Session HT4: Poster Session I (4:00-6:00pm, JST)

Convergent close-coupling calculations of electron scattering on HeH+

The helium hydride molecular ion HeH⁺ is comprised of the two most universally abundant elements, hydrogen and helium. Along with the hydrogen molecule, HeH⁺ is expected to form in the cooler edge and divertor regions of fusion reactors, where it is well-known that electron collisions with molecular species play an important role in governing the plasma dynamics.

Understanding the important influence of electron collisions with molecules in plasmas requires accurate cross-section data for many reactions over a broad range of collision energies. Here we apply the molecular convergent close-coupling method to study electronic excitation and ionization from the ground (electronic and vibrational) state of HeH⁺.

Previous calculations for this collision system have been almost exclusively limited to low-energy rovibrational excitation, and the only available measurements are for helium-ion production following electron-impact dissociation of HeH⁺. In this poster, we present cross sections for ionization, and excitation of the first 18 excited electronic states, which we use to estimate the He⁺ production cross section for comparison with the experiment.

    

The integral cross-section for electron-ion ionization collisions with an optical selection of the target’s quantum state.

We propose a method to measure integral cross sections for electron-ion collisions. The study is focused on the ionization of ions – creating ions of an electric charge higher than one. The technique is based on an electron-bombardment of an optically Doppler-cooled set of calcium ions inside a Paul trap. By analyzing the time dynamics of depletion of the ions, the desired cross-sections can be deduced. The method allows distinguishing the scattering of electrons from the ions in various initial quantum states.

The details of the apparatus and the experimental procedure will be presented. The first, preliminary results will be shown and compared with available experimental and theoretical data sets.

    

Forming a pulsed beam of anions via electron dissociative attachment to diatomic molecules

Negative ions (anions) are of great importance in various research fields, including studies of matter-antimatter interactions. Forming such ions is a much more difficult task than creating positive ions, as the additional electron must be attached to an atom or molecule, losing most of its kinetic energy. Such an attachment is possible in a well-known phenomenon of electron dissociative attachment to a molecule.

To provide a pure set of anions, diatomic molecules should be used in such experiments.

In the case, when an intense, pulsed beam of anions is required, the anions may be produced continuously inside an ion trap and stored for minutes and subsequently released in a controlled way in a single pulse.

We are designing an apparatus allowing the production of such negative ion beams. Details of the design will be discussed. Numerical analysis of the expected efficiency and purity of the anion sets will be presented and discussed.

    

The Ps- ion and e--Ps Scattering

There exists a discrepancy between a prior calculation for the photodetachment cross section of Ps^-  using variational wave functions [1-3] with calculations that use the close-coupling [4] and hyperspherical close-coupling [5] wave functions. In preparation for calculating the photodetachment cross-section of Ps^-, we have used a Hylleraas-type trial function to determine the ¹S bound-state energy of Ps^-. We have determined the ¹P phase shifts for e^--Ps scattering using the inverse Kohn variational method and a trial wave function with Hylleraas-type terms. We have also computed the ^1,3S and ³P phase shifts and parameters for the ¹S, ¹P, and ³P resonances. Our calculations are an extension of prior work [1-3]. We have re-optimized the nonlinear parameters and mostly increased the number of linear parameters in the trial wave functions.

    

Quantum vortices in ionization processes by impact of positrons and ions.

The appearance of quantum vortices in the ionization of atoms and molecules due to the impact of electrons, positrons, protons, and photons, has been theoretically studied, and experimentally observed for the first of these cases. In general, restrictive geometries were used to analyze these varieties of codimension 2 in quadruple differential cross sections. On the other hand, in the case of proton impact, only the evolution of the electronic distribution for a fixed impact parameter and slow velocities was analyzed. In this communication we present a comparative study of quantum vortices free of any restrictive geometries, both for the case of positrons and ions, and in similar ranges of impact velocities. Finally, we discuss strategies to experimentally observe these fascinating quantum structures.   

Data-driven discovery of electron continuity equation and its application to measurement of electron transport coefficients in argon

We develop a novel method for determining electron transport coefficients from measured spatiotemporal distribution of electrons n(t, z). In the hydrodynamic equilibrium regime, n(t, z) satisfies the electron continuity equation, and we discover the continuity equation from measured n(t, z). Electron transport coefficients can be acquired as a series of coefficients in the discovered equation. Our method allows us to determine the ionization rate coefficient, Townsend first ionization coefficient, center-of-mass drift velocity, mean-arrival-time drift velocity, longitudinal diffusion coefficient, and high order transport coefficients. These electron transport coefficients in Ar are determined from n(t, z) measured by the scanning drift tube experiment recently developed by Korolov et al. [Rev. Sci. Instrum. 87, 063102 (2016)]. It is found that considering high order transport coefficients, which are often ignored in the traditional drift-tube experiments, is important to determine longitudinal diffusion coefficient properly at middle and high reduced electric fields.   

Scanning drift tube measurements and kinetic computations of electron swarm parameters in CO

We present scanning drift tube measurements and modelling results of electron swarm transport coefficients in CO as a function of the reduced electric field E/N at room temperature. The measurements are performed under time-of-flight (TOF) conditions: a cloud of electrons is initiated by short UV laser pulses and its evolution is traced by detecting the electrons beyond a drift region whose length is varied. This way, so-called swarm maps are recorded, which are used to extract the transport coefficients of the electrons by fitting the current signals. Using the most recent set of cross-sections for electron scattering in CO from Magboltz (v11.11), modelling results are obtained by Monte Carlo (MC) simulations and by solving the Boltzmann equation using a multi-term approach and the density gradient expansion procedure. The measured and computed values of electron swarm transport coefficients agree very well, except for E/N<5 Td. For such low values of E/N the more advanced model for the strongly forward-peaked nature of rotational excitation is required. Measurements and calculations under TOF conditions are augmented by MC simulations of electron transport for idealized steady-state Townsend (SST) conditions. We found a ‘window’ of E/N where the SST transport properties of the electrons exhibit oscillatory behaviour as they relax towards the equilibrium state far downstream from the electron emitting boundary.

    

Data-Homology(DH) Applied-Topology(AT) in/of Solutions of Wide-Class of Maze-Search/Sorting Analog-(Visible)-Computing Approach via/for Microfluidics-Chips via Glow-Discharges

Ghrist/Silva[Notices.AMS 9,1(07)] sensors(DH AT) applies easily VS. harder digital-computing! Solutions of wide-class of maze-searching/sorting analog-(visible)-computing approach via/for microfluidics-chips via glow-discharges[Reyes/Ghanem/White-sides/Manz[Nature m(27/5/02);Lab on a Chip,2(2):113 (02)]] solutions of certain sorting/mapping-problems demonstrate how to build a mini-map giving tourists luminous-route-indicators by glass slide- -etching a city-center-plan make one-inch city-map-chip;flat-lid over top turns streets into hollow/connected tubes, filled-with He-gas with inserted-electrodes at key-tourist-hubs. Voltage applied between two-points, electric-current J obeying (0 =/= [div J – dJ/dt)) = 0] aka extent VS. evolution non-(=/=)-conservation VS. (=)-conservation Noether-(16)-theorem [whose asymptotic-limit-antipodes(ALAS) any/all-symmetries-restorings aka dis-order/random-ness/un-certainty VS. Ramsey-(30)-theory/theorem/numbers/Motzkin!!!/Calude!!!] naturally runs-through streets along shortest-route from A to B – gas-glows like tiny glowing-strip-light, novel visible analog computing approach, to which Ghrist/Silva SA DH applies easily to analog VS. harder digital-computing requiring noise-induced-transitions (NITS).   

Development of a surface wave probe to examine intermediate pressure plasmas

Examination of intermediate pressure (1-5 Torr) plasmas is difficult because often the collision frequency is on the order plasma frequency. In this poster, we will examine how surface waves can be used to measure plasma density, temperature, and collision frequency in discharges at 1-5 Torr. Surface waves can be excited along the sheath of a small antenna by coupling electromagnetic waves into the plasma via the electron plasma frequency. This coupling can be understood by examining the dielectric constant of the plasma. When electron-neutral collision becomes non-negligible the dielectric constant becomes a function of both the plasma frequency and the collision frequency. Typically, the collision frequency is small and thus is approximated as zero. However, a simple analysis of typical electron thermal velocity, electron-neutral collision cross-section, and neutral density shows above 1 Torr, the momentum transfer collision frequency is on the order of the plasma frequency. Thus, in the regime of 1 Torr and above, an attempt to measure the plasma density with interferometry or a single wire probe such as the Langmuir probe is futile. However, with the simultaneous use of three sensitive plasma absorption probes, it is possible to measure not only the density and temperature but also the collision frequency of the plasma. In this poster, we will examine the basic models needed to accomplish this task as well as preliminary measurements from plasmas in the 0.5 to 2.5 Torr regime.   

Characterization of low-pressure ExB plasmas generated by e-beam and non-thermal electrons in 0.1-10 torr air and nitrogen

Electron beam generated plasmas are promising for applications requiring efficient generation of ions and radicals in low pressure environments [1–3]. In this work, we discuss measurements of plasma properties and production of radical species by an electron beam generated plasma in low pressure (0.1-10 mTorr) air and nitrogen. We investigate low temperature plasmas in an applied magnetic field with magnetized electrons and unmagnetized ions. These plasmas are generated by two different sources: i) a high energy electron beam in the keV range produced by an ion-induced secondary electron emitting cathode [2] and ii) a lower energy non thermal electron source (< 100 eV) produced by a hot-filament thermionic cathode. We compare electron kinetic properties and chemical composition of the plasmas created by these two sources. In addition, we analyze plasma instabilities in both plasma systems and compare them with theoretical predictions for beam-plasma and electron gradient drift instabilities [4,5].

[1] Zhao F et al 2021 Carbon 177 244–51

[2] Walton S G et al 2015 ECS J. Solid State Sci. Technol. 4 N5033–40

[3] Zolotukhin D B et al 2017 Physics of Plasmas 24 093502

[4] Fernsler R F et al 1998 Physics of Plasmas 5 2137–43

[5] Rodríguez E et al 2019 Physics of Plasmas 26 053503   

Intermittent variation of electron temperature in converging field following a magnetic beach ECR plasma source

In converging field configuration of cylindrical plasma following electron cyclotron resonance plasma source, intermittent variation of electron temperature is observed. The phenomenon is characterized by higher electron temperature, negative spike in floating potential, exponential distribution of occurrence probability as a function of waiting time, and radially finite and axially elongate correlation. These are similar to a phenomenon observed in diverging field but the first time observation in converging filed configuration.

Experiments were performed using a linear plasma device NUMBER. A 2.45 GHz microwave injected from an end of chamber produces helium plasma by the electron cyclotron resonance in the magnetic beach configuration, B < 0.1 T. The plasma expands along the magnetic field into a higher field test region up to 0.3 T. Electron temperature is obtained from current - voltage characteristics of Langmuir probe using conditional sampling triggered by negative spike in floating potential monitored by another probe.

Electron temperature rising of 2 - 10 eV is observed during negative spikes in floating potential. Cross correlation coefficients between two floating potentials measured at different position indicate correlation along the field line.   

Fast electron heating due to the interplay of electron-and ion-acoustic waves in a current-driven turbulence

We report a one-dimensional particle-in-cell simulation of fast electron heating in collisionless current-driven turbulence due to the coupling between electron- and ion-acoustic waves. We used a condition for the ramp-up phase in the tokamak startup, but similar physics should apply to e.g. plume of hollow cathode at low pressure. PIC simulation results show that the drift velocity difference between electrons and ions excites the ion-acoustic waves, which interact with slow electrons and modify the local distribution function, thereby leading to the marginal instability that generates fast electron-acoustic waves. Electron-acoustic waves slow down the high-energy electrons, and then, ion-acoustic wave further traps these decelerated electrons in the wave trough and thus creates a giant electron hole. In this hole, strong phase mixing is generated, producing strong heating to the electrons. The numerical simulations are consistent with the measurements and provide insight into the key processes responsible for electron heating and the generation of nonlinear waves in a collisionless current-driven instability.

    

Investigation of Single-particle Motion in the X-point of Two-wire Model

A numerical analysis with the single-particle simulation is performed to investigate various phenomena of single-particle motion in the two-wire model (TWM). TWM is the configuration of magnetic field generated by two parallel current-carrying wires [1]. The field lines form an ideal figure ‘8’ shape (Cassini oval), which models simple single-null diverted tokamak configuration, that can be analytically analyzed. The wire current and the distance between the wires shape the profile of the TWM field. And two parameters mainly determine the single-particle motion of charged particles; they are canonical momentum or constant flux surface value, and total energy, which are both invariant. Magnetic moment, an adiabatic invariant, may change if field gradient is not small enough [2]. We investigate this phenomenon, the evolution and the resultant distribution of magnetic moment of particle motion near X-point null region. Also, due to finite gyro-radius, particles with constant flux surface inside separatrix may cross the X-point region to migrate to the other side of the TWM field. This phenomenon makes it possible for particles inside separatrix to reach divertor without collisional transport. We derive this crossing condition and the crossing confinement time of particles. In addition, we show that, with the single-particle simulation including Monte-Carlo collision, particles prefer to reside closer to the separatrix for relatively longer times. And finally, with the analyses of the aforementioned phenomena, we predict the expected profiles of collisionless magnetized plasmas in TWM.

[1] A. Reiman, Physics of Plasmas, 3, 906 (1996) [2] C. Stephens, Physics of Plasmas, 24, 102517 (207)   

Physical Regimes of Electrostatic Wave-Wave nonlinear interactions generated by an Electron Beam Propagation in Background Plasma

Electron-beam plasma interaction has long been a topic of great interest. Despite the success of Quasi-Linear (QL) theory and Weak Turbulence (WT) theory, their validities are limited by the requirement of sufficiently dense mode spectrum and small wave amplitude. In this paper, by performing a large number of high resolution two-dimensional (2D) particle-in-cell (PIC) simulations and using analytical theories, we extensively studied the collective processes of a mono-energetic electron beam emitted from a thermionic cathode propagating through a cold plasma. We confirm that initial two-stream instability between the beam and background cold electrons is saturated by wave trapping. Further evolution occurs due to strong wave-wave nonlinear processes. We show that the beam-plasma interaction can be classified into four different physical regimes in the parameter space for the plasma and beam parameters. The differences between the different regimes are analyzed in detail. For the first time, we identified a new regime in strong Langmuir turbulence featured by what we call Electron Modulational Instability (EMI) that could create a local Langmuir wave packet growing faster than ion frequency. Ions do not respond to EMI in the initial growing stage. On a longer timescale, the action of the ponderomotive force produces very strong ion density perturbations, and eventually the beam-plasma wave interaction stops being resonant due to strong ion density perturbations. Consequently, in this EMI regime, electron beam-plasma interaction is a periodic burst (intermittent) process. The beams are strongly scattered, and the Langmuir wave spectrum is significantly broadened, which gives rise to the strong heating of bulk electrons.   

Coupled oscillations of the cathode temperature and the sheath in self-sustained arcs

Previous works showed that externally heated cathodes with fixed thermionic emission can have a sheath that attracts ions or repels ions [1,2]. Here, we show that when the thermionic emission is self-sustained by plasma-induced heating, a persistent oscillation between sheath structures is possible. Starting with a classical ion-accelerating sheath, the ion bombardment heating of the cathode raises the thermionic emission until a potential well forms at the surface. Charge-exchange ions accumulate in the well and restructure the sheath until ion bombardment is minimized. The cathode then undergoes net cooling from the loss of thermoelectrons, reducing the emitted flux and restoring a classical cathode sheath. Characteristics of the oscillation will be demonstrated in 1D kinetic continuum plasma simulations under development. Although the simulations are not a complete model of arc physics, the resulting oscillation likely has implications on stability, current flow, and cathode erosion in certain regimes of arcs. [1] F. Greiner, et al., Phys. Plasmas 2, 1810 (1995). [2] M. D. Campanell and M. V. Umansky, PSST 26, 124002 (2017).   

Simulations of stochastic heating induced by RF biased sheath in inductively coupled plasmas

In this study, a 2D(two-dimensional) fluid/EMC(electron Mento Carlo) hybrid model is established to investigate the stochastic heating in an argon ICP(inductive coupled plasma) with an RF capacitive bias power applied on the subplate. The RF coil current of 13.56 MHz is adjusted to keep the inductive power at 60 W. The pressure is fixed at 3 mTorr, so the capacitive ohmic heating is negligible. The bias voltage is turned between 50-500 V, and the frequency of the bias power is set to be 13.56 MHz, 27.12 MHz and 40.68 MHz, respectively. In the simulation, the dc self-bias is adjusted by balancing the electron flux and ion flux at the subplate during one RF cycle. Electron velocity distribution function in vertical direction(EVDF) is collected, and the stochastic heating power is calculated by the classical hard wall model. The results show that the peak of the areal stochastic heating appears when the sheath starts to expand, and the power raises faster with the increase of the bias frequency. Besides, the inaccuracy caused by the inconsistent of the electrostatic field with the macroparticle distribution in EMC is analyzed.   

Progress on development of optical tomography as a plasma diagnostic

Recent advances in laser diagnostics and models have been able to investigate well-controlled idealized plasmas in 2D fashion, but the spatially complex structure in actual plasmas requires a technique than can provide a more complete 3D picture. To address this limitation, a plasma tomographic optical imaging capability is being developed at Sandia National Labs. The system includes four intensified cameras that measure angular projections of the light source. An algebraic reconstruction technique (ART) is used to determine the light intensity at each voxel using the method of projections onto convex sets. Initial efforts will focus on 3-D optical emission spectroscopy. The primary development challenge includes achieving sufficient 3-D spatial and temporal resolution to resolve features of interest. A nanosecond pulsed atmospheric pressure plasma jet is investigated as an initial demonstration of the technique.   

Optics-based measurements of temporal evolution of currents through a load of X-pinch system using Tb-doped optic fiber

An X-pinch system is capable of generating high energy density plasmas with pulsed and large currents through a load which typically consists of a pair of crossed thin wires. Temporal evolution of the currents through the load can be used to identify various stages of the X-pinch system such as ablation of the wires, formation of plasmas, or emission of X-rays from the hot-spot. To measure such currents on the X-pinch system, we construct an optics-based diagnostic system capable of measuring up to a few MA scale of currents with fast response time. The sensing material is an optic fiber wound around the load to detect the Faraday rotation induced by the currents through the load. More specifically, Tb-doped optic fiber is used since its sensitivity to the Faraday rotation is approximately an order of magnitude greater than general optic fibers. As a result, the required fiber length becomes shorter, thus, providing us better temporal resolution. We present configuration of our optics-based current diagnostic system and the measurement results of the temporal evolution of the currents through the load of the X-pinch system.   

Experimental verification of laser-induced fluorescence based on the velocity distribution of helium ash in multi-dipole device

Future fusion devices, such as ITER, based on the fusion of deuterium and tritium (D-T), will have to deal with helium as the 'ash' byproduct. Continuous operation is not possible at the maximum helium content in the plasma, where significant dilution of the D-T fuel and radiation of plasma thermal energy occur. Where helium ash is produced by the D-T process, the confinement period of helium ash is dominated by edge transport, recycling, and edge pumping rate rather than core transport. Consequently, the discharge of helium ash in the edge region, particularly the discharge of helium neutral particles in the divertor region, is a crucial factor determining the future stability of nuclear reaction processes. The non-invasive diagnostic method of laser-induced fluorescence (LIF) is expected to be used extensively to study the dynamics of helium metastable neutral particles via direct measurement of the neutral particles velocity distribution function (NVDFs) in order to monitor helium ash exhaust in fusion devices. A multi-dipole chamber in this work was employed to simulate the existence of helium neutral particles near the divertor of a fusion reactor. In addition, the laser wavelength calibration, data analysis automation,  and the required tunable range of a laser to cover the NVDF and efforts to extend the tunable range to meet this requirement were discussed in detail. These works will enable us in developing a feasible LIF diagnosis technology scheme suitable for the divertor region of the future fusion reactor, determining the velocity distribution of helium neutral particles near the divertor, and monitoring helium ash exhaust.

    

TALIF of atomic hydrogen in the divertor simulator NAGDIS-T

Reducing the heat load on the divertor by plasma detachment is essential for the establishment of a fusion reactor, and atomic and molecular processes involving neutral particles are important for understanding and controlling such processes. The ground states and metastable atoms which have long lifetimes are considered to play an important role in the divertor region. However, detailed measurements have not yet been fully implemented in the divertor environment. In the detached plasmas, where plasma recombination can dominate the processes, passive spectroscopy cannot be applied to measure the ground state densities. In this study, we measure hydrogen (H) atomic density by two photon absorption laser induced fuluorescense (TALIF). A dye laser (LIOPTEC, LIOPStar) pumped by a second harmonic of the YAG laser is used for the TALIF. The third harmonic of 615 nm (205 nm) is used to excite H atoms (n=1→3, 205 nm×2 photons) and detect the emission at 656 nm. The H signals were detected using a photomultiplier tube in the divertor simulator NAGDIS-T, where H plasmas can be produced in steady state using DC arc discharge. Also, for the density calibration, we introduced krypton (Kr) gas to the chamber and measured Kr TALIF signal.   

Development and construction of a Laser-Induced Fluorescence (LIF) diagnostics system for a low temperature multidipole plasma device with X-point magnetic configuration, MAXIMUS

A laser-induced fluorescence (LIF) based diagnostics system is being developed to investigate the plasma properties inside a multidipole chamber producing low temperature plasmas. The system utilizes a continuous wave, tunable diode laser for fine modulation of the laser frequency, as well as a lock-in amplifier based measurement system. The implementation of the lock-in amplifier allows the LIF signal to be recovered from overwhelming background emission of the plasma, even at low laser power (~5 mW). The ion velocity distribution functions (IVDF) are measured under various plasma conditions. From the IVDFs, the plasma flow velocities and the ion temperatures are deduced by assuming Maxwellian-shaped distribution function. Present results, capabilities, and limitations of the LIF system are discussed along with future applications on IVDF measurements in an X-point magnetic configuration.

    

Development of a polarization-resolved spatial heterodyne spectrometer for high wavelength resolution and high throughput measurement of near-infrared atomic emission lines in magnetically confined toroidal plasmas

For spectroscopic diagnostics of magnetically confined toroidal plasmas, a viewing-chord-integrated emission line spectrum can be spatially inverted using the correspondence between the given magnetic field profile along the viewing chord and the Zeeman effect appearing on the spectrum. Observation of near-infrared emission lines is advantageous for this method owing to a relative increase in the magnitude of the Zeeman effect compared with the line width. To apply this method to hydrogen atom Paschen-β (wavelength 1282 nm) and helium atom 2³S-2³P (wavelength 1083 nm) emission lines, we designed a novel spatial heterodyne spectrometer (SHS) ^[1]. SHS is an interference spectrometer that can simultaneously achieve high wavelength resolution, high throughput, and no moving parts, so it has been rapidly developed and widely used in the fields of trace components in the atmosphere, environmental gas, and magnetic fusion diagnostics. For enhancing its advantage in weak signal detection, in our design, we reduced light loss from a conventional design by resolving polarization using polarizing beam splitters, quarter waveplates, and double gratings in each interference arm. According to the theoretical evaluation, the expected wavelength resolving power is more than 10⁵. The throughput is approximately 2 times greater than conventional SHS and the etendue is 8 times greater than the Czerny-Turner spectrometer used in measuring helium atom 2³S-2³P emission lines.

 

[1] W Zhang, et al. Microchemical Journal 166 (2021): 106228.

    

Development of microwave plasma diagnostics for various plasma devices

Plasmas are widely used in science and engineering, including magnetic confinement plasmas for nuclear fusion, plasmas for electric propulsion, and process plasmas for semiconductor manufacturing. However, few plasma diagnostic systems such as electron density and electron temperature are commercially available, and researchers are burdened with the burden of building their own measurement devices.

The author has developed a number of microwave and millimeter-wave plasma measurement devices, and is promoting their use in the various fields mentioned above. For example, microwave reflectometer and interferometer can measure electron density profile and fluctuation, electron cyclotron emission diagnostics can measure electron temperature and fluctuations. We have developed an extremely easy-to-use transmitter and receiver system using superheterodyne and frequency multiplication techniques, and applied them to many plasma device such as tokamak and helical plasma devices, linear plasma devices, Hall thrusters and RF plasma thrusters.

In this presentation, these microwave devices and its applications will be shown.   

A novel spectral element method for modelling streamer discharges and its comparison with the conventional finite-element method.

Simulating streamers in atmospheric pressure plasmas is a challenging problem due to the multiscale nature of this discharge type. The need to simulate streamers in complex geometries and the interaction of streamers with plasma-facing surfaces motivates the search for new numerical approaches to this problem. In particular, it is important to implement efficient methods for elliptic problems, which have to be dealt with in streamer simulations. A promising approach is to use a hierarchical Poincare-Steklov (HPS) scheme for solving the resulting elliptic problems [1]. The HPS scheme belongs to the family of composite spectral collocation methods and has a number of attractive features. As a proof of concept, the application of the HPS scheme to the simulation of streamer discharges has been demonstrated in [2] for the case of regular grids. In this contribution, we continue this work and compare the proposed HPS scheme with the conventional finite-element method (implemented within the FEniCS framework) in the context of streamer simulations. A suite of test problems is considered and the performance of both methods is assessed in terms of the required computational time and the number of discrete unknowns being involved. [1] P. G. Martinsson, J. Comput. Phys. 242, 460 (2013). [2] I. L. Semenov, K.-D. Weltmann, arXiv:2111.06210.   

A novel Monte Carlo simulations code for electrons and ions including efficient variance reduction techniques

Fluid and kinetic models for non-equilibrium, low-temperature plasmas require accurate calculations of electrons and ions distribution functions, transport coefficients, and reaction rate coefficients from cross section data. These parameters are typically obtained from numerical solution of the Boltzmann equation using approximations for the distribution functions or from Monte Carlo simulations. In this work, we present a new open-source Monte Carlo simulations code for electrons and ions swarms. The implementation includes efficient variance reduction techniques, such as the Monte Carlo Flux, as well as dynamic particle weighting in non-conservative collisions, a modified time step algorithm, energy ranges with different null-collision frequencies and multi-core parallelization.In this work, we compare the computational cost of different variance reduction techniques typically used to simulate electrons and ions swarms. Moreover, the accuracy of the results is assessed by code benchmarking against conventional Monte Carlo simulations or multi-term solutions. Ultimately, the code can be used to check the validity of two-term solutions or directly coupled with plasma models.   

High Accuracy Interatomic Potential Model for Binary Collision Approximation and Its Application into Sputtering Yield Estimation

To investigate the plasma-surface interaction (PSI) from the viewpoint of atomic-scale dynamics, binary collision approximation (BCA) and molecular dynamics (MD) are often used. An important factor common to these simulations in plasma irradiation is the model of the interatomic potential which can represent atomic collisions in the energy range above 10 eV. The famous model developed for the BCA is the Ziegler-Biersack-Littmark (ZBL) potential. Since the ZBL potential was formed as fitting function with 8 parameters, it requires low computational cost. The ZBL potential model is often used as the repulsive two-body term connecting to multi-body potential model for MD simulation. However, due to its small number of parameters, the reliability of the ZBL potential model is insufficient by comparison with the modern simulations such as machine-learning potential for MD. In the present work, higher accuracy interatomic potential model was proposed, which is named ReGenerated ZBL (ReGZ) potential. The ReGZ potential function was analytically derived from the spherical electron density of an independent atom which is expanded by the small number of terms. As a result, the number of atomic pairs is the square of the number of atomic elements, while the number of parameters required for the potential function is on the order of the number of atomic elements. Therefore, not only the error of potential energy is small but also computational cost is kept low. Furthermore, in the BCA simulation the difference from the ZBL potential appears on the sputtering yield and the reflection coefficient.   

Electromagnetic Wave Analysis in Collisional Discontinuous Galerkin Particle-in-Cell Simulations

Discontinuous Galerkin particle-in-cell (DG-PIC) models are used to simulate kinetic plasma dynamics. They solve Maxwell's equations via a discontinuous Galerkin (DG) scheme to calculate the electric and magnetic fields and solve the Boltzmann equation via a particle-in-cell scheme (PIC). DG schemes maintain high-order accuracy within each cell while having explicit, semidiscrete forms that allow wave solutions to develop for unstructured meshes. PIC schemes allow for non-Maxwellian velocity distributions to develop by simulating charged species as discrete macroparticles, which can give rise to waves and instabilities. Coulomb collisions can be significant in highly ionized plasmas and may dampen wave behavior as they relax velocity distributions towards a Maxwellian. This work will show the use of a collisional DG-PIC scheme to study electromagnetic radiation created from hypervelocity impacts created spacecraft, illustrating its benefits in plasma-wave analysis.   

Moments approach to compare a particle-in-cell simulation with a fluid model for RF capacitively coupled plasmas

The particle-in-cell (PIC) and fluid simulations are commonly used in low-temperature capacitively coupled plasma (CCP) equipment utilized in semiconductor manufacturing processes. As the PIC simulation calculate the transient motion of all charged particles, the energy distribution functions are computed without assumptions, and all terms of fluid equations can be compared with moments obtained from the PIC simulation. We compare every term of moment equations in the PIC and fluid simulations under the same simulation conditions. Therefore, closures of a fluid model are verified by the PIC simulation, and the role of the viscosity of the pressure tensor is investigated. As a result, we suggest the criterion for valid fluid models in a CCP.   

Numerical Analysis of Fundamental Properties in Sub-atmospheric Pressure He/CH4 Pulsed Plasmas for Hard Coating of Diamond-Like Carbon Thin Films

Diamond-Like Carbon (DLC) films have various properties such as high hardness, low-friction coefficient, and gas barrier properties, and have been used in industrial and medical fields. In recent years, the demand for deposition on materials such as polymer materials has increased, and the technology for deposition in low-temperature environments has strongly been desired. One of the deposition techniques at low-temperatures is the plasma-enhanced chemical vapor deposition (PECVD) method. Conventional PECVD has been used in low-pressure and low-density plasmas, which results in low-deposition rate of DLC films and requires vacuum systems. Thus, a high-rate deposition technique without the use of vacuum systems have been highly required.

In this study, in order to examine the deposition technique of hard coating DLC thin films at sub-atmospheric pressure condition, we newly developed a spatially one-dimensional fluid model of hydrocarbon (He/CH₄) plasmas at sub-atmospheric pressure condition, and then discussed the influence of the applied voltage waveforms (i.e. square- and RF-modulated pulsed voltage waveforms) on the plasma characteristics.   

Influence of Electrode Structure on Ion Beam Extraction in Cold-cathode Ion Source

In this study, we investigated the influence of electrode structure on ion beam extraction in a cold-cathode ion source. To investigate this influence, we developed a two-dimensional ion ray-tracing code for studying the ion beam extraction. Simulations were performed for three typical structures of cold-cathode ion sources using the developed code. The simulation results show that the potential structure influenced by the electrodes and sheath on the electrodes play an important role in determining the divergence and current of the ion beam. We showed the change of the ion beam according to the electrode structure in terms of current extraction efficiency and ion beam divergence. The results were validated through a series of experiments with a small-sized cold-cathode ion source.   

Importance of C3Hy and C3Hy+ in Modeling of Radio Frequency Methane Plasma

Methane is widely used in not only semiconductor industry but also diamond-like carbon and carbon nanotubes production. Radio frequency methane plasma was studied by Tachibana in 1984 [1], and some other researchers presented different models. In their models, however, species and related reaction steps were limited up to C₂H_y, C₂H_y⁺, and C₃H₈, although mass spectroscopy analysis showed that various C₃H_y/C₃H₅⁺ species were detected in the plasma. In the present study, methane plasma was studied by a capacitively coupled plasma modeling with 21 neutrals and 19 ions in the gas phase reaction, including some C₃H_y and C₃H_y⁺. One- and two-dimensional (1-D and 2-D respectively) simulation models were prepared to calculate two different gases: CH₄ and CH₄/H₂ (8/92 %). In the case of pure CH₄, CH₅⁺, C₂H₅⁺, and C₃H₅⁺ were abundant ions; among them, C₃H₅⁺ was the most major ion which was omitted in the past reports. It was proved that including C₃H_y and C₃H_y⁺ into the reaction set was essentially important to reproduce the experimental results. Time evolution of major ions and neutrals showed similar trends in both the 1-D and 2-D models, and continuous decrease of CH₄ in the plasma for a longer process time was similar to the experiment results by [1].

    

Complex network analysis of low-temperature plasma reaction systems

Research on low-temperature plasmas is not only in physics but expanding into biology and medicine. The plasma reaction systems, however, tend to be complicated to understand as the number of chemical species and reaction equations considered increases. To solve this problem, it has been attempted to visualize chemical reaction equations graphically and analyze the plasma reaction systems through graph theory[1,2]. In this research, we analyzed the plasma reaction systems at different scales based on the graph theory to reveal their unique properties. Consequently, we revealed the primary species and fast reaction paths in the plasma reaction systems by visualizing their reaction network considering their reaction probability and found that there was a similar network structure even in the different plasma reaction networks. [1] O. Sakai, et al. AIP Adv. 5, 107140 (2015). [2] T. Murakami and O. Sakai, Plasma Sources Sci. Technol. 29, 115018 (2020).

    

Fokker-Planck-Boltzmann Model for Low-Pressure Plasmas

The common description of kinetic effects in low-pressure plasmas is based on the Boltzmann equation. This applies especially to the description of Ohmic (collisional) and stochastic (collisionless) electron heating, where the Boltzmann equation is the starting point for the derivation of the corresponding heating operator. Here, it will be shown that an alternative and fully equivalent approach for describing the interaction between electrons and fields can be based on the Fokker-Planck equation in combination with the corresponding Langevin equation. While the final results are the same in both cases, the procedure is entirely different. The Fokker-Planck approach provides physical insights in a very natural way, which in contrast requires some effort to extract when starting with the Boltzmann equation. The alternative concept is introduced and applied to the combined Ohmic and stochastic heating in inductively coupled plasmas. Further, a generalization of the plasma dispersion function is introduced, for arbitrary forms of the distribution function and for velocity-dependent elastic collision frequencies. Combined with the Fokker-Planck heating operator, a fully self-consistent description of the plasma and the fields is realized. Finally, an outlook on how to integrate the operator in a standard local Boltzmann solver will be provided.

    

Simulations of Cathode Plasma Expansion in Vacuum

Vacuum arcs are a concern in many high voltage applications: accelerator cavities, spacecraft charging, ion sources, and pulsed power systems. In many cases an arcing event is a failure mode, while in others it may be the intended mode of operation. Understanding the initiation of a vacuum arc is critical to mitigating or controlling them. This work aims to clarify some of the elements of initiating arcs that are not well-understood.

 

A typical description of vacuum arc discharge involves a cathode (K) plasma forming that propagates to an anode (A), creating a conducting pathway, or arc. The timescale for some of the processes is quite fast (single ns’s) and there is indication that the speed of neutral and/or ion emission from the cathode is ~20,000 m/s. Furthermore, there is evidence that the cathode and anode might be acting in coordination but without an intervening plasma.

 

PIC-DSMC simulations are performed to investigate the speed of AK gap closure and ask questions related to cathodic material transiting the gap to the anode. The simulations are performed with Aleph, a massively parallel electrostatic low temperature plasma simulation capability. The numerical requirements for simulating such a large range of densities and high speeds are enormous and will be discussed.

    

Features of DC gas breakdown between electrodes with variable gap

The results of experimental researches of gas breakdown features between electrodes with variable gap are presented. The breakdown curves of the discharge between the cylindrical electrodes with a hemispherical apex were measured in the range of nitrogen pressures of 0.03 – 200 Torr for different values of the interelectrode gap. It is established that on the curves of the dependence of the breakdown voltage on the gas pressure there are long horizontal sections for distances smaller than the diameter of the electrode. The principle of gas breakdown curve formation between electrodes with variable gap is formulated: the breakdown curve consists of three parts, including horizontal section with minimum voltage, where gas breakdown occurs with the optimal path, which is automatically selected so that the breakdown voltage is the lowest (Stoletov's point). A numerical theoretical model has been developed that allows calculating the breakdown voltage of a gas between electrodes of arbitrary curvilinear shape. It is shown that in the case of flat electrodes for a sufficiently large distance between the electrodes, the Paschen similarity law is not satisfied. It is concluded that the reason for this is the loss of charged particles on the dielectric walls of the discharge chamber due to radial diffusion and the defocusing geometry of the electric field lines. For the system with hemispherical electrodes it is shown that the physical reason for the formation of a horizontal section on the breakdown curve is the change in the breakdown path when the gas pressure changes, diffusion processes expand the breakdown channel, and the trajectory of particles in a curved electric field does not repeat the shape of the force line due to the inertia of motion. It is shown that due to the accumulation of surface charge, the losses of charged particles are reduced, and breakdown becomes possible at much lower voltages.   

Generation of 2.45-GHz microwave plasma filament in sub-atmospheric pressure

Plasma photonic crystals (PPCs), which are periodically arranged plasmas, are expected to have engineering applications in electromagnetic wave control devices. Our objective is to construct filamentary structures using 2.45-GHz microwave discharge plasma that can generate high-density plasma and to understand their plasma characteristics while keeping in mind PPCs for controlling electromagnetic waves near 10 GHz. This work explores the pressure and power characteristics of plasma filaments produced by helium quasi-atmospheric microwave discharges. We measured the length characteristics of plasma filaments within a power range of 150–900 W and the pressure range of 200–900 hPa. In the transition region, the filament length is proportional to power and increases from 5 mm to the saturation value regardless of pressure. The filament length in the saturated region decreases from approximately 50 mm to approximately 40 mm in the pressure range of 200–900 hPa. According to spectroscopic measurements, the electron density of the filament plasma was 4.81×10⁹–4.02×10¹⁰ cm^-3.   

Analysis of a radiofrequency plasma reactor for etching

Microelectronics are a core component of modern technology. Production of microelectronics relies on plasma processing for etching fine features in semiconductors. While the plasma etch process is relatively mature, as microelectronic structures become smaller, plasma properties that lead to non-uniformities in the etch must be explored and addressed. To this end, we couple experimental and modeling efforts to characterize a radiofrequency argon plasma reactor operating in the 10 MHz regime with input power on the order of 100 W and 14 cm diameter electrodes. Experimental plasma diagnostics include Langmuir probe measurements, laser-induced fluorescence, optical tomography, and a retarding field analyzer to measure the ion energy distribution function (IEDF). The modeling effort consists of a one-dimensional PIC/DSMC model that mimics the experimental configuration. Cross sections for electron-neutral collisions include elastic, excitation, and ionization, as well as elastic and charge exchange collisions for ions. Furthermore, we analyze the IEDF reaching the electrode surface as a function of pressure and input power. The outlook for this work is to train and use machine learning models using measured and simulated data to predict optimized plasma parameters for semiconductor etch and other relevant scenarios.   

Electron power absorption in capacitively coupled plasmas operated in gas mixtures containing oxygen

Capacitively Coupled Plasmas (CCPs) operated in gas mixtures containing oxygen are studied by Particle-in-Cell/Monte Carlo Collisions (PIC/MCC) simulations. The changes of the discharge characteristics are investigated by adding oxygen to noble gases. In the PIC/MCC simulations, a complex model is applied, which includes the cross processes between the various species of the gas components and accounts for the surface processes. This model has been verified by Phase Resolved Optical Emission Spectroscopy (PROES) measurements of neon-oxygen mixtures, providing a good agreement between the excitation rates of the neon 2p1 state. The electron power absorption dynamics in such mixtures is found to change significantly as the mixing ratio of the two gases and the pressure is varied. As the pressures or the oxygen concentration is increased, the Ohmic heating becomes the dominant mechanism of electron power absorption, while the contribution of electron power absorption from the ambipolar electric field becomes minor.   

Numerical simulation of discharge mode in capacitively coupled plasma with beam injection

In recent years, the capacitively coupled plasmas (CCP) is modulated by electrical asymmetry effect (EAE), magnetical asymmetry effect (MAE), electron beam (EB) injection et al. Some reports describe the energy electron provided by EB can directly take part in the ionized reaction in plasma to increase the electron density. In this work, we used 1D implicit particle-in-cell/Monte Carlo collisions (PIC/MCC) to study the modulation of CCP with the beam injection. The results indicate that the trend of electron density in DF-CCP does not always increase with EB energy (10 eV<ε<300 eV), which is due to the lower ability of the bulk plasma to capture high-energy electrons. Moreover, the asymmetric sheath could bring up different influences of gamma mode to control the self-bias voltage. When the Ar⁺ beam injects into SF-CCP, the discharge mode can be changed with the increase of ion-induced secondary electron emission coefficient γ and discharge pressure. Furthermore, the secondary electrons emitted by ions from the injected electrode increase the electron density and result in the high-energy tail of EEPF. Therefore, the IB injection could be expected to be one of the effective methods to modulate the CCP.   

Experimental and numerical investigation of the plasma characteristics and mode transition in dual-frequency capacitively coupled argon plasmas: effects of low-frequency source and gas pressure

Capacitively coupled rf plasmas biased by a low-frequency (LF) source in the medium frequency range (0.3−3 MHz) have recently received growing attention due to high ion energy and narrow ion angular distribution, which are beneficial for the deep reactive ion etching. In this work, we report on a combined experimental-numerical investigation of the effects of the LF parameters (i.e. frequency and voltage) and gas pressure on the plasma characteristics and mode transition in dual-frequency (DF) capacitively coupled argon plasmas. The LF source is varied from 40 kHz to 2.72 MHz while the high-frequency (HF) source is fixed at 27.2 MHz. A hairpin probe is used to measure the plasma density while a phase-resolved optical emission spectroscopy technique is used to measure the spatio-temporal distributions of the electron-impact excitation rate; meanwhile, a 1d3v PIC/MCC simulation is employed to reproduce the experimental data. Results indicate that i) the modulation effect of LF source on the HF oscillation becomes enhanced (weakened) with increasing the LF voltage (HF voltage) and ii) the discharge might experience an α-to-γ mode transition with increasing the LF voltage/gas pressure or decreasing the LF frequency.

    

On plasma parameters with changing chamber size

In general, the chamber size in an inductively coupled plasma is very important because it affects the electron heating mechanism and the effective area of particle loss. In this work, we investigate the change in plasma parameters by changing the radius and length in a cylindrical chamber. The electron energy distribution functions, plasma density, electron temperature, etc are experimentally measured and compared with the calculation results. Both results showed good agreement with each other. The work is discussed along with the relevant physical mechanism.

    

Investigations of the INductively Coupled Array (INCA) discharge

Operation of inductively coupled plasmas at low pressures and over large areas is desirable. However, upscaling of standard sources is problematic and in addition at low pressures standard Ohmic heating becomes inefficient. A novel concept is the INductively Coupled Array (INCA) discharge which provides tailored collisionless, stochastic electron heating at low pressures. The concept is based on a large lattice of vortex fields, which heats electrons at certain carefully designed resonances in velocity space. Experimentally the concept was realized using a 6x6 array of small planar coils. Due to the lattice nature, upscaling of the discharge is easily possible. Its operation was tested in atomic and molecular gases, including electronegative, processing gases. The plasma parameters were measured using a combination of diagnostic methods. Electrical properties of the discharge (current-power characteristic, electric field structure) were also studied with the goal of investigating and optimizing the efficiency of the discharge. Here an overview of the various results on this discharge and its key properties are presented.   

Spectroscopic measurement of a compact helium ECR discharge produced in a simple cusp field

In the plasma-based chemical and physical vapor deposition processes, the enhanced ionization degree of up to more than several tens of percent enables better controllability of the homogeneity, composition, and chemical structure of deposited films. For this purpose, pulsed power discharge techniques have been utilized to produce highly ionized plasmas. The pulsed power discharges are, however, limited in duty ratio, and the production of a large volume and uniform plasma is an ongoing research topic. As an alternative approach, an increase in ionization in an ECR discharge by imposing a simple cusp field has been proposed. The cusp field can confine electrons within the ECR surface by magnetic mirror force enhanced by the ECR heating and ions by the ambipolar electric field. Although better confinement can be obtained using a combination of the mirror and multi-cusp fields, the simple cusp field has better plasma accessibility, which would be an important advantage for industrial applications. We constructed a compact device with approximately 30 cm on each side and evaluated plasma parameters and ionization degree of helium plasmas using optical emission spectroscopy.

    

Investigating the influence of ion mass on plasma characteristics in low temperature ExB plasmas using 2D-3V PIC-MCC simulations

Presence of drifts and instabilities in low-temperature plasma with an inhomogeneous magnetic field in different E×B discharge devices leads to complex plasma characteristics. The plasma profile varies with input conditions (such as gas chemistry, bias voltage, magnetic fields, power and pressure). Previously [1], we have studied instabilities in the background plasma in the case of Hydrogen in a simplified geometry (in the context of negative ion source using a 7 mT Gaussian shape transverse magnetic field) using the 2D-3V PIC-MCC kinetic model. In this work, using the same geometry, we have performed a comprehensive computational investigation to investigate how ion mass affects the plasma characteristics and plasma transport. Using the same initial conditions, comparison is made for two different gases - Hydrogen and Xenon. Ions are un-magnetized in the case of the Xenon, but ions are partially magnetized in the case of the Hydrogen. Compared to the Hydrogen case, we observe high plasma density and low electron and ion temperature in the case of Xenon due to high ionization rate than Hydrogen. In the Xenon case, the potential gradient present in the system is small compared to the Hydrogen, implying a weak electric field. E×B drift instabilities are visible in the case of Hydrogen, however, we do not observe any such instabilities in the case of Xenon. Plasma transport across the magnetic field is significantly influenced by these instabilities [1]. Our investigations suggest that ion mass plays a significant role in plasma transport across magnetic fields.

[1]. The feasibility of resonance induced instabilities in the magnetic filter region of low temperature plasma based negative ion sources, AIP Conference Proceedings 2373, 080003 (2021)

    

Comparison of space and time-resolved electric fields from experiment and simulation in packed bed dielectric barrier discharges

Packed bed dielectric barrier discharge (PBDBDs) have been extensively investigated for gas conversion applications. Significant past work has been performed using space-resolved modeling. Due to complex geometry, complex chemistry, and atmospheric pressure operation, the models need to rely on several assumptions which influence their accuracy. The improved confidence in model prediction and their utilization requires a quantitative comparison between the experiments and modeling which has been largely missing. In this work, we have compared the 2D electric field, space and time-resolved electron impact excitation and streamer speed in a packed bed plasma reactor operated in helium from both experiments and computations using nonPDPSIM plasma hydrodynamic modeling [1]. The electric field was measured using the helium line ratio method [2] with 2D spatial resolution in an experimental setup pioneered by [3]. The apparent electric field reached values up to 100 kV/cm. The 2D electric fields compare well with the experiments during the surface streamer phase of the discharge and differ for the streamer approach towards the dielectric pellet. The discrepancies have been resolved by changes in the model and/or have been explained based on the limitations of the experimental technique and model.

[1] J Kruszelnicki et al. J. Phys. D: Appl. Phys. 50 025203 (2017)

[2] S.S. Ivković et al. Journal of Physics D: Applied Physics, 47(5): p. 055204 (2014).

[3] J. Čech Plasma Sources Sci. Technol. 27 105002 (2018)

    

Observation of the stripe and filamentary self-organized structure of atmospheric pressure nitrogen microgap dielectric barrier discharge

A dielectric barrier discharge (DBD) is a discharge caused by an alternating voltage applied to a pair of electrodes, one or both of which are covered in the dielectric material. Microplasmas generated by DBD have been observed to self-organize their discharge structure under certain conditions. Self-organization is an undesirable nonuniform discharge in several industrial applications; however, the periodic discharge structure generated by self-organization has recently been applied to plasma photonic crystals. With short-time observation techniques, hexagonal lattice structures and stripe structures in helium microgap DBDs are visually observed, and stripe structures are shown to be composed of hexagonal structures. There are several examples of observations of stripe structures in the case of N2 discharge, but no observations of discharge phases on a short-time scale have been reported.

In this study, the discharge phase of an atmospheric pressure nitrogen microgap dielectric barrier discharge self-organized structure was observed. Stripe structures were observed in discharge patterns with two peaks of discharge current under conditions of long exposure time and a high number of integration times. In the discharge region, with an exposure time of 1 µs and several integrations of 1, discharges with a random arrangement of large and small filaments were observed. The average movement speed of the filament was estimated to be at least 62.5 mm/s.   

Production of large-volume atmospheric-pressure dielectric barrier discharge using high-rate helium flow

It is desirable to increase the volume of atmospheric-pressure plasma to expand the range of applications. In this work, we attempted to generate a large-volume dielectric barrier discharge at atmospheric pressure. The discharge tube had a diameter of 2.7 cm and a length of 12 cm. An end of the discharge tube was closed, whereas the other end was open to air and it was used as the inlet and the outlet of helium. The tube was not evacuated. Two ring electrodes were attached on the outside of the discharge tube, and they were connected to a high-voltage power supply at a frequency of 1-5 kHz and the electrical ground. The generation of the discharge was possible if we used a helium flow at a rate of 500-4000 sccm. We observed both the filament and glow discharge modes [S. Okazaki, et al., J. Phys. D: Appl. Phys. 26, 889 (1993)]. The glow discharge mode was obtained at a higher flow rate. The phase-resolved optical emission pattern observed using a gated ICCD camera showed the propagation of the discharge from the instantaneous cathode to the anode. We currently examine the generation of the discharge in the presence of liquid water inside the discharge tube.   

Application of Sliding Discharge with Tri-Electrode Dielectric Barrier Discharge for Formation of Planar Atmospheric Pressure Plasma

Atmospheric pressure low temperature plasma has been formed by a tri-electrode dielectric barrier discharge configuration. This electrode configuration is used for sliding discharge in plasma actuators. The plasma is applied on formation of functional thin film with metal organic plasma decomposition. The first electrode, which is covered with an insulator layer, was grounded. The second electrode, which is attached on the insulator layer, was powered by low frequency AC high voltage power supply. The third electrode, which is also attached on the insulator layer, was powered by the other low frequency Ac high voltage power supply. In this experiment, two-dimensional plasma with 20 mm x 20 mm in area has been formed by feeding helium gas.

    

DC glow discharge - Fluidized bed reactor for CO2 recycling

The increase in global temperature is attributed to the greenhouse gas effect especially from carbon dioxide (CO₂) emissions. Non-thermal plasmas (NTP) can provide the highly energetic environment needed for CO₂ conversion. Therefore, a plasma-catalysis approach could offer a significant advantage by improving conversion, selectivity and energy efficiency. Fluidized bed reactors increase the surface contact area with the gas phase and improve the heat transfer. A DC glow discharge ignited in a fluidized bed with and without Alumina (Al₂O₃) particles is investigated with aid of Optical Emission Spectroscopy (OES) at low pressure. It is observed a decay in Oxygen atom density through the fluidization of the material and an increase in the intensity of CO systems, specifically 3^rd positive system which could be due CO density, electron density and/or electric field, in comparison to the plasma alone. This indicates that fluidized particles indeed cause a reduction in the O presence and could lead to an increase in CO density. In addition, temperature of rotation was calculated by CO Angstrom system. The results confirmed the decrease in gas temperature in the positive column in comparison to plasma without catalyst. This effect is mostly attributed to increase radial heat transport towards the reactor walls in the fluidized bed. The plasma-assisted catalytic behavior was further investigated by FTIR for the characterization of the downstream gas from glow discharge/FBR resulting on superior performance than the glow discharge alone. The development of this innovative route is crucial to understanding the enhancement of plasma-surface interaction for CO₂ recycling.

    

Upscaling of a Surface Dielectric Barrier Discharge for Air Purification

Hazardous microorganisms or toxic volatile organic compounds (VOCs) are common pollutants in industrial or ambient air. These contaminants can be a risk for the environment and human health and are currently challenging to remove. Conventional air purifying systems that are used in the industry or as indoor cleaners have still several disadvantages and, therefore, there is a demand for new methods.

 

A twin surface dielectric barrier discharge (SDBD) is well studied in the literature and shows great potential for air cleaning. In this study, a novel scaled-up SDBD reactor is presented, which is more suitable for air purification at industrial scales than previous used SDBD reactors. This reactor is capable of treating gas flows of up to 500 slm (standard litres per minute) and measurements with special regard to the VOC conversion and bacteria inactivation are performed.

 

The conversion of butoxyethanol and n-butane as a function of the energy density is investigated. Complementary schlieren images show that the discharges perturb the overflowing air and that additional metal plate inserts influence the flow dynamics. Additionally, airborne Micrococcus luteus bacteria can be inactivated by our system, opening up further potential applications in the field of hygiene.

    

Analysis of Electron Temperature and Heavy Particle Temperature at Vacuum Arc Cathode Spot as a Function of Ambient Pressure

The vacuum arc cathode spot is characterized by a fast-moving electron emission spot with a high current density and low voltage. The factors that cause the movement of the vacuum arc cathode spot have not yet been elucidated, and simulations based on the assumption of local thermal equilibrium have been performed in previous research. However, since the non-equilibrium between electrons and heavy particles is remarkable in high vacuum discharges, it is necessary to consider the spatial and thermal non-equilibrium between electrons and heavy particles in order to elucidate the movement factor of the cathode spot. In this calculation, thermal non-equilibrium is considered by solving the conservation of energy equations for electrons and heavy particles, respectively. In this research, electron temperature and heavy particle temperature at vacuum arc cathode spot as a function of ambient pressure was analyzed by three-dimensional electromagnetic thermal fluid simulation with consideration of thermal non-equilibrium.   

Contribution to Bead Width Using Welding Torch Feedback Control with Real-time AI Discrimination

TIG arc welding is often used because this welding has high quality and strength. However, when the distance between electrodes is changed, the arc length and arc voltage are displaced, and the heat transfer rises or falls, causing welding defects. Therefore, it is necessary to adjust the interelectrode distance, although there is a limit to how much an artisan can adjust and control it. The welding torch was controlled longitudinally using feedback control based on the results of the classification. The bead width generated at this time was also calculated. Specifically, the image of the arc captured by the camera was classified by supervised learning, and the image was classified by a predetermined labeling. Based on this, the welding torch was driven and controlled to keep the interelectrode distance constant. The measured arc voltage confirmed the displacement of the interelectrode distance, and the resulting bead width confirmed the change in heat input to the base metal with AI control of the welding torch. As a result, the AI discrimination controlled the interelectrode distance to be constant and suppressed the variation of bead width.

    

Research on current filament mechanism of nonlinear semi-insulation GaAs photoconductive semiconductor switches

   With the rapid development of pulsed power technology, the GaAs photoconductive semiconductor switch (PCSS) with power and band width arouses broad attention. On the nonlinear mode, the current filament phenomena will occur in the GaAs PCSS. Because the phenomena are so complicated and uncertain, up to now the latent mechanism is not very clear.

   The paper proposes a new streamer model to explain current filament phenomenon. The model includes three aspects: the streamer generation, the streamer development and the streamer flowing through PCSS. There are two conditions for streamer formation: one is the carrier density which should be not less than 10¹⁷cm^-3, and another is the electric field which should be much higher than 250 kV/cm. Avalanche ionization and compound radiation are the physical factors of the streamer development. When the current filament flows through PCSS, Lock-on effect is caused by dynamic distribution of transient voltages of PCSS and the load. On a micro level, Lock-on effect is caused by dynamic change of production rate and recombination rate of carries. Based on this streamer mode, the current filament velocity is calculated as about 2×10⁸cm/s. Then the current filament phenomenon is explained by Franz-Keldysh effect.

    

Development of an alternative differential pumping system by virtual vacuum interface plasma window

Plasma window (PW) is an innovative technology, by which atmospheric pressure and vacuum can be separated. The PW is expected to be useful for an alternative differential pump. Thus, we focus on the reduction of pumps of He gas cell charge stripper in the RIKEN heavy ion beam accelerator, where the gas cell is kept to be 7 kPa while beam transport line is 10^-6 Pa by using 5 stage differential pumps installed both sides. Our goal is to remove two differential pump stages by PWs with an opening of 8 mm.

The apparatus we developed is a cascade arc He discharge source. Although we have succeeded in separating 2.2 kPa and 50 Pa by using our PW without large pumps, we have not achieved the requirement for the differential pumps for ion accelerator. Therefore, we improve the electrode structure, especially, LaB₆ cathode heated by a C/C heater is employed. Cylindrical LaB₆ rings yield lots of electrons due to hollow cathode effect. We expect that this structure contributes to a high-performance PW. In this presentation, we will introduce the new electrode in detail, comparing with the previous apparatus.

    

Spectrum Intensity and Temperature of Cu I and Cu II Measurement of Vacuum Arc Cathode Spot as a Function of External Transverse Magnetic Field

Vacuum arc discharge is a low-voltage, high-current-density discharge phenomenon that forms a highly intense electron emission spot. The cathode spot is characterized by rapid and irregular movement, and the factors that cause the cathode spot to move have not been elucidated. In order to elucidate the cathode spot movement factors, it is necessary to elucidate the physical phenomena in the cathode spot. In this research, spectrum intensity and temperature of Cu I and Cu II at the cathode spot during the changes of external transverse magnetic field is measured by multi-spot spectroscopic measurements. Specifically, Cu I and Cu II emitted from the cathode spot were kept attention, and multi-spot spectroscopic measurements were performed using a spectrometer and a high-speed video camera. The temperature was calculated using the Boltzmann plot method. This research elucidated the temperature of the cathode spot during the transverse magnetic field change and the particles preceding the cathode spot just before the cathode spot moves.

    

Measurement of Temperature at Anode Spot Affected by External Magnetic Field Just Before Re-strike in Magnetic Driven Arc

Magnetic driven arc is a technology used as a DC circuit breaker in the field of electric railroads. To elucidate the breakdown factor of magnetic driven arc, the research has been conducted the focusing on the high-temperature metal vapor that exists between the electrodes just before restrike. Few researchers have researched about the spectrum intensity that emits at the next restrike spot just before the restrike. Temperature at anode spot affected by external magnetic field just before re-strike in magnetic driven arc is measured by spectrometer. Specifically, Cu I emitted from anode spot is paid attention to, and multi-spot spectroscopic measurements were performed using a spectrometer and a high-speed video camera. The temperature was calculated using the Boltzmann plot method. This research elucidates temperature at anode spot affected by external magnetic field just before re-strike in magnetic driven arc.   

Fluid Density Dependence of Electrical Discharges Generated Using Carbon Nanotube as Electrode in Liquid, Supercritical, and Gaseous Nitrogen

Plasmas in high-density media, including liquids and supercritical fluids (SCFs), have been attracting attention. We have previously reported the successful generation of stable low-voltage discharges in high-density N₂ and Ar using carbon nanotubes as electrodes, which are thought to act as field emitters [1]. In supercritical and liquid N₂, the strong auroral green emission derived from ON₂ excimer was observed, suggesting that the discharge was reactive. However, little is known about the plasma chemistry with CNT electrodes, since no detailed measurements have been made so far. In this study, we discuss the fluid density dependence of optical emissions by taking more than several thousand spectra while continuously varying the density in the range of two to three orders of magnitude from gases, SCFs, and liquids at cryogenic temperatures. With increasing density, the emission intensity of ON₂ initially decreased, but began to increase after ~10²¹ cm⁻³. At high densities, not only the emission intensity but also the emission area was wider, which was clearly different from the density dependence of normal discharges. Further details including spectral broadening and discussion will be presented.

[1] H. Muneoka et al, Plasma Sources Sci. Technol. 28, 075014 (2019).   

The effect of electrolyte concentration on the microdischarge behaviour during plasma electrolytic oxidation (PEO) on aluminium and titanium

The production of oxide-ceramic coatings on light metals aluminium, magnesium and titanium can be achieved by plasma electrolytic oxidation (PEO). High corrosion resistance and a good adhesion of the coating to the substrate are the main advantages of this method. During the process, anodic dielectric breakdowns in form of short-living microdischarges are generated in a conductive liquid.

To analyse the effect of the electrolyte concentration on these microdischarges, the active anode surface is reduced to the tip of a wire with a diameter of 1 mm. The electrolyte consists of varying concentrations of potassium hydroxide (1 - 4 g/l) in distilled water.

Fast optical measurements with a high-speed camera are carried out for a better understanding of the development and evolution of single microdischarges and the accompanying gas evolution.

Optical emission spectroscopy allows the determination of electron densities with Stark broadening of H­­­_α and scanning the treated wire tips with an electron microscope (SEM) enables to investigate the morphology of oxide layer. The measurements are performed in galvanostatic DC Mode with a current density of 1.27A/cm².   

Supercontinuum spectroscopy for studying the production of solvated electrons

Contamination of groundwater by poly-fluoroalkyl substances (PFAS) is increasingly recognized as a major environmental issue. These compounds bioaccumulate, can cause adverse health outcomes, and are difficult to break down. Low temperature plasma (LTP) offers promising avenues to remediation, especially via the production of free electrons dissolved in water ("solvated electrons"). We present the design of recently-funded experiments focused on understanding this process. We plan to measure the time-dependent concentration of solvated electrons produced by atmospheric pressure plasma at the water's surface using a pulsed supercontinuum light source. Supercontinuum transient absorption spectroscopy is a well-established method for studying time-resolved chemical and physical processes, but it has not yet been applied to LTP or to the plasma/water interface. By coupling supercontinuum transient absorption spectroscopy with the total internal reflection geometry of Rumbach, et. al (2015), we hope to obtain surface-sensitive measurements with nanosecond time resolution over a wide absorption band. We will describe the current diagnostic design, and discuss the expected challenges, workarounds, and enhancement techniques that may be necessary to improve the signal to noise ratio.   

Characteristics of DC discharges with a liquid cathode and a metal anode

We will present the characteristics of atmospheric pressure direct current (DC) discharges generated between a liquid cathode and a metallic anode. Plasma is generated by purging helium (He) gas through a tungsten tube. The discharge characteristics are investigated by varying applied voltage, U, gas flow rate, inter-electrode separation, gas mixture, and cathode properties (distilled water and sodium chloride as the cathodes). At He flow rates of 800-1600 sccm and voltage U up to 3.5 kV, the glow has the form of a circle on the surface of distilled water. At higher applied voltages, a conical shape is formed. The circular and conical shapes disappear with increasing the electrode separation and the He flow rates. Mixing N₂ or air from the surrounding tube flow results in the disappearance of the circular and conical shapes formed on the water's surface. With sodium chloride solution as the cathode, the discharge spreads in the form of a circle at a lower applied voltage. It transforms to a yellowish conical shape at higher voltages. Liquid cathode discharges are often subject to hydrodynamic instabilities, which result in the ejection of liquid droplets into the plasma. Our experimental observation could give helpful insight into the mechanism of droplet generation and the transport of metal atoms from the solution to the plasma. Modeling some aspects of the discharge has been performed using computational tools available to the team. A comparison of simulation results with electrical and optical measurements is being conducted to explain the observed characteristics.

    

Production of reactive oxygen species in an atmospheric-pressure pulsed He+H2O plasma: Effect of pulse repetition frequency

The controlled production of reactive oxygen species (ROS) in atmospheric-pressure plasmas is key in many of their applications. In this work, the production pathways of ROS are studied numerically in a nanosecond-pulsed pin-pin He+H₂O discharge as a function of pulse repetition frequency (1-500 kHz). The plasma is simulated using the 0D plasma-chemical kinetics model GlobalKin. The pulse shape (80 ns duration, 3 ns rise time, 2.3 kV amplitude) is kept constant in all cases, such that the afterglow duration is dependent on the repetition frequency. Analysis of the bulk plasma chemistry shows that short-lived species such as atomic hydrogen and oxygen increase with increasing repetition frequency, because these species are predominantly produced during the plasma pulse and decay in the subsequent afterglow. With decreasing afterglow time, their density at the start of the pulse increases, increasing the overall density within the plasma pulse. However, for some long-lived species such as H₂O₂ and O₃, there is a decrease of density at high repetition frequencies because these species are mainly produced during the afterglow from a conversion of short-lived species. A careful consideration of both pulse and afterglow period needs to be made when optimising the production of long-lived ROS through pulse repetition rate.   

Plasma-chemical kinetics in a parallel plate capillary plasma jet operated in He/H2O/O2 mixtures

Radio-frequency driven atmospheric pressure plasma jets are suitable sources for reactive species that can be used in a variety of applications in biomedicine and chemical conversion. In these applications, achieving selective and energy efficient species production is often essential. In this work, the plasma-chemical kinetics in a parallel plate plasma jet operated with a glass capillary between the two electrodes are investigated by zero-dimensional simulations.

The glass capillary serves as dielectric and yields a wide range of plasma operating conditions compared to similar sources operating without a dielectric barrier. Consequently, the utilisation of higher power and higher molecular gas admixtures are possible.

Here, we focus on identifying the optimum conditions for the production of H₂O₂ in this source. To do this, the plasma-chemical pathways and energy efficiency of H₂O₂ production are studied under a wide range of operating parameters, such as H₂O- and O₂-admixture, gas flow rate, and power deposition, including the use of pulsed power.

Simulated gas phase H₂O₂ concentrations are compared with those measured in plasma treated liquids using the same source.   

Fundamental processes in CO2-CH4 plasmas: a comparison of experimental and numerical results

Dry Reforming of Methane (DRM), which converts CO2 and CH4 into value-added products, knows a spike of interest due to it’s potential for energy storage.

Despite extensive literature on pure CO2 or pure CH4 plasmas, the main mechanisms controlling the evolution of CO2-CH4 plasma are not well understood. This works comparing experiments and simulations aims at providing insights on these mechanisms.

To do so, an RF discharge in various mixtures at a few Torr is ignited in a dedicated reactor in static configuration (without any gas flow). The RF voltage is controlled by a signal generator bursting trains of pulses of a few ms with controllable duty cycle ratio. The evolution of the densities of IR active species is monitored between the trains of pulses by FTIR spectroscopy.

These measurements were used to validate a 0D kinetic model using the LoKI simulation tool.

The excited state of dissociation fragments of CO2 and CH4 are found to be crucial, leading to further dissociation. The O(1D) state seems to control the production of water in the plasma via production on OH through oxidation of CH4

    

Princeton Collaborative Low Temperature Plasma Research Facility (PCRF)

Low-temperature plasmas (LTPs) have a wide range of applications in industry, medicine, aerospace, and sustainability. In 2019, the Department of Energy (DOE), Office of Science, Office of Fusion Energy Sciences established Princeton Collaborative Low Temperature Plasma Research Facility (PCRF, http://pcrf.pppl.gov) at the US DOE Princeton Plasma Physics Laboratory (PPPL). The PCRF provides the plasma science community and industry access to state-of-the-art research capabilities, including advanced diagnostics, plasma sources and computational codes, theory support, and expertise for comprehensive characterization of LTP properties with the goal of advancing methods of predictive control of LTP. Since the beginning of the facility operation, 64 collaborative users from the plasma and other scientific communities, including from universities, national labs, and industry, were awarded with runtime at the PCRF. These user projects cover: i) plasma-liquid and plasma-solid interactions, ii) plasma transport, iii) collective phenomena in LTP, iv) use of LTP in modern applications including but not limited to plasma processing and synthesis of materials; plasma medicine; plasma-assisted combustion and catalysis; and aerospace. In this presentation, we will discuss PCRF research capabilities and opportunities for collaboration.

    

Measurement of Bead Width Using Feedback Control During Welding Speed Change in TIG Welding

TIG arc welding is a technique to melt and join base metal with heat input using arc discharge. This technology has the advantages of no spatter during the welding, welding in all positions, and applicability to all joint shapes. In recent years, with the decrease in the number of skilled workers, the digital twin of TIG arc welding has been desired. However, the automation cannot be applied to the distorted base metal. Therefore, in this research, as a first step of digital twin, a drone camera was used to capture the arc shape, and feedback control was performed using a Raspberry Pi. Specifically, TIG arc welding was automated by taking pictures of the arc state, sending them to the Raspberry Pi, extracting only the arc state using OpenCV, measuring the arc length from the arc state, and automatically controlling the distance between electrodes. As a result, the bead width was observed using feedback control during welding speed change in TIG welding.   

Feasibility study for monitoring of tendency of particle generation in plasma etching by load impedance measurement

Particles in plasma etching process decrease the production yield and the overall equipment effectiveness (OEE) at LSI mass production line. In this study, we have tried monitored the tendency of particle generation in plasma etching by load impedance measurement. The reactive ion etching (RIE) equipment is used as the experimental apparatus and the particles, which originate from etching reaction product deposited on a gas shower type electrode, are investigated under the condition that a Si wafer is etched by SF₆ plasma. The particle is observed during each etching process by using the laser light scattering (LLS) method. The detection system introduces a sheet-like laser beam into the process chamber approximately 4 mm below the electrode. Scattered light by particle is detected by charge-coupled device (CCD) camera. In addition to the particle detection, the impedance monitoring system measures load impedance from a 50 ohm transmission line during the etching process. The measurement results of load impedance clarify that the imaginary part of the impedance decreases with the increase in the number of particles observed by the LLS method. We have demonstrated that load impedance monitoring method can monitor the tendency of particle generation caused by the flaking off of the deposited film. This non-invasive impedance measurement can be easily applied to the process equipment at mass-production line and contribute to improvement of the production yield and the OEE.   

Measurement of thickness of silicon carbide using multi-frequency analysis in the inductively coupled plasma

 The focus ring plays an essential role in improving the plasma uniformity at the edge of the wafer in the semiconductor etching. However, as etching proceeds, erosion of the focus ring occurs by ion bombardment. When the focus ring is eroded, the tilting of the incident ions distorts the wafer profile. Silicon carbide (SiC), which has high heat resistance and durability, is used as material for the focus ring. To measure the thickness of SiC, we applied multi-frequency sinusoidal voltages to a metal plate in contact with SiC in plasma. The impedances are measured from the multi-frequency currents, and we can obtain the thickness using the capacitance of SiC from a SiC equivalent circuit model. The measurement is in good agreement with the actual SiC thickness. Our result is expected to be applied to erosion monitoring of the focus ring in plasma etching processing.

    

A study on the effect of ultra-low electron temperature on the etching of MoS2 layer

We investigated the effect of ultra-low electron temperature (ULET, electron temperature < 1 V) on MoS₂ layer etching_. Oxygen ULET (~0.6 V) plasma was generated by using a grid. When a negative voltage (-20 V) is applied to the grid, an electron beam is generated, and ionization occurs by the electron beam [1]. Since the electron produced from the ionization have low energy, ULET plasma can be generated. When MoS₂ was exposed to conventional plasma, which has 3.9 V of electron temperature (T_e) and has about 2x10⁸ cm^-3 of electron density during 60 sec, MoS₂ layers are fully etched. However, MoS₂ layers are less etched in the ULET plasma despite the same density and exposure time as the conventional plasma. It is because the sheath voltage is significantly lower at ULET plasma than that of the conventional plasma due to low T_e. The removal of MoS₂ layer is verified through Raman spectroscopy and T_e are obtained by measuring electron energy probability function (EEPF). It is expected that plasma induced damage caused by ion bombardment can be reduced because of much lower sheath voltage in the ULET plasma.

    

Time-dependent Measurement of Ion Composition in Pulse-operated Ar/C4F8/O2 Dual-frequency Capacitively-Coupled Plasma

In the HAR etching, transport of neutral radicals to the hole bottom becomes difficult due to large incident angle and high surface chemical reaction probability and ionic fluorocarbon becomes important as the precursor of etching reaction. In pulse-operated plasma, ion flux composition to the hole bottom might be time-varying during the pulse-on period and, as the C/F ratio influences the etching performance, control of ion composition during the pulse-on period is important. In this study, time-dependent ion composition in an Ar/C₄F₈/O₂ dual-frequency pulsed capacitively coupled plasma is measured by a mass spectrometer with an energy analyzer. For the precise time-resolved measurement, ion transit time through the mass spectrometer is carefully evaluated. By integrating energy distribution of each ionic species, ion flux to the mass spectrometer is evaluated. In the pulsed plasma, temporal variation of the ion composition, i.e., gradual increase of fluorocarbon ions after turning on the pulse, is observed. High electron temperature at a moment of plasma ignition in the pulse is considered as the origin of this phenomenon with higher Ar ionization rate rather than C₄F₈ ionization rate. Gradual change of the ion composition for ~100 ms is explained by ion diffusion loss from the plasma.

    

Synthesis of diamond-like carbon thin film via multi pulse high-power impulse magnetron sputtering

Diamond-like carbon (DLC) thin films were prepared via a multi pulse high-power impulse magnetron sputtering (HiPIMS) at an Ar gas pressure of 0.6 Pa and an average power of 65 W. The first HiPIMS plasma temporally evolved during a pulse width of 12 μs and the discharge current reached 50 A corresponding to 1.8 A/cm². After the application of the first-pulse voltage for HiPIMS, multi-pulse voltages were applied to generate the multi pulse HiPIMS. The film structure and the film density were analyzed using the Raman spectroscopy and X-ray reflectivity (XRR) measurement. The Raman parameters, such as G peak position, intensity ratio of G band and D band, and FWHM of G band, of the films prepared by multi-pulse HiPIMS were not strongly dependent on the pulse number, but were different from those of the film prepared by single pulse HiPIMS. The film density was calculated using the XRR patterns of the prepared films with the film thickness of 60-80 nm. The estimated film density of the DLC film prepared by single pulse HiPIMS was about 2.36 g/cm³, whereas that of the films prepared by the multi-pulse HiPIMS ranged between 2.41 and 2.46 g/cm³.

    

Plasma discharge characteristics for balanced magnetron sputtering cathode

The sputtering technique is valuable for film deposition processes such as semiconductor manufacturing and surface modification and functionalization. Most commonly used magnetron sputtering cathodes (MCs) are categorized as unbalanced MCs whose magnetic field lines (B-lines) diverge to the substrate surface from the target material surface for obtaining an efficient plasma irradiation to the substrate surface. In contrast, MCs with a non-divergent B-field are called balanced MCs which has an advantage for plasma confinement. In this study, B-field lines of unbalanced MC have been modified to be an almost completely balanced MC that loops B-field lines near the target and produces magnetic-mirror confinement on crossing area of B-field lines on target surface, namely the magnetic mirror-type magnetron cathode (M3C)[1]. The M3C is low power and low gas pressure operations available because permanent magnets inside the cathode form a B-field with the magnetic-mirror ratio of ~25 near the target surface. In this presentation, we will present the experimental results for the Langmuir probe measurement and the plasma discharge performances in a low RF or DC power of ~30 W and a gas pressure operation of ~0.1 Pa.

[1] T. Motomura and T. Tabaru, AIP Adv. 7, 125225 (2017).   

Phase-Resolved Analysis of an Inductively Coupled Plasma with a Dual-Frequency Bias Using a Two-Dimensional Particle-in-Cell Simulation

A two-dimensional (2D) particle-in-cell (PIC) Monte Carlo collision (MCC) simulation has the advantage of investigating the energy and angle distribution of incident ions on the wafer by directly calculating the dynamics and collisions of each particle. Inductively coupled plasmas (ICPs) have antenna coils on the upper plate to generate plasmas by skin effect and apply a bias to the wafer to pull ions. The plasma profiles and the bias affect the ion energy and angle distributions (IEADs). This study examines the phase-resolved ion energy and angle distributions by applying a dual-frequency RF bias in addition to the ICP source using a 2D PIC simulation. The ICP power is directly related to the ion flux, the low-frequency bias directly affects the ion energy, and the angle distribution is affected by the potential profile.   

Elemental gradient functional thin film production for hydrogen entry prevention using powder target

Nickel-doped stainless steel thin films with high hydrogen-entry resistance were prepared on a metal and Si surface via sputter deposition. Mixed nickel oxide and stainless steel powders were used as the sputtering target. The experimental results indicated that nickel-doped stainless steel thin films could successfully be prepared both on the stainless steel and Si substrate surface. Deposition rate was dependent on the processing conditions such as input RF power, and the thin film Ni/SUS304 concentration ratio strongly depended on the powder target composition.   

Characteristics of DLC films deposited by pseudo-spark discharge PE-CVD with different substrate bias voltages

Diamond-like carbon films were deposited on silicon wafers by a Pseudo-spark discharge (PSD) plasma-enhanced CVD method. Process gases were hydrogen and methane. For basic characteristics of the PSD, pressure range of the discharge was measured. As a result, it was confirmed that the discharge is formed on the left-hand side of the minimum point of the Paschen curve. The effect for the bias voltage of substrate on film structure, deposition rate and film hardness were also investigated by Raman spectroscopy, SEM and nano-indentation. In the Raman spectra, disorder (D) and graphite (G) are observed at around 1370 and 1580 cm^-1, respectively. The G-peak position and the area ratio of D-band to G-band (I(D)/I(G)) were decreased slightly as the bias voltage increased from -250 V to -325 V. The maximum deposition rate of 0.22 μm/h was obtained at the bias voltage of -300 V. Furthermore, Deposited film at -325 V exhibited the film hardness of 3.55 GPa. By increasing the substrate bias voltage, it was confirmed that the film characteristics was improved, too.

    

TiN Film Formation by Linear and Novel Winding Filtered-Arc Deposition

Protective coatings on cutting tools are expected to improve machining efficiency and tool life. A vacuum arc deposition (VAD) system has been used in industry as forming protective coatings. VAD system produces dense films with excellent adhesion due to the highly ionized ions in the plasma, but the macro-particles (MPs) adhesion sometimes causes low-quality.  For reduction of MPs adhesion, a filtered arc deposition (FAD) system with magnetically plasma transportation function has been employed.

In this study, we attempted to form TiN films by two types of FAD systems having an electromagnetic coil on the outside of the connection duct (duct coil). One was a traditional linear-FAD (L-FAD) with a cylindrical anode as a part of FAD system. The other was newly designed winding-FAD (W-FAD) with a winding shape anode. The anode was composed of C-ring-shaped copper plates assembled into a coiled shape. Therefore, the anode in W-FAD generated magnetic field by arc current itself to transport the vacuum arc plasma tunneling the anode as well as to block the flying MPs. As a result, it was found that the number of MPs on the 1-micron thick film prepared by W-FAD were excellently few compared to that on the film prepared by L-FAD.   

Deposition of nitrogen doped amorphous carbon film using high power impulse magnetron sputtering

A thin film having electrical conductivity and corrosion resistance is required for a separator of a polymer electrolyte fuel cell or an electrode of lithium ion battery. An amorphous carbon (a-C) thin film shows good adhesion to the substrate and high corrosion resistance, and exhibits an electrical conductivity by doping nitrogen. The a-C film deposited by high power impulse magnetron sputtering (HiPIMS) shows high film density and good adhesion due to high plasma density. In this study, the effect of nitrogen doping to a-C film ​​deposited by HiPIMS was investigated.

A negative pulse voltage with a frequency of 400 Hz was applied to the carbon target. The pulse duration was 10 µs and the peak power density was 1.2 kW/cm². The flow rate ratio of N₂ gas to Ar gas was varied from 0 to 20% at the pressure of 0.5 Pa and the total flow rate of 10 sccm.

I_D/I_G ratio, which is due to disorder and graphite band, was evaluated by Raman scattering spectroscopy. I_D/I_G ratio increased and the G peak position shifted to the higher wavenumber with increasing the nitrogen gas flow rate ratio. These results indicate that the incorporation of nitrogen induces an increase and clustering of sp2 bond.

    

Investigation of Material Properties of Fluorocarbon Films Deposited by Plasma-Enhanced Chemical Vapor Deposition

Energy-related research at phase boundaries has attracted much attention in recent years. In particular, the boundary between electronic materials and liquid water is interesting from application viewpoints. Since the potential gradient of the liquid phase are affected by the properties of solid materials, controlling the properties of solid materials in the presence of liquid is an important factor in improving their output performance of application. In this study, we focused on polar fluorocarbon films and investigated their physical properties. C4F8 molecule was used as a precursor gas to deposit film by plasma-enhanced chemical vapor deposition. We investigated the effect of deposition pressure on bonding state formation and physical properties of the films. Composition ratio of the film is changed to fluorine rich at high pressure that is confirmed by x-ray photoelectron spectroscopy. It is considered that the dissociation of precursor in the plasma and polymerization reactions on the substrate affect the film composition. Similarly, the surface resistivity is decreased, and the zeta potential changed to negative value. This is due to reduction of sp2 carbon which contributes conductivity at high pressure. Thus, composition ratio of bond state can be controlled by discharge condition. These results are considered useful to maximize output performance of the devices at phase interface.

    

Thin plasma-polymerised layers on PET-substrates under the influence of NaOH solution

Plasma-enhanced chemical vapor deposition coatings are already used to reduce the gas permeation of polyethylenterephthalat-bottles. To apply the coatings for refillable bottles, a corrosion protection has to be developed to protect the barrier coating and the bottle. This is necessary because the SiO_x-barrierlayer has a low resistance to the NaOH solution used for the washing process.

For the experiment, PET-foils and Si-Wafers are coated. Therefore, a possible interlayer (3 nm) and barrierlayer (30 nm) are deposited with a pulsed microwave plasma. The HMDSO-based protection layer has a thickness of 30 nm. The process parameters for the corrosion protection are varied to identify a correlation between the parameters and the resistance. To assess the resistance to NaOH solution, an analysis of oxygen permeation, Fourier-transformation measurement, and SEM-examination are used. Optical emission spectroscopy is done to analyse the electron density, electron temperature, and gas temperature.

The presented results correlate the power and pulsing during the deposition process with the layers’ resistance against NaOH solution. A lower power increases the resistance. With pulsing of 2 ms/160 ms compared to 1 ms/160 ms a layer is deposited, which has a higher resistance to NaOH solution.   

Design and Preliminary Performance Assessment of a Porous Dielectric Barrier Discharge Reactor for Ammonia Synthesis

Ammonia is the second largest industrial chemical compound, mainly used for agricultural fertilizers. The Haber-Bosh process, the main ammonia production method, relies on large-scale and energy-intensive operations leading to 1-2% of energy consumption and 3% of CO₂ emissions globally. Ammonia synthesis using non-thermal plasma operated at atmospheric pressure conditions could be a viable approach to decarbonize ammonia production, especially in distributed and on-demand operations. The design and characterization of a porous Dielectric-Barrier Discharge (DBD) reactor are presented. The porous DBD reactor design is aimed at plasma catalytic-membrane processes, which allow integrated product separation, enabling process intensification and potentially greater efficiency, while addressing catalyst packing issues commonly faced by packed-bed reactors. The reactor allows three operation modes: conventional DBD with non-porous dielectric, porous DBD with porous dielectric, and catalytic-membrane DBD with porous dielectric loaded with catalyst particles. Reactor characterization and performance assessment without catalyst are presented. Expected operational characteristics are evaluated using computational thermal-fluid modeling together with electrical, optical, and chemical analyses. Ammonia production and process efficiency as function of design and operational parameters are investigated.

    

Effect of discharge parameters on the shock wave pretreatment of wood flour for enzymatic saccharification

Woody bioethanol is expected to be a new biomass fuel because it can reduce CO₂ emissions while avoiding conflict with food issues. The woody bioethanol is obtained by saccharifying cellulose and fermenting the produced sugar. However, saccharification efficiency of cellulose is insufficient since the reaction area of the saccharifying enzyme is limited by a large amount of hydrogen bonds between cellulose molecules. In this work, we applied underwater discharge shock waves to wood flour to physically crush the flour and expand the area of enzymatic reaction by breaking hydrogen bonds.

Shock waves were generated in the water with wood flour dispersed by spark discharges produced by applying a pulsed high voltage via spark gap switch to pin-to-pin electrodes. The shock wave pressure was 2 MPa at 17 mm distant from the electrodes, suggesting that the shock wave mechanically destroyed wood flour. Increasing the discharge frequency from 3.3 to 7.3 Hz improved the obtained glucose sugar from 427 to 518 ppm. However, increasing the discharge energy from 0.85 to 1.13 J/pulse at constant discharge frequency of 3 Hz did not strongly affect the saccharification efficiency. These results indicate that the number of shock wave irradiations is important for improving the pretreatment effect.   

Increased energy efficiency by optimization of the separation processes in waste incineration plants by means of Gemini

Germany has one of the highest recycling standards in the world. Nevertheless, enormous amounts of waste accumulate that cannot be recycled - i.e. cannot be reprocessed. For this type of waste, combustion is a sensible and environmentally friendly solution that also contributes to the energy transition, as it produces electricity from renewable energy sources. However, the combustion process produces large quantities of dust, which must be filtered out of the exhaust gas. One established solution are electrostatic precipitators (ESP) that remove dust and aerosols from exhaust gases. They consist of housings containing electrodes and strong electrostatic fields to generate a plasma to accelerate the electrically charged particles which are then deflected from the gas stream and collected on the precipitation electrodes. From there, the collected particles are removed from the electrostatic precipitator and then transported to landfills. In the power plant of the GMVA (Gemeinschafts-Müll-Verbrennungsanlage Niederrhein), enormous improvements in waste gas purification, as well as energy savings, could be achieved with the help of this process.

This contribution provides an insight into the real process of exhaust cleaning and the optimisation possibilities with the new controller generation “Gemini”. Direct comparisons of the regulation process show an enormous reduction in dust levels.

    

Decomposition of high-density toluene in water-vapor-mixed Nitrogen/Air using dielectric barrier discharge

Volatile organic compounds (VOCs), one of the major air pollutants, are emitted from various industrial fields. Typical measures for processing VOCs are the combustion and the adsorption method. However, both have limitations including large energy consumption or high running costs. Alternatively, decomposition by a non-thermal plasma is regarded as an effective solution for pollutants treatment, due to the abundance of energetic electrons and reactive species. Therein, dielectric barrier discharge (DBD) is suitable for decomposing pollutants in the air, due to it can generate large-flow plasma.

Since oxygen-induced active species are considered to play a major role in the decomposition process in the air, in this study, decomposition experiments using air and pure nitrogen DBD plasma, were conducted. For the experiment, a DBD device was developed with a processing area of 200 mm long, 100 mm wide and 2 mm high. As a target gas, toluene was mixed in the air, nitrogen, and water-vapor-wetted nitrogen and air, at 100 ppm and 30 L/min. As a result, air plasma regardless of whether wet or not showed a higher decomposition rate than nitrogen. This indicates that active species derived from oxygen in the air contribute significantly to the decomposition of toluene.

    

Development of an experimental system for cell viability assays of yeasts using gas-temperature controllable plasma jets

When model organisms such as fission yeast and budding yeast are treated by direct plasma irradiation, it is essential to maintain the irradiated target at an appropriate temperature. Therefore, we have developed a room temperature atmospheric pressure helium plasma jet device utilizing a Peltier element to cool the feeding gas [1]. Here we report on the realization of an experimental system for cell viability assays of yeasts using this device. Good reproducibility of cell viability was observed when gas temperature, gas flow rate, applied high voltage, and irradiation distance remained fixed, and only irradiation time was used as a parameter. This allows for a variety of experiments, such as the search for plasma-resistant mutants that will contribute to the identification of genes involved in resistance to direct plasma irradiation. Details of this system, including the density of ground-state oxygen atoms in the plasma plume that was measured by vacuum ultraviolet absorption spectroscopy, the molar concentration of hydrogen peroxide produced upon irradiation to the culture medium, and the time development of medium temperature during plasma irradiation, will be reported at the conference.

[1] S. Yoshimura et al., Jpn. J. Appl. Phys. 58, SEEG03 (2019).   

New Plasma Device for Selective Generation of Dinitrogen Pentoxide from Air and Its Applications

Atmospheric-pressure plasmas (APPs) have been of great interest and have been widely applied. In particular, air APP devices, working only with air and electricity, can potentially allow for ubiquitous supply of reactive species, which can be used widely. Recently, we developed a new air APP device, enabling to supply reactive species (e.g., N₂O₅, O₃, NO_x, …) with fine control and good reproducibility. Specifically, its ability to generate high density (up to 300 ppm) N₂O₅ with high selectivity is quite unique and could accelerate scientific and industrial N₂O₅ applications, which has not been used widely due to its ordinary synthesis methods requiring multiple hazardous raw materials (requiring handling with much care). N₂O₅ is well known as a powerful oxidizing and nitrating agent and can potentially be bioactive. However, there are no previous studies using N₂O₅ for bioapplications. This air APP device can partially convert air molecules into N₂O₅ and can easily spray on biological targets. Thus, we have explored the effects of N₂O₅ exposure on various biological samples (e.g., bacteria, virus, cultured cells, plants). Based on the experimental results, we will show a scheme of selective N₂O₅ generation from air and future perspectives for bioapplications using N₂O₅.   

Antitumor effects on mouse colorectal Colon-26 tumors in mice induced by normal tissue treatment using streamer discharge

We previously reported antitumor abscopal effects in mice induced by tumor treatment using streamer discharge (K. Mizuno et al, J. Phys. D, 50, 12LT01, 2017). When one of the tumors in mice with two mouse melanoma B16F10 tumors was irradiated with streamer discharge, antitumor effects was observed in the other tumor, which was not irradiated with discharge, as well as in the irradiated tumor. This remote antitumor effect is called abscopal effect, which is well-known phenomena in radiotherapy. In the current study, the abscopal effect induced by normal tissue irradiation, not by tumor irradiation, is reported. When the discharge was irradiated on the upper left back of mice, which have mouse colorectal Colon-26 tumors in the right flank, the tumor growth was delayed. The distance between the tumor and irradiation point was 2 to 3 cm. The abscopal effect induced by normal tissue treatment was not observed in immunodeficient mice. This suggests that the abscopal effect is related to some adaptive immune responses.   

Comprehensive analysis of gene expression in PAL-treated glioblastoma cells

We have previously developed plasma-activated medium (PAM) and plasma-activated Ringer’s lactate solution (PAL). PAM induced more intracellular reactive oxygen species (ROS) than PAL did on glioblastoma cells. Gene expression analysis using microarray revealed that PAM induced oxidative-stress dependent cell death on glioblastoma cells. In this study, we have performed microarray analysis of PAL-treated glioblastoma cells. Many histone cluster genes were upregulated more than 2-fold in PAL-treated glioblastoma cells. Histones play a critical role in epigenetic regulation, and histone genes are usually expressed in the S phase (synthesis phase) of cell cycle. Flow cytometry analysis suggest that PAL induced cell cycle arrest on the G2/M and S phases on glioblastoma cells. These results suggest that PAL upregulated histone genes because it induced cell cycle arrest on the S phase on glioblastoma cells.

    

Enhancement of cytokine production and differentiation from sensitized EL4 T-cell by using atmospheric plasma irradiation

In recent years, immunotherapy for cancer has become popular as an alternative of conventional methods. However, the period required to culture immune cells up to the enough activity for cancer treatment is as long as 3 weeks. To reduce this period, the purpose of this study is to activate immune cells with plasma and enhance the release of cytokines and differentiation from precursor cells. Oxygen plasma generated by the torch-type atmospheric DBD was supplied to the cells. The oxygen gas flows at 0.6 L/m with discharge voltage 4.2 kV. The EL4 T-cell that is one of kinds of lymphocytes was used, and the initial cell concentration is 5×10⁵ cells/ml. After 24 h from plasma irradiation, T-cells are sensitized and after another 24 h, amounts of IFN-γ and IL-4, which are kinds of cytokine released by Th1 and Th2 cells, respectively, are measured using the ELISA kits. The amount of IFN-γ released by one cell was increased by 2 times, which is equivalent to the total cell number increase by the plasma irradiation. The amount of IL-4 released by one cell was increased by 10 times. Therefore, the ability of the IL-4 production by EL4 T-cell was enhanced by the plasma irradiation. This result suggests that EL4 T-cells were differentiated to Th2 cells, which is one kind of CD4+ T-cells.

    

Role of short-lived nitrogen species generated at low-pressure RF plasma on the germination and seedling growth

Plasma agriculture has been gaining tremendous attention in recent years to increase seed germination, seedling growth, and yield [1-3]. Although in some cases, the negative or no effects of plasma treatment was also observed [1]. Several factors influence the negative and positive effects of plasma on seeds, but plasma generated reactive species are crucial in plasma agriculture. To unlock the role of mores especially reactive nitrogen species (RNS) species, on the seeds, we treated the radish seeds with low-pressure RF plasma with N₂ feed gas for various time intervals. Additionally, we used the radish seeds with two different seed coat colors and treated them under two other conditions (dry and wet treatment). We can understand the possible role of seed coat color and the effect of humidity on seed germination, seedling growth, and changes in phytohormone (Abscisic acid) and antioxidant (γ-tocopherol) levels after plasma treatment. Further, we also performed the electron paramagnetic resonance spectroscopy (EPR) analysis to find any possible changes in the paramagnetic species of the radish seeds under low-pressure plasma. We also included 1D simulation using COMSOL Multiphysics software to determine how the potential RNS generated in plasma affects the seeds. Our results clarify that seed coat color and humidity influence plasma effects on seed germination, phytohormone, and antioxidant levels.

[1] P. Attri, et al., Processes 2020, 8, 1002.

[2] P. Attri, et al., Agronomy 2022, 12, 482.

[3] K. Koga, et al., Japanese Journal of Applied Physics 2020, 59, SHHF01.   

The effect of plasma-activated water in enhancing seeds germination, plant growth, and its use as a nitrogen source for algae growth

In the present work, we studied the effect of plasma-activated water (PAW) treatment on pea seeds and the use of PAW as a nitrogen source for freshwater algae. The PAW treatment removes the wax from the seeds surface confirmed by morphology and energy dispersive X-ray analysis. The removal of wax from seeds surface makes the seeds surface hydrophilic and enhances the water-absorption capacity of seeds. Therefore, a faster germination rate, higher plant growth, and higher fresh and dry weight have been observed in PAW treated seeds in comparison to control. Moreover, the use of PAW as a nitrogen source significantly increases the algae growth compared to control.

            The biochemical analysis showed a higher concentration of chlorophyll ‘a’, sugar, and protein in PAW-grown plants and algae compared to control. Also, the electrolytic and phenolic leakage, and H₂O₂ concentration do not show any significant difference in PAW and control-grown plants and algal species. Moreover, no external oxidative stress is created in PAW-grown plants and algae compared to control shown in antioxidant enzyme activities. In conclusion, PAW has an enormous potential to be used in agriculture field to enhance seeds germination, plant growth, and its use as a nitrogen source for algae growth.   

Dependence of depth in liquid and gas-flow-rate ratio irradiated with nitric-oxide radicals on proliferation of fibroblast cells

The reaction of NO• with water produces various reactive oxygen and nitrogen species (RONS) with different lifetimes. In this study, we have investigated the dependence of gas-flow-rate ratio and focused on this difference in the lifetime and investigated the promotion of fibroblast proliferation as a function of depth from the air-liquid interface to the cell surface in liquids irradiated with RONS. In the experiment of dependence of gas-flow-rate ratio, the depth in liquid was fixed at 1.0 mm, and Ar gas set to 4.0 slm and N₂ / (N₂ + O₂) was varied from 40, 60, 70, and 80%. In the experiment of dependence of depth in liquid, N₂ / (N₂ + O₂) was fixed at 80%, and the depth in liquid was set to 1.0, 1.3, 1.7, and 2.0 mm. After that, fibroblasts viabilities were measured. The results of the gas-flow-rate ratio dependence showed that the proliferation ratio had a peak at a gas-flow-rate ratio of around 60%. In the dependence of depth in liquid, the maximal promotion was obtained at the shallowest depth of 1.0 mm. These results suggest that NO• or other short-lived RONS generated in liquid is a key factor for proliferation promotion of fibroblast.   

Application of heavy ion plasma to understand treatment mechanism of heavy ion cancer therapy

Over one century passes since plasma is thought to be produced in swift heavy ion irradiation, theoretically. However, it is still unclear if this irradiation forms plasma or not. The problem of the presence or the absence of the plasm is important to estimate the radial dose distribution because the presence of plasma increases radial dose, that is, decreases this rate. Here, the radial dose is the dose as a function of the distance from the incident ion path and is employed in the treatment planning system for the cancer therapy to estimate the survival rate of cells.  To study this plasma, we developed a new simulation model as follows. In heavy ion irradiation, huge number of molecules are ionized along the path of the incident ion. We expect the molecular ions produced here to form strong electric field. We incorporated the effect of this electric field on the motion of secondary electrons in our model.Not that there is a measurement of the secondary electron yield that indicated that this electric field traps the motion of secondary electrons. These trapped secondary electrons are expected to form plasma. Only our simulations almost reproduce the trend of this measurement. Using plasma theory, We derived a new formulation which almost reproduced our simulations.

    

Effect of alcohol addition on radical production

Atmospheric pressure plasma can produce many kinds of reactive oxygen radicals, which make important role in plasma application in technological, chemical and biological field. Radical production occurs mainly in discharge gas and in the gas-liquid interface. Discharge performance depends on the composition of discharge gas. Reduction of the breakdown voltage is reported to become smaller by adding ethanol. In this work, effect of alcohol addition to argon gas on this radical production. Radical monitering methods with heat flux measurement and chemical probe will also reported.

Compared with pure argon case, plasma jet length shrinks but plasma heat flux becomes larger by ethanol addition. This means heat flux is carried by radicals produced and released on the target surface through the recombination process. So this results indicates radical enhancement occurs.

Our group has proposed that polyvinyl alcohol(PVA) - iodine potassium (KI) could be a new chemical probe. Color change of PVA-KI is enhanced by small amount of alcohol addition. There is an optimum value of alcohol for radical production. Although decompose of alcohol becomes source of oxygen radicals, alcohol itself works as radical scavenger.   

Changes in the permeation characteristics of ROS through biological membranes by discharge plasma-Induced electric field

  In plasma medicine, the oxidation reaction of active species in vivo (biostimulation) is known to be an important factor. However, the actual cell penetration mechanism is complex, and electronic behaviors such as surface charging and membrane potential change are likely to contribute. Therefore, the purpose of this study is to clarify the basic mechanisms of plasma medicine by focusing on the interaction with cells through the supply of electric field and charge, apart from the chemical reactions of reactive species that have been studied.

  In this paper, the free energy and permeability coefficients of the cell membrane to plasma-induced reactive species when the electric field is varied are numerically analyzed by molecular dynamics (MD). The cell membrane model was constructed with a lipid bilayer consisting of 128 POPC phospholipids and a 0.15 M KCl solution. Hydrogen peroxide () was used as the active species. The electric field was varied from 0 to 0.4 V/nm.

  The free energy at the center of the membrane was reduced by about 40% at the maximum by the application of the electric field, and the permeability of the membrane increased sharply by a factor of about 8 from 0.3 to 0.4 V/nm. The influence of electric field directions on those properties was also discussed.   

Improvement of gene transfer efficiency for establishing cells with higher safety for gene therapy by using surface discharge

We investigate the easy method of safe cell establishment for gene therapy using surface discharge. The experimental results show that gene transferred cells efficiently express target protein and suggest that surface discharge is an efficient gene transfer method for primary cells. In addition, This method induces low/no random integration in the genome of the established cells.

It is difficult to establish safe cells for gene therapy with conventional gene transfer methods that are not free from causing damage to cells. Therefore, we investigate the safety of the established cells with surface discharge by comparing them with conventional gene transfer methods.

As a result, more expressed cells were produced efficiently by one treatment. In addition, the surface discharge method causes few random integrations into chromosomes. We reported that the complex stimuli of electrical and chemical factors generated by a discharge plasma promote gene introduction via endocytosis. 

The surface discharge method enables us to collect efficiently expressed cells in only one treatment and reduce random integration. The surface discharge method is expected to bring a safe gene therapy.

 

    

Mechanism of macromolecular introduce into plant cells by plasma treatment.

Gene transfection is one of the methods of plant breeding. Since a plant cell is covered with a cell wall, introducing genes directly into it is difficult. We have been developing the gene introduction method for plant cells with discharge plasma treatment. In this study, macromolecules such as plasmids were introduced into plant cells with multi-time plasma treatments.

The callus was placed statically in a 3.5 cm dish on a GND electrode. A needle electrode was placed above the callus, and plasma was irradiated directly to the cells by applying a sinusoidal voltage. Twenty-four hours after plasma treatment, the callus was stained with X-Gluc to observe the expression of GUS. GUS expression was observed on the cell treated two times, though no GUS expression was observed for single-time treatment. After the plasma treatment, the cell surface of the callus was observed by FE-SEM. The callus treated two times scraped the cell wall and other parts of the introduction barrier more deeply than the callus treated only one time.

These results suggest that by multi-time plasma treatment, the target macromolecules, such as GUS plasmid, reach the cell membrane through the cracks in the cell wall, then they are introduced into cells by endocytosis mediated by the complex stimuli of plasma.   

Effect of non-equilibrium atmospheric pressure plasma (APP) on adipocyte browning via modulations of TRPV1 and TRPA1 channels

Complex mixtures of active agents produced by APP can regulate various cellular functions by triggering complex biochemical reactions, and APP thus emerges as being applicable to several clinical therapies. However, the details of the mechanisms underlying APP-inducible biological responses remain ill-defined. Recently, we revealed that exposure of 3T3L1 fibroblasts to APP-irradiated buffer raised cytoplasmic Ca²⁺ concentrations ([Ca²⁺]_c) by activating TRPV1 and TRPA1. We herein examined the effects of APP exposure on 3T3L1 adipocyte, focusing on its browning via APP-inducible TRPV1/A1 modulations. In 3T3L1 adipocytes, APP increased [Ca²⁺]_c. APP irradiation also increased browning-involved gene expressions, UCP1 and PRDM16. The siRNA-mediated silencing of TRPV1/A1 inhibited both the [Ca²⁺]_c transients and adipocyte browning, implying essential roles of these TRP channels in APP-inducible actions. Consistent with in vitro results, in vivo experiments on mice intraperitoneally injected with APP-irradiated buffer showed increased UCP1 mRNA expression in white adipose tissues, indicating promotion of their browning. These findings demonstrate that APP contributes to adipose tissue browning and suggest novel therapeutic applications benefiting energy homeostasis. 

    

Effect of plasma-generated gaseous nitrogen on plant growth

Nitrogen (N) content in the soil is a determinant for crop yield. Therefore, the demand for use of N fertilizer is increasing. Currently, most of N fertilizer production relies on the non-eco-friendly Haber-Bosch process. To develop the eco-friendly N fertilizer, nitrogen fixation by plasma is considered to be as one of promising way. Here, we developed a device to selectively produce dinitrogen pentoxide (N₂O₅) from air by plasma. N₂O₅ could be used as a nitrogen source for plant growth owing to high water solubility of N₂O₅ and rapid conversion to nitrate ions in water. We observed that plasma-generated N₂O₅ gas dissolved in a hydroponic medium successfully recovered the growth of N-deficient plants. We supplied the plasma-generated N₂O₅ gas to plants in two ways; 1. direct exposure and 2. indirect exposure to plants. The method 1 impaired leaves and reduced the fresh weight, but prevented anthocyanin accumulation caused by N deficiency. The method 2 showed that plants with N₂O₅ gas allowed the plants to have six leaves, whereas the mock treatment stopped growing. Based on the results, the second method is better because it minimized injuries and maximized benefits. In summary, this study proved that the plasma-generated N₂O₅ gas can be applied as N fertilizer for plant growth.   

Analysis of Intracellular Nucleic Acid Damage Induced by Cold Atmospheric Pressure Plasma Irradiation

In recent years, applications of cold atmospheric pressure plasma (CAP) in life science, medicine, and agriculture have gained increasing interest. It has been reported that plasma irradiation to cancer cells induces apoptosis, a programmed cell death, and is expected to be applied to cancer therapy. Recent progress in the analysis of the cell death mechanism has revealed that reactive oxygen and nitrogen species generated by CAP induces oxidative damage in biomolecules such as nucleic acids and proteins. Nucleic acid damage is an important indicator in evaluating genotoxicity and carcinogenicity. Therefore, analysis of plasma-induced nucleic acid damage may contribute to elucidating the mechanism of antitumor effects and evaluating the safety of plasma medicine. In this study, we analyzed the damage of intracellular RNA and mitochondrial DNA after plasma irradiation, focusing on 8-oxoguanine (8-oxoG), one of the representative modified bases. As a result, it was suggested that 8-oxoG was generated in intracellular RNA and mtDNA by CAP treatment of A549 cells.

    

Introduce gene into many cells by creepage discharge method

Gene therapy, including cell medicine, is an excellent method that offers new therapy against incurable diseases. However, since general gene transfer methods are not free from cell damage, the establishment of safe gene therapy cells is desired.

In addition, gene therapy requires tens of millions of transduced cells. Therefore, there is a need for a technology that enables high-efficiency, high-volume batch processing.

We are studying the surface discharge method, which can introduce genes into many cells by plasma processing, with a view to practical applications in advanced medicine such as gene therapy. We have already established the surface discharge method, which is capable of the process for many cells in a 3.5 cm dish with high-introducing efficiency. 

In this study, we tried to optimize the electrode arrangement and examined the introduced area to expand the introduced area of the surface discharge method. We attempted to introduce a plasmid expressing AcGFP (green fluorescence protein) into L-929 in a 10 cm dish by the surface discharge method. As a result, we achieved a six times larger introduction area with a 10 cm dish than a 3.5 cm dish. Through this research, we expect that the surface discharge method can easily produce a large number of cells for gene therapy.

    

Density profile control of a magnetically expanding plasma and its impact on a plasma thruster

Inductively coupled plasmas (ICPs) generated by a radio-frequency (rf) power under a magnetic field have been vigorously investigated for industrial and space applications. In terms of plasma processing technologies, the uniform density profile is highly required. Moreover, the shape of the plasma density profile in the magnetic nozzle is expected to have a significant impact on the performance of a magnetic nozzle rf plasma thruster. Therefore, the control of the plasma density profile is highly desired for the fundamental studies and thruster development.

In this study, it is demonstrated that the radial density profile in the magnetic nozzle rf plasma thruster can be controlled by adjusting the rf powers to two rf antennas wound around the source tube. In addition, the thrust component corresponding to the Lorentz force onto the magnetic nozzle is solely measured to clarify the impact of the density profile on the imparted thrust. The results show the controllability of the density profile of the ICP in the magnetic nozzle and indicates the possible impact of the density profile on the thrust.

    

Forced van der Pol oscillator modeling of Hall-thruster's externally modulated breathing mode

Signatures of entrainment, frequency, and wavenumber pulling, excitation thresholds, and particle transport, all associated with observed patterns of self-organizing dynamics of instabilities are examined in the archived data from PPPL's cylindrical Hall-Thruster. The objective is explaining spatio-temporal plasma behavior of "spokes" observed in modulated breathing oscillations in E×B plasma discharges using a combination of forced van der Pol oscillator equations, electrostatic gradient drift instabilities, the modified Simon-Hoh instability, and the influence of ionization instability. These signatures, yet to be fully explained, resemble limit-cycle behavior, thoroughly characterized in other discharge plasmas and electronic nonlinear-oscillator circuits, so the anticipated validation of an improved model of spoke frequency scaling with the pressure and magnetic field for Xenon and other gases is underway. Archived evidence includes time-resolved laser-induced-fluorescence measurements that were performed in the two identified thruster-response regimes: linear and nonlinear. In linear regime, the ion velocity distribution function was observed in all phases of the discharge and, with this methodology, precise kinetic evidence of ion sloshing during the evolution of the discharge current was acquired. A key gap issue is the theory of Modified Simon-Ho instability does not explain the observed pressure and magnetic field effects even though the suppression of spoke rotation frequency with the pressure correlates with the condition E is approximately zero (seemingly in agreement with MSHI). Furthermore, the observed spoke frequency scaling is different from known scaling for conventional sputtering E×B magnetron discharges and thrusters.   

Size controlled synthesis of gold nanoparticle/carbon nanotube composites by atmospheric-pressure microplasma

Gold nanoparticles (AuNPs) have a superior catalytic activity. It has been reported that AuNPs with a diameter of 10 nm or less exhibit not only CO oxidation at room temperature but also glucose oxidation in liquid phase. On the other hand, carbon nanotubes (CNTs) have excellent electrical conductivity, chemical stability and large surface/ volume ratio. Gold nanoparticle/carbon nanotube composites (AuNPs@CNT), therefore, have attracted much attention as a high-sensitive biosensor. AuNPs@CNT have been synthesized by chemical reduction or plasma reduction, due to electrons and H radicals, of a HAuCl₄ aqueous solution using -COOH functionalized CNTs. However, it is an important issue to synthesize size-controlled and monodispersed AuNPs. In this study, AuNPs@CNT are synthesized by atmospheric-pressure He microplasma irradiation to a small amount of a HAuCl₄ aqueous solution (0.80 mM/L) of 100μL with adding CNTs dispersed ethanol of 200 µL. A microplasma was generated by a RF (13.56 MHz) driven narrow metal pipe electrode with He gas flow rate of 550 sccm. Plasma irradiation time was varied from 1 min to 5 min. AuNPs@CNT with an average AuNPs size of 5.3 nm and a standard deviation of 2,6 nm have been successfully synthesized at a plasma irradiation time of 1 min. An average size and standard deviation of AuNPs increase with increasing plasma irradiation time. Both the volume of CNTs dispersed HAuCl4 solution and plasma irradiation time play an important role in the size controlled synthesis of AuNPs.   

Growth of metal-organic frameworks in solution influenced by laser-induced plasma at the early stage

Metal-organic frameworks (MOFs) in which organic molecules are coordinated to metal ion clusters has been attracting much attention. MOFs have nano-sized pores in their structure and have a large specific surface area, whose size can be widely tunable via combinations of metal elements and organic ligands. Sometimes by functionalization with functional groups, various features of MOFs can be adjusted. Due to these characteristics, MOFs are expected to be applied to various applications such as gas adsorption/storage/separation, catalysts, and sensors. While conventional MOF synthesis methods include solvothermal method, microwave irradiation method, and electrochemical method, MOF synthesis with laser-induced plasma on metal targets in liquid has been reported recently. However, there is insufficient research on the effect of pulsed-laser irradiation on MOF synthesis. In this study, we irradiate raw materials of HKUST-1 by pulsed-laser to initiate plasma in liquid only at the early stage of the synthesis, to see how the initial plasma treatment can control the following growth process. After the plasma generation, MOF grows dependently on the initial plasma-treatment conditions. The details as well as the kinetic analysis with a Gualtieri model will be presented at the conference.   

A 2D Particle-In-Cell model of an Electron Cyclotron Resonance plasma for the purpose of lifetime tests

Stable and high transmission of the optical elements in lithography machines is crucial for high-volume manufacturing of semiconductor devices. This requires understanding and solving of the impact of EUV and of the EUV-induced plasma on the optical elements. An elaborate set of tests is realized in a low-pressure ECR plasma environment to ensure the endurance of the optical elements. An implicit energy-conserving electromagnetic 2D particle-in-cell code is utilized to calculate the plasma properties and the load to the optical samples. The simulations exhibit a rotationally varying electron energy distribution function in the magnetized region and a bi-Maxwellian electron energy distribution function in the afterglow. The ion energy flux distribution function on the optical samples is calculated and their relation to their lifetime is discussed.

    

Treatment of Polyethylene Terephthalate using low-temperature atmospheric pressure helium plasma jet for improvement of adhesion

A treatment using low-temperature atmospheric pressure plasmas can control the surface properties of materials. In this study, Polyethylene Terephthalate (PET) films were treated by helium plasma jet to improve adhesion between the films. To investigate a role of reactive species on the change of surface property a chamber was used, where argon, dry air, and ambient air were used as surrounding gas. By the plasma treatment for 60s, the contact angle of water on the PET surface was decreased from 82 to 25, 22, and 11 deg. using ambient air, dry air, and argon as surrounding gas, respectively. To observe the effect of plasma treatment on adhesion between the PET films (0.2 mm thick), a standard T-peel test was carried out. On PET films of 15 mm × 90 mm, the plasma treatments were applied for 60 s. The effective plasma-treated area was about 5 mm in radius. After the plasma treatments, the plasma-treated PET films were pressed with their irradiated surfaces faced under a pressure of 83.3 kg/cm² at 100 °C for 60 s. Using argon as ambient gas, the highest peel strength over 20 N was obtained. In the conference, surface property and morphology of films treated in different ambient conditions will be discussed.     

    

Effect of Biasing Voltage on Fiber-Form Nanostructured Tungsten Formation by Collisional Helium Arc Plasma Irradiation

Recently, it has been found that fiber-form nanostructures are formed on metal surfaces such as tungsten (W) by irradiation with helium (He) plasma. We are conducting experiments to produce fiber-form nanostructures on W surfaces by He arc discharge plasma irradiation under a gas pressure of 5 kPa. The biasing voltage (V_b) applied between the W substrate and the vacuum vessel was varied from -52 to -242 V to control the incident energy E_i of He ions to the W substrate. Under such high neutral gas pressure, collisions between ions and neutral particles in the sheath cannot be neglected and E_i has a distribution. The results show that as V_b increases, the diameter of the W fiber at the top surface of the fiber-form nanostructure layer becomes smaller. Furthermore, surface blackening due to the nanostructure formation appeared more quickly when V_b was increased. These results suggest that the thickness of the fiber-form nanostructured layer increased with increasing in V_b. Although He ions with E_i above the sputtering threshold of W were also irradiated at V_b = -162 and -242 V, it is suggested that the growth of fiber-form nanostructures is dominant over sputtering erosion in the present experiment.

    

Dependence of ground-state NH radical fluorescence in atmospheric-pressure pulsed-arc plasma jet on operating gas composition

We are developing our original nitriding technology with the atmospheric-pressure pulsed-arc plasma jet. Since atmospheric-pressure plasmas do not require vacuum equipment, the processing time becomes shorter, the capital cost becomes lower, and the operation becomes easier than those of the conventional low-pressure plasma nitriding. These advantages show the potential to realize high-mix low-volume production technology. However, the mechanism of nitriding has not yet been elucidated. In this experiment, the laser-induced fluorescence observation of ground-state NH radicals in the atmospheric-pressure plasma jet plume was performed in order to investigate the elementary process of NH radicals and the correlation between NH density and nitriding efficiency. We succeeded in detecting the fluorescence of NH radicals at 336 nm and 337 nm by irradiating the jet plume with a pulsed laser of ca. 305 nm, where a mixed gas of nitrogen and hydrogen is used as the operating gas. We found that increasing the hydrogen fraction in the operating gas from 0 to 0.5% increased the fluorescence intensity of NH, while increasing it from 0.5% to 5% decreased the fluorescence intensity. This tendency is qualitatively consistent with the nitriding efficiency of the plasma jet for steel surface.   

Low-temperature nitrocarburizing by pulsed-DC discharge of N2-H2-C2H2 for surface engineering of austenitic stainless steel

Nitrocarburizing is widely applied for case hardening and improved corrosion resistance of steels. The operational temperature of the process is above 550 ^oC causing the formation of ε-Fe2-3N, γ'-Fe4N, carbides of Fe, and nitrides of alloying elements present in the steel which provide the surface with greater hardness and improved corrosion resistance. When the process is applied to stainless steels, the temperature causes Cr₂N, CrN, and Cr carbides to form, enhancing the surface hardness but compromising corrosion resistance. So low-temperature nitrocarburizing process utilizing plasma is developed at 400 ^oC which forms S-phase and no Cr compounds. Gas mixtures of N₂-H₂-C₂H₂ with varied % N₂ were utilized to create pulsed-DC discharges and the processing durations were varied. The plasmas and the time durations tailored the surface properties differently and a surface property characterization using Vicker's microhardness indentation, XRD, XPS, SEM, Glow Discharge Optical Emission Spectrometry (GDOES) to measure concentration-depth profiles along the S-phase layers, and potentiodynamic polarization testing for corrosion study are displayed in the presented work. Optical emission spectroscopy of the discharges showed the presence of N₂⁺, N₂^*, CH-, and CN- species.    

Initial growth of graphene on copper foil in non-equilibrium atmospheric pressure remote plasma CVD

Graphene has attracted much attention due to its excellent properties.

Initial growth of graphene on copper foil in microwave-excited non-equilibrium atmospheric pressure remote plasma (MN-APP) using a CH₄/H₂/He gas mixture has been investigated in this study.

Since the growth of graphene in the plasma enhanced chemical vapor deposition (PEVD) is driven by surface reactions of radicals generated by the plasma, it is important to identify the radicals in order to elucidate its growth process. Therefore, measurement of C₂ radicals generated in the MN-APP was also carried out using absorption spectroscopy.

At low CH₄ flow rate, the variation in graphene size at each process time was very small, although the variation increased with the increase in the process time at the high CH₄ flow rate. Therefore, at low CH₄ flow rate, the flux of precursors supplied from the MN-APP are relatively low and mainly consumed for the planar growth of graphene without inducing subsequent nucleation after the initial nucleation. However, the excess supply of precursors would induce subsequent nucleation after the initial nucleation.

In these conditions, the C₂ radical density was estimated to be less than the order of 10¹² cm^-3 by absorption spectroscopy.

    

Fabrication of Amorphous Carbon Nitride Films with High [N]/([N]+[C]) Ratios Using the Plasma Chemical Vapor Deposition of the Gas Mixture of C2H2 with N2 : The Possibility to obtain the [N]/([N]+[C]) Ratio of >0.5

Hydrogenated amorphous carbon nitride films with the high [N]/([N]+[C]) ratios of >0.5 were fabricated using the microwave (MW) plasma chemical vapor deposition (CVD) of the gas mixture of N₂ with a trace amount of C₂H₂. This gas mixture was excited by a MW discharge (2.45 GHz, 50-150 W). The partial pressures of them were in the ranges of 26-266 Pa and 2.7-13.3 Pa with the deposition time of 1 h. The X-ray photoeletron spectra (XPS) were measured for the films etched with the Ar-cluster ion beams. Under the deposition conditions of the partial pressures of N₂ and C₂H₂ of 26 Pa and 2.6 Pa, respectively, the [N]/([N]+[C]) ratios of the film surface (without etching) were 0.15-0.49, and those of the film inside (with etching) 0.16-0.55, both of which were the increasing functions of the MW power. The ratios of [N]/([N]+[C])>0.5 were obtained for the inside of the films formed under the conditions of the MW power of >100 W. From the decomvolution analysis of the high-resolution XPS profiles of the C(1s) and N(1s) spectra, the network structure of films was converted from C-C to C-N. As a result, the [N]/([N]+[C]) ratios of >0.5 may be possible for the inside of the films obtained with suppressing the partial pressure of C₂H₂ compared with that of N₂ and with the MW power of >100 W.

    

Machine learning-based prediction of process conditions in atmospheric-pressure microwave plasma reactor from plasma images

Atmospheric-pressure plasma is thermal and has been used to decompose gases with high global warming potential and strong chemical bonds based on its high temperature and abundance of radicals. These process gases were in typical diluted with inert molecular nitrogen prior to the decomposition process, and oxygen-containing species such as molecular oxygen and water were added to prevent the decomposed species from recombination into the original gases. The concentration and flowrate of the process gas vary in time and it is necessary to monitor these process conditions to operate the system optimally. In this study, we proposed a method that predicts the process conditions of process gas concentration and flowrate from a plasma plume image using a convolutional neural network. The type of the plasma used is the microwave plasma and methane and molecular oxygen were selected as the process gas and reaction agent, respectively. Both the concentration of the process gas and the total flowrate were predictable to within ±2% of their full operating ranges with 95% accuracy (i.e., ±100 ppm and ±2 slpm, respectively).   

    

Hot carrier dynamics in LSPR tuneable plasmonic TiN at the interface of p and n type semiconductors

The field of plasmonics has grown tremendously in the last decade and with the progress of this field, new materials with some exceptional plasmonic properties need to be found out to replace conventional plasmonic materials such as Au and Ag. In this work, we have successfully synthesized a very promising alternative plasmonic material, Titanium Nitride (TiN) by magnetron sputtering, with the tuning of LSPR from visible to NIR region. Stoichiometry as well as the morphology & distribution of the synthesized nanoparticles are found to be the major factors behind this tuning. To understand the hot carrier dynamics of TiN plasmon at the interface of both the p-type as well as n-type semiconducting medium, two different devices are fabricated with the synthesized TiN samples. We have shown that both the hot holes and hot electrons can be useful for photocurrent generation if the TiN fermi level gets the favorable energy band alignment with the associated semiconductor. The photo-electrical characterization of the devices is carried out and the charge transport mechanism is thoroughly studied.

    

Separating Critical Materials using an Electromagnetic Centrifuge

Rare earth elements and platinum group elements (REE/PGEs) are required to produce sustainable energy technologies and drive a transition to renewable energy sources. However, the vast majority of the world's REE/PGEs are imported from China and are subject to supply chain instabilities. Current commercial extraction of REE/PGEs from ores uses chemical solvents which produce hazardous waste, have high operational costs, and result in low extraction rates and efficiencies due to the chemical similarities between REE/PGEs. An electromagnetic centrifuge (EMC) extracts REE/PGEs without these concerns. Instead, extraction is driven by the molecular weight and ionic charge of REE/PGEs. EMCs act as an all-in-one platform capable of both extracting REE/PGEs from unconventional sources such as coal fly ash and separating the ensuing components. Unlike a traditional centrifuge, an EMC enables high separation efficiency as electromagnetic forces drive high rotational velocities. Operation in a continuum regime trades separation efficiency and high vacuum requirements for higher yields and lower vacuum requirements. Separation results using two-component gas mixtures of Nitrogen, Argon, and Helium will be presented alongside variational studies with respect to pressure, RF power, DC voltage, and radial and axial sampling positions to detail the relevant physics and tuning parameters. Additionally, the platform's ability to separate multi-component materials will be demonstrated using Zirconium Silicate with SEM imaging and emission and Raman spectroscopy diagnostic data. Lastly, a techno-economic analysis is provided that outlines the commercial viability of the technology.