold current gain is obtained with sulfur hexafluoride. Carbon tetrafluoride furnishes up to 4-fold gain, while nitrogen and oxygen show the typical current gain of 1--2. We suggest the physical explanation of the current gain phenomenon in the pulsed discharge according to which the current gain at the start of the plasma phase of the pulsed discharge is observed due to the diffusion filling of the cathode sheath with charged particles in the afterglow phase. The current gain increase in electronegative gases is attributed to the slower plasma decay rate in this case because of the lower value of the ambipolar diffusion coefficient in the plasma with negative ions.   

Applicability of the Child-Langmuir laws versions for describing the glow discharge cathode sheath in CO$_{\mathrm{2}}$

This work is devoted to the determination of the law that may be applicable to the description of the cathode sheath in CO$_{\mathrm{2}}$. To this end three versions of the Child-Langmuir law have been considered -- a collision free one (for the ions moving through a cathode sheath without collisions with gas molecules) as well as two collision- related versions-- one for a constant mean free path of positive ions and one for a constant mobility of positive ions. The current-voltage characteristics and the cathode sheath thickness of the glow discharge in carbon oxide have been simultaneously measured in the pressure range from 0.05 to 1 Torr and with the discharge current values up to 80 mA. The inter-electrode distance has been chosen such that the discharge consists only of the cathode sheath and a small portion of the negative glow, i.e. the experiments have been performed in short tubes. In this case the voltage drop across the cathode sheath is equal approximately to the voltage drop across the electrodes. In the whole range of the discharge conditions we have studied the cathode sheath characteristics are found to obey correctly only to the Child-Langmuir law version with a constant ion mobility. The reason for this phenomenon may be related with a significant conversion of carbon dioxide molecules.   

Effects of partially covered metallic wave guide on the linear microwave plasma source with TE-TEM power coupling

A linear microwave plasma system with TE-TEM power coupling has been used for large scale PECVD processing. In this system, plasma acts as outer conductor of TEM waveguide and microwave power is broadly absorbed around the quartz tube. Due to large power absorption rate, plasma density decreases along the waveguide. For low input power condition, plasma column cannot reach the end of waveguide. Limiting power absorption area by simple partially covered metallic waveguide can improve plasma uniformity and make longer plasma column. When half of the plasma waveguide area is covered by metal in 450 mm long waveguide, local plasma density increases about 20 {\%} and density non-uniformity along waveguide decreases from 13{\%} to 7{\%} for pressure 100 mTorr, input power 300W, Ar plasma case.   

Radial Distribution of Plasma Parameters in an Asymmetric Coaxial Capacitive Discharge

It has been shown that plasma processing is a promising technique for material removal from the inner surface of superconducting radiofrequency (SRF) cavities used in large particle accelerators. A radiofrequency (rf) Capacitive Coupled Plasma (CCP) is created in a coaxial setup with the powered electrode inside a hollow cylindrical cavity. While a great deal of knowledge has been gathered on effective plasma etching criteria in Ar/Cl$_{\mathrm{2\thinspace }}$discharge such as pressure, temperature, rf power, dc bias voltage, and experiment construction, little is known about important plasma specific parameters. The determination of plasma parameters is important due to the unique cylindrical geometry of the plasma defined by the SRF cavity geometry. This configuration leaves many questions regarding the structure and distribution of the discharge as it relates to radial position. Presented here are the diagnostic methods and subsequent results for both electropositive (Ar) and electronegative (Ar/Cl$_{\mathrm{2}})$ discharges in a cylindrical coaxial rf CCP. Optical Emission Spectroscopy in conjunction with a robust kinetic model of Argon produces electron temperatures and metastable populations with respect to radial positions of the discharge.   

Charged particle dynamics and process control in capacitive RF discharges driven by tailored voltage waveforms in mixtures of Argon and CF$_4$

The electron power absorption dynamics and the Electrical Asymmetry Effect (EAE) are computationally investigated for Argon-CF$_4$ gas mixtures in geometrically symmetric capacitively coupled plasmas. Simulations are performed for both single- and triple-frequency tailored voltage waveforms at 20 and 60 Pa, using a fundamental frequency of 13.56 MHz and its consecutive harmonics. The results at 60 Pa show electron power absorption mode transitions between the Drift-Ambipolar (DA) mode and the $\alpha$-mode induced by varying the admixture of Ar to CF$_4$, which leads to a change of the plasma chemistry. In the triple-frequency cases small argon admixtures (of the order of 10 $\%$) strongly affect the electron power absorption dynamics and the symmetry of the discharge. The change of the electrical generation of a DC self-bias via the EAE, the ion flux-energy distribution functions of different ion species at the electrodes, and the excitation of resonance effects are studied as a function of the mixing ratio of these two gases. The results are expected to be highly relevant for plasma processing, where such gas mixtures are often used.   

PIC/MCC simulation for magnetized capacitively coupled plasmas driven by combined dc/rf sources

Hybrid dc/rf capacitively coupled plasma (CCP) sources have been popular in substrate etching due to their simplicity in the device structure and better plasma property. In this work, the characteristics of magnetized capacitively coupled plasmas driven by combined dc/rf sources are described by a one-dimensional Particle-in-cell/Monte Carlo collision (PIC/MCC) model. The simulation is using a rf source of 13.56MHz in argon and at a low pressure of 50mTorr. The effects of dc voltage and magnetic field on the plasmas are examined for 200--400V and 0--200Gs. It is found that, to some extent, dc voltage will increase the plasma density, but plasma density drops with increasing dc voltage. The magnetic field will enhance the plasma density significantly, due to the magnetic field will increase the electron life time and decrease the loss to the electrodes. In the bulk plasma, electron temperature is increased with the magnetic field but decreased with the dc voltage. The electron temperature in sheath is higher than in bulk plasma, due to stochastic heating in sheath is greater than Ohmic heating in bulk plasma under low gas pressure.   

Frequency dependence of the Electrical Asymmetry Effect in electronegative capacitive RF discharges driven by Tailored Voltage Waveforms

Capacitively coupled RF plasmas operated in CF$_{\mathrm{4}}$ at 80 Pa and driven by voltage waveforms composed of four consecutive harmonics are investigated for different fundamental driving frequencies (2.86 - 13.56 MHz) using PIC/MCC simulations and an analytical model. In contrast to previous findings in electropositive discharges the absolute value of the DC self-bias generated via the Electrical Asymmetry Effect for peak waveforms is found to increase as the fundamental frequency is reduced, providing an increased range over which it can be tuned by phase control. The analytical model reveals that this increased DC self-bias is caused by changes in the spatial profile and the mean value of the net charge density in the grounded electrode sheath induced by varying the fundamental driving frequency for peak waveforms. The spatio-temporally resolved simulation data show that as the frequency is reduced the grounded electrode sheath region becomes electronegative. This strongly affects the electron power absorption dynamics and the discharge symmetry.   

Spatial structure of plasma density and electron temperature in capacitive RF discharges with a single ring-shaped narrow trench of various depths

Capacitive RF plasmas are used for a variety of technological applications, but suffer from low plasma densities and poor lateral uniformity. This limits the system throughput. Here, the effect of implementing a single narrow trench of 2 mm width and various depths (5 -- 15 mm) into the powered electrode on the spatial structure of the electron density and temperature is studied experimentally by probe measurements. The plasma is driven at 13.56 MHz in Argon at a fixed pressure (\textasciitilde 50 Pa) and power (20 W). The plasma density is found to increase in the presence of the trench and its radial profile shows a peak above the trench. The density becomes homogeneous further away from the electrode at all trench depths and the electron temperature distribution remains uniform. The measured radial density profiles are in good agreement with a diffusion model for all trench depths. Under the conditions investigated the trench of 10 mm depth is found to result in the highest density at various axial and radial positions. The results show that the radial uniformity of the plasma density at various axial positions can be improved by using structured electrodes of distinct depths.   

Effect of Electron Energy Distribution on the Hysteresis of Plasma Discharge: Theory, Experiment, and Modeling

Hysteresis, which is the history dependence of physical systems, indicates that there are more-than-two stable points in a given condition, and it has been considered to one of the most important topics in fundamental physics. Recently, the hysteresis of plasma has become a focus of research because stable plasma operation is very important for fusion reactors, bio-medical plasmas, and industrial plasmas for nano-device fabrication process. Interestingly, the bi-stability characteristics of plasma with a huge hysteresis loop have been observed in inductive discharge plasmas Because hysteresis study in such plasmas can provide a universal understanding of plasma physics, many researchers have attempted experimental and theoretical studies. Despite long plasma research, how this plasma hysteresis occurs remains an unresolved question in plasma physics. Here, we report theory, experiment, and modeling of the hysteresis [Sci. Rep. 5, 15254 (2015)]. It was found experimentally and theoretically that evolution of the electron energy distribution (EED) makes a strong plasma hysteresis. In Ramsauer and non-Ramsauer gas experiments, it was revealed that the plasma hysteresis is observed only at high pressure Ramsauer gas where the EED deviates considerably from a Maxwellian shape. This hysteresis was presented in the plasma balance model where the EED is considered. Because electrons in plasmas are usually not in a thermal equilibrium, this EED-effect can be regarded as a universal phenomenon in plasma physics. e-mail:lhc@kriss.re.kr   

Coupling mechanisms in inductive discharges with RF substrate bias driven at consecutive harmonics with adjustable relative phase

Hybrid combinations of inductive and capacitive RF discharges are commonly used for plasma etching because the inductive coupling ensures a high plasma density, while the capacitive coupling allows the control of the ion bombardment energy at the substrate. We experimentally study the coupling mechanisms between the two driving-voltage sources in such a plasma driven inductively at 13.56 MHz and capacitively at 27.12 MHz in argon and neon at low pressure. We find that the resulting DC self-bias can be controlled via the Electrical Asymmetry Effect by adjusting the relative phase between the two driving harmonics in the E-mode. Langmuir probe measurements and Phase Resolved Optical Emission Spectroscopy (PROES) reveal that the addition of the applied RF-bias in the plasma acts as a catalyst for the transition between E- and H-mode. PROES measurements generally show that the electron power absorption dynamics are affected by the relative phase between the two driving voltage waveforms and by the ratio of the inductive to the capacitive driving powers. Finally, the ion flux-energy distribution function is measured at the RF-powered electrode and found also to be affected by coupling effects.   

Effects of hydrogen as sheath gas on properties of inductive coupled thermal plasma for B$_{\mathrm{4}}$C/Cu functionally gradient material preparation

B$_{\mathrm{4}}$C/Cu functionally gradient material (FGM) is a promising candidate for plasma facing material (PFM) in fusion device. Thermal Plasma spraying technology is supposed to be suitable for the B$_{\mathrm{4}}$C/Cu FGM preparation. In this work, inductively coupled thermal plasma (ICTP) is used to prepare the B$_{\mathrm{4}}$C/Cu FGM. However, the high gas temperature of pure Ar plasma can damage the torch and it's thermal conductivity is low. The hydrogen has high thermal conductivity, so the mixture gas of Ar and hydrogen is used as sheath gas to protect the ICP torch and enhance the thermal conductivity of the plasma. Optical emission spectroscopy is used to diagnose the properties of ICTP to determine the optimum condition for preparation process. In addition, to control the preparation process, some atomic emission lines and prepared material changed with experiment conditions will be studied. All the plasma properties would give us an insight on the mechanism and the possibility of improving the process.   

Electron heating in the inductive discharge array (IDA): theoretical concept and first measurements

Besides the common stochastic heating effect in inductively coupled plasmas, recently a novel heating mechanism was identified theoretically by Czarnetzki and Tarnev [1]. It considers the movement of electrons in a plane parallel to the induced electric field lines, in contrast to the well-known case, when the considered electrons move perpendicular. To enable the possibility of non-local energy gain for electron in this parallel plane, a periodically structured electric field was proposed. To experimentally verify this hypothesis a new plasma source was designed and assembled. This source is named Inductive Discharge Array (IDA). The special spatial field structure is realized by a large electrode with an array of 6 x 6 small plane inductive coils. Due to the use of two separate electric circuits, both electric field structures mentioned in [1] can be achieved. Here the theoretical background and the relevant design considerations are presented. In addition first \mbox{experimental} results are shown.\\[1] [1] U. Czarnetzki and Kh. Tarnev, \textit{Phys. Plasmas} \textbf{21}, 123508 (2014).   

Generation of off-axially localized tail electrons in helical antenna produce cylindrical plasma.

Off-axially localized tail electrons are observed in helical antenna produce cylindrical radio frequency (RF) plasma. Although, tail electrons are commonly observed in capacitive and inductive plasmas, localization of their population only at the off-axis of a cylindrical RF system is very unique. Moreover, we are reporting the generation of tail electrons even in absence of double layer in expanding helical antenna produce plasma. It is also shown that the confinements of these tail electrons are restricted only at the off-axis at Argon fill pressure bellow 1x10$^{\mathrm{-3}}$mbar. Experimental results will be presented to show that the tail electrons which generate off-axially in the source chamber are also found at the expansion chamber. External axial diverging magnetic field lines are bringing them from narrow source to large expansion chamber. To understand the underline mechanism of these tail electrons generation, role of (a) RF electric fields via changing RF source power and (b) their off-axial confinement with rising magnetic fields are discussed. Quantitative discussion on self-consistent model for collisionless RF power coupling with edge electrons will also be presented.   

Plasma Parameters Characterization of Large Diameter Inverted Cylindrical Magnetron Discharge.

In this study, magnetically enhanced large cathode diameter inverted magnetron discharge is characterized for its plasma properties. The current-voltage characteristics at different operating pressure and magnetic field is studied and compared with post cathode configuration. The radial profile of plasma potential, floating potential, plasma density, electron temperature is measured by emissive probe, double langmuir probe etc. The effect of magnetic field on the plasma properties is studied for different operating pressure and discharge voltages. The higher anode fall is observed in case of inverted magnetron for lower operating pressure. It is also found that at very low operating pressure and higher magnetic field, the discharge is transformed to high impedance low current discharge confined near the anode. The oscillations in floating potential are also studied when the discharge is operating in this low current mode.   

Time resolved ion energy distribution functions of non-reactive and reactive high power impulse magnetron sputtering of titanium

High power impulse magnetron sputtering (HiPIMS) is a technique for thin film deposition and can be operated in reactive and non-reactive mode. The growth rate of HiPIMS in non-reactive mode reduces to 30\% compared to direct current magnetron sputtering (dcMS) at same average power. However, the quality of the coatings produced with HiPIMS is excellent which makes these plasmas highly appealing. In reactive mode target poisoning is occurring which changes the plasma dynamics. An advantage of reactive HiPIMS is that it can be operated hysteresis-free which can result in a higher growth rate compared to dcMS. In this work thin films are deposited by a HiPIMS plasma which is generated by short pulses of 100 $\mu$s with high power densities in the range of 1 kW/cm$^2$. Ar and Ar/N$_2$ admixtures are used as a working gas to sputter a 2'' titanium target. The particle transport is analysed with time resolved ion energy distribution functions which are measured by a mass spectrometer with a temporal resolution of 2 $\mu$s. Phase resolved optical emission spectroscopy is executed to investigate the particle dynamics of different species. The time and energy resolved particle fluxes in non-reactive and reactive mode are compared and implications on the sputter process are discussed.   

Fast optical and electrical diagnostics of pulsed spark discharges in different gap geometries

Spark discharges in different electrode configurations and with various electrode materials were ignited in air at atmospheric pressure using a custom build pulse charger with $\approx$ 1~$\mu$s voltage rise time (up to 28~kV) in single shot operation. Fast voltage and current measurements were combined with iCCD imaging with high spatial resolution (better than 10 $\mu$m) on pin-to-pin, pin-to-half-sphere and symmetrical half-sphere tungsten electrodes and symmetrical half-sphere brass electrodes for electrode gaps of 0.1 to 0.7~mm. Breakdown voltages, consumed electrical energies and the discharge emission structures as well as the discharge diameters were obtained. Because of the synchronization of the electrical measurements and the iCCD imaging (i.e. one complete data set for every shot), it was possible to estimate the current density and the change of the discharge pattern, such as single or multiple channels, for all cases.   

Self-organized plasmas formed by accumulated charge in dielectric barrier discharge

Atmospheric pressure dielectric barrier discharges (DBDs) have been widely applied to various research fields, such as bio-medical treatment, toxic decomposition and so on. However, the details of DBD have not been understood yet. Because the phenomena occur in nanosecond time scale under atmospheric pressure. It is known that DBDs are significantly affected by accumulated charges on dielectrics, but the distributions and development of accumulated charges are not known for years. To clarify the distributions and the developments of accumulated charges on dielectrics and electron behavior in the vicinity of dielectrics, DBDs in atmospheric pressure oxygen have been simulated using a two dimensional fluid model with relatively high electron emission coefficient. In this condition, DBD simulation results are obtained in so called self-organized form. As a result, the locations of highly accumulated charges are at where the primary streamers reached in a half cycle. And the charges on the dielectrics become almost zero by the electrons after the change of discharge voltage polarity. The electron distribution in the vicinity of the dielectric forms similar to that of accumulated charges to compensate the charges. Excess electrons in front of dielectric become the seed electrons for next half cycle. This continuation makes discharge in self-organized form.   

The effect of humidity on ionic wind velocity in ambient air

Due to the evolution of portable electronics and LED lightning system, advances in air cooling technologies must also keep pace. Active cooling by ionic wind, which is usually generated by corona discharge, can greatly reduce the noise and lifetime issues compared to the mechanical fans. The wind is induced when a gas discharge is formed, and neutral molecules gain their energy by the momentum transfer of ion-neutral collisions. However, there is few discussion about the effect of gas composition such as humidity on the wind generation and the physical mechanism is not clear. In the experiment, a positive 5-20 kV DC voltage is applied to the needle-cylinder electrodes with separation of 20 mm. The ionic wind velocity is measured by hot wire anemometry. As the relative humidity (RH) in the ambient air increases, the velocity is found to be severely inhibited. The current is also measured between the cylinder electrode and earth. The results show that the DC component of corona current decreases when RH increases. Since both the discharge current and the ion mobility are reduced when RH increases, their combined effects determine the ionic wind velocity.   

Multiple surface DBD electrode system for efficient and controlled generation of ozone

Electrical characteristics and ozone production measurements were performed to evaluate the efficiency of ozone generation using an amplitude-modulated AC Surface Dielectric Barrier Discharge (SDBD) in dry synthetic air and pure oxygen at atmospheric pressure. To increase the concentration and production of ozone we used the multiple SDBD electrode system consisting of several identical elements in parallel configuration. Each SDBD element is made of a thin alumina plate (10cm x 10 cm x 0,065cm) with metallic strips deposited on the upper side as a HV electrode and full square or strips on the opposite side as a ground electrode. An influence of a photocatalyst on ozone production was studied as well by inserting thin alumina plates coated with titanium dioxide thin films between SDBD electrodes. Alternatively, the SDBD electrodes directly coated with titanium dioxide were tested either. Dependence of ozone production on the discharge duty cycle and gas flow rate of 0,8 slm -- 10 slm were evaluated.   

Effects of pulse-to-pulse residual species on discharges in repetitively pulsed discharges through packed bed reactors

Atmospheric pressure dielectric barrier discharges (DBDs) sustained in packed bed reactors (PBRs) are being investigated for conversion of toxic and waste gases, and CO$_{\mathrm{2}}$ removal. These discharges are repetitively pulsed having varying flow rates and internal geometries, which results in species from the prior pulse still being in the discharge zone at the time the following discharge pulse occurs. A non-negligible residual plasma density remains, which effectively acts as preionization. This residual charge changes the discharge properties of subsequent pulses, and may impact important PBR properties such as chemical selectivity. Similarly, the residual neutral reactive species produced during earlier pulses will impact the reaction rates on subsequent pulses. We report on results of a computational investigation of a 2D PBR using the plasma hydrodynamics simulator \textit{nonPDPSIM}. Results will be discussed for air flowing though an array of dielectric rods at atmospheric pressure. The effects of inter-pulse residual species on PBR discharges will be quantified. Means of controlling the presence of residual species in the reactor through gas flow rate, pulse repetition, pulse width and geometry will be described. Comparisons will be made to experiments.   

Effects of oxygen concentration on atmospheric pressure dielectric barrier discharge in Argon-Oxygen Mixture

A dielectric barrier discharge (DBD) can generate a low-temperature plasma easily at atmospheric pressure and has been investigated for applications in trials in cancer therapy, sterilization, air pollution control, etc . It has been confirmed that reactive oxygen species (ROS) play a key role in the processes. In this work, we use a fluid model to simulate the plasma characteristics for DBD in argon-oxygen mixture. The effects of oxygen concentration on the plasma characteristics have been discussed. The evolution mechanism of ROS has been systematically analyzed. It was found that the ground state oxygen atoms and oxygen molecular ions are the dominated oxygen species under the considered oxygen concentrations. With the oxygen concentration increasing, the densities of electrons, argon atomic ions, resonance state argon atoms, metastable state argon atoms and excited state argon atoms all show a trend of decline. The oxygen molecular ions density is high and little influenced by the oxygen concentration . Ground state oxygen atoms density tends to increase before falling. The ozone density increases significantly. Increasing the oxygen concentration, the discharge mode begins to change gradually from the glow discharge mode to Townsend discharge mode.   

Spatial profiles of the cathode layer parameters in the strongly constricted and diffuse atmospheric pressure glow discharges.

In high-current atmospheric pressure glow discharges without limiting walls of discharge chamber, the degree of positive column contraction should be determined by the relation of its diameter to the dimension of negative glow which are defined by transverse electron concentration or light emission profiles. In this report, it is shown experimentally that parameters of the cathode fall in normal atmospheric pressure glow discharge with diffuse positive column more or less fit the scaling laws. Radially limited heat flow from the strongly constricted positive column to the cathode results in inhomogeneous distribution of the reduced electric field along the cathode surface. At that, the reduced electric field decreases radially and the cathode fall parameters mismatch the scaling laws at the center of the cathode fall. It is established that the cathode heating resulting from the discharge current flow leads in such a discharge to the increase in the cathode fall. On the contrary, additional heating of cathode by external heat source decreases the cathode fall. The gas heating at the edge of cathode fall happens mainly due to both the heat transfer from hot cathode and the current flow. Spatial profiles of current flow lines in cathode region are discussed.   

1D fluid model of the dielectric barrier discharge in chlorine

The 1D fluid model of the dielectric barrier discharge (DBD) in pure chlorine$_{\mathrm{\thinspace }}$is developed. The discharge is excited in 8 mm gas gap between quartz dielectric layers of 2 mm thickness covered metallic electrodes. The source voltage $U_{S}=U_{0}$\textit{sin}$\omega t$ with a frequency 100 kHz and amplitude 8 kV is applied to the electrodes. Chlorine pressure is varied from 15 to 100 Torr. At pressure of 15 Torr a breakdown appears with voltage drop across the discharge gap about 1 kV whereas at 100 Torr it appears with voltage drop about 2 kV. After the first current spike some lower current spikes are observes with chlorine pressure of 100 Torr and large in number current spikes of about identical magnitude are observed with the pressure of 15 Torr. The maximal current density at all pressures reaches about 4 mA/cm$^{\mathrm{2}}_{\mathrm{.}}$ Total density of surface charge deposited on the electrodes during a half-cycle decreases with chlorine pressure because duration of the current spike discharge phase reduces with chlorine pressure. The average power density inputted in the discharge is 2.5-5.8 W/cm$^{\mathrm{3}}$ per a cycle. The Cl$_{\mathrm{2}}$ plasma is electronegative, the most abundant ions are Cl$_{\mathrm{2}}^{\mathrm{+}}$ and Cl$^{\mathrm{-}}$. It is shown, that ions get about 95{\%} of the discharge power as electrons get about 5{\%} of the discharge power. 67-97{\%} of the electron power is spending for dissociation and ionization of Cl$_{\mathrm{2}}$ molecules. Emission of Cl$^{\mathrm{\ast }}$ atoms and Cl$_{\mathrm{2}}^{\mathrm{\ast }}$ molecules is weak.   

Investigation of atmospheric pressure glow microdischarge between flat cathode and needle anode in helium and argon.

DC atmospheric-pressure glow microdischarge was generated between a flat cathode and needle anode with a diameter of 100 $\mu $m in a special chamber with helium or argon. Dependences of discharge parameters on an interelectrode gap was investigated with an original experimental setup based on a movable arm on the hinge joint which allowed changing the gap with a step of 5 $\mu $m. The gap was varied from 5 to 700 $\mu $m. Discharge current was 1-21 mA. Such discharge cell has a very low interelectrode capacitance and provides increasing the stability of the discharge against arc formation (transition to RC oscillations mode) at low currents of 1 mA. A weak dependence of discharge voltage across the gap was revealed in helium at 100-250 $\mu $m between the electrodes (normal discharge). In contrast to this, glow microdischarge in argon has a descending current-voltage characteristic and unstable nature. The discharge voltage depending on the gap changes significantly slower than in helium. According to our estimations, the strength of electrical field of positive glow in argon is 5 times lower than in helium.   

Metastable densities in rf-driven atmospheric pressure microplasma jets in argon and helium

Rf-driven atmospheric pressure microplasma jets ($\mu $-APPJ) are usually operated in the homogeneous glow mode ($\alpha $-mode). At higher powers the glow discharge becomes unstable due to thermal instabilities and turns into a constricted $\gamma $-like discharge (constricted mode), which can damage the jet due to the significantly increased temperature in this operation mode. To prevent these instabilities, rf-driven $\mu $-APPJs are predominantly operated in helium since it provides a better thermal conductivity than argon. However, since argon is much more cost-effective, it is worthwhile to achieve a stable operation of the $\mu $-APPJ using argon as feed gas. Metastable atoms play an important role in the stability of atmospheric pressure discharges, since they pose an important source of electrons via stepwise ionization and penning ionization. To understand the basic processes that lead to the transition from $\alpha $- to the constricted mode, helium and argon metastable densities have been determined in the $\mu $-APPJ in different operation modes using tunable diode laser absorption spectroscopy (TDLAS). Supported by DFG within (FOR1123).   

OH rotational temperature measurements via a two temperature distribution analysis in plasma with water microdroplets

We study plasma processing with water/solution microdroplets for a new nanoparticle synthesis method. In the process, it is important to know gas temperature (Tg) for understanding the mechanism of the particle growth and controlling its properties. Since OH emissions are naturally observed in such plasma, the rotational temperature (Tr) of OH (A-X) is estimated and compared with Tr from N$_{\mathrm{2\thinspace }}$(C-B). The plasma is generated by dielectric barrier discharges in He with N$_{\mathrm{2}}$ (2.6 {\%}) gas flow, and microdroplets are generated by an ultrasonic atomizer and carried into He/N$_{\mathrm{2}}$ plasma. Optical emission spectroscopy revealed that with the increase of voltage and frequency of plasma generation, the Tr of N$_{\mathrm{2}}$ increases. While the good theoretical spectrum fit on N$_{\mathrm{2}}$ experimental spectrum could be achieved, it was hard to obtain a reasonable fit for the OH spectrum with a single rotational energy distribution. On the other hand, two rotational distribution analysis could reproduce the experimental spectrum of OH and the lower Tr agrees to Tr by N$_{\mathrm{2}}$. The results suggest that the lower Tr obtained with the two rotational temperature analysis of OH spectrum represents Tg of the environment.   

Reactive Microplasma Discharge for In-Situ Study of Surface Modification

In-situ evaluation of surface modifications induced by reactive plasma-surface interactions is an important part of the fundamental studies of the processes at plasma-surface interfaces. We have developed a microdischarge cell for use inside an Environmental Scanning Electron Microscope (ESEM). Plasma is ignited inside hollow cylindrical electrode and interacts with ta grounded substrate. The substrate is interchangeable and the plasma gasses CO$_{\mathrm{2}}$, N$_{\mathrm{2}}$, water vapor, are consistent with the requirements of the ESEM. The microdischarge cell has been characterized at 2-8 torr and tested in an ESEM in a hollow cathode (MHC) or a hollow anode (MHA) configuration. The electrical measurements show that the MHC configuration has lower reduced field and higher plasma density than MHA. The optical emission spectra of the CO and N$_{\mathrm{2}}$ bands and H lines were used to find the rotational temperature of 450 K in both configurations, and the vibrational temperature of 3700 K for the MHC and 4500 K for the MHA. The electron excitation temperature is higher in the MHA configuration. MHA can potentially offer a better controllability of the electron energy distribution function, which is useful for micro plasma applications.   

Interaction between a microplasma array and an adjacent dielectric surface

Microplasma pixel devices are interesting for applications such as surface modification. A representative is the metal grid array, which is a stable alternative to silicon-based arrays and consists of a dielectric, a grounded electrode and a metal grid with symmetrically arranged cavities. Typically, microplasma arrays are operated close to atmospheric pressure with noble gases like argon and helium. By applying a bipolar triangular voltage waveform with an amplitude of 700 V peak-to-peak and a frequency of 10 kHz to the metal grid, the discharge is ignited in the cavities having a diameter of about 200 and depth of 50 \textmu m. For future applications, such as coating and catalysis, the interaction between the array and a dielectric surface positioned at close distance (\textless \textasciitilde 200 \textmu m) is of great importance. By application of phase resolved optical emission spectroscopy, the phase dependent expansion of the emission out of the cavities has been observed. Here, we present results of investigations on the dependence of emission structures of the cavities (individually or as group) on pressure, applied voltage and distance between grid and dielectric.   

Net Emission Coefficients for Copper and Iron Plasmas

Radiative heat transfer is an important mechanism for heat transport in electrical arcs, e.g.\ in electrical switchgear. An exact description of this phenomenon is important (i) for the energy balance of the arc itself, and (ii) for the estimate of the escaping radiation that leads to evaporation of polymer nozzles; the evaporated material and its flow have a strong effect on the arcs. For low voltage arcs, the plasma composition within the arc is dominated by the contact material. In the present study, we compare copper and iron. Especially, we discuss the calculation of absorption and emission spectra and their characterisation by net emission coefficients. The latter describe well the effective power balance at the centre of the arc. We show that in addition to the net emission coefficients, it is important to characterise the radiation that is emitted from the arc core.   

Characterization, mechanical, and corrosion properties of chromium carbide films by using a 90$\circ $bend magnetic filtered cathodic vacuum arc (FCVA) method

The 90$\circ $bend magnetic FCVA that equipped with the target of Cr (99.95{\%}) and C$_{\mathrm{2}}$H$_{\mathrm{2}}$/Ar gas mixture deposited a high quality of chromium carbide films on the AISI D2 steel and Si wafer. The FCVA has been employed to eliminate the macroparticles during the film deposition. Various deposition temperatures of ambient temperature, 300, and 500${^\circ}$ and negative substrate bias voltages ranging from -50 to -550V were applied. The microstructure of chromium carbide films was investigated by GIXRD and HRTEM. The atomic concentrations of C and Cr were measured by AES. The chemical bonding was elucidated by XPS, showing that the total C-Cr bond contents increased with increasing deposition temperature. As the substrate bias voltage increased from -50 to -550V, the phase transformed from amorphous to crystalline Cr$_{\mathrm{3}}$C$_{\mathrm{2}}$. The mechanical properties were evaluated by nanoindetation, nanoscratch, and scratch test. The surface roughness decreased from 2.05 to 0.34nm and the friction coefficient decreased from 0.28(amorphous) to 0.22(crystalline) as the substrate bias voltage increased from -50 to -550V. The corrosion resistance showed that the Cr$_{\mathrm{3}}$C$_{\mathrm{2}}$ coated steel had the noticeable increasing with the negative bias voltage up to -550V, and the pitting corrosion did not appear on the Cr$_{\mathrm{3}}$C$_{\mathrm{2}}$ coated steel.   

Arc Conductance and Flow Velocity Affected by Transient Recovery Voltage

Recently, the stable supply of electric power is indispensable. The GCB (Gas Circuit Breaker) can prevent the spread of the fault current. However, it should have the reliability more. Therefore the GCB has been researched for performance improvement of the arc interruption of abnormal fault current without the fail. Therefore, it is important to prevent the breakdown such as the re-ignition and thermal re-ignition of arc after the arc interruption. It is necessary to reduce the arc conductance in order to prevent the re-ignition of arc. The arc conductance is derived from the temperature distribution and the volume of the arc. The temperature distribution of the arc is formed by convection. In this research, the arc conductance and flow velocity affected by transient recovery voltage are elucidated. The flow rate and temperature distribution of the arc is calculated with changing transient recovery voltage. In addition, the arc conductance is calculated in order to know the extinguish arc ability. As a result, when the transient recovery voltage increases, the probability of re-ignition increases. Therefore, the arc temperature and the arc conductance were increased.   

Distribution of Argon Arc Contaminated with Nitrogen as Function of Frequency in Pulsed TIG Welding

TIG arc welding is the high-quality and much applicable material joining technology. However, the current has to be small because the cathode melting should be prevented. In this case, the heat input to the welding pool becomes low, then, the welding defect sometimes occurs. The pulsed TIG arc welding is used to improve this disadvantage This welding can be controlled by some current parameters such as frequency However, few report has reported the distribution of argon arc contaminated with nitrogen It is important to prevent the contamination of nitrogen because the melting depth increases in order to prevent the welding defects. In this paper, the distribution of argon arc contaminated as function of frequency with nitrogen in pulsed TIG welding is elucidated. The nitrogen concentration, the radial flow velocity, the arc temperature were calculated using the EMTF simulation when the time reached at the base current. As a result, the nitrogen concentration into the arc became low with increasing the frequency The diffusion coefficient decreased because of the decrement of temperature over 4000 K. In this case, the nitrogen concentration became low near the anode. Therefore, the nitrogen concentration became low because the frequency is high   

Study of transfer efficiency in a screw pinch plasma

A screwpinch for UV generation and possible FAIR plasma stripper applications was designed based on a previous development. The intention was to increase the applicable voltage and as a result the electron density. Research was conducted into the transfer efficiency for plasma generation providing preliminary values up to 66 \%. The device operates at a frequency of approximate 14 $kHz$ and is characterised by a capacity of 34 $\mu F$, a thyratron switch with a maximum current rise time of 5 $\textsc{MA}/\mu s$ as well as a variable set of up to 5 planar coils with diameters of 172 and 208 $mm$. A cylindrical volume of 4 $l$ can thusly be ignited within the region of 5$\cdot$10$^{-3}$ to 1 $mbar$ with an applied voltage of 6 to 13 $kV$, resulting in peak electron densities of 4$\cdot$10$^{16}$ electrons per $cm^3$.   

Removal of Oxide Layer Using Vacuum Arc Cathode Spot with Transverse External Magnetic Field

A remarkable characteristic of a cathode spot in a vacuum arc is that the cathode spot moves around the metal at high speed. Cathode spots of vacuum arc have been used for cleaning metal oxide surface. In addition, the adhesion strength increases in the case of thermal spraying because the roughness on the metal surface is formed. However, the removal of oxide layer is not enough and the re-melting occurs because a cathode spot moves with random manner on the metal surface. In this paper, the removal of oxide layer was observed in order to control the cathode spot movement with transverse external magnetic field. Experiment were performed using a SS400 cathode work piece. A high-speed video camera recorded the cathode spot. Then, the obtained images were analyzed by plasma image processing. As a results, the cathode spot moves with retrograde motion under removing the oxide layer when a magnetic field was applied. Then, the moving speed of cathode spot increases with increasing the magnetic field.   

Applying a laser-induced incandescence (LII) diagnostic to monitor nanoparticle synthesis in an atmospheric plasma, \textit{in situ}

A DC arc discharge with a consumed graphite anode is commonly used for synthesis of carbon nanoparticles, including carbon nanotubes (CNTs) and graphene flakes [1-3]. The graphite electrode is physically vaporized by high currents (20-60 A) in a buffer gas at 100-600 torr, leading to nanoparticle synthesis in a low temperature (\textgreater 1 eV), plasma. Utilizing arc plasma synthesis technique has resulted in the synthesis of higher quality nanomaterials [3]. However, the formation of nanoparticles in arc discharge plasmas is poorly understood. A particularly interesting question is where in the arc the nanoparticles nucleate and grow. \quad In our current work we show the results of studying the formation of carbon nanotubes in an arc discharge, \textit{in situ}, using laser-induced incandescence (LII). The results of LII are discussed in combination with \textit{ex situ} measurements of the synthesized nanoparticles and modeling, to provide an insight into the physics behind nanoparticle synthesis in plasma. 1. C. Journet \textit{et al. Nature} \textbf{388}, 756-8 (1997); 2. A. J. Fetterman \textit{et al. Carbon} \textbf{46} 1322-6 (2008); 3. M. Keidar \textit{et al. Phys. Plasmas} \textbf{17}, 057101 (2010);   

Pulsed picosecond and nanosecond discharge development in liquids with various dielectric permittivity constants

The dynamics of pulsed picosecond and nanosecond discharge development in liquid water, ethanol and hexane were investigated experimentally. It is shown that the dynamics of discharge formation fundamentally differ between liquids with low and high dielectric permittivity coefficients. The difference in the nanosecond discharge development in liquid dielectrics may be explained by the formation of micro-discontinuities in the media during the electrostriction compression/rarefaction stage in liquids with high dielectric permittivity. Three possible mechanisms for the propagation of discharge in liquids play a different role depending on the pulse duration. The first is the formation of low density channels in liquid. In the second case the electrostatic forces support the expansion of nanoscale voids behind the front of the ionization wave; in the wave front the extreme electric field provides a strong negative pressure in the dielectric fluid due to the presence of electrostriction forces, forming the initial micro-voids in the continuous medium. Finally, in the third case, when a picosecond electric pulse is utilized, the ionization in the liquid phase occurs as a result of direct electron impact without undergoing a phase transition.   

Evolution of Spatial pH Distribution in Aqueous Solution induced by Atmospheric Pressure Plasma

Discharge plasma at gas-liquid interface produces some active species, and then they affect chemical reactions in aqueous solution, where pH of aqueous solution is changed due to redox species. The pH change of aqueous solution is an important factor for chemical reactions. However, spatial pH distribution in a reactor during the discharge has not been clarified yet. Thus, this work focused on spatial pH distribution of aqueous solution when pulsed discharge plasma was generated from a copper electrode in gas phase to aqueous solution in a reactor. Experiments were conducted using positive unipolar pulsed power. The unipolar pulsed voltage at $+$8.0 kV was applied to the copper electrode and the bottom of the reactor was grounded. The size of the reactor was 80 mm wide, 10 mm deep, and 40 mm high. The electrode was set at distance of 2 mm from the solution surface. Anthocyanins were contained in the aqueous solution as a pH indicator. The change pH solution spread horizontally, and low pH region of 10 mm in depth was formed. After discharge for 10 minutes, the low pH region was diffused toward the bottom of the reactor. After discharge for 60 minutes, the pH of the whole solution decreased.   

Emission spectroscopyp of single buble sonoluminescence using argon mixuture water

If the liquid has been degassed and is irradiated with a standing acoustic wave, a single bubble sonoluminescence(SBSL) can be generated. It is think that very high temperature and pressure environment is generated inside a SBSL bubble with emission of light. However, little is known about the SBSL emission mechanism. The study is intended as an elucidation of SBSL emission mechanism. In this study, to generate SBSL, the Ar mixture pure water in the 200 ml round bottom flask was irradiated with ultrasound at about 24.1 kHz. SBSL emission of light was detected by PMT. We will acquire the emission spectrum of SBSL using spectroscope and ICCD camera next.   

Imaging diagnostics of pulsed plasma discharges in saline generated with various sharp pin powered electrodes

Plasmas formed by 1 ms pulses of between 180 and 300 V applied to sharp pin-like electrodes immersed in saline solution have been imaged with a Photron SA-X2 fast framing camera and an Andor iStar 510 ICCD camera. Stainless steel, Tungsten and Gold electrodes were investigated with tip diameters of 30 $\mu$m, 1 $\mu$m and $< 1 \mu$m respectively. As previously observed, a vapour layer forms around the electrode prior to plasma ignition [1]. For gold and stainless steel lower voltages were required to minimize electrode damage. Preliminary anlaysis indicates at lower voltages for all tips the fast framing results show that light emission is normally centred on a single small volume, which appears to move about, but remains close to the tip. In the case of Tungsten with higher voltages or longer pulses the tip of the needle can heat up to incandescent temperatures. At higher voltages shock wave fronts appear to be observed as the vapour layer collapses at the end of the voltage pulse. Backlighting and no lighting to observe bubble/vapour layer formation and emission due to plasma formation were employed. Sometimes at higher voltages a thicker vapour layer engulfs the tip and no plasma emission/current is observed. 1. Schaper, L. et al. Plasma Sources Sci. Tech., 20 (2011) 034003   

Investigation Of The High-Voltage Discharge On The Surface Of Gas-Liquid System

This paper describes an experimental setup for study of physical processes in the high-voltage discharge on the surface of gas-liquid system at atmospheric pressure. Measurements of electrical and optical characteristics of the high-voltage discharge in gas, at the surface of the gas-liquid system and in the electrolyte are obtained. The parameters of the high-voltage discharge and the conditions for its stable operation are presented. Investigations with various electrolytes and cathode assemblies of various materials and sizes were carried out. The installation can be used for the processing and recycling of industrial and chemical liquid waste.   

Experimental study of low-temperature plasma of electrical discharges with liquid electrodes

Results of the experimental research of discharge between the liquid jet cathode (LJC) and the metal anode are presented. The discharge was studied over the voltage range $U=100-600$~V, discharge current range $I=0.1-0.25$~A, external pressure range $P=10^5$~Pa, discharge power $P_d=10-150$~W. We used the techniques of infrared thermography and spectral measurements. Schlieren's photography is applied for describing the processes in liquid and gas phase. Results of the experimental researches of discharge current-voltage characteristic (CVC), the surface temperature distribution both on the LJC and the metal anode, a spectral measurements are showed. Effects of action both of breakdown and discharge on the jet flow as well as on the air flow near the discharge are described. It is found that the discharge CVC has an ascending behavior due to increase of plasma current density. The discharge is generated on the borders between the LJC and the metal anode as well as along the LJC misshaping this one. It is established that both the convection streams and an electrolyte drops are formed during the discharge burn. It is found that the discharge temperature in the vicinity of electrode surface reaches $T \approx 348$~K.   

Synthesis, transport, and retention of tin nanodroplets in a magnetron sputtering source combined with a capacitively-coupled plasma

The intention of this work was the development of a method for coating metal nanodroplets with thin films having high melting temperatures. To realize this process technology, we combined a magnetron sputtering plasma for synthesizing metal nanoparticles with a capacitively-coupled plasma (CCP) for retaining and heating synthesized nanoparticles. The magnetron sputtering source with a tin target was operated at a high pressure of 400 mTorr. The high pressure induced the condensation of tin atoms in the gas phase, resulting in the formation of tin nanoparticles. The nanoparticles were transported downward, and were trapped in the sheath electric field near the planar electrode for the CCP discharge. The formation, the transport, and the retention of nanoparticles were monitored by laser light scattering. Collected tin nanoparticles did not have agglomerated shapes, suggesting that tin nanoparticles were melted when they were stored in the CCP discharge. The surfaces of tin nanoparticles were oxidized. When we introduced methane before the collection, we observed core-shell nanoparticles without oxidization. Tin nanoparticles were coated with amorphous carbon films by plasma-enhanced chemical vapor deposition of methane.   

Dust formation and dynamic in magnetized and non-magnetized microwave discharge.

Dusty plasmas studies are conducted for several decades to answer to various issues from microelectronic, nanotechnology, astrophysics and thermonuclear fusion devices. These studies are usually conducted in RF discharges at low pressure in which the major physics concerning dust formation mechanisms and dynamic is now well known. In our case, we focus on dust formation and dynamic in (i) microwave plasma under typical pressure conditions of RF discharges (50 Pa) and (ii) in magnetized (ECR: Electron Cyclotron Resonance) microwave plasma under very low pressure condition (0.1 to 1 Pa). The aim of this study is not only for fundamental purpose but also for respond to some issues concerning dust in fusion devices. Thus, we investigate the dust formation mechanisms and dynamic using laser extinction method and laser light scattering imaging coupling with SEM imaging in hydrocarbon plasma and with PVD system with using tungsten target (according to fusion device). We observed that dust formation occurs even if the very low pressure conditions are generally not suitable for nucleation growth in gas phase (the influence of the magnetic field will be discussed). We will also discuss about the particular dust dynamic behavior in microwave discharge in comparison with RF discharge.   

Hybrid simulation of nanoparticle growth and transport in a pulsed RF CCP sustained in silane

A pulsed RF silane plasma is studied numerically by adopting a self-consistent one-dimensional fluid/MC model. The large anions (typically Si12H25-$^{\mathrm{\thinspace }}$and Si12H24-) in the discharge are the main precursors in the pathways leading to particle formation in a nucleation process. In order to study detailed growth of nanoparticles, an aerosol general dynamics equation is introduced and self-consistently coupled to the plasma fluid model, in which spatial distribution of nanoparticles, from several to tens of nm in diameter, is investigated. The numerical results show that, the ion drag force on smaller nanoparticles could to some extent exceed the electrostatic force in the plasma bulk, making the nanoparticles generally move towards the plasma boundaries. So the axial spatial distribution of nanoparticles is like a bimodal structure. With increase of the particle size, the distance between two peaks gradually becomes larger, reflecting the appearance of void in the plasma. At the same time, the presence of nanoparticles can lead to a decline of the electron density and a rise of the potential. In addition, by pulsing the RF source, size-controlled nanoparticles are expected to be extracted from the bulk plasma during the afterglow period.   

Numerical studies from quantum to macroscopic scales of carbon nanoparticules in hydrogen plasma

Dusty plasmas take part in large scientific domains from Universe Science to nanomaterial synthesis processes. They are often generated by growth from molecular precursor. This growth leads to the formation of larger clusters which induce solid germs nucleation. Particle formed are described by an aerosol dynamic taking into account coagulation, molecular deposition and transport processes. These processes are controlled by the elementary particle. So there is a strong coupling between particle dynamics and plasma discharge equilibrium. This study is focused on the development of a multiscale physic and numeric model of hydrogen plasmas and carbon particles around three essential coupled axes to describe the various physical phenomena: (i) Macro/mesoscopic fluid modeling describing in an auto-coherent way, characteristics of the plasma, molecular clusters and aerosol behavior; (ii) the classic molecular dynamics offering a description to the scale molecular of the chains of chemical reactions and the phenomena of aggregation; (iii) the quantum chemistry to establish the activation barriers of the different processes driving the nanopoarticule formation.   

Interaction of UV laser pulses with reactive dusty plasmas

This contribution deals with the effects of UV photons on the synthesis and transport of nanoparticles in reactive complex plasmas (capacitively coupled RF discharge). First measurements showed that the irradiation of a reactive acetylene-argon plasma with high-energy, ns UV laser pulses (355 nm, 75 mJ pulse energy, repetition frequency 10Hz) can have a large effect on the global discharge characteristics. One particular example concerns the formation of a dust void in the center of the discharge. At sufficiently high pulse energies, this formation of a dust free region - which occurs without laser irradiation---is totally suppressed. Moreover the experiments indicate that the laser pulses influence the early stages of the particle formation. Although the interaction between the laser and the plasma is not yet fully understood, it is remarkable that these localized nanosecond laser pulses can influence the plasma on a global scale. Besides new insights into fundamental problems, this phenomenon opens also new possibilities for the controlled manipulation of particle growth and particle transport in reactive plasmas.   

Mode transitions and electronegativity in oxygen CCP and ICP.

Mode transitions in 13.56 MHz oxygen radio frequency plasmas (CCP, ICP) and their impact on the electron heating mechanisms and electronegativity were studied by advanced plasma diagnostics. In particular, Langmuir probe measurements, Gaussian beam microwave interferometry (160 GHz) coupled with laser photodetachment of negative oxygen ions, as well as the (phase resolved) optical emission and VUV absorption spectroscopy, and ion mass spectrometry are taken into consideration. With increasing RF power a transition between high and low electronegativity was found both in CCP and ICP discharge configuration. Thereby, the changed electron heating mechanisms, e.g., the alpha-gamma mode transition in CCP and the E-H mode transition in ICP is combined with the change of electronegativity. In strongly asymmetric CCP at moderate pressure the emission of secondary negative ions at the powered electrode have to be considered, too. Thereby, pseudo secondary electrons may be produced due to collision detachment of negative ion by metastables. During the E-H mode transition in oxygen ICP, the increasing gas temperature and the metastables influences significantly the oxygen kinetics.   

Neutrosophic Triplet as extension of Matter Plasma, Unmatter Plasma, and Antimatter Plasma

A {Neutrosophic Triplet}, is a triplet of the form: {\textless a, neut(a), and anti(a) \textgreater , } where neut(a) is the neutral of a, i.e. an element (different from the identity element of the operation *) such that a*neut(a) $=$ neut(a)*a $=$ a, while anti(a) is the opposite of a, i.e. an element such that a*anti(a) $=$ anti(a)*a $=$ neut(a). Neutrosophy means not only indeterminacy, but also neutral (i.e. neither true nor false). For example we can have neutrosophic triplet semigroups, neutrosophic triplet loops, etc. As a particular case of the Neutrosophic Triple, in physics one has \textless Matter, Unmatter, Antimatter\textgreater and its corresponding triplet \textless Matter Plasma, Unmatter Plasma, Antimatter Plasma\textgreater . We further extended it to an {{m-}}{valued refined neutrosophic triplet}, in a similar way as it was done for T$_{\mathrm{1}}$, T$_{\mathrm{2}}$, ...; I$_{\mathrm{1}}$, I$_{\mathrm{2}}$, ...; F$_{\mathrm{1}}$, F$_{\mathrm{2}}$, ... (i.e. the refinement of neutrosophic components). We may have a {neutrosophic m-tuple} with respect to the element ``a'' in the following way: ( a; neut$_{\mathrm{1}}$(a), neut$_{\mathrm{2}}$(a), ..., neut$_{\mathrm{p}}$(a); anti$_{\mathrm{1}}$(a), anti$_{\mathrm{2}}$(a), ..., anti$_{\mathrm{p}}$(a) ), where m $=$ 1$+$2p, such that: - all neut$_{\mathrm{1}}$(a), neut$_{\mathrm{2}}$(a), ..., neut$_{\mathrm{p}}$(a) are distinct two by two, and each one is different from the unitary element with respect to the composition law *; - also a*neut$_{\mathrm{1}}$(a) $=$ neut$_{\mathrm{1}}$(a)*a $=$ a, a*neut$_{\mathrm{2}}$(a) $=$ neut$_{\mathrm{2}}$(a)*a $=$ a, \textellipsis , a*neut$_{\mathrm{p}}$(a) $=$ neut$_{\mathrm{p}}$(a)*a $=$ a; - and a*anti$_{\mathrm{1}}$(a) $=$ anti$_{\mathrm{1}}$(a)*a $=$ neut$_{\mathrm{1}}$(a), a*anti$_{\mathrm{2}}$(a) $=$ anti$_{\mathrm{2}}$(a)*a $=$ neut2(a), \textellipsis , a*anti$_{\mathrm{p}}$(a) $=$ anti$_{\mathrm{p}}$(a)*a $=$ neut$_{\mathrm{p}}$(a); - where all anti$_{\mathrm{1}}$(a), anti$_{\mathrm{2}}$(a), ..., anti$_{\mathrm{p}}$(a) are distinct two by two, and in case when there are duplicates, the duplicates are discarded.   

Efficient Technique to Evaluate the Lindhard Dielectric Function.

Since the pioneering work of Lindhard, the dielectric response function obtained in [1] from first principles within the Random Phase Approximation (RPA) has been and is widely used in many areas of Physics such as Plasma Physics, Atomic Physics in plasmas, Solid State Physics, Plasmonics and Nuclear Physics. Indeed, the dielectric function is the fundamental ingredient for many theories related to the response of matter to an external perturbation characterized by a wavenumber k and a frequency $\omega $. In all the above applications the Lindhard dielectric function has to be evaluated many times (for given temperature T, and given values of a real parameter which depends on k and $\omega )$. It is notorious that the integral defining its real part presents a logarithmic divergence which renders the numerical calculation delicate and time consuming. Through a simple but very efficient mathematical trick we are able to remove the singularity and obtain a useful integral expression which is trouble-free, i.e., it can be dealt with any standard numerical quadrature [2]. Our analytical expression greatly facilitates the computation of the dielectric function. [1] J. Lindhard, K. Dan. Vidensk. Selsk. Mat. Fys. Medd., 28(8), 1 (1954). [2] L.U. Ancarani and H. Jouin, Eur.Phys. J.-Plus, 131, 114 (2016).   

Discharge-pumped XUV source.

We have built two experimental devices (CAPEX and CAPEX-U) working as XUV sources, which are based on the fast, pinching capillary discharge. On both these devices we have observed lasing at 46.9 nm (Ne-like Ar line). However, besides lasing at the above mentioned relatively long wavelength, they are also used for testing a possibility of amplification at the wavelengths below 20 nm that have more practical applications. Particularly, at present nitrogen-filled capillary (?4 mm x 90 mm) discharge is studied for the development of XUV (soft X-ray) laser based on recombination pumping scheme: the fully stripped nitrogen nuclei recombine to hydrogen-like atoms, where Balmer-alpha transition (wavelength 13.4 nm) is - according to theoretical predictions - capable of creating population inversion. The modified electrical parameters (peak current \textasciitilde 60 kA with quarter period of \textasciitilde 45 ns) meet the necessary theoretical conditions. The only question remains, if suitable pre-pulse can suppress the capillary-wall-ablation, which in all presently known cases has quashed the amplification. In this paper the recent results obtained from both these discharge systems (argon-, nitrogen-filled capillaries) will be presented.   

Energy flux to substrate in high-power impulse magnetron sputtering measured by using optical low-coherence interferometry

The substrate during the plasma irradiation is heated by charged species, neutral species, and the heat radiation and the substrate temperature is determined by energy flux to the substrate. High-power impulse magnetron sputtering (HiPIMS) using short-pulse high-voltage promotes the ionization of sputtered atoms and realizes high density plasma. In this study, we measured the silicon substrate temperature with non-contact type substrate temperature measurement method using optical low-coherence interferometry(LCI) and elucidated the heating mechanisms of the substrate temperature in HiPIMS. The target material was Ti and the distance between the substrate and the target was 60mm. Ar is used as the sputtering gas. The pulse width was from 50 to 300µsec, the pulse frequency was from 100 to 500Hz. Applied voltages were changed to be from -400V to -900V. Measurement accuracy of contact-type thermocouples and that of noncontact-type LCI were within 2 degree C and 0.7 degree C, respectively. The heat influx to the substrate was calculated from the temporal variation of substrate temperature base on the energy balance equation and increased with increasing applied voltage. The emission intensity of Ti ion increased with increasing applied voltage even though that of Ti atom was constant. These results suggested that main contribution of substrate heating is Ti ion bombardment.   

Network structural analysis using directed graph for chemical reaction analysis in weakly-ionized plasmas

We visualize complicated chemical reaction systems in weakly-ionized plasmas by analysing network structure for chemical processes, and calculate some indexes by assuming interspecies relationships to be a network to clarify them. With the current social evolution, the mean size of general data which we can use in computers grows huge, and significance of the data analysis increases. The methods of the network analysis which we focus on in this study do not depend on a specific analysis target, but the field where it has been already applied is still limited. In this study, we analyse chemical reaction systems in plasmas for configuring the network structure. We visualize them by expressing a reaction system in a specific plasma by a directed graph and examine the indexes and the relations with the characteristic of the species in the reaction system. For example, in the methane plasma network, the centrality index reveals importance of ${\mathrm{CH}}_{3}$ in an influential position of species in the reaction [1]. In addition, silane and atmospheric pressure plasmas can be also visualized in reaction networks, suggesting other characteristics in the centrality indexes. [1] O. Sakai, K. Nobuto, S. Miyagi and K. Tachibana, AIP Advances \textbf{5}, 107140 (2015).   

Simplification of the laser absorption process in the particle simulation for the laser-induced shockwave processing

To reduce the computational cost in the particle method for the numerical simulation of the laser plasma, we examined the simplification of the laser absorption process. Because the laser frequency is sufficiently larger than the collision frequency between the electron and heavy particles, we assumed that the electron obtained the constant value from the laser irradiation. First of all, the simplification of the laser absorption process was verified by the comparison of the EEDF and the laser-absorptivity with PIC-FDTD method. Secondary, the laser plasma induced by TEA CO2 laser in Argon atmosphere was modeled using the 1D3V DSMC method with the simplification of the laser-absorption. As a result, the LSDW was observed with the typical electron and neutral density distribution.   

Plasma Treatment of Metal Surface by Runaway Electrons Preionized Diffuse Discharge

In this work we present experimental results on the generation of diffuse discharge initiated by runaway electron beam and X-rays in pulsed-periodic mode in nitrogen at atmospheric pressure, and its application for metal surface modification. The aim of this work is to investigate the possibilities of surface modification of copper, stainless steel, aluminum, niobium and titanium in the plasma of REP DD, formed in nitrogen flow. The study shows that REP DD treatment after exposure of 100000 shots provides ultrafine surface cleaning of all metals from carbon contamination. At the same time, it is found that all materials subjected to REP DD are involved in surface oxidation. Moreover, the surface energy of the treated specimens increased up to 4 times, whereas the other surface properties like microhardness or roughness remain almost unchanged. Thus, plasma treatment by runaway electron preionized diffuse discharge has enabled us to create an optimum metal surface without mechanical damages that is important for further coating, printing, painting, and adhesive bonding.   

Plasma Etching of superconducting radio frequency cavity by Ar/Cl$_{\mathrm{2}}$ capacitively coupled Plasma

We are developing plasma processing technology of superconducting radio frequency (SRF) cavities. The formation of dc self-biases due to surface area asymmetry in this type of plasma and its variation on the pressure, rf power and gas composition was measured. Enhancing the surface area of the inner electrode to reduce the asymmetry was studied by changing the contour of the inner electrode. The optimized contour of the electrode based on these measurements was chosen for SRF cavity processing. To test the effect of the plasma etching on the cavity rf performance, a 1497 MHz single cell SRF cavity is used, which previously mechanically polished, buffer chemically etched afterwards and rf tested at cryogenic temperatures for a baseline test. Plasma processing was accomplished by moving axially the inner electrode and the gas flow inlet in a step-wise manner to establish segmented plasma processing. The cavity is rf tested afterwards at cryogenic temperatures. The rf test and surface condition results are presented.   

Investigation of the synthesized graphene on the copper foil depending on the plasma condition.

In this study, direct growth of graphene nano-walls (GNWs) synthesized by electron cyclotron resonance (ECR) plasma on Cu foil at low temperature. The direct growth method is simplified manufacturing process and avoid damages and contaminants from graphene transfer process. The density and temperature of plasma were measured using Cylindrical Langmuir probe analysis. Using the Residual Gas Analyzer (RGA, SRS200) for the generated gas analysis by plasma conditions. The morphologies and structures of GNWs were characterized by field-emission scattering electron microscope (FESEM), Transmission electron microscopy (TEM), 3D optical measurement system and Raman spectra measurement.   

Solid coatings deposited from liquid methyl methacrylate via Plasma Polymerization

The polymerization of methyl methacrylate via plasma discharges is well known today. Usually, plasma-enhanced chemical vapor deposition (PECVD) is used to deposit polymer coatings. Solid coatings are formed out of the liquid phase from methyl methacrylate via dielectric barrier discharge. The formation of the coating proceeds in the gas and the liquid phase. To learn more about the reactions in the two phases, the coatings from MMA monomer will be compared to those from MMA resin. Finally, attenuated total reflection infrared spectroscopy, confocal laser scanning microscopy and X-ray photoelectron spectroscopy are employed to characterize the solid coatings. In conclusion, the plasma enhanced chemical solution deposition is compared to the classical thermal polymerization of MMA.   

Measurement of dielectric-film thickness at low density plasma

The measurement system of dielectric-film thickness was improved to measure thin-film at low density plasma. There are three improvements than previous method, which is electrical measurement of dielectric-film thickness using R-C sheath model. First, the frequency of input voltage was decreased to reduce the ratio of the dielectric-film impedance to sheath impedance. Second, three different frequencies were used to overcome the inaccuracy of measured phase; only amplitudes of measured current were used to obtain a film thickness. Third, the notch filter was used for sensing current instead of the resistor to improve the signal to noise ratio. Using this method, dielectric-film thickness was well measured at low density plasma (thickness: \textasciitilde 300¡Ê, sheath impedance: 100\textasciitilde 200 k$\Omega )$.   

Temporally resolved plasma spectroscopy for analyzing natural gas components

Temporally resolved plasma spectroscopy has been carried out in two different hydrocarbon gas mixtures (CH$_{\mathrm{4}}$/Ar and C$_{\mathrm{2}}$H$_{\mathrm{6}}$/Ar) to explore the possibility of a new gas sensor using plasma emission spectral analysis. In this experiment, a nanosecond-pulsed plasma discharge was applied to observe optical emissions representing the initial molecular structure. It is found that a CH emission intensity in CH$_{\mathrm{4}}$/Ar is higher than that in C$_{\mathrm{2}}$H$_{\mathrm{6}}$/Ar. On the other hand, C$_{\mathrm{2}}$ intensities are almost the same degree between CH$_{\mathrm{4}}$/Ar and C$_{\mathrm{2}}$H$_{\mathrm{6}}$/Ar. This finding indicates that the emission intensity ratio of CH to C$_{\mathrm{2}}$ might be an effective index for a gas analysis. In addition, a time for the highest emission intensities of CH and C$_{\mathrm{2}}$ is several nanoseconds later than that of Ar. This result suggests that spectra from the initial molecular structure may be observed at the early stage of the discharge before molecules are fully dissociated, and this is currently in progress.   

Conversion of high-pressure carbon dioxide by laser-induced plasma

In the conversion process of CO$_{\mathrm{2}}$ -\textgreater CO $+$ 1/2 O$_{\mathrm{2}}$ by means of plasma, an atomic oxygen is often observed as the intermediate state. As the following reaction forming 1/2 O$_{\mathrm{2}}$ from O is exothermic, unless the energy is reused, the existence of O atoms results in a lower conversion efficiency of the process. Thus, we are trying to find a pathway which forms 1/2 O$_{\mathrm{2}}$ directly, by contribution of the high pressure, which hopefully boosts the conversion efficiency. In this study, we produce plasma by nanosecond-pulsed laser focused on various metallic targets (Sn, Zn and Cu) in pressurized CO$_{\mathrm{2}}$ environments. The results indicate that the energy conversion efficiency depends on the pressure. In addition, applying a target results in a higher energy conversion efficiency than that without targets, and the efficiency depends on the target materials. We currently believe that the target materials modify the initial density of plasma and the pressure controls the following plasma dynamics. The details will be presented at the conference.   

The purification mechanism of wastewater by underwater discharge

There is a continuing need for development of effective, cheap and environmentally friendly processes for purification of wastewater. In this regard, the plasmas can be a promising candidate for next-generation method to purify the wastewater. It is well known that the plasmas generate many reactive species and thus they are predominant for degradation of organic pollutants from water. In order to generate plasma in wastewater, the capillary electrodes are used with ac power supply. After plasma treatment, the coagulants are added to purify the wastewater. The efficiency of coagulation is significantly improved by plasma treatment of wastewater. These results may come from the reactions among radicals of plasma-treated water, electron reduction and oxidation of ions in waste water, and coagulant. In order to verify the hypothesis, we measured characteristics changes of water by underwater discharge. In this study, we propose the purification mechanism of wastewater by underwater discharge. We expect that the underwater discharge can be applied to purify wastewater in near future.   

EMF generation in low-temperature plasma

EMF generation in plasma created by an $e$-beam in electropositive gases at atmospheric pressure was investigated experimentally and numerically. It was found that propagation of 120~keV $e$-beam with cross-section $1.2\times2$ cm$2$ and current of 240~$\mu$A through argon at $10^5$~Pa gas pressure between an aluminum exit window and an iron collector was followed by 360~$\mu$A current of opposite direction. A numerical modeling of the current flux was performed in an one-dimensional approximation along the axis $z$ in the direction of $e$-beam propagation. It is seen, that the current density grows with increasing the ionization rate and the largest effect takes place in argon. The discovered effect of the current flux is determined by nonuniform gas ionization resulting in different diffusion electron fluxes near different electrodes and, therefore, in different near-electrodes potential falls. This difference creates a steady current flux in the inter-electrode gap. The mechanism of EMF generation is analogous to the Dember effect at the nonuniform photoexcitation of semiconductors.   

Effect of sheath gas in atmospheric-pressure plasma jet for potato sprouting suppression

Recently, low-temperature atmospheric-pressure plasma jets (APPJs) attract much interest for medical and agricultural applications. We try to apply APPJs for the suppression of potato sprouting in the long-term storage. In this study, we investigated the effect of sheath gas in APPJ on the suppression efficiency of the potato sprouting. Our APPJ was composed of an insulated thin wire electrode, a glass tube, a grounded electrode which was wound on the glass tube, and a sheath gas nozzle which was attached at the end of the glass tube. The wire electrode was connected to a rectangular-waveform power supply at a frequency of 3 kHz and a voltage of $\pm 7$ kV. Helium was fed through the glass tube, while we tested dry nitrogen, humid nitrogen, and oxygen as the sheath gas. Eyes of potatoes were irradiated by APPJ for 60 seconds. The sprouting probability was evaluated at two weeks after the plasma irradiation. The sprouting probability was 28\% when we employed no sheath gases, whereas an improved probability of 10\% was obtained when we applied dry nitrogen as the sheath gas. Optical emission spectroscopy was carried out to diagnose the plasma jet. It was suggested that reactive species originated from nitrogen worked for the efficient suppression of the potato sprouting.   

Cysteine as a Biological Probe for Comparing Plasma Sources

A large variety of plasma sources are available in the plasma medicine community. While enabling to choose the most promising source for a certain biomedical application, comparison of the different sources with a focus on their effect on biological targets is rather challenging. To allow for better comparison of various sources, the recent European COST action MP1101 was used to design the COST reference microplasma jet\footnote{J. Golda \textit{et al}., \textbf{J. Phys. D} 49, 084003}. Cysteine is a promising candidate investigate the impact of plasma from various sources on a standardized biological molecule, which is especially relevant for the investigations on a molecular level after plasma treatment. The simple structure of cysteine allows for a more in-depth analysis of each chemical group after plasma treatment and enables a comparison between different plasma sources and treatment parameters on each chemical group. The model itself has already been successfully established using a dielectric barrier discharge\footnote{F. Kogelheide \textit{et al}., \textbf{J. Phys. D} 49, 084004}. Here, additional plasma sources are compared by the means of their impact on cysteine samples, showing e.g. the influence of feed-gas variations by adding oxygen or nitrogen admixture   

Promotion of cell proliferation using atmospheric-pressure radical source

In this study, we have focused on the effects of neutral radicals on cell proliferation and treated budding yeasts and mouse fibroblast cells in solutions using neutral radical source, which can selectively supply neutral radicals without charged species and optical emissions. The activation and inactivation effects of neutral oxygen or nitrogen-oxide radicals on cells were investigated using a cell count and a colony count method, respectively. The radical densities supplied from the radical source were measured using VUVAS and UVAS. Based on the measurements of free residual chloride and hydrogen peroxide concentrations in the solutions treated with radicals, we have investigated their effects on the activation and the inactivation. From these results, we have concluded that the main factor for the inactivation in PBS solutions is due to the hypochlorous acid generated in the PBS irradiated with oxygen radicals. On the other hand, we have found that the main factor for the promotion is not the hypochlorous acid but other radicals.   

Ion acceleration in electrodeless plasma thrusters

Since electrodeless plasma thrusters do not use biased electrodes or grids to accelerate ions, it is unclear what determines the magnitude of the ``accelerating voltage'' and hence what the ion beam energy is. In this work a combined theoretical/experimental study of the relationship between the electron temperature and the ion energy was performed to provide such an answer. Experimental measurements show that the ion energy and electron temperature are strongly correlated, and demonstrate that the driving force for the plasma expansion in magnetic nozzles is the electron pressure: in complete analogy to chemical rockets with physical nozzles. Because there are no electrodes or applied voltages, the plasma that exits the thruster must be current-free, and we show that this establishes a strong criterion that determines the maximum accelerating potential that self-forms in the plasma. This maximum accelerating potential (which is between about 4-6 times the electron temperature) is similar to that which develops for a floating sheath, and depends on the electron velocity distribution function. Based on plasma loss considerations inside the thruster cavity, and the drop-off of the ionization cross section for large electron energies in most gases, we predict a theoretical maximum achievable ion beam energy of about 400 eV for argon and xenon propellants.   

Dynamic stall control by plasma actuators with combined energy/momentum action.

Increased interest in plasma assisted flow control is reflected by a dramatic increase in publication rate over the past decade, including numerous demonstrations of plasma-assisted flow control. Many of these have been summarized in several topical reviews published recently. As an alternative to AC voltage inputs, nanosecond pulse driven plasma actuators in which voltage is applied in pulses at a specific frequency and with a specific on-time have been proposed for separated flow control. Nanosecond pulsed periodic dielectric barrier devices have been experimentally demonstrated to affect separated flows over a range of Mach numbers ({\$}0.03 $\backslash $ge $M \quad \backslash $ge 0.85{\$}) and Reynolds numbers ({\$}10\textasciicircum 4 $\backslash $ge \textit{Re} $\backslash $ge 2$\backslash $times10\textasciicircum 6{\$}) that are consistent with retreating blade flows. Furthermore, the nanosecond pulsed actuators tested to date have required less than 10 Watt per cm. of wing span, and therefore are energy efficient.   

Plasma guiding and deflection of high speed projectiles.

The deposition of energy in the air in front of a high-speed projectile can lead to both the reduction of drag and the production of steering moments. Modeling has shown that the major contributor to the drag reduction and the steering moment is the high temperature, low density region that is produced by the energy addition. If the energy addition is off axis, it leads to a non symmetric pressure distribution on the projectile as it passes through this region, producing steering control authority that increases nonlinearly with Mach number. Experiments with a tethered projectile and subsequently with a rotating projectile using pulsed laser energy addition were reported. More recent experiments with a 30-mm diameter projectile in M$=$3.5 flow have been undertaken using a nozzle driven by a pulsed shock tunnel 9.5 m in length and 100 mm internal diameter. Energy was deposited by Nd-YAG laser with pulse energy of about 3 Joules at 1064nm. The laser pulse duration was 5-6 ns. Preliminary results indicate that the laser spark -- flow interaction changes the angular momentum of the model for with a laser pulse energy of 2.85 J, the angle between laser spark axis and the flow 30\textasciicircum 0 and a flow speed 1100 m/s.   

Effect of segmented electrode length on the performances of Hall thruster

The influences of the low-emissive graphite segmented electrode placed near the channel exit on the discharge characteristics of Hall thruster are studied using the particle-in-cell method. A two-dimensional physical model is established according to the Hall thruster discharge channel configuration. The effects of electrode length on potential, ion density, electron temperature, ionization rate and discharge current are investigated. It is found that, with the increasing of segmented electrode length, the equipotential lines bend towards the channel exit, and approximately parallel to the wall at the channel surface, radial velocity and radial flow of ions are increased, and the electron temperature is also enhanced. Due to the conductive characteristic of electrodes, the radial electric field and the axial electron conductivity near the wall are enhanced, and the probability of the electron-atom ionization is reduced, which leads to the degradation of ionization rate in discharge channel. However, the interaction between electrons and the wall enhances the near wall conductivity, therefore the discharge current grows along with the segmented electrode length, and the performance of the thruster is also affected.   

Effect of plasma distribution on propulsion performance in electrodeless plasma thrusters

A helicon plasma thruster consisting of a helicon plasma source and a magnetic nozzle is one of the candidates for long-lifetime thrusters because no electrodes are employed to generate or accelerate plasma. A recent experiment, however, detected the non-negligible axial momentum lost to the lateral wall boundary, which degrades thruster performance, when the source was operated with highly ionized gases. To investigate this mechanism, we have conducted two-dimensional axisymmetric particle-in-cell (PIC) simulations with the neutral distribution obtained by Direct Simulation Monte Carlo (DSMC) method. The numerical results have indicated that the axially asymmetric profiles of the plasma density and potential are obtained when the strong decay of neutrals occurs at the source downstream. This asymmetric potential profile leads to the accelerated ion towards the lateral wall, leading to the non-negligible net axial force in the opposite direction of the thrust. Hence, to reduce this asymmetric profile by increasing the neutral density at downstream and/or by confining plasma with external magnetic field would result in improvement of the propulsion performance. These effects are also analyzed by PIC/DSMC simulations.   

Measurement of Velocity Induced by a Propagating Arc Magnetohydrodynamic Plasma Actuator

Plasma actuators can substantially improve the maneuverability and e?ciency of aerial vehicles. These solid state devices have low mass, small volume, and high bandwidth that make them excellent alternatives to conventional mechanical actuators. In particular, a Rail Plasma Actuator (RailPAc) has the potential to delay ?ow separation on an aerodynamic surface by generating a large body force. A RailPAc consists of parallel rails and an electrical arc that propagates along the rails with a self-induced Lorentz force. The motion of the arc transfers momentum to the surrounding neutral air. A study was conducted to understand how the motion and shape of a propagating arc couples with the ?uid momentum. In particular, we used Particle Imaging Velocimetry (PIV) and seedless PIV based on Background Oriented Schlieren (BOS) technique to measure the induced velocity of a propagating arc in one atmosphere. Results obtained provide insight into how the ?ow ?eld responds to the passage of a RailPAc electrical arc. A complete description of the RailPAc actuation mechanism can be obtained if the ?uid momentum measurements from PIV and seedless PIV are compared to the transit characteristics of an arc.   

A Study of Impedance Relationships in Dual Frequency PECVD Process Plasma

Commercial plasma process reactors are commonly operated with a very limited suite of on-board plasma diagnostics. However, as process demands advance so has the need for detailed plasma monitoring and diagnosis. The VI probe is one of the few instruments commonly available for this task. We present a study of voltage, current, impedance and phase trends acquired by off-the-shelf VI probes in Dual Frequency (DF) 400 kHz/13.56MHz capacitively-coupled plasma (CCP) as typically used for Plasma Enhanced Chemical Vapor Deposition (PECVD). These plasmas typically operate at pressures from 1 to 5 Torr and at RF power levels of \textasciitilde 3 W/cm$^{\mathrm{2}}$. Interpretation of DF VI probe impedance trends is challenging. Non-linear interactions are known to exist in plasma impedance scaling with low and high frequency RF power. Simple capacitive sheath models typically do not simultaneously reproduce the impedance observed at each drive frequency. This work will compare VI probe observed DF CCP impedance tends with plasma fluid simulation. Also explored is the agreement seen with sheath models presently available in the literature. Prospects for the creation of useful equivalent circuit models is also discussed.