Session LT2: Poster Session II (4:30pm - 6:30pm)

Spatial and time evolution of N$_{\mathrm{2}}$(C), N$_{\mathrm{2}}$(B) and N in atmospheric nitrogen plasma created by nanosecond repetitively pulsed discharges

Atomic nitrogen sources are essential for nitridation processes and for other applications such as nanomaterial synthesis or biomedical engineering. Nitrogen plasmas produced by nanosecond repetitively pulsed (NRP) discharges at pressures above atmospheric are considered as a potential source of atomic nitrogen owing to their high energy efficiency and atomic yield. In the present work, we are operating in pure molecular nitrogen at atmospheric pressure. We have measured during the first nanoseconds of the discharge down to 100 us in the postdischarge, the density of N2(C), N2(B) using absolutely calibrated 2D optical emission spectroscopy. The density of ground state N is deduced from the density of N2(B) whose certain vibrationnal states (here v$=$11) are in partial kinetic equilibrium with the ground state of N.   

Spatial distribution of emission in low pressure DC discharges in water and alcohol vapours

Here we show the spatial distributions of emission in the DC breakdown in water vapour and alcohol vapours, for electrode gaps 1.1 and 3.1 cm. We recorded the 2D side-on distributions of emission using an ICCD camera. For the same \textit{pd}, at different electrode gaps $d$, the anatomy of the discharge changes. The radial discharge width is smaller at larger electrode gaps, as shown for the case of a gap of 3.1 cm, as compared to the gap of 1.1 cm. This difference is most noticeable at low pressures (large $E/N)$.At given electrode diameter to gap ratio the increased diffusional losses at lower pressures will violate \textit{pd} scaling, however, it is interesting that processes induced by heavy particles are more sensitive to changes in geometry of the discharge than electron induced processes. Under the investigated conditions, most of the emission comes from the cathode region due to the heavy-particle excitation. As the electrode gap increases the radial distribution of emission near the cathode becomes significantly narrower than that near the anode. We propose that the smaller radial discharge widths at larger gaps may be due to the change of electric field distribution in the vicinity of the cathode edges, in addition to the diffusion losses of charged particles near the discharge chamber walls. Consequently, processes of excitation and ionization in this region would be less prominent.   

Rarefaction Flow in Bounded Plasma with Adiabatic Electrons

We study, by numerical and analytical means, the evolution of a collisionless plasma initiated between absorbing walls. The ensuing flow is described by rarefaction waves that travel inward from the boundaries, interact, and eventually vanish after crossing through, leading up to the asymptotic stage of the decay. Particle simulations indicate that the kinetic evolution strongly resembles one found in isentropic gas dynamics. Namely, a very gradual density profile forms in the expanding central region where the rarefaction waves interact, with an accompanying linear velocity profile. Asymptotically, the density falls off as $1/t$. The density and the flux at the boundary show little variation over the period when rarefaction waves still exist. Plasma potential, on the other hand, drops quite rapidly (on the underlying ion-acoustic time scale) to less than $T_e$ when over 70\% of the particles still remain in the system. This is due to electron kinetics being governed by conservation of adiabatic invariant in a slowly varying potential well. Analytical model of the velocity distribution is presented to explain the simulations. The results have implications for afterglow plasmas used in material processing and also for ion-extraction devices.   

DOLI-II upgrades at UW-Madison

The upgrade of the triple plasma device at the University of Wisconsin-Madison is close to being finished. The device consists of two outer plasma source chambers and a central chamber with biased grids placed in between. Plasma is produced in each outer chamber by thermionic electrons which are emitted from negatively biased filaments. Permanent magnet line cusps were placed around each chamber to trap the ionizing electrons. The device can be used to study double layer structures and nonlinear wave propagation. Diagnostic tools such as Langmuir probes and emissive probes are used to take measurements of plasma parameters. New upgrades include laser-induced fluorescence and Mach probe diagnostics to directly measure ion speed. The initial design and construction of this device was discussed by Justin Kim et al. at the 56th APS-DPP conference. The latest device status will be presented at this meeting.   

Does the discrepancy between Langmuir Probe and emissive probe measurements of plasma potential depend on ion flow and sheath formation?

It has recently been shown that emissive probes (EPs) measure plasma potential profiles correctly in plasma presheaths, and that Langmuir probes (LPs) do not, in low temperature, low pressure plasma. It has been argued that these differences are thought to be caused by inherent, diffuse, ion flow in the presheath region toward the negatively biased electrode, characteristic of sheath formation. One of the roles of experiment is that of suggesting models and to spur theory formation. In pursuit of this goal, we test the hypothesis that ion flow to the boundary plays an essential role by examining the difference between plasma potential measurements made by Langmuir probes and emissive probes in single ion species plasmas of differing mass number. The Bohm speeds for Xe, Kr, Ar, Ne, and He plasmas are known to vary. Experiments are performed in unmagnetized discharges in the parameter regimes, $ 0.1\leq P_n \leq 1mTorr$, with $ 1 \leq T_e \leq 5 eV,$ and $1 \times 10^9 \leq n_e \leq 1 \times 10^{10} cm^{-3}$. Results are discussed.   

An algebraic RF sheath model for a wide range of excitation waveforms and amplitudes, and levels of collisionality

The boundary sheath of a low temperature plasma comprises often only a fraction of its volume but is responsible for many aspects of the macroscopic behavior. Reliable models of the sheath are therefore of theoretical and practical interest. Particularly interesting are "algebraic" models which describe the particle densities and the electrical field of the sheath in closed analytical form. This contribution presents an algebraic model of the RF modulated boundary sheath which applies for a wide range of excitation waveforms and amplitudes, and levels of collisionality. A comparison with the results of a kinetic Particle-in-Cell/Monte Carlo Collisions simulation is conducted.   

Feasibility study on various optics design for X-pinch high voltage measurement system using the Pockels electro-optic effect

We develop an optics-based high voltage sensor applicable to X-pinch plasma source using a Pockels cell which changes the refractive index according to the intensity of externally applied electric fields. The developed voltage sensor has a nsec temporal resolution allowing us to follow dynamics of X-pinch plasmas. In addition, since the sensor works as a polarimetry, it makes no electrical contacts with the X-pinch system where a high voltage (\textasciitilde 100 kV) with the fast rising (\textasciitilde nsec) time is applied. Various optics configurations for the sensor are designed and examined on the X-pinch system without plasmas by applying a voltage up to a few tens of kV with a rising time of \textasciitilde 10 nsec. We present the investigated optics configurations and discuss their performance.   

Measuring ion flow velocity toward negatively biased electrode using optical vortex beams

We have been developing a new diagnostic method for measuring ion flow velocity using Laguerre-Gaussian beams, which is known as optical vortices. To measure ion velocity toward a negatively biased electrode immersed in a plasma by conventional laser-induced fluorescence (LIF) method, a laser path perpendicular to the electrode is indispensable. However, a path parallel to the electrode can be chosen when the propagation mode of light is converted into optical vortices, where an atom feels an additional Doppler effect in the azimuthal direction. Although the azimuthal Doppler shift is generally small compared with the longitudinal one, it may deform the shape of the LIF spectrum in case of high ion flow velocity with a focused higher order optical vortex beam, which is numerically shown in ref [1]. A proof-of-principle experiment using a liner ECR plasma device, HYPER-I, is now underway, where the optical vortex beam of which topological charge is ten is produced by the holographic method using a spatial light modulator. Some initial results on modification of the LIF spectrum due to the azimuthal Doppler shift will be presented at the conference. \\ \\ $[1]$ S. Yoshimura et al., Jpn. J. Appl. Phys. $\bf{59}$, SHHB04 (2020).   

Measurement of two-dimensional distribution of electric fields in collisional environments.

In this presentation we describe a picosecond pump-probe method termed laser induced fluorescence dip (LIF-dip) spectroscopy for interrogating electric fields in medium pressure (200 Torr) plasma discharges. We employ an intense picosecond pump laser to drive two-photon absorption on a Kr seed immersed in a helium environment. Specifically, we utilize \textasciitilde 202.3 nm to excite the Kr from the ground state to the 5p$^{\mathrm{\mbox{'}}}$[\textonehalf ]$_{\mathrm{0}}$ state. Shortly after excitation, a second tunable picosecond laser excites the 5p$^{\mathrm{\mbox{'}}}$[\textonehalf ]$_{\mathrm{0}}$ state to (Stark shifted) n $=$ 11 to n $=$ 25 Rydberg states causing a depletion in the observed fluorescence of the excited state to the 5s'[3/2]$_{\mathrm{1}}$. The electric field dependence of the Stark shifted Rydberg states is experimentally measured for a few select Rydberg states to provide electric field detection spanning \textasciitilde 500 V/cm to 10 kV/cm and then utilized to asses electric field distributions. We also discuss the use of a least-squares fitting procedure with additive gaussian noise-based approach to assign observed LIF-Dip profiles electric field values.   

Estimating electron density, electron temperature, and signal-to-noise ratio from laser-collision induced fluorescence data by treating the measurement as a stochastic process

This communication presents a parameter estimation framework and its application to the laser-collision induced fluorescence (LCIF) diagnostic problem. An advantage of the approach is a capability to determine plasma parameters that may have a complicated or nonlinear relationship to the expected value of the measurement. Furthermore, by modeling the measurement as a stochastic process with additive Gaussian noise the experimental signal-to-noise ratio (SNR) can be quantified and the detection limits understood. We present estimation of the electron density and electron temperature in 2-D from camera measurements of LCIF from He within the plasma expansion region of a cathodic arc (50 mTorr) and the positive column of a DC discharge (1 Torr). Additionally, the SNR as defined by the measurement noise model is determined in 2-D or for each camera pixel by estimating the signal variance. Sandia National Laboratories is managed and operated by NTESS under DOE NNSA contract DE-NA0003525.   

High-speed stereoscopic LIBS measurement of fuel droplet and laser induced plasma interaction

Investigation of flame kernel development of a single fuel micro-droplet gives insight into flame kernel development in spray combustion ignition. In this study, an electrodynamic balance is used to trap an electrically charged fuel droplet in space and with a 10 ns Q-switched Nd:YAG laser, a breakdown is localized either directly on the droplet or at a precisely selected nearby location. A stereo-imaging optical configuration allows for simultaneous LIBS diagnostics and high-speed imaging of two orthogonal views of the laser-droplet interaction. Spatially and temporally resolved spectral profiles are captured on an imaging spectrometer and applied toward measurements of electron number density and electron temperature. The resulting dataset is analyzed to understand the variation in plasma parameters as a function of the relative distance between the laser breakdown and micro-droplet. In addition, high-speed imaging of OH* chemiluminescence is used to study the incipient flame kernel evolution and the correlation between the peak OH* concentration and the rate of heat release for different separation distances between the plasma and droplet.   

Atmospheric Pressure Plasma Jet Modeling with Smoothed Particle Hydrodynamics

In previous work,$^{\mathrm{1}}$ a pulsed-dc atmospheric-pressure plasma jet (APPJ) was characterized across varied voltage, pulse width, frequency, and flow rate operating conditions. However, the gas flow structure, along with the spatial temperature and density of the electrons and ions of the APPJ is difficult to measure due to the small sizes. To better understand the relations between the operating conditions, we use we use a multi-step Smoothed Particle Hydrodynamics (SPH) code with Maxwell equation solver called SPFMaX to study the plasma discharge. First, the steady state helium flow structure is studied with a pure fluid simulation. Then, a pulsed power circuit is turned on to ionize the flowing gas, allowing calculations of the plasma characteristics. The simulation is run for a maximum of 200 ms to study the steady state, and 2000 ns to study a single bullet. While the characterization is the primary indented contribution of this model, having a detailed model will allow for predictions of additional operating conditions. Additionally, to the best of our knowledge, this is the first use of SPH to model APPJs. 1. DOI: 10.1109/TPS.2019.2942576   

1D model of the DBD discharge in Ar-S2 mixtures

Spectrum of a discharge in mixtures of argon with sulfur vapor in the visible part of spectrum is similar to the solar spectrum due to strong emission of S2 molecules. This paper presents the one-dimensional fluid model of the dielectric barrier discharge (DBD) in Ar-S2 mixtures. At first kinetics of excited sulfur molecules in a discharge under various Ar-S2 mixture compositions was studied using a global model by calculating time profiles of the plasma species densities under voltage pulses at various Ar-S2 mixture compositions . The DBD was simulated in 3 mm gas gap between dielectric layers covered metallic electrodes. The spatio-temporal characteristics of the DBD in Ar with additive of 0.1 -1 {\%} S2 were simulated at pressure 300 Torr at applying to the electrodes sinusoidal voltage with frequency 20 kHz and amplitude 8 kV. The simulations shown that the discharge radiation includes the S2* and Ar2* bands as well as Ar* lines, radiation of S* lines is small. The radiation efficiency of sulfur dimer bands strong depends on S2 content in the mixture. It is found that the Ar-S2 plasma is very electronegative even at S2 fraction of 0.1{\%} in Ar-S2 mixture, that makes the great impact on the discharge properties.   

Numerical analysis of chemical reactions in a single droplet in atmospheric-pressure nonequilibrium plasma

Plasma-induced liquid-phase reactions primary occur at the plasma-liquid interface by the incident species in the plasma. The use of microdroplet is advantageous to enhance plasma-liquid interaction due to their large specific surface. Because the electron and ion fluxes onto the droplet surface depend on the local electric field, the droplet charging and the reactions in the droplet can have a spatial distribution. In this study, we modeled the plasma-droplet interaction for a single droplet in a homogeneous atmospheric pressure He plasma by continuity equations for charged/neutral species coupled with Poisson's equation. The droplet is a silane solution with a radius of 4 µm and the analysis time is about 10 ms. The simulation results show that the charge of the droplets in the plasma is not uniform but angular dependency. The flux from the plasma to the droplet is approximately constant after the surface charging of the droplet has reached steady state. The chemically active region in the droplet is in a thin layer near the interface. Most of the products inside the droplet is started to be formed near the interface due to chemical reactions, and then they diffuse into the interior of the droplet.This work was supported by JSPS KAKENHI Grant Number JP18H01207.   

Fast computations in atmospheric pressure plasma jets

A framework for the computation of plasma-generated species densities in atmospheric pressure plasma jets is developed and applied to kINpen device with Ar feed. The first component of the framework is a turbulent flow model, namely the large eddy simulation (LES) model [1]. The second component is a global model being solved at transient state; a transformation from time to space (distance from nozzle exit) is achieved by using the fluid velocity. The model is implemented with a suitably modified version of $\pi $lasma-R code [2] which allows very fast calculations (some minutes for a set of 764 reactions and 84 species). The electron energy distribution function is calculated by Bolsig$+$. The assumptions frequently used for global models are evaluated in order to examine the extent of their limits. Atomic oxygen measurements [3] are used for the evaluation of the results. LES model is necessary for high accuracy. Simple turbulence models (e.g. realizable k-$\varepsilon )$ can lead to significant inaccuracies for specific plasma-generated species. [1] Passaras D et al. 2020 J. Phys. D: Appl. Phys. 53 26 [2] www.plasma-r.com [3] Reuter S et al. 2012 Plasma Sources Sci. Technol. 21 2.   

Semi-Classical Reaction Network Generation from Localized Molecular Orbital DFT for Arbitrary Network Complexities

When modeling atmospheric-pressure plasma sources, one of the main challenges is on the construction of the reaction network describing the plasma chemistry of the system. Reaction network size scales exponentially with the number of atom identities and molecular bonds in the network precursors, prohibiting time-intensive multivariate transition state calculations to find branching ratios for dissociative processes. The size of the network alone may be too large to handle, with $10^6$-$10^9$ or more potential reactions for large precursor molecules. In this work we show a method to generate reaction networks using established classical and semi-classical relations, combined with information from ML-assisted localized molecular orbital DFT, including electron-driven excitation, scattering, and dissociation, as well as prominent heavy-species reactions such as de-excitation and charge exchange. We assess accuracy and precision of the method via a comparison of the density and reaction rate outputs of a 0D model from both an automatically-produced and hand-produced network of some common network chemistry (e.g. Ar, He/N, Si/H). Furthermore, we show strategies for complexity mitigation, including runtime application of network reduction strategies on networks from larger precursors.   

Two-dimensional modeling of the ignition phase of an atmospheric-pressure argon DBD using FEniCS

The ignition of a single-filament dielectric barrier discharge (DBD) in argon at atmospheric pressure is analysed by means of the two-dimensional finite element discharge modelling code FEDM. This new open source code is developed on the basis of FEniCS (https://fenicsproject.org) and implements balance equations for the relevant plasma species, the electron energy balance equation, Poisson's equation for the electric potential and a surface charge balance equation. The present study focuses on the initial streamer propagation phase in an asymmetric single filament DBD with hemispherical electrodes. The metallic high-voltage electrode is directly exposed to the plasma, while the grounded electrode is covered by a dielectric. The obtained results show that the properties of the positive streamer breakdown sensitively depend on the description of the electron component. In comparison with standard models, faster streamer propagation is predicted by the consistent drift-diffusion approximation, which is based on a four-moment model.   

Gas Kinetic Model for Titan Atmospheric Entry Plasma

The Huygens mission to Titan showed us a great deal of interesting things about the atmosphere of Titan. During the atmospheric entry phase for the lander, most of the kinetic energy of the probe was lost in the form of thermal ionization of the atmosphere, i.e. a plasma is formed around the probe during entry. One interesting idea for replenishing the battery power for any probe landing on Titan is to use a magnetohydrodynamic (MHD) generator to extract energy from the plasma that forms during the entry phase. In this presentation we will describe the chemical composition, density, and pressures of the atmosphere at different altitudes. We will then show a basic gas kinetic model to determine how the composition changes during the thermal ionization process, which is important for determining the amount of energy produced by an MHD generator.   

Particle-in-cell simulations of ion acoustic turbulence and energy exchange between ions and electrons in collisionless plasmas

We report a one-dimensional particle-in-cell (1D PIC) simulation of ion acoustic turbulence (IAT) and energy exchange between ions and electrons in collisionless plasmas. We used a condition for the ramp-up phase in the tokamak startup, but similar physics should apply to, for example, a plume of the hollow cathode at low pressures. During a typical current ramp-up stage in tokamak, plasma is collisionless; and electrons and ions keep accelerating in the inducted toroidal electric field. In this study, making use of a 1D electrostatic PIC code EDIPIC, we observed an anomalous rate of the energy exchange between ions and electrons due to IAT. From the simulations, a quasi-periodic pattern of IAT is observed with IAT on and off all the time. IAT causes the heating of electrons and ions, increase of the friction force and decrease of the relative velocity between ions and electrons. The ion temperature rise damps the instability until relative velocity increases and IAT reemerges. Surprisingly, we observed that only one soliton with a hole structure in the ion phase forms during the decay phase of IAT.   

Fast 2D computations in the COST reference jet with an 1D plasma model

A novel approach for fast 2D computations in capacitively coupled atmospheric pressure plasma jets is applied to the COST reference jet [1] with He/O2 feed. The keystone of this approach is the simplification of the 2D equations for the mass and electron energy balances of the plasma fluid model by a) the assumption of negligible variation of the plasma potential along the flow direction and b) the exploitation of the high values of Peclet numbers and/or the low values of the diffusive Damkohler numbers for the species and the electron energy. These simplifications allow the solution of a small number of 1D problems instead of solving a 2D problem and reduce significantly the computational cost. The latter is further reduced by using a time-slicing technique and a simplified reaction set [2]. The results of this approach are validated through a comparison to the results of a 2D plasma model. The effect of the feed on the results is investigated and the use of 1D plasma models [3] for the COST jet is evaluated. [1] J. Golda, et al., J. Phys. D: Appl. Phys. 49, 8 (2016). [2] M. Turner, Plasma Sources Sci. Technol. 25, 1 (2016). [3] J. Waskoenig, et al., Plasma Sources Sci. Technol. 19, 4 (2010).   

Sensitivity analysis of PIC/MCC simulation results on the parameters of a realistic model for electron-surface interaction in low-pressure capacitively coupled radio-frequency discharges

Recently, PIC/MCC simulations have been performed to study the role of electron-induced secondary electrons ($\delta$-electrons) on the plasma parameters in low-pressure capacitively coupled radio-frequency (CCRF) discharges. In these studies, a realistic model has been used to describe the interaction of electrons with the boundary surfaces, which takes into account the elastic and inelastic reflection of electrons, as well as the emission of $\delta$-electrons upon primary electron (PE) impact; the emission coefficients of these processes are functions of the energy and angle of incidence of PEs and depend on the surface properties. For this model, a number of material specific input parameters have to be specified, e.g. the maximum emission, the threshold energies for elastic reflection and $\delta$-electron emission, etc., which have some uncertainty. However, these input parameters can have significant influence on the simulation outcome. Here, a sensitivity analysis of the numerical results on the parameters of the realistic electron-surface model is provided for low-pressure argon CCRF discharges.   

Machine Learning methods for studying plasma instabilities in DC magnetized plasmas

The transport of ions extracted from hot cathode DC magnetized ion sources is often limited by the presence of plasma instabilities and stochastic noise. These ion sources are commonly used in high current implanters to generate ion beams. At very high current densities experiments show increase in the stochastic noise and amplitude in frequency spectra associated with the ExB and diamagnetic instabilities. If we assume that the noise is due to ExB rotation, i.e. pronounced when we have both magnetic and electric field in the source, the increase of the electric field in the bulk would result in rotation enhancement. The higher magnetic field leads to stronger potential gradients in the plasma, so ExB effects will most likely increase at higher magnetic fields. Higher gas pressure decreases the electric field in the plasma therefore less noise is expected at high current densities. In this work, we use artificial neural networks (NN) trained on theoretical models and simulations to predict experimental observations of the stochastic noise and rotation at the characteristic frequencies in hot cathode DC magnetized ion sources.   

Basic research of electron dynamics in low pressure capacitively coupled plasmas

At low pressures, capacitively coupled plasmas operate in a ‘nonlocal’ regime. Electrons interact with the space- and time dependent electric field and traverse a certain distance without any collisions. Consequently, the EVDF becomes anisotropic and the discharge exhibits complex electron dynamics. In the last decades, various terminologies have been introduced which are based on theoretical, experimental and computational work in order to study the electron power absorption at low pressure. The goal of this work is to demonstrate a basic strategy of how to investigate the electron dynamics for a universal electropositive CCRF discharge scenario (3 Pa, argon). 1D3V PIC/MCC simulations provide spatio-temporal diagnostic methods in order to illustrate the interplay between the fundamental plasma parameters, such as densities, fields, currents and temperatures. The analysis shows how current conservation is ensured and how it is linked to the generation of electrostatic waves. Electron distribution functions are discussed and the importance of energetic beam-like electrons is addressed. A detailed analysis of the momentum balance equation shows, how the electron power absorption really works. [1] S. Wilczek et al. (2020) Journal of Applied Physics, 127(18), 181101.   

Experimental investigation of capacitively coupled discharge in air at 1 Torr at frequencies near the series resonance

Capacitively coupled radio-frequency (CCRF) discharges are typically operated at frequencies much lower than that at which a series resonance occurs. In this work, we studied CCRF with 5 cm diameter parallel-plate electrodes and 2 cm interelectrode gap operating in air at 1 Torr. An ultrawideband amplifier was used to reach the series resonance and study the CCRF discharge behavior at near-resonant frequencies. Using voltage and current probes, the current-voltage characteristics were captured, and the plasma electron density was measured with a 58.1 GHz microwave interferometer. The resonant frequency, at which the phase shift between the current and voltage is zero, was found to be about 160 MHz. Below and above that frequency, the discharge exhibits capacitive and inductive behavior, respectively. At near-resonant frequencies, the current-voltage characteristics were found to be parabolic, and the effective momentum-transfer collision frequencies were inferred from these parabolic characteristics using Godyak's theory. The collision frequencies were found to be several times lower than those at electron temperatures on the order of 1 eV. This could imply an unusual electron heating regime at this relatively high pressure, which requires further studies.   

Mode transition in an oxygen low-pressure, very high frequency (162MHz), multi-tile electrode capacitively coupled plasma

An experimental investigation of electrode voltage/discharge current, plasma density including negative ions and ion flux, and ion energy distributions (IEDs) is performed in a low-pressure oxygen discharge excited by a multi-tile electrode, very high frequency (162 MHz) capacitively coupled plasma system. The results show a mode transition versus RF power. An inflection point is observed in the measured electrode voltage and current near to the mode transition. The negative ion density inferred from measured electron density and ion flux using resonance hairpin probe and planar probe respectively shows an initial increase and then decrease after mode transition. The IED shows a symmetric narrow distribution and the mean energy first increases up to the transition point and then decreases with further increase in RF power. A change in the current coupling mechanism and variation in the discharge impedance due to the presence of negative ions are responsible for the observed mode transition.   

Investigation of the driving frequency dependence in the inductively coupled plasma

In this study, we investigate the dependence of the power transfer efficiency with the driving frequency in an inductive coupling plasma (ICP). The power transfer efficiency depends on driving frequency in the ICP according to the transformer model. The experimental results show that the efficiency increases as driving frequency increases because the power loss in the plasma decrease as driving frequency increase. Furthermore, at higher driving frequency, the electron density and the electron temperature are higher than lower driving frequency. We compare the derivative of absorbed power and total energy loss with respect to the driving frequency from the power balance equation. The power transfer efficiency depending on the driving frequency is measured by the current sensor. To verify the frequency dependence, electron temperature and electron density were measured through EEPF.   

Control of microwave propagation property with metamaterials and tuned plasma.

Metamaterial researches have been reported on the validity of not only cloaking devices but also layers of absorption, reflection and scattering. This study demonstrates tunability of plasma-metamaterials switchable for these wave propagation properties using experimental and numerical results. Our first aim is to verify a cloaking phenomenon, which has been one of the successful principles based on transformation optics [1]. General metamaterial cloaks have difficulties in terms of the complexity and the excessive thickness of cloaking layers because the layers require elaborate designs of the refractive index. To solve it, we combined solid metamaterials with gaseous plasma [2]. Plasma has inherent gradients, which brings simplification and miniaturization to the layers. In addition, permittivity of plasma is tunable by controlling electron density in plasma. Turning on/off and tuning plasma power enables physical wave reactions to switch instantly with one device for a cloak, an absorber, a reflector, and scatterer, which is our second aim in this study. [1] J. Pendry, D. Schurig, D. Smith, Science, 314, 977 (2006). [2] O. Sakai, S. Yamaguchi, A. Bambina, A. Iwai, Y. Nakamura, Y. Tamayama, S. Miyagi, Plasma Physics and Controlled Fusion, 59, 014042 (2017).   

Radio-frequency plasma capacitor can increase rates of seeds imbibition and germination

Cold atmospheric pressure plasma jets are used in agriculture for the treatment of seeds and plants. Here we found that the treatment of seeds by a plasma ball or flat plasma panel can also accelerate seed imbibition, germination, and radicle growing rates. Generated by the plasma lamps, high-frequency electromagnetic fields and photons can penetrate seed coats and modify their surface properties. The plasma ball treatments of seeds produce hydrophilization of seed coats and decrease the apparent contact angle between a water drop and the seed surface, thereby improving the wetting properties of seeds surfaces. High-frequency electromagnetic fields and light emitted by plasma lamps accelerate the germination of seeds. Treatment using a plasma lamp is not as effective as treatment by a cold atmospheric pressure plasma jet. Plasma lamps can be used in agriculture for the acceleration of seed germination, increasing growth of plant seedlings, and corrugation of the bio-tissue surfaces without side effects of reactive oxygen and nitrogen species generated by plasma jets.   

Plasma decay around control grid apertures of medium voltage direct current circuit breaker with a thermionic hollow cathode

We report recent progress on modeling the interruption phase in a medium voltage direct current (MVDC) circuit breaker with a hollow thermionic cathode. When switching to the interruption phase, a negative bias is applied to the control grid, discharge is not sustained any more at the anode side and plasma starts decaying. Plasma decay is governed by the formation of an ion matrix sheath in the grid apertures, competing with ongoing ionization. Prior work indicates maximal interruption current density j$_{\mathrm{max}}$ \textasciitilde 5 A/cm$^{\mathrm{2}}$ but for thermionic cathode, the expectation is that a higher density \textasciitilde 15 A/cm$^{\mathrm{2}}$ can be achieved. Our model can help find optimal grid dimensions that don't compromise breaker operation in the closed phase and guarantee its successful opening phase. A two-dimensional asymmetric model of plasma decay around the control grid aperture was implemented into the 3D particle-in-cell code WARP. It has been modified to simulate particle collisions needed for plasma modeling and validated against previously developed 1D EDIPIC code and experimental data.   

Diagnostics of a magnetized plasma by PDO radiation

We investigated the spectral properties of a radiation emitted from a plasma dipole oscillation (PDO) in a magnetized plasma and its use for plasma diagnostics. The PDO is a noble concept of generating a localized bunch of electrons, which oscillate in-phase with the plasma frequency. It resembles the pedagogical slab model of the plasma oscillation, but actually has much richer nonlinear physics such as high harmonics. The PDO can be generated from two colliding laser beams. Originally the PDO was devised for use as strong high-frequency radiation sources (Kwon et al., SREP 2017). We soon found that the PDO can be used as a novel diagnostic method for pin-point detect of plasma density and reconstruction of density profiles (Kylychbekov et al., PSST 2020). In this work we studied the PDO in a magnetized plasma, using two-dimensional particle-in-cell simulations. The magnetized PDO is found to oscillate with right- and left-handed cut-off frequencies of the X-mode. The frequency of the radiation emitted from the PDO exactly corresponds to the cut-off frequencies of the X-mode, from which the informations of local magnetic field and density can be extracted. The polarization property of the radiation is analyzed by 3D PIC simulations.   

Instabilities in magnetised plasma columns

In the International Thermonuclear Experimental Reactor, the two neutral-beam injectors (NBI) are designed to deliver a joint power of 33MW with 1MeV deuterium injection. The neutralisation of positively charged deuterium is inefficient at such energies which requires to generate, accelerate and neutralise negatively charged deuterium ions which is central for developing DEMO. The magnetised plasma column source is a serious candidate for the generation of negative ions [New J. Phys. \textbf{18} 125005(2016)] while the beam-driven plasma neutraliser concept has been proposed as a good trade-off between cost and yield [E. Surrey, AIP Conf. Proc. \textbf{1515} 532 (2013)]. These linear plasma devices are prone to instabilities in the plane perpendicular to the magnetic field which affects plasma transport and eventually their performance. We follow these instabilities by running particle-in-cell simulations [G. Fubiani et al., New J. Phys. \textbf{19}, 015002 (2017)] for magnetic fields values that span regimes in which the ions are either magnetised or unmagnetised together with its transition. We characterise them via Fourier analysis and compare the modes with a dedicated dispersion relation [S. Sadouni et al. APS Meeting Abstracts (2018)] that lead to the non linear regime.   

The behavior of floating potential and electronegativity during the E-H transition in an electronegative inductively coupled plasma

This study experimentally investigates the evolutions of floating potential and electronegativity during the E-H transition of an inductively coupled oxygen plasma. The electronegativity, plasma potential, floating potential, electron temperature and electron energy probability function (EEPF) are measured by means of a small cylindrical Langmuir probe. The electronegativity can be deduced by applying the orbital-motion-limited (OML) theory to the positive ion current portion of the I-V characteristic curve, and assuming that the electron current is balanced with the positive ion current at the floating potential. When the plasma absorption power increases in the range of 8.2 W to 185 W, the EEPFs do not escape from the Maxwellian distribution, and the electron temperature remains constant around 3.5 eV. Moreover, the floating potential tends to decrease in the E mode, be minimized in the EH transition region, and rise in the H mode. Contrariwise, the electronegativity peaks in the transition region and decreases towards E and H modes. This behavior has never been observed theoretically or experimentally in electronegative plasma.   

RF distortion effect by the external load on the floating harmonic method.

It is well known that the floating potential is far from the plasma potential due to the self-bias. So, in the floating Langmuir probe, I-V curve around floating potential is difficult to be distorted by RF fluctuations. However, there are situations in which RF distortion occurs even with small RF fluctuations on the floating harmonic method. RF distortion which arises from the amplification of RF fluctuations owing to the series resonance of the probe sheath and the external load is expected. To verify this series resonance effect, changes of RF fluctuations around the probe with choke filters of $\omega_{\mbox{RF}} ,\mbox{\thinspace }\omega_{\mbox{RF}} \pm 1,\mbox{\thinspace }\omega_{\mbox{RF}} \pm 2\mbox{\thinspace }\;\mbox{[MHz]}$ were observed. In cases of choke filters except for driving and its harmonic frequencies, it is found that RF fluctuations can be greatly affected and high electron temperatures are measured. This phenomenon can be explained by the series resonance of equivalent circuit between the probe sheath and the choke filter. Choke filters with incorrectly compensated for driving and its harmonic frequencies can overestimate electron temperatures than even without a filter.   

Particle-in-cell simulation of multi-frequency capacitively-coupled plasmas at low pressure: a 2D perspective

Multi-frequency capacitively coupled plasmas (MFCCPs) are one of the key technologies enabling forefront of current etching process in 3D NAND and FinFET manufacturing. These processes rely crucially on the precise control of plasma density profile, uniformity of ion/radical fluxes and ion energy distribution (IED) in MFCCPs. With a rapidly expanding process space due to puling of RF sources, computational modeling has become an important tool in conjunction with experimental diagnostics in understanding the intricate physical mechanisms in MFCCPs. In this paper, a 2D particle-in-cell (PIC) plasma model is used to study the kinetic behavior of low pressure (1 -- 10's mTorr) MFCCPs in Ar. The low frequency RF source is at 100's kHz while 10's MHz is used for the high frequency. Simulation results show a shift of the plasma density profile from center-peak to edge-peak over pressure ranging from 2 -- 20 mTorr. Simulation results are compared with experimental measurements of plasma density, fluxes and IED over a range of pressure, frequency and RF voltages.   

Singular-point reproducibility for physical phenomenon in supercritical plasma analyzed by network model

A complex network is one of the techniques for analyzing and evaluating a complexity in uniform low-temperature plasma [1]. The previous studies on chemical reaction networks revealed the roles of species by using centrality indices derived from the macroscopic structure of the networks. However, when plasma reaches a supercritical state, another structural property emerges; it becomes necessary to consider not only chemical reactions but also physical spatial structures with density fluctuations. Consequently, macroscopic physical phenomena having singular points, such as thermal conductivity and discharge breakdown voltage, are observable in experiments [2]. In this regard, if physical network structures in non-uniform plasma have feedback loop(s) in their information flows, the loops can create singular point(s) in calculations of a network model which is similar to a recurrent neural network, which is demonstrated in this study. To reproduce a singular point in our theoretical model, a specific range of the physical parameters is a key issue, which is comparable to those in experimental setting. [1] O. Sakai. K. Nobuto, S. Miyagi and K. Tachibana, AIP Adv. 5, 107140 (2015). [2] T. Ito, H. Fujiwara and K. Terashima, J. Appl. Phys. 94, 5411 (2003).   

Effect of slit electrode structure on electron heating and sheath dynamics in capacitively RF discharges

In a capacitively coupled plasma (CCP), hollow structure showerheads are commonly used for the control of uniform gas distribution. In this study, we investigate the effect of a slit electrode structure on electron heating and transport in a CCP using a two-dimensional particle-in-cell simulation. As the streamlines of the electric fields near the slit structure are alternated, not only the sheath size but also the particle transport become asymmetric between the powered and the ground electrode. The conventional discharges with asymmetric electrode sizes show that sheath expansion and a negative DC bias occur in front of a smaller electrode. With the slit structure, however, it is possible to control the sheath size as well as the DC self-bias by varying the number of slits and the configuration. Furthermore, it is also possible to control the radial plasma uniformity.   

Experimental investigation on capacitively coupled plasma source using a passive resonant antenna

We developed a capacitively coupled plasma source using a passive resonant antenna. The source consists of a transmitter coil (powered antenna) connected to the power supply and a receiver coil (resonant antenna) connected to the electrode plate, and the distance between the two coils is 5 cm. At non-resonance, the powered antenna current is very high and there is no plasma discharge in the chamber. However, at resonance, the powered antenna current rapidly decrease and plasma discharge is maintained in the chamber. Plasma density distribution is measured by varying RF power and argon gas pressure, and the results are discussed along with the matcher efficiency.   

The effect of discharge current, pressure and magnetic field on the radial distribution of the formation of particles and flows in a dusty plasma of a positive column of glow discharge.

Interest in the interaction of plasma with dust particles results from numerous applications in fundamental science and technology. Recently, the effect of dust grains on the gas discharge plasma parameters and the space distributions of particles, fluxes and fields has been studied actively. To investigate this problem, we developed numerical axisymmetric modified extended fluid model of the uniform PC dusty argon plasma of glow discharge. Obtained results allowed us to determine the possibility of strong influence of the spatial distributions of dust particle's density on the plasma parameters and identify the influence of the gas pressure, the current of discharge, and the magnetic field values on the spatial profiles of charged particle's densities, radial fluxes and electric field. For example, different values of the discharge current, as well as the different values of the dust particles density, can lead to the realization of different scenarios of the radial profiles formation, including similar (parallel) and even nonmonotonic particles density profiles, accompanied by the zeroing and reversal of the sign of the ambipolar electric field. At the same time, the mechanisms of transition and realization of nonmonotonic density profiles at high discharge currents and high dust densities are different.   

Low Power Flex Dielectric Barrier Plasma Source for Surface Decontamination.

A surface dielectric barrier discharge using a flexible printed circuit design is investigated for surface decontamination for bacterial and non-biological contaminants. The device operates in ambient air without any additional gas flow and power density of \textless 0.5 W/cm2. Using e-coli as a model bacteria we demonstrate a 4log10 reduction of the bacterial load on an inoculated glass surface in direct contact with the device. We also demonstrate a novel use of the device to improve the effect of a common disinfectant. Using a 3{\%} hydrogen peroxide as a model disinfectant we demonstrate an improvement from a 2.4log10 with hydrogen peroxide alone to \textgreater 6log10 with the addition of the plasma output from the dielectric barrier discharge. The synergistic action of the plasma bio active properties and hydrogen peroxide result in a dramatic improvement of surface disinfection. This opens new possibilities for using the low power flexible dielectric barrier plasma sources for surface disinfection and decontamination.   

Scalability of Nanosecond Pulsed Non-Equilibrium Plasma Reactor for Heavy Oil Upgrading

New technologies developed in the laboratory often need to be scaled up to pilot scale and additional investigations are required to ensure that the large-scale unit is still able to replicate the results produced in the lab. A multi-phase non-thermal plasma reactor was developed by LTEOIL that uses nanosecond pulsed electrical discharges to partially upgrade oil with methane and hydrogen at ambient pressure. The single gap reactor was then extended to a continuous flow reactor by adding multiple spark gaps. The reaction in the plasma reactor is governed by the direct reaction of species on the interface and mass transfer between gas and liquid modeled by chain scission reaction. Mass and heat transfer processes are highly scale-dependent but since reaction kinetics and thermodynamics are relatively size independent and the reaction zones are identical in the scale up process, the hypothesis is such that the conversion will also be similar. A mathematical model has been developed to calculate the number of reactors required to process X BBL/day by controlling the input parameters. Results show that to process 1BBL/day with a specific energy input of 500KJ/Kg and 10{\%} conversion, 7 reactors are required with each containing 20 spark gaps and pulsing at a frequency of 200Hz.   

Correlation of plasma impedance and plasma parameters in inductively coupled Ar and CF$_{\mathrm{4}}$ plasma

VI probe has been used to measure electrical impedances in plasma etcher. When an impedance was measured at the port after RF bias matcher, which was connected to an electrostatic chuck, the information of transmission lines and ceramic parts were included in the measured impedance. The impedances of chamber parts had to be excluded because these components interfered with obtaining genuine plasma impedance. We analyzed the correlation between the genuine plasma impedance such as a resistance, an inductance of plasma and total sheath capacitance and plasma parameters such as ion density and electron temperature in Ar plasma and CF$_{\mathrm{4}}$ plasma, respectively. When the genuine plasma impedances were applied, the coefficient of determination between the resistance of plasma and the square root of electron temperature divided by ion density were improved from 0.606 to 0.955 in Ar plasma and from 0.368 to 0.554 in CF$_{\mathrm{4}}$ plasma. In addition, the etching rates of SiO$_{\mathrm{2}}$ and Si$_{\mathrm{3}}$N$_{\mathrm{4}}$ using CF$_{\mathrm{4}}$ plasma were directly proportional to the total sheath capacitance. Monitoring genuine plasma impedance can be used to non-invasively inspect plasma parameters in real-time.   

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Double Langmuir probe (DLP) is an important diagnostic tool that can be used to characterize various types of plasmas. Unlike the single Langmuir probe (SLP), DLP withdraws no net current from the plasma; therefore, it can be used to do measurements on plasmas that are small and time-varying. However, DLP can measure fewer parameters than SLP. With appropriate data analysis, electron temperature ($T_{e})$, positive ion density ($n_{i})$ and the local field can be determined using DLP measurements. For the analysis of measured I-V curves, one must choose an electron energy distribution function (EEDF) \textit{a priori}, and in literature, the methods of analysis generally assume a Maxwellian EEDF. In this work, we review and discuss available theories to reliably evaluate $T_{e}$ and $n_{i}$ from DLP I-V curves and incorporate those theories into a new and free analysis software named DLPGUI. DLPGUI acts both as a theoretical compendium and a flexible analysis tool. With a graphical user interface, DLPGUI allows the user to choose from a list of ion flux models of varying collisionality and different EEDFs before performing analysis of I-V data. The software also provides an approach to extract $n_{i}$ from measurements conducted in gas mixtures.