Session MW6: Poster session II

Negative ion formation in potassium-adenine collisions

We have devoted experimental studies to time-of-flight negative ion formation in electron transfer experiments from neutral potassium atoms with neutral adenine molecules$^{\mathrm{1}}$. Total partial cross sections have been obtained as a function of the collision energy, together with branching ratios for the most relevant fragment anions. Additional set of measurements in adenine derivatives have been performed in order to probe the role of negative ions as well as to probe whether site- and bond-selective excision is also a prevalent mechanism within electron transfer in atom-molecule collision experiments.   

Electron cross-sections and transport in liquids and biomolecules

Modelling of electron induced processes in plasma medicine and radiation damage is reliant on accurate self-consistent sets of cross-sections for electrons in tissue.~ These cross-sections (and associated transport theory) must accurately account not only the electron-biomolecule interactions but also for the soft-condensed nature of tissue.~ In this presentation, we report on recent swarm experiments for electrons in gaseous water and tetrahydrofuran using the pulsed-Townsend experiment, and the associated development of self-consistent cross-section sets that arise from them.~ We also report on the necessary modifications to gas-phase cross-sections required to accurately treat electron transport in liquids.~ These modifications involve the treatment of coherent scattering and screening of the electron interaction potential as well as the development of a new transport theory to accommodate these cross-sections.~ The accuracy of the ab-initio cross-sections is highlighted through comparison of theory and experiment for electrons in liquid argon and xenon.   

QDB: Validated Plasma Chemistries Database

One of most challenging recurring problems when modelling plasmas is the lack of data. This lack of complete and validated datasets hinders research on plasma processes and curbs development of industrial Applications [1]. We will describe the QDB project which aims to fill this missing link by provide a platform for exchange and validation of chemistry datasets. The database will collate published data on both electron scattering and heavy particle reactions and also facilitates and encourages peer-to-peer data sharing by its users. This data platform is rigorously supported by the validation methodical validation of the datasetsan automated chemistry generator employed; this methodology identifies missing reactions in chemistries which although important are currently unreported in the literature and employs mathematical methods to analyze the importance of these chemistries. Gaps in the datasets are filled using in house theoretical methods.\\ \\$[1]$.J. Kushner {\&}k. Bartschat (2016) proc. Nat. Acad. Sci. (In press)   

Electron-impact excitation and ionization of boron.

We present a comprehensive study of electron collisions with neutral boron atoms [1]. The calculations were performed with the $B$-Spline $R$-matrix (close-coupling) method [2], by employing a parallelized version of the associated computer code [3]. Elastic, momentum-transfer, excitation, and ionization cross sections were obtained for all transitions involving the lowest 11 states of boron, for incident electron energies ranging from threshold to 100 eV. A multi-configuration Hartree-Fock method with non-orthogonal term-dependent orbitals was used to generate accurate wavefunctions for the target states. Close-coupling expansions including 13, 51, and 999 physical and pseudo-states were set up to check the sensitivity of the predictions to variations in the theoretical model. The cross-section dataset generated in this work is expected to be the most accurate one available today and should be sufficiently comprehensive for most modeling applications involving neutral boron. \par\noindent [1] K. Wang, O. Zatsarinny, and K. Bartschat, \par\noindent ~~~~Phys. Rev. A~{\bf 93} (2016)~052715. \par\noindent [2] O. Zatsarinny and K. Bartschat, J. Phys. B {\bf 46} (2013) 112001. \par\noindent [3] O. Zatsarinny, Comp. Phys. Commun. {\bf 174} (2006) 273.   

Vibrational excitation and vibrationally resolved electronic excitation cross sections of positron-H$_2$ scattering

Vibrational excitation and vibrationally resolved electronic excitation cross sections of positron-H$_2$ scattering have been calculated using the single-centre molecular convergent close-coupling (CCC) method. The adiabatic-nuclei approximation was utilized to model the above scattering processes and obtain the vibrationally resolved positron-H$_2$ scattering length. As previously demonstrated [1], the CCC results are converged and accurately account for virtual and physical positronium formation by coupling basis functions with large orbital angular momentum. Here vibrationally resolved integrated and differential cross sections are presented over a wide energy range and compared with previous calculations and available experiments. [1] M. C. Zammit $et$ $al$. J. Phys. Conference Series $\textbf{635}$, 012009 (2015).   

CCC calculated differential cross sections of electron-H$_2$ scattering

Recently we applied the molecular convergent close-coupling (CCC) method to electron scattering from molecular hydrogen H$_2$ [1]. Convergence of the major differential cross sections has been explicitly demonstrated in the fixed-nuclei approximation. A large close-coupling expansion that coupled highly excited states and ionization channels proved to be important to obtain convergent results. Here we present benchmark elastic and electronic excitation differential cross sections for $b^3 \Sigma^+_u$, $a^{3} \Sigma^{+}_g$, $c^{3} \Pi_u$, $B^{1}\Sigma^{+}_u$, $EF^{1}\Sigma_g^{+}$, $C^{1}\Pi_u$, and $e^{3}\Sigma^{+}_u$ states and compare with available experiment and previous calculations. \newline [1] M. C. Zammit et al. Phys. Rev. Lett. accepted (2016)   

Elastic Electron Scattering from Hexafluoropropene

Low energy, experimental and theoretical elastic electron scattering differential cross sections (DCS) are presented. The experimental DCSs are obtained at incident electron energies from 0.5eV to 20eV and for scattering angles from 10 to 130 degrees using the relative flow method with helium as a reference standard. Our model uses the multi-channel Schwinger method with polarization effects included.   

Study of inelastic e-Cd and e-Zn collisions

Electron-photon coincidence experiments are well known for providing more detailed information about electron-atom collision than any other technique [1-3]. The Electron Impact Coherence Parameters (EICP) values obtained in such studies deliver the most complete characterization of the inelastic collision and allow for a verification of proposed theoretical models [4]. We present the results of Stokes and EICP parameters characterising electronic excitation of the lowest singlet P-state of cadmium and zinc atoms for various collision energies [5-7]. The experiments were performed using electron-photon coincidence technique in the coherence analysis version. The obtained data are presented and compared with existing CCC [8] and RDWA [9] theoretical predictions. [1] D. Dyl et al 1999 J. Phys. B: At. Mol. Opt. Phys. 32, 837–844 [2] M. Piwinski et al 2006 J. Phys. B: At. Mol. Opt. Phys. 39, 1945–1953 [3] L. Klosowski et al 2009 Phys. Rev. A 80, 062709 [4] N. Andersen et al 1988, Phys. Rep. 165, 1–188 [5] M. Piwinski et al 2002 J. Phys. B: At. Mol. Opt. Phys. 35, 3821–3827 [6] M. Piwinski et al 2012 Phys. Rev. A 86, 052706 [7] M. Piwinski et al 2015 Phys. Rev. A 91, 062704 [8] M. Berrington et al 2012 Phys. Rev. A 85, 042708 [9] T. Das et al 2014 Phys. Lett. A 378, 641   

Efficient thermoelectric trap for metal vapours suitable for high-vacuum system

Atomic beams are widely used in various collisional experiments [1-3]. Typically, cold traps are used to prevent the investigated atoms from spreading within the vacuum chamber and contaminating the system. Usually such a trap consists of a vacuum feedthrough with metal element cooled with liquid nitrogen or dry ice on the atmosphere side and a metal trap in the vacuum [4]. Using liquid nitrogen or dry ice is relatively inconvenient due to high costs of operation and a need of periodically refilling the reservoir of the cold medium. We present a new thermoelectric cold trap composed of water--cooled vacuum feedthrough with Peltier modules placed at the high vacuum end. The present system ensures the cold trap temperature below -20$^{\circ}$C, low enough to efficiently catch the atoms of interest. The new cold trap was characterised and compared with typical LN$_{2}$ trap [2,5]. [1] S. Napier et al. Phys. Rev. A 79, 042702 (2009) [2] M. Piwinski et al. Phys. Rev. A 91, 062704 (2015) [3] B.P. Marinkovic et al. Rad. Phys. and Chem. 76, 455 (2007) [4] M. Piwinski et al. Phys. Rev. A 86, 052706 (2012) [5] M. Piwinski et al. J. Phys. B: At. Mol. Opt. Phys. 35, 3821–3827 (2002)   

Coulomb crystal as a detector in electron impact ionization experiment

Ensembles of ions in trap at sufficiently low temperature can form a structure called Coulomb crystal. Some species of such ions can be optically cooled and observed using CCD camera. Number of ions composing the crystal can be determined with high accuracy. Other, invisible species of ions can be sympathetically cooled and detected indirectly by observation of their influence on visible ones. Thus, the efficiency of ionization processes leading to Coulomb crystal formation can be determined. We present preliminary results for electron-impact-ionized molecules forming a multi-species Coulomb crystal in a linear segmented Paul trap together with atomic calcium ions.   

Calculations of non-coplanar ionization of helium

Nixon et al [1] have measured the triple differential cross sections for electron ionization of the noble gases in the case where the direction of the incident electron is perpendicular to the plane containing the outgoing electrons which have equal energies. Miller et al [2] have carried out non-relativistic distorted-wave Born approximation calculations in these cases. In preparation for a study of all of these cases we have carried out calculations of the ionization of helium using a relativistic distorted-wave model. The evaluation of the relativistic distorted waves representing the outgoing electrons is based on a program [3] which produces relativistic coulomb waves which has been modified to take account of the finite size of the helium nucleus. The calculations are based on an integral equation approach as given in [4] and an asymptotic correction has been applied to account for the integration over an infinite interval. Convergence in the sum over partial waves has been obtained and a preliminary evaluation of the explicit inclusion of post-collision interaction has been carried out. [1] K. L. Nixon, A J. Murray and C. Kaiser, J. Phys. B 43 085202 (2010) [2] F. K. Miller, H. R. J. Walters and C. T. Whelan, PRA 91 012706 (2015) [3] F. Salvat, J. M. Fernandez-Varea and W. Williamson, Jr. CPC 90 151 (1995) [4] T. Zuo, R. P. McEachran and A. D. Stauffer, JPB 24 2853 (1991)   

A non-iterative treatment of the non-local exchange terms in the Complex Optical Potential method

Non-local exchange terms in atomic scattering equations are usually treated iteratively. This method normally works well but there can be problems with convergence, either requiring a large number of iterations or converging to a spurious value. It has long been known [1] that these terms can be treated non-iteratively but at the cost of expanding the number of equations needed to be solved. With the vastly increased memory and speed of modern computers, this approach is now feasible even for heavier targets. We have decided to implement this method in our calculations of electron elastic scattering from atoms using the Complex Optical Potential (COP) method [2] which is based on the relativistic Dirac equations. This method accounts for incident flux lost to the elastic channels through inelastic processes (excitation and ionization) via the imaginary part of the optical potential and also provides a value for the total cross section for these processes. The basis for the method will be given along with sample calculations where the iterative method fails. [1] Marriott R, Proc. Phys. Soc. A \textbf{70} 288 (1957), \textbf{72} 121 (1958) Marriott R 1958 Proc. Phys. Soc. A 72 [2] Chen S, McEachran R P and Stauffer A D, J. Phys. B: At. Mol. Opt. Phys. \quad \textbf{41 }025201 (2008)   

An out-of-plane ($e,2e$) study of He autoionization from 80 to 488 eV incident energies

We report out-of-scattering-plane $(e,2e)$ measurements on helium $2\ell2\ell'$ auto\-ionizing levels for 80, 120, 150, and 488eV incident electron energies, and scattering angles 60$^\circ$, 45$^\circ$, 39.2$^\circ$, and 20.5$^\circ$, respectively. The kinematics are the same in all cases: ejected electrons are detected in a plane that contains the momentum transfer direction and is perpendicular to the scattering plane, and the momentum transfer is 2.1 a.u.. The 80eV results complete our sets of measurements at low, intermediate,\footnote{http://meetings.aps.org/link/BAPS.2015.DAMOP.Q1.123} and high,\footnote{B.A. deHarak, K. Bartschat, and N.L.S. Martin, Phys. Rev. Lett. \textbf{ 100}, 063201 (2008)} incident energies. The results are presented as $(e,2e)$ angular distributions energy-integrated over each level, and are compared with our theory calculated for 488eV incident electron energy. The 120eV, 150eV and 488eV experiments are characterized by recoil peaks appropriate to each autoionizing level. However, for the 80eV angular distributions, these recoil peaks are largely absent for all levels, including the $^3P$ level observed at this energy.   

Convoluted Quasi Sturmian basis for the two-electron continuum.

In the construction of solutions for the Coulomb three-body scattering problem one encounters a series of mathematical and numerical difficulties, one of which are the cumbersome boundary conditions the wave function should obey. We propose to describe a Coulomb three-body system continuum with a set of two-particle functions, named Convoluted Quasi Sturmian (CQS) in [1]. They are built using recently introduced Quasi Sturmian (QS) functions [2] which have the merit of possessing a closed form. Unlike a simple product of two one-particle functions, by construction, the CQS functions look asymptotically like a six-dimensional outgoing spherical wave. The proposed CQS basis is tested through the study of the double ionization of helium by high-energy electron impact in the framework of the Temkin-Poet model [3]. An adequate logarithmic-like phase factor is further included in order to take into account the Coulomb interelectronic interaction and formally build the correct asymptotic behavior when all interparticle distances are large. With such a phase-factor (that can be easily extended to take into account higher partial waves) rapid convergence of the expansion can be obtained. [1] Zaytsev A S, Ancarani L U and Zaytsev S A (2016) Eur. Phys. J. Plus 131, 48. [2] Del Punta J A et al (2014) J. Math. Phys. 55, 052101. [3] Gasaneo G et al (2013) Phys. Rev. A 87, 042707.   

Electron impact ionization--excitation of Helium.

We calculate triple differential cross sections (TDCS) for the process of ionization--excitation of Helium by fast electron impact in which the residual ion is left in the n$=$2 excited state. We chose the strongly asymmetric kinematics used in the experiment performed by Dupr\'{e} et al. [1]. In a perturbative scheme, for high projectile energies the four--body problem reduces to a three--body one [2] and, within that framework, we solve the time- independent Schr\"{o}dinger equation with a Sturmian approach [3]. The method, based on Generalized Sturmian Functions (GSF), is employed to obtain the initial ground state of Helium, the single-continuum state and the scattering wave function; for each of them, the GSF basis is constructed with the corresponding adequate asymptotic conditions. Besides, the method presents the following advantage: the scattering amplitudes can be extracted directly in the asymptotic region of the scattering solution, and thus the TDCS can be obtained without requiring a matrix element evaluation. [1] C. Dupr\'{e} et al. (1992) J. Phys. B 25, 259. [2] M.J. Ambrosio et al. (2014) Phys. Rev. A 89, 012713. [3] G. Gasaneo et al. (2013) Adv. Quantum Chem. 67, 153.   

Ionization of small molecules by electron impact: a Sturmian approach.

The Sturmian approach [1], using Generalized Sturmian Functions (GSF), has been applied successfully to study (e,3e) and ($\gamma $,2e) processes in helium. A first extension of the method to molecular systems has been developed for the study of single photoionization [2,3]. In this contribution, we use the tool to look at ionization by electron impact of small molecules. In particular, we are interested in (e,2e) processes on CH$_{\mathrm{4}}$, NH$_{\mathrm{3}}$ and H$_{\mathrm{2}}$O under sufficiently asymmetrical kinematical conditions as to ignore exchange between the two escaping electrons. Within a single active electron approximation, we solve the time-independent, first-order perturbative, Schr\"{o}dinger equation, expanding the scattering wave function in a GSFs basis set. The adequate asymptotic behavior of all basis elements allows us to extract the transition amplitudes directly from the expansion coefficients, without requiring any evaluation of a matrix element. From the amplitudes, we calculate triply differential cross sections (TDCSs) and compare them with theoretical and relative experimental data. [1] G. Gasaneo et al, Adv. Quantum Chem. 57, 153 (2013). [2] C. M. Granados--Castro et al, Adv.Quantum Chem. 73, 3 (2016). [3] C. M. Granados-Castro, Ph.D. thesis, Universit\'{e} de Lorraine, Metz (2016).   

Double ionization of helium by impact of fast protons.

In comparison with the electron impact case, fully differential cross sections for the double ionization of helium by proton impact have been little investigated. The reasons are quite simple: experimentally, the measurement requires a long time as the count rates are very low; theoretically, the full four-body problem poses a formidable challenge. The present theoretical investigation is a contribution towards understanding this process. We performed [1] ab initio first Born calculations for proton impinging with 6 MeV, in the experimental configuration investigated in [2]. We solve a three-body scattering driven equation with the Generalized Sturmian Functions method [3]. Using the asymptotic behavior of the solution, we extract directly the transition matrix and thus the corresponding fully differential cross section. A detailed comparison with the relative experimental data will be presented at the conference. [1] Ambrosio M J et al (2015) Phys. Rev. A 92, 042704. [2] Fischer D et al (2003) Phys. Rev. Lett. 90, 243201. [3] Gasaneo G et al (2013) Adv. Quantum Chem. 57, 153   

Single- and two-center interference effects in fully differential cross sections for dissociative capture in 75 keV p H$_{\mathrm{2}}$ collisions

We have measured fully differential cross sections (FDCS) for dissociative capture in 75 keV p $+$ H$_{\mathrm{2\thinspace }}$collisions for coherent and incoherent projectiles. Data were obtained for a kinetic energy release of 1 eV, so that only vibrational excitation in the H$_{\mathrm{2}}^{\mathrm{+}}$ ion contributes to dissociation. The FDCS were analyzed for various molecular orientations (relative to the transverse momentum transfer direction q$_{\mathrm{x}})$ as a function of the scattering angle. In the ratio between the coherent and incoherent FDCS for a molecular orientation perpendicular to q$_{\mathrm{x}}$ a structure was observed, which is due to single-center interference. For a parallel orientation a structure of different shape is observed, which is due to a combination of single- and two-center interference. Furthermore, the data provide an additional independent confirmation that differences between coherent and incoherent cross sections are not merely due to the experimental resolution.   

X-ray emission in charge-exchange ion-atom and ion-molecule collisions

Charge exchange between highly-charged ions in solar winds and cometary neutrals is the primary process in many observed radiation phenomena in astrophysical environments. The present work examines this process using the two-center basis generator method within the independent electron model. We consider single capture in C$^{6+}$-He collisions from low to intermediate impact energies and benchmark our results with measurements by Defay \textit{et al}. [1,2]. We also consider single and multiple capture in O$^{6+}$-Ar collisions, where impact energies more closely resemble solar wind speeds (approximately from 1.17 to 2.33 keV/u). These results are compared with measurements and calculations by Machacek \textit{et al}. [3]. An outlook on collision-induced radiative processes involving ion-molecule collision systems will be given as well. [1] X. Defay \textit{et al.}, Phys. Rev. A \textbf{88}, 052702 (2013). [2] A.C.K. Leung and T. Kirchner, Phys. Rev. A \textbf{93}, 052710 (2016). [3] J.R. Machacek \textit{et al.}, Astrophys. J. \textbf{809}, 75 (2015).   

Fragmentation of methane molecules by antiproton impact

Extending previous work for proton impact [1], we have investigated the fragmentation of methane molecules due to collisions with antiprotons in the 25 keV to 5 MeV impact energy range. The multi-center nature of the problem is addressed by using a spectral representation of the molecular Hartree-Fock-level Hamiltonian and a single-center expansion of the initially populated molecular orbitals. The two-center basis generator method (TC-BGM) is used for orbital propagation. Electron-removal cross sections obtained from the TC-BGM solutions are complemented with a dynamical decay-route fragmentation model [2] to calculate cross sections for the production of fragment ions. Good agreement with the available experimental data [3] is observed for CH$_4^+$, CH$_3^+$, CH$_2^+$ and CH$^+$. [1] A. Salehzadeh and T. Kirchner, J. Phys. Conf. Ser. \textbf{635}, 032077 (2015). [2] H. Luna \textit{et al.}, J. Phys. B \textbf{36}, 4717 (2003). [3] H. Knudsen \textit{et al.}, J. Phys. B \textit{28}, 3569 (1995).   

Spin-density functional theory treatment of He$^{+}$-He collisions

The He$^{+}$-He collision system presents an interesting challenge to theory. On one hand, a full treatment of the three-electron dynamics constitutes a massive computational problem that has not been attempted yet; on the other hand, simplified independent-particle-model based descriptions may only provide partial information on either the transitions of the initial target electrons or on the transitions of the projectile electron, depending on the choice of atomic model potentials. We address the He$^{+}$-He system within the spin-density functional theory framework on the exchange-only level. The Krieger-Li-Iafrate (KLI) approximation is used to calculate the exchange potentials for the spin-up and spin-down electrons, which ensures the correct asymptotic behavior of the effective (Kohn-Sham) potential consisting of exchange, Hartree and nuclear Coulomb potentials. The orbitals are propagated with the two-center basis generator method. In each time step, simplified versions of them are fed into the KLI equations to calculate the Kohn-Sham potential, which, in turn, is used to generate the orbitals in the next time step. First results for the transitions of all electrons and the resulting charge-changing total cross sections will be presented at the conference.   

An independent atom model description of ion-molecule collisions including geometric screening corrections: application to biomolecules

Recently, we proposed to calculate electron removal cross sections for ion-molecule collisions in an independent atom model that accounts for geometric screening corrections [1]. The correction coefficients are obtained from using a pixel counting method (PCM) for the exact calculation of the effective cross sectional area that emerges when the molecular cross section is pictured as a structure of (overlapping) atomic cross sections. This structure varies with the relative orientation of the molecule with respect to the projectile beam direction and, accordingly, orientation-independent total cross sections are obtained from averaging the pixel count over many orientations. In this contribution, we apply the PCM to proton collisions from amino acids and DNA and RNA nucleobases. The strength of the screening effect is analyzed by comparing the PCM results with Bragg additivity rule cross sections and with experimental data where available. [1] H.J. L\"udde \textit{et al.}, Eur. Phys. J. D \textbf{70}, 82 (2016).   

Path Integral Approach to Atomic Collisions

The Path Integral technique is an alternative formulation of quantum mechanics that is based on a Lagrangian approach. In its exact form, it is completely equivalent to the Hamiltonian-based Schr\"{o}dinger equation approach. Developed by Feynman in the 1940's, following inspiration from Dirac, the path integral approach has been widely used in high energy physics, quantum field theory, and statistical mechanics. However, only in limited cases has the path integral approach been applied to quantum mechanical few-body scattering. We present a theoretical and computational development of the path integral method for use in the study of atomic collisions. Preliminary results are presented for some simple systems. Ultimately, this approach will be applied to few-body ion-atom collisions.   

Determination of Collisional Quenching Rate Coefficient of N$_2$(A$^3\Sigma_u^+)$ by H$_2$O

The effective lifetimes of metastable excited molecule N$_2$(A$^3 \Sigma_u^+$) in N$_2$/10.2ppm H$_2$O and N$_2$/103ppm H$_2$O mixtures were measured by waveform analysis$^{(1)}$ of the transient ionization current after interruption of the initial electron from the cathode in the Townsend discharge region. The collisional quenching rate coefficient of N$_2$(A$^3 \Sigma_u^+$) by H$_2$O was determined together with the diffusion coefficient of N$_2$(A$^3 \Sigma_u^+$) in nitrogen and the reflection coefficient of N$_2$(A$^3\Sigma_u^+$) at the cathode surface with the procedure based on the diffiusion equation analysis$^{(2)}$. The obtained collisional quenching rate coefficient of N$_2$(A$^3 \Sigma_u^+$) by H$_2$O is 5.7$\times$10$^{-13}$ cm$^3$/s. This value is ten times as large of the value reported by Callear and Wood$^{(3)}$. (1) S. Suzuki, H. Itoh, H. Sekizawa and N. Ikuta, J. Phys. Soc. Jpn., 62, No.8, 2692-2697 (1992) (2) S.Suzuki and H.Itoh, J. Phys D: Appl. Phys., 49,185202(14pp) (2016) (3) A. B. Callear and P. M. Wood, Trans. Faraday Soc., 67, 598-600 (1971)   

Excess Thermal Energy In The Cell Loaded With Mixture of Ni Powder And Li[AlH$_{\mathrm{\mathbf{4}}}$\textbf{]}

Interest has significantly increased in the study of Low-Energy Nuclear Reactions. Especially after the publication of the test results from Andrea Rossi's high-temperature heat source that operates on a mixture of Nickel powder and Lithium Aluminum Hydride. Initial experiments showed that the reaction is unstable; occurring in a narrow temperature range (in practice, it is outside the melting temperature of the fuel materials). In this work we describe the design of the heat generator, the calorimetric method for measuring the amount of heat energy. The results demonstrate excess heat during heating the powder mixture of Ni and Li[AlH$_{\mathrm{4}}$] to temperatures ranging from 1030---1140 \textdegree C. The generator did not expose an X-ray photographic emulsion.   

Ternary recombination of H$_{3}^{+}$, H$_{2}$D$^{+}$, HD$_{2}^{+}$, and D$_{3}^{+}$ with electrons in He/Ar/H$_{2}$/D$_{2}$ gas mixtures

The temperature dependence of the ternary recombination rate coefficients of H$_{2}$D$^{+}$ and HD$_{2}^{+}$ ions has been studied in the temperature range of 80--150 K at pressures from 500 to 1700 Pa in a stationary afterglow apparatus equipped with a cavity ring-down spectrometer. Neutral gas mixtures consisting of He/Ar/H$_{2}$/D$_{2}$ (with typical number densities $10^{17}/10^{14}/10^{14}/10^{14}$ cm$^{-3}$) were employed to produce the desired ionic species and their fractional abundances were monitored as a function of helium pressure and the [D$_{2}$]/[H$_{2}$] ratio of the neutral gas. In addition, the translational and the rotational temperature and the ortho to para ratio were monitored for both H$_{2}$D$^{+}$ and HD$_{2}^{+}$ ions. A fairly strong pressure dependence of the effective recombination rate coefficient was observed for both ion species, leading to ternary recombination rate coefficients close to those previously found for (helium assisted) ternary recombination of H$_{3}^{+}$ and D$_{3}^{+}$.\footnote{R. Johnsen \textit{et al.}, J. Phys. Chem. \textbf{117}, 9477 (2013).}   

Stationary-Afterglow measurements of dissociative recombination of H$_{2}$D$^{+}$ and HD$_{2}^{+}$ ions

Binary recombination rate coefficients of H$_{2}$D$^{+}$ and HD$_{2}^{+}$ ions have been measured at a temperature of 80 K in an afterglow plasma experiment in which the fractional abundances of H$_{3}^{+}$, H$_{2}$D$^{+}$, HD$_{2}^{+}$, and D$_{3}^{+}$ ions were varied by adjusting the [D$_{2}$]/([D$_{2}$] $+$ [H$_{2}$]) ratio of the neutral gas. The fractional abundances of the four ion species during the afterglow and their rotational states were determined in situ by continuous-wave cavity ring-down absorption spectroscopy (CRDS), using overtone transitions from the ground vibrational states of the ions. The experimentally determined recombination rate coefficients will be compared to results of advanced theoretical calculations and to the known H$_{3}^{+}$ and D$_{3}^{+}$ recombination rate coefficients. We conclude that the recombination coefficients depend only weakly on the isotopic composition. Astrophysical implications of the measured recombination rate coefficients will be also discussed.   

Computational Study on Dissociation Properties of C$_{\mathrm{4}}$F$_{\mathrm{6}}$ Molecules

Saturated or unsaturated perfluorocarbons(PFCs) have been used extensively in dry etching processes due to their relatively low global warming potential and their high CF$_{\mathrm{2}}$ radical levels in commercial plasma processes. Many experimental and theoretical studies of these species have been performed for useful information about physical and chemical properties of PFCs. Recently, it was reported that the $\omega $B97X-D/aVTZ method is strongly recommended as the best practical density functional theory(DFT) for rigorous and extensive studies of PFCs because this theoretical level shows the high performance and reliability especially for van der Waals interactions. Among various PFCs, this study focuses on C$_{\mathrm{4}}$F$_{\mathrm{6}}$ molecules including c-C$_{\mathrm{4}}$F$_{\mathrm{6}}$, 1,3-C$_{\mathrm{4}}$F$_{\mathrm{6}}$, and 2-C$_{\mathrm{4}}$F$_{\mathrm{6}}$ isomers. All the feasible isomerization and dissociation paths of C$_{\mathrm{4}}$F$_{\mathrm{6}}$ molecules were investigated mainly at the $\omega $B97X-D/aVTZ level. Their reaction rate constants were computed by using variational transition-state theory for a deep insight into C$_{\mathrm{4}}$F$_{\mathrm{6}}$'s reaction mechanism. Fates and roles of C$_{\mathrm{4}}$F$_{\mathrm{6}}$ molecules and their fragments in plasma phases could be explained based on our theoretical results and data.   

Transport coefficients of He$^{\mathrm{+}}$ ions in helium

New experimental mobilities of $^{\mathrm{4}}$He$^{\mathrm{+}}$ in $^{\mathrm{4}}$He at 298.7 K, as a function of E/N, have been determined. Uncertainties in the mobilities were reduced to about 1{\%} by using a shuttered drift tube. Comparison with previously measured values show that only one set of previous data is reliable. We demonstrate that the mobilities and diffusion coeffcients of $^{\mathrm{4}}$He$^{\mathrm{+}}$ in $^{\mathrm{4}}$He can be calculated over wide ranges of E/N with high precision if accurate potential energy curves are available for the X$^{\mathrm{2}}\Sigma_{\mathrm{u}}^{\mathrm{+}}$ and A$^{\mathrm{2}}\Sigma_{\mathrm{g}}^{\mathrm{+}}$ states, and if one takes into account resonant charge transfer and corrects for quantum-mechanical effects. Potentials, obtained by extrapolation of results from d-aug-cc-pVXZ (X$=$6,7) basis sets using the CASSCF$+$MRCISD approach were found to be in exceptionally close agreement with the best potentials available (separately) and with experiment, and those were subsequently used in a new computer program to determine semi-classical phase shifts and transport cross sections, from which the gaseous ion transport coefficients are determined. A new set of data for the mobilities of alpha particles (He$^{\mathrm{2+)}}$ ions was obtained as a byproduct of the experiment, but the transport theory has not yet been completed.   

Transport Parameters For Positive IONS In Pure H$_{\mathrm{2}}$O DC Discharge

Transport properties of positive ions originating from H$_{\mathrm{2}}$O (H$_{\mathrm{2}}$O$^{\mathrm{+}}$, OH$^{\mathrm{+}})$ in DC fields and at the room temperature were calculated by using Monte Carlo simulation technique. Initially, the relevant cross section sets were assessed by using Denpoh-Nanbu theory for resolving between elastic and reactive collision events and then resolving contribution of exothermic processes from available experimental data. Newest experimentally or theoretically determined cross sections were compiled and included wherever possible. \quad We present transport coefficients for low and moderate reduced electric fields $E/N $ ($N$-gas density) accounting for non-conservative processes.   

Third order transport coefficients for electrons and positrons in gases

Third order transport coefficients (the skewness tensor) of the electron and positron swarms, in atomic and molecular gases, are investigated. The knowledge of the skewness tensor is necessary for the conversion of the hydrodynamic transport coefficients to the arrival time and steady-state Townsend transport data as well as for the determination of the deviations of the spatial density profiles from an ideal Gaussian. In this work, we investigate the structure and symmetries along individual elements of the skewness tensor by the group projector method. Individual components of the skewness tensor are calculated using a Monte Carlo simulation technique and multi term theory for solving the Boltzmann equation. Results obtained by these two methods are in excellent agreement. We extend previous studies by considering the sensitivity of the skewness components to explicit and implicit effects of non-conservative collisions, post-ionization energy partitioning, and inelastic collisions. The errors of the two term approximation for solving the Boltzmann equation are highlighted. We also investigate the influence of a magnetic field on the skewness tensor in varying configurations of electric and magnetic fields. Among many interesting points, we have observed a strong correlation between the skewness and diffusion.   

Transport properties of electrons and transition of an electron avalanche into a streamer in atomic liquids

A Monte Carlo simulation technique is developed and used to calculate transport coefficients of electron swarms in non-polar atomic liquids. We employ the two model processes in which only momentum and energy are exchanged, respectively, to account for structure dependent coherent elastic scattering at low energies. The validity of the code is confirmed by comparison with results of previous authors. We apply two scenarios for higher energy cross sections. In the first scenario excitations in the liquid phase are approximated by excitations in the gas phase. In the second scenario excitations are completely neglected. Ionization threshold is reduced to values which are suggested in the literature, in both scenarios. Transport coefficients in these two scenarios, as well as transport coefficients for gas and liquid phases are compared. Special attention has been given to the structure induced negative differential conductivity (NDC), which has been observed both in this work, and in previous publications. Spatially-resolved electron transport properties are calculated in order to understand this phenomenon. The important aspect of this work is modeling of the transition of an electron avalanche into a streamer. Calculations are performed using 1D and 1.5D fluid models. Streamer properties in scenarios with and without excitations are compared.   

Monte Carlo simulations of electron transport in strongly attaching gases

Extensive loss of electrons in strongly attaching gases imposes significant difficulties in Monte Carlo simulations at low electric field strengths. In order to compensate for such losses, some kind of rescaling procedures must be used. In this work, we discuss two rescaling procedures for Monte Carlo simulations of electron transport in strongly attaching gases: (1) discrete rescaling, and (2) continuous rescaling. The discrete rescaling procedure is based on duplication of electrons randomly chosen from the remaining swarm at certain discrete time steps. The continuous rescaling procedure employs a dynamically defined fictitious ionization process with the constant collision frequency chosen to be equal to the attachment collision frequency. These procedures should not in any way modify the distribution function. Monte Carlo calculations of transport coefficients for electrons in SF$_{\mathrm{6}}$ and CF$_{\mathrm{3}}$I are performed in a wide range of electric field strengths. However, special emphasis is placed upon the analysis of transport phenomena in the limit of lower electric fields where the transport properties are strongly affected by electron attachment. Two important phenomena arise: (1) the reduction of the mean energy with increasing E/N for electrons in SF6, and (2) the occurrence of negative differential conductivity in the bulk drift velocity of electrons in both SF$_{\mathrm{6}}$ and CF$_{\mathrm{3}}$I.   

Electron transport in mercury vapor: magnetic field effects, dimer induced NDC and multi-term analysis

A multi term theory for solving the Boltzmann equation and Monte Carlo simulation technique are used to investigate electron transport in varying configurations of electric and magnetic fields in mercury vapor. Using different sets of cross sections for electron scattering in mercury as an input in our Boltzmann and Monte Carlo codes, we have calculated data for electron transport as a function of reduced electric and magnetic fields. A multitude of kinetic phenomena in electron transport has been observed and discussed using physical arguments. In particular, we discuss two important phenomena: (1) for certain values of electric and magnetic field, we find regions where swarm mean energy increases with increasing magnetic field for a fixed electric field, and (2) the occurrence of negative differential conductivity (NDC) for higher pressures and temperatures. In particular, NDC is induced by the presence of mercury dimers. The measured drift velocities agree very well with our Monte Carlo results only if the superelastic collisions are included in our calculations. Spatially-resolved electron transport properties are calculated using a Monte Carlo simulation technique in order to understand these phenomena.   

Dependence of ion drift velocity and diffusion coefficient in parent gas on its temperature.

The results of Monte Carlo calculations of the ion drift characteristics are presented: ions of noble gases and Ti, Fe, Co, Cs, Rb, W and mercury ions in case of constant and uniform electric field are considered. The dependences of the ion mobility on the field strength and gas temperature are analyzed. The parameters of the drift velocity approximation by the Frost formula for gas temperatures of 4.2, 77, 300, 1000, and 2000 K are presented. A universal drift velocity approximation depending on the reduced electric field strength and gas temperature is obtained. In the case of strong electric fields or low gas temperatures, the deviation of the ion distribution function from the Maxwellian one (including the shifted Maxwellian one) can be very significant. The average energies of chaotic motion of ions along and across the electric field can also differ significantly. It is analyzed the kinetic characteristics of ion drift in own gas: ion diffusion coefficient along the field and across the field; thermal spread of velocities (temperature) along the field and across the field. The unexpected and nontrivial fact takes place: collision with backscattering represent only 10-50{\%} of the total number of collisions. This calculation can be used when analyzing experiments with dusty plasma under cryogenic discharge, ultracold plasma.   

On the role of helium molecules in atmospheric pressure discharges

Despite their intrinsic simplicity, helium plasma kinetics are still not fully understood and quantitatively described. This is particularly the case at high pressures when various molecular helium species (i.e. ions, excimer(s) and Rydberg states) are formed. In this contribution, the absolute density of helium Rydberg molecules is measured for the first time by a combination of laser photo-ionization and Thomson scattering experiments. The experiments are performed on a parallel plate, nanosecond pulsed, DC discharge at 700 mbar. The results are combined with electron and helium metastable densities measurements and compared with a kinetic model of the discharge. The source of He$_{\mathrm{2}}$ molecules in the discharge and afterglow phases are identified and discussed. The present experimental data and kinetic model solve several inconsistencies between reaction paths proposed in the literature.   

IST-LISBON database with LXCat: Electron scattering cross sections for oxygen and carbon dioxide

This work proposes sets of electron scattering cross sections compiled for kinetic energies up to 1 keV, as part of the IST-LISBON database with LXCat, for both molecular and atomic oxygen and for carbon dioxide. The complete and consistent sets of cross sections for O$_{\mathrm{2}}$, O and CO$_{\mathrm{2}}$ are validated using the two-term Boltzmann solver embedded in LoKI (LisbOn KInetics) numerical code to calculate swarm parameters, yielding fairly good agreement with the available experimental data. It is evidenced that the inclusion of rotational transitions in O$_{\mathrm{2}}$ and superelastic collisions with CO$_{\mathrm{2}}$(010) molecules is essential to reproduce the experimental values of the swarm parameters for $E/N$\textless 1 Td. Further improvement can be achieved by deconvolution of the current vibrational excitation cross sections and/or the inclusion of additional vibrational excitation channels, which would also contribute to improve our knowledge of O$_{\mathrm{2}}$ and CO$_{\mathrm{2}}$ plasmas.   

The electron heating mode transition by the change of driving frequency in atmospheric pressure dielectric barrier discharges

Over the past twenty years, micro plasma~technology including dielectric barrier discharges (DBDs) brought great enhancement of stable and high density plasma sources in atmospheric pressure environment. However, the experimental diagnostics are difficult to use in atmospheric pressure micro plasmas, and thus the particle-in-cell (PIC) simulation is a good tool to investigate the nonlinear and kinetic effects of the plasma dynamics. In this study, PIC simulation results show that time-dependent parameters compare well with theoretical estimates like energy diffusion theory in the RF frequency ranges up to 500 MHz~in atmospheric pressure plasmas for a set of controllable input parameters. Here, alpha-gamma heating mode transition is observed when the driving frequency matches the maximum of energy relaxation frequency by electron impact excitation. The inflection point in a semi-log scaled electron energy probability function (EEPF) is also explained by energy diffusion theory, which corresponds to a transition point of heating mode. Moreover, it was found that extra results in low gas pressure have the same solution at lower input frequency. For this reason, temporal differential term generates non-stationary EEPF in a specific energy range in Boltzmann kinetics.   

Azimuthal ExB drift of electrons induced by the radial electric field flowing through a longitudinal magnetic channel with non-magnetized ions

To examine of the effect of the radial electric field on the azimuthal electron motion under $E\times B$ field for plasmas with magnetized electrons and non-magnetized ions, an experimental study is conducted by a stationary plasma flow. The argon plasma flow is generated by a DC arc generator under atmospheric pressure, followed by a cw expansion into a rarefied gas-wind tunnel with a uniform magnetic field $\sim 0.16$ T. Inside one of the magnets, we set a ring electrode to apply the radial electric field. We applied an up-down probe for the analysis of the electron motion, where one of the tips is also used as a Langmuir probe to measure electron temperature, density and the space potential. We found that the order of the radial electric field is about several hundred V/m, which should be caused by the difference in the magnetization between electrons and ions. Electron saturation current indicates the existence of the $E\times B$ rotation of electrons, whose order is about $2000 - 4000$ m/s. The order of the observed electron drift velocity is consistent with the theoretical value calculated from the applied magnetic field and the measured electric field deduced from the space potential.   

Statistical Physics of Electron Temperature of Low-Pressure Discharge Nitrogen Plasma with Non-Maxwellian EEDF

We reconsider electron temperature of non-equilibrium plasmas on the basis of thermodynamics and statistical physics. Following our previous study on the oxygen plasma in GEC 2015, we discuss the common issue for the nitrogen plasma. First, we solve the Boltzmann equation to obtain the electron energy distribution function (EEDF) $F(\epsilon)$ of the nitrogen plasma as a function of the reduced electric field $E/N$. We also simultaneously solve the chemical kinetic equations of some essential excite species of nitrogen molecules and atoms, including vibrational distribution function (VDF). Next, we calculate the electron mean energy as $U=\langle\epsilon \rangle=\int_0^\infty \epsilon F(\epsilon) \mathrm{d}\epsilon$ and entropy $S=-k\int_0^\infty F(\epsilon)\ln [F(\epsilon)] \mathrm{d} \epsilon$ for each value of $E/N$. Then, we can obtain the electron temperature as $T_\mathrm{e}^\mathrm{stat} = [\partial S/\partial U]^{-1}$. After that, we discuss the difference between $T_\mathrm{e}^\mathrm{stat}$ and the kinetic temperature $T_\mathrm{e}^\mathrm{kin} \equiv (2/3) \langle\epsilon\rangle$, as well as the temperature given as a slope of the calculated EEDF for each value of $E/N$. We found $T_\mathrm{e}^\mathrm{stat}$ is close to the slope at $\epsilon \sim 4$ eV in the EEPF.   

Inter-comparison of calculation techniques of the electron Boltzmann equation for the analysis of swarm parameters in CO$_{\mathrm{2}}$

The plasma-based CO$_{\mathrm{2}}$ conversion is a promising route for achieving the reduction of fossil fuel consumption and of CO$_{\mathrm{2}}$ emission. An accurate description of the electron kinetics by solving the electron Boltzmann equation (EBE) is necessary for this application. This work is dedicated to the inter-comparison between various calculation techniques of the EBE (two-term, multi-term and space gradients of the electron density) and the Monte-Carlo reference technique for the analysis of swarm parameters and their comparison with previously available and present experimental data. We adopt the complete set of electron--impact cross sections for CO$_{\mathrm{2}}$, to be published on the IST-LISBON database with LXCat. Results show that despite the fact that the IST-LISBON cross sections were derived to fit measured swarm parameters when used in a two-term expansion Boltzmann code, good agreement with the other solution and simulation techniques is generally obtained for the electron swarm parameters under consideration.   

Effects Of Electrons and Heavy Particles On Halpha Emission In Pure H$_{\mathrm{2}}$O DC Discharge At High E/N (E-Electric Field, N-Gas Density)

In this work we present results of Monte Carlo simulations for spatially resolved emission due to the transport of electrons and heavy particles (fast H, H$^{\mathrm{+}}$, OH$^{\mathrm{+}}$, H$_{\mathrm{2}}$O$^{\mathrm{+}}$,H$_{\mathrm{3}}$O$^{\mathrm{+}})$ in pure H$_{\mathrm{2}}$O for the conditions used in plasma assisted technologies. Monte Carlo technique, already used for similar discharges in nitrogen, argon and hydrogen is used to obtain spatially resolved Halpha emission in H$_{\mathrm{2}}$O. Data for anisotropic scattering of electrons, ions and fast neutrals are used to obtain contribution to Halpha spatially resolved emission. Agreement with experimental data for drift velocities for all charged particles and effective electron ionization for the conditions of moderate E/N allowed us to study production of heavy particles and subsequently spatial emission as a consequence of their transport.   

Numerical Investigation of Propagation and Decay of Fast Ionization Waves Generated by Nanosecond Pulsed Discharge

Fast ionization waves (FIW) are an effective tool for studying plasma kinetics in nanosecond pulsed discharges. A numerical investigation of FIWs in air having high energy deposition was conducted in capillary tubes having different diameters using a two dimensional model. Continuity equations for charged and neutral species, the electron energy equation and Poisson's equation were implicitly integrated together with a propagator model for photoionization which includes both ionizing and non-ionizing absorption. The species and reactions included in the study were selected on the basis of a sensitivity analysis. The main goals of this work are to quantify how system parameters (e.g., pressure, voltage and specific energy deposition) affect the properties of the plasma in the early afterglow (tens to hundreds of nanoseconds) following the FIW.   

Three-body recombination and dynamics of electrons and excited states in the low-pressure argon afterglow

The afterglow phase occurs naturally during the power-off period in pulsed low-pressure plasmas and in atmospheric pressure ns discharges. During that period the electron energy rapidly declines and the charged particles are lost due to diffusion and recombination. In low-pressure discharges the dominant process is three-body recombination (TBR) of Ar$^{+}$ ions with electrons. It leads to complex dynamics of the excited states, dominated by collisional-radiative cascades that eventually repopulate the metastable states. In this contribution the afterglow dynamics of an argon discharge is analyzed in detail to elucidate the roles played by the various processes. An analytical model for the fast drop of the electron energy by evaporative cooling and electron-ion collisions is combined with a time-dependent collisional radiative model for the atomic excited states that numerically solves the electron energy and density balance equations. By including further gas heating and cooling, the model leads to excellent agreement with experiments utilizing different diagnostic techniques, and hence gives insight into the interplay of the various processes in the afterglow \footnote{Ts V Tsankov, R Johnsen, and U Czarnetzki, \textit{PSST} \textbf{24} (2015) 065001.}   

The effect of realistic surface coefficients on the electron dynamics and process control in simulations of capacitive RF plasmas

In most PIC simulations of capacitively coupled plasmas (CCPs), only an ion induced constant secondary electron emission coefficient, $\gamma$, is used, which is usually guessed to be 0.1. Similarly, a constant electron reflection coefficient, $\rho$, is typically used and assumed to be 0 - 0.2. Here, we utilize an ion and atom induced energy-dependent $\gamma$ for ''dirty'' and ''clean'' surfaces in a single frequency 13.56 MHz CCP. Its effects on electron heating mode transitions are analyzed as a function of pressure. By utilizing the same energy-dependent $\gamma$, its effects on the separate control of the ion flux and mean ion energy are studied for a dual frequency (df) CCP with frequencies of 2 and 27 MHz. The results are compared to multiple simulations with constant $\gamma$ and significant differences are found. Finally, $\rho$ is varied in a single frequency CCP and its effects on plasma parameters such as the sheath width and electron density are studied. Strong effects of using different reflection probabilities at both electrodes on the discharge symmetry are found. A df CCP is modeled to understand the coupling between the electrical asymmetry effect (EAE) and a discharge asymmetry induced by different electron reflection coefficients for the two electrodes.   

Second harmonic wave generation from a nonlinear combination of volume wave and overdense plasma in negative permeability space

We clarify the relation between second harmonic wave (SH wave) and plasma generation in various experimental conditions by detecting properties of propagating electromagnetic waves (EM waves). Plasma has a nonlinear reaction against EM wave, generating harmonic waves which depends on electron density $n_{\mathrm{e}}$. In the case with increased $n_{\mathrm{e}}$, EM wave comes to be prevented from going into plasma with negative permittivity $\varepsilon_{\mathrm{p}}$. Double-split-ring resonators (DSRRs), one of metamaterials, make permeability $\mu _{\mathrm{D}}$ negative. We have shown that EM wave being volume wave can propagate into the combination of overdense plasma and DSRRs because of real negative value refractive index $N$. In our previous paper [1], we have confirmed enhanced SH wave (4.9 GHz) generation in the composite with 2.45-GHz input. In this report, we show the dependence of the SH wave emission with plasma generation on plasma parameters and gas conditions of plasma. Furthermore, we show the phase change with $N$ variation of the composite space in the case with various input power as the proof of the negative index state. [1] A. Iwai, Y. Nakamura, and O. Sakai, Phys. Rev. E, 92 (2015) 033105.   

Thermoelectric instability of radio-frequency discharges

Spatio-temporal instabilities (``striations'') are a ubiquitous feature of direct current discharges, understood to be closely linked to the drift of electrons towards the anode. Similar phenomena are observed in high-frequency discharges, but cannot be understood in the same way, because the drift is absent. Explanations in terms of nonlinear ionization mechanisms, such as multi-step ionization, have been suggested but appear to conflict with some observations. In this work we suggest that a usually neglected transport process may be partly or wholly responsible for these instabilities. This process is known as the thermoelectric effect, which is commonly included in the energy transport equation for semiconductor charge carriers, but not usually in moment models of low-temperature plasmas. An investigation of the the moment equations shows that such an instability is possible, and can be observed in simulations.   

Analysis of heavy particle processes in low current dc discharge in water vapor

Results presented in our recent paper [1] show that heavy particles -- positive ions and fast neutrals (created in charge transfer processes) -- can have significant contribution to the processes of excitation at moderate and high reduced electric fields ($E/N)$. In the case of water vapor, hydrogen ions and fast atoms are the most probable candidates, as the lightest products in water vapor discharges. In order to identify dominant heavy species in water vapor discharge, we analyzed discharge parameters in low current Townsend regime. Based on the model developed by Phelps and coworkers in 1993. [2] we were able to estimate transit time of ions from experimentally determined frequency of damped oscillations and parameters of electrical circuit. Furthermore, we compared calculated transit times with transit times of hydrogen ions (H$^{\mathrm{+}}$, H$_{\mathrm{2}}^{\mathrm{+}}$, H$_{\mathrm{3}}^{\mathrm{+}})$ [3]. Initial analysis indicates that H$_{\mathrm{2}}^{\mathrm{+}}$ is dominant ion in the range of moderate $E/N$ (\textasciitilde 2~kTd). Calculations were done for the discharge initiated at electrode gap of 1.1 cm and pressure ($p)$~x~gap ($d)$ of 0.6~Torrcm, which corresponds to the conditions of the minimum of Paschen curve. In the next step we will extend the analysis to wider range operating conditions. [1] Sivo\v{s} J \textit{et al J. Phys. }D\textit{: Appl. Phys}. \textbf{48 }(2015) 424011; [2] Petrovi\'{c} Z Lj \textit{et al} \textit{Phys. Rev}. E 47 (1993) 2825; [3] Phelps A V, \textit{J. Phys. Chem. Ref. Data} 19 (1990) 653   

Instabilities in Hall $\bf E \times \bf B$ discharges with complex magnetic field configurations.

Hall plasmas with electron ${\bf E} \times {\bf B}$ drift exhibit wide range of unstable modes affecting operation and performance of various devices, e.g. such as magnetrons and Hall thrusters. The plasma density, magnetic field and temperature gradients are important source of various gradient-drift and lower-hybrid instabilities across wide range of spatial and temporal scales. The electron response is critically sensitive to the electron dynamics along the magnetic field which has a characteristic frequency scale $k_{\Vert }v_{Te}$. The motion across magnetic field occurs on a slower time scale due to $E\times B$ drift/magnetic drift and/or electron inertial/collisional drifts. The ratio of the transverse to the parallel times scale affects electron dynamics and change the conditions and nature of the instabilities in a significant manner. The nonlinear fluid 3D model has been developed that takes into account both perpendicular and parallel dynamics. Few illustrative examples are considered for the instabilities in various regimes and results of nonlinear simulations in 3D geometries are presented.   

Quantitative Defect Analysis of PAN-based Carbon Fibers Treated by Single and Dual HF RF-CCP

This work states the effects of single (40.68 MHz) and dual (40.68/2.1 MHz) RF-CCPs on defect structure of the PAN-based carbon fibers. The fibers were treated between two identical aluminum electrodes with R$\sim $200 mm in a 78.5 L stainless steel cylindrical reactor (R$\sim $500 mm, H$\sim $400 mm). The gap distance was 4 cm. In SRF mode, P$_{\mathrm{HF}}=$50-200 W, p$=$0.3, 0.5, 0.75 and 1 Torr, t$=$30, 60 and 90 min. In DRF mode, P$_{\mathrm{LF}}=$50-200 W, p$=$ 0.1-0.9 Torr and t$=$15, 30, 45 and 60 min at fixed P$_{\mathrm{HF}}=$50 W. The structural analyses of the treated fibers were done by using high sensitive confocal Raman spectroscopy and the surfaces were excited by 532 nm-100 mW He-Ne (2.33 eV) laser. The average defect size and density of the treated fibers were calculated according to the following formulas; L$_{\mathrm{D\thinspace }}$(size) $=$ (1.8\texttimes 10$^{\mathrm{-9}}\lambda _{\mathrm{L}}^{\mathrm{4}}$I$_{\mathrm{G}}$/I$_{\mathrm{D}})^{\mathrm{\thinspace 1/2}}$ and n$_{\mathrm{D}}$ (density) $=$ (1.8\texttimes 10$^{\mathrm{22}}$/$\lambda_{\mathrm{L}}^{\mathrm{4}})$ \texttimes I$_{\mathrm{D}}$/I$_{\mathrm{G\thinspace }}$where $\lambda_{\mathrm{L}}$ is the laser wavelength, I$_{\mathrm{D}}$ is the intensity of D-band (\textasciitilde 1350 cm$^{\mathrm{-1}})$ and I$_{\mathrm{G\thinspace }}$is the intensity of G-band (\textasciitilde 1580 cm$^{\mathrm{-1}})$.   

Numerical and analytic results showing the suppression of secondary electron emission from velvet and foam, and a geometric view factor model to guide the development of a surface to suppress SEE.

The technique of suppressing secondary electron emission (SEE) from a surface by texturing it is developing rapidly in recent years. We have specific and general results in support of this technique: We have performed numerical and analytic calculations for determining the effective secondary electron yield (SEY) from velvet, which is an array of long cylinders on the micro-scale, and found velvet to be suitable for suppressing SEY from a normally incident primary distribution. We have performed numerical and analytic calculations also for metallic foams, which are an isotropic lattice of fibers on the micro-scale, and found foams to be suitable for suppressing SEY from an isotropic primary distribution. More generally, we have created a geometric weighted view factor model for determining the SEY suppression of a given surface geometry, which has optimization of SEY as a natural application. The optimal surface for suppressing SEY does not have finite area and has no smallest feature size, making it fractal in nature. This model gives simple criteria for a physical, non-fractal surface to suppress SEY. We found families of optimal surfaces to suppress SEY given a finite surface area.   

Computational study of sheath structure in oxygen containing plasmas at medium pressures.

Plasma mixtures containing active species are used in many plasma-assisted material treatment technologies. The analysis of such systems is rather difficult, as both physical and chemical processes affect plasma properties. A combination of experimental and computational approaches is the best suited, especially at higher pressures and/or in chemically active plasmas. The first part of our study of argon-oxygen mixtures was based on experimental results obtained in the positive column of DC glow discharge. The plasma was analysed by the macroscopic kinetic approach which is based on the set of chemical reactions in the discharge. The result of this model is a time evolution of the number densities of each species. In the second part of contribution the detailed analysis of processes taking place during the interaction of oxygen containing plasma with immersed substrates was performed, the results of the first model being the input parameters. The used method was the particle simulation technique applied to multicomponent plasma. The sheath structure and fluxes of charged particles to substrates were analysed in the dependence on plasma pressure, plasma composition and surface geometry.   

Optical emission spectroscopy of OH lines in N$_{\mathrm{2}}$ and Ar plasma during the treatments of cotton fabric

Low pressure non-equilibrium plasmas are proven to be irreplaceable tool in material processing. Among other fields their applications in treatments of textiles are still diversifying, but the main role of plasma is activation of the surface of treated sample. After, or during, the treatments these surfaces can be covered with different materials or species (such as microcapsules) that enhance properties of the fabric. In order to investigate mechanisms how active species from plasma interact with the cotton surface, we studied both plasma and surface properties. Bleached cotton samples were treated in low-pressure nitrogen and argon plasma in a chamber with parallel-plate electrodes. The effect of the plasma treatment on the cotton samples was investigated with the colorimetric measurements on dyes absorption by a spectrophotometer. Optical emission spectroscopy was performed by using spectrometer with a sensitive CCD camera. We have recorded the evolution of the maximum of the intensity of OH and N$_{\mathrm{2}}$ second positive band lines. Measurement were done with and without samples in the chamber and comparison between the lines intensity was made. The parameters for optimal plasma treatment conditions were determined.   

An Investigation of the Role of Near-Anode Plasma Conditions on Anode Spot Self-Organization in Atmospheric Pressure DC Glows

In previous work, plasma self-organization patterns were experimentally observed on both liquid surface and metal anode surface in atmospheric pressure glows. However, the origin of the self-organized pattern formation is still poorly understood and is currently under study. In this work, it was observed that the discharge current is the dominant parameter controlling the onset of the self-organization of the plasma attachment on a liquid anode. On the other hand, it is observed that interelectrode spacing is the key parameter that controls plasma self-organization on metal anodes. Presented here are experiments aimed at understanding how these parameters control conditions at the anode surface which ultimately result in self-organization. Here we determine the effects of space charge at the anode surface and also estimate the anode fall voltage in response to discharge parameter variations. Additionally, electron microscopy is used to assess anode morphological changes resulting from the self-organization plasma attachments.   

Tomographic reconstruction for plasma diagnostic

A tomographic system originally designed for high intensity ion beam diagnostic to reconstructed 3D light density of photons produced by interactions between ion beam and residual gas has been developed and commissioned. In contrast to the classical approach where the beam is rotated in front of the camera, the camera was rotated around the beam and images were taken at multiple angles. The reconstructed light density of each voxel is proportional to the beam particle density at this point. With a inverse radon transformation these images can be transformed to 2D slices through the observed volume. Additionally, a maximum entropy algorithm was implemented due to the potentially very limited number of angles. This concept can be transferred to the diagnostics of classical optical thin plasma. If combined with a shutter camera a suitable monochromator or spectral filters, it should be possible to reconstruct a time and spectral dependent 3D volume of the plasma.   

High-resolution TALIF measurements of atomic oxygen: determination of gas temperature and collisional broadening coefficients

Two-photon Absorption Laser-Induced Fluorescence (TALIF) is a well-established technique to measure relative (and with appropriate calibration techniques, absolute) densities of atoms in plasmas and flames. The excitation line profiles can provide additional information, but this is usually overlooked due to the mediocre spectral resolution of commercial pulsed dye laser systems. We have investigated O-atom TALIF excitation line profiles using a house-built narrow line-width pulsed UV laser system, based on pulsed Ti:Sa ring laser seeded by a cw infrared diode laser. The observed Doppler profiles allow unambiguous measurement of gas temperature with high precision in O$_{\mathrm{2}}$ and CO$_{\mathrm{2}}$ DC glow discharges. Sub-Doppler measurements, performed by reflecting the laser beam back through excitation zone, allow the pressure-broadened line shapes to be observed, both in a pure O$_{\mathrm{2}}$ DC discharge (up to 10 Torr pressure) and in an atmospheric pressure RF plasma jet in He/O$_{\mathrm{2}}$. Pressure broadening coefficients of the 3p$^{\mathrm{3}}P_{J}$ state of O were determined for O$_{\mathrm{2}}$ and He bath gases, and were found to be an order of magnitude bigger than that predicted from the measured quenching rate.   

Dual frequency diffuse dielectric barrier discharge in atmospheric-pressure air-like gas mixture for thin film deposition

A dual frequency (DF) diffuse discharge was obtained in an atmospheric-pressure dielectric barrier discharge reactor in air-like gas mixtures. By adding a radio frequency (RF) voltage to a low frequency (LF) voltage, we aim to increase the plasma power density. In this study, the discussion is mainly focused on the discharge characteristics and the thin film deposition. According to the spatio-temporal emission, the discharge shows a glow-like structure with both LF and DF voltages. By fitting the spectral lines of the second positive system of N$_{\mathrm{2}}$, the gas temperature was estimated which does not obviously increase with the extra RF signal. Moreover, SiO$_{\mathrm{2}}$-like film was deposited from TEOS using the DF power supply. Thin film properties such as surface morphology, microstructure and stoichiometry were analyzed by AFM, FTIR and XPS, respectively. Because of the higher plasma power density, the DF power supply can be an efficient approach to improve the properties and to increase the throughput of the thin film deposition.   

Laser absorption velocimetry using an optical vortex beam

A plain-wave-like beam, or a Hermite-Gaussian mode, has been used for conventional laser spectroscopy. Since the Doppler shift in frequency of light absorbed by a moving atom is given by the dot product of the wave vector of the light beam and an atomic velocity, it is essentially a one-dimensional measurement. It has a merit that the interpretation of the result is clear and straightforward; however, it simultaneously poses a limitation that the measurable velocity component is confined to the projection along the wave vector. This limitation may be overcome by using an optical vortex beam, or a Laguerre-Gaussian mode, which has helical phase fronts associated with orbital angular momentum of light. Due to its three-dimensional phase structure, the Doppler shift for an atom moving in the optical vortex beam has three components. Therefore, the laser measurement method that has a sensitivity even for transverse motion across the beam is possible to be achieved. We have performed laser absorption measurements using optical vortex beams as a proof-of-principle experiment, where an additional frequency shift in the absorption spectra of metastable argon neutrals in a plasma has been observed. The details of experimental results will be discussed in the conference.   

Characterisation of a dielectric barrier surface twin discharge using defined gas mixtures

In the last decades extensive study has been performed on dielectric barrier discharges (DBDs) in several fields of applications of non-thermal atmospheric pressure plasmas. Their applicability ranges from health-promoting effects to the human skin to air decontamination combined with a rather good scalability [1,2]. Further insight into their physical and chemical properties is mandatory for a proper configuration of plasma sources for a given application. In our case a dielectric barrier surface twin discharge is ignited in different gas mixtures. The surface discharge electrode is made of an Al2O3 plate working as a dielectric barrier and grid-structured copper traces on each side of the plate. The electrode is connected to a HV-HF plasma generator with external transformer. The plasma parameters are determined via OES using an absolutely calibrated Echelle-spectrometer [3]. \textbf{References} [1] U. Kogelschatz, B. Eliasson, W. Egli, \textit{J. Phys. IV France}, \textbf{7}, C4-47-C4-66 (1997) [2] A. M. Vandenbroucke, R. Morent, N. De Geyter \textit{et al.}, \textit{Journal of Hazardous Materials}, \textbf{195}, 30-54 (2011). [3] N. Bibinov, H. Halfmann, P. Awakowicz \textit{et al.},\textit{ Measurement Science Technology}, \textbf{18}, 1327-1337 (2007).   

Characterization of a dielectric barrier discharge in controlled atmosphere

Non-thermal atmospheric-pressure plasmas are advantageous for various biomedical applications as they make a contact- and painless therapy possible [1]. Due to the potential medical relevance of such plasma sources further understanding of the chemical and physical impact on biological tissue regarding the efficacy and health-promoting effect is necessary. The knowledge of properties and effects offers the possibility to configure plasmas free of risk for humans. Therefore, tailoring the discharge chemistry in regard to resulting oxidative and nitrosative effects on biological tissue by adjusting different parameters is of growing interest. In order to ensure stable conditions for the characterization of the discharge, the used dielectric barrier discharge was mounted in a vessel. Absolutely calibrated optical emission spectroscopy was carried out to analyze the electron density and the reduced electric field [2]. The rather oxygen-based discharge was tuned towards a more nitrogen-based discharge by adjusting several parameters as reactive nitrogen species are known to promote wound healing [3]. Furthermore, the impact of an ozone-free discharge has to be studied [4]. References: [1] Emmert et al., Clin. Plas. Med., 1, 24-29 (2013) [2] Bibinov et al., Meas. Sci. Technol., 18, 1327-1337 (2007) [3] Heuer et al., Nitric Oxide, 44, 52-60 (2015) [4] Kogelheide et al., J. Phys D: Appl Phys., 49, 087004 (2016)   

Two-photon absorption laser induced fluorescence measurement of atomic oxygen density in an air atmospheric pressure plasma jet.

Two-photon Absorption Laser Induced Fluorescence (TALIF) is used to measure atomic oxygen number density [O] in an air Atmospheric Pressure Plasma Jet (APPJ). A novel technique based on photolysis of O2 is used to calibrate the TALIF system ensuring the same species (O) is probed during calibration and measurement. As a result, laser intensity can be increased outside the TALIF quadratic laser power region without affecting calibration reliability as any high intensity saturation effects will be identical for calibration and experiment. Higher laser intensity gives stronger TALIF signals helping overcome weak TALIF signals often experienced at atmospheric pressure due to collisional quenching. O2 photo-dissociation and two-photon excitation of the resulting [O] are both achieved within the same laser pulse. The photolysis [O] is spatially non-uniform and time varying. To allow valid comparison with [O] in a plasma, spatial and temporal correction factors are required. Knowledge of the laser pulse intensity I0(t), and wavelength allows correction factors to be found using a rate equation model. The air flow into the jet was fixed and the RF power coupled into the system varied. The resulting [O] was found to increase with RF power.   

Heated probe diagnostic inside of the gas aggregation nanocluster source.

Gas aggregation cluster sources (GAS) usually operate outside common working conditions of most magnetrons and the size of nanoparticles created in GAS is below that commonly studied in dusty plasmas. Therefore, experimental data obtained inside the GAS are important for better understanding of process of nanoparticles formation. In order to study the conditions inside the gas aggregation chamber, special ``diagnostic GAS'' has been constructed. It allows simultaneous monitoring (or spatial profiling) by means of optical emission spectroscopy, mass spectrometry and probe diagnostic. Data obtained from Langmuir and heated probes map the plasma parameters in two dimensions - radial and axial. Titanium has been studied as an example of metal for which the reactive gas in the chamber starts nanoparticles production. Three basic situations were investigated: sputtering from clean titanium target in argon, sputtering from partially pre-oxidized target and sputtering with oxygen introduced into the discharge. It was found that during formation of nanoparticles the plasma parameters differ strongly from the situation without nanoparticles. These experimental data will support the efforts of more realistic modeling of the process.   

Coherent Rayleigh-Brillouin scattering for in situ detection of nanoparticles and large molecules in gas and plasma

Laser-based diagnostics methods, such as Spontaneous and Coherent Rayleigh and Rayleigh-Brillouin scattering (SRBS and CRBS), can be used for in-situ detection and characterization of nanoparticle shape and size as well as their concentration in an inert gas atmosphere [1]. We recently developed and tested this advanced diagnostic at PPPL. It was shown that the signal intensity of the CRBS signal depends on the gas-nanoparticle mixture composition, density and the polarizabilities of the mixture components [2]. The measured results agree well with theoretical predictions of Refs [1,2]. In this work, we report the application of this diagnostic to monitor nucleation and growth of nanoparticles in a carbon arc discharge. In support of these measurements, A time-dependent density functional theory was used to compute the frequency-dependent polarizabilities of various nanostructures in order to predict the corresponding Rayleigh scattering intensities as well as light depolarization [3]. Preliminary results of measurements demonstrate that CRBS is capable to detect nanoparticles in volume. 1. M. N. Shneider, S. F. Gimelshein, Appl. Phys. Lett. 102, 173109 (2013). 2. A. Gerakis, M.N. Shneider, B.C. Stratton, submitted to Appl. Phys. Lett. (2016). 3. B. Santra, M. N. Shneider, R. Car , to be submitted   

Actinometry of Atomic Fluorine in a Capacitively Coupled Discharge with a mixture of SF$_6$ and O$_2$

Actinometry is a well-known technique for determining neutral species densities in reactive plasmas using optical emission spectroscopy. The basic idea is to compare the intensity of two emission lines, the first from a species of known density, and second from a species whose density is to be determined. Clearly, this approach succeeds only if the relationship between the relevant line intensities and the species densities is known. Actinometric measurement of atomic fluorine density is of special interest because alternative techniques, such as laser induced fluorescence, are impractically difficult for most purposes. In this paper we discuss actinometric measurement of fluorine densities in mixtures of SF$_6$ and O$_2$. In this system, the atomic neutral radical densities are a complex function of the gas mixture. Comparison of the actinometric data with mass spectrometric measurements shows that under many conditions they are proportional, so that ideal actinometric behaviour is observed. We show that this behaviour is predictable on the basis of a recently calculated excitation cross section for fluorine emission line. Hitherto, actinometry of fluorine has depended on difficult empirical calibration procedures.   

Registration of RF Plasma Radiation in Ultra-Violet Range by Solar-blind Photoreceptor

A spectrum response of a photoreceptor to the RF plasma radiation is determined in the present work by means of a spectrophotometer utilizing a gas-filled photoreceptor. A continuous radiation spectrum was observed in the wavelength interval of 190 -- 270 nm. The photoreceptor allows measuring of absolute radiation taking into account the spectral sensitivity of the photoreceptor and the values of quantum output for the given wavelength. A continuous spectrum was observed in all three orders of magnitude of diffraction. Develop and test a technique for measuring the intensity of the plasma radiation in the UV wavelength range measured amount of discharge pulses can be used to determine the spectral sensitivity range of UV radiation receivers.   

Microwave techniques for electron density measurements in low pressure RF plasmas

Results of the experimental studying of RF plasma jet at low pressure in the range of 10 - 300 Pa is presented. The electron density distribution both in inductive and in capacitive coupled RF discharges was measured at 1.76 MHz and 13.56 MHz consequently. We used three independent microwave diagnostic techniques such as free space (the "two-frequency" and "on the cut-off signal") and a resonator. It is found that the electron density in the RF plasma jets is by 1-2 orders of magnitude greater than in the decaying plasma jet, and by 1-2 orders of magnitude less than in the RF plasma torch. Thus the RF plasma jet is similar to the additional discharge between the electrodes or the coil and the vacuum chamber walls. As a consequence, the formation of the positive charge sheath near the specimen placed in plasma stream is observed. It is found that the maximum of ionization degree as well as more uniform electron density distribution across the stream is observed in the range of the gas flow rate $G_g = 0.06 - 0.12$~g/s and the discharge power $P_d = 0.5 - 2.5$~kW.   

Investigation of adaptive signal processing methods for denoised I-V curve of Langmuir probe

It is an important issue to obtain a clear second derivative of Langmuir probe I-V curve which involves the electron energy distribution function. Therefore, noise suppressions against random walk of charges are required in the experimental data. Proper numerical methods including fitting, digital smoothing, digital filtering with window function should be used to remove each types of noise to determine electron energy distribution. The calculation of electron energy distributions demands sequential algorithm of several numerical methods to reduce the noise in I-V curve. In this presentation, a new noise suppression method is suggested to achieve advanced Langmuir probe diagnostics. Combined utilization of nonlinear curve fitting and low pass filter with window function shows more precise results than the utilization of smoothing only. Therefore, results including noise analysis algorithm give new guideline of probe diagnostics.   

A self-consistent global model of surface wave discharges with cylindrical or co-axial structures: Ar or O2 fed with continuous or pulse-modulated power input

A series of cylindrical and co-axial surface wave discharges fed with either argon or oxygen is modelled by a zero-dimensional global modelling approach. Compared to a recent study of the cylindrical surface-wave discharges (Kemaneci et al 2015 J. Phys. D: Appl. Phys. 48 435203), a self-consistent estimation of the edge-to-center ratios are analytically defined and extended to include the co-axial structures. The simulation results are compared with the experimental data of the considered discharges for continuous and pulse-modulated power input and a good agreement is obtained.   

Monte Carlo simulation of radio-frequency breakdown in oxygen and air

Radio frequency discharges have been used extensively in the materials processing industry. Nevertheless, processes taking place during the breakdown are still poorly understood. Part of the reason is in large displacement current which prohibits measurements and leaves all to modelling. We have performed detailed simulations using Monte Carlo code, tested in our group, that allows also verification against RF and DC benchmarks and treatment of temporal spatial non-localities. This work contains simulation results of the breakdown voltage curves, Paschen curves. Background gases are oxygen and synthetic air. Electrons were initialized at the point in the middle of the gap and their distribution is evolved in time under the effect of the applied field through Monte Carlo approach. Results qualitatively agree with the available experimental and simulation results. In order to get better insight of the processes leading to the breakdown, spatial distribution of electron energy and concentration as well as rates of ionization and elastic scattering are discussed.   

Plasma Chemistry Reduction from ILDM

Numerical simulation of plasma models involving large number of species and reactions is computationally expensive. One of the solutions of this problem is to employ Chemical Reduction Techniques (CRT) used in combustion research. The CRT that we apply here is Intrinsic Low Dimensional Manifold (ILDM). ILDM uses the fact that the reaction system is not evenly sensitive to all the reactions, but some reactions are fast and attain steady state in a short interval of time. Based on this information, the ILDM method finds the lower dimensional space inside a complete state-space. After a short time interval the fast processes have relaxed and the densities evolve on a low dimensional manifold of the solution space. Construction of such a lower dimensional manifold allows the reaction space to be described in terms of fewer parameters and it becomes possible to tabulate the results in terms of those parameters. By generating a look-up table for given values of controlling parameters, the remaining parameters are computed explicitly. In this work we apply the ILDM method to argon containing 78 levels. The results are compared with the full simulation. The method is validated by comparison of the results with a traditional collisional radiative model.   

Heating dominated inception of pulsed discharges.

We simulate the inception of pulsed discharges with heating as the driving agent that leads to spark formation. To understand the phenomenon, we developed a 2D-cylindrically symmetric model that couples the electric discharge dynamics with the background gas dynamics. To capture the ion dynamics well, we reduced the classical drift-diffusion-reaction model of electric discharges to the timescale of ion motion. Additionally, we include secondary emission of electrons from the cathode. We employed the model to study electrical breakdown in air at STP conditions between planar electrodes under the application of pulsed voltages. Our model captures space-charge effects, thermal shocks and induced pressure waves. We observe a cycle of discharge pulses heating the gas and the thermal expansion helping the discharge. This cycle might either lead to spark formation or to discharge decay.   

Discontinuous model with semi analytical sheath interface for radio frequency plasma

Sumitomo Heavy Industries, Ltd. provide many products utilizing plasma. In this study, we focus on the Radio Frequency (RF) plasma source by interior antenna. The plasma source is expected to be high density and low metal contamination. However, the sputtering the antenna cover by high energy ion from sheath voltage still have been problematic. We have developed the new model which can calculate sheath voltage wave form in the RF plasma source for realistic calculation time. This model is discontinuous that electronic fluid equation in plasma connect to usual passion equation in antenna cover and chamber with semi analytical sheath interface. We estimate the sputtering distribution based on calculated sheath voltage waveform by this model, sputtering yield and ion energy distribution function (IEDF) model. The estimated sputtering distribution reproduce the tendency of experimental results.   

Block matrix based LU decomposition to analyze kinetic damping in active plasma resonance spectroscopy

``Active plasma resonance spectroscopy'' (APRS) is a widely used diagnostic method to measure plasma parameter like electron density. Measurements with APRS probes in plasmas of a few Pa typically show a broadening of the spectrum due to kinetic effects. To analyze the broadening a general kinetic model in electrostatic approximation based on functional analytic methods has been presented $[1]$. One of the main results is, that the system response function $Y(\omega)$ is given in terms of the matrix elements of the resolvent of the dynamic operator evaluated for values on the imaginary axis. To determine the response function of a specific probe the resolvent has to be approximated by a huge matrix which is given by a banded block structure. Due to this structure a block based LU decomposition can be implemented. It leads to a solution of $Y(\omega)$ which is given only by products of matrices of the inner block size. This LU decomposition allows to analyze the influence of kinetic effects on the broadening and saves memory and calculation time.\\ $[1]$ J. Oberrath and R.P. Brinkmann, Plasma Sources Sci. Technol. {\bf 23}, 045006 (2014).   

A Global PLASIMO Model for H2O Chemistry

Global warming is one of the critical contemporary problems for mankind. Transformation of CO$_2$ into fuels, like CH$_4$, that are transportable with the current infrastructure seems a promising idea to solve this threatening issue. The final aim of this research is to produce CH$_4$ by using microwave plasma in CO$_2$-\H$_2$O mixture and follow-up catalytic processes. In this contribution we present a global model for H$_2$O} chemistry that is based on the PLASIMO plasma modeling toolkit. The time variation of the electron energy and the species’ densities are calculated based on the source and loss terms in plasma due to chemical reactions. The short simulation times of such models allow an efficient assessment and chemical reduction of the H$_2$O chemistry, which is required for full spatially resolved simulations.   

A Robust Compressible Flow Solver for Studies on Solar Fuel Production in Microwave Plasma

n order to simulate the dissociation of CO$_2$ with H$_2$O admixture by microwave plasma for the production of solar fuels, we need a multicomponent solver that is able to capture the complex nature of the plasma by combining the chemistry, flow, and electromagnetic field. To achieve this goal, first we developed a robust finite volume compressible flow solver in C++. The solver is implemented in the framework of the PLASIMO software and will be used in complete plasma simulations later on. Due to the compressible nature of the solver, it can be used for simulation of dissociation of CO$_2$ with H$_2$O admixture by supersonic expansion in microwave plasmas. A spatially second order version of this solver is able to reveal the vortex flow structure of the plasmas. Capabilities of this solver are presented by benchmarking against well-established analytical and numerical test cases.   

Influence of SF6/O2 inductively coupled plasma parameters on Si etching rates and profiles

A hybrid 2D Monte Carlo---fluid model, called hybrid plasma equipment model (HPEM) developed by Kushner and co-workers is employed in the present investigation. The model calculates the electron and heavy particle densities, energies and fluxes by solving self-consistently a set of equations including the Maxwell equations, fluid equations, a Monte Carlo procedure or the Boltzmann equation. An additional module, called plasma chemistry Monte Carlo simulation, is used to calculate plasma species fluxes and energy distributions to the substrate to produce detailed information at the substrate level. An analytical module within this hybrid code is addressed to predict the etch rate based on the calculated fluxes and kinetic energies of the different plasma species arriving at the silicon wafer. A detailed SF6/O2 chemistry set is used to simulate the electron -- heavy particle collisions and a number of reactions. The influence of operating parameters on plasma characteristics, etch rates and profiles is investigated. It is aimed to understand how the etch process influences the contact hole local critical dimension uniformity.   

Verification of high performance two-dimensional particle-in-cell simulations of low-temperature plasmas

We discuss a two-dimensional implementation of the particle-in-cell algorithm with Monte Carlo collisions. This implementation is designed for multiprocessor environments in which each processor is assumed to offer vector capabilities and multiple execution threads. An appropriate implementation therefore combines OpenMP to exploit multithreading with MPI to coupled computing nodes. This approach promises to achieve accelerations of a least a factor of several hundred, relative to to a simple serial implementation. However, the complexity involved also offers many opportunities for error, and makes correctness demonstrations especially desirable. In this presentation we discuss the characteristics of this parallel implementation, and we describe a suite of verification tests that collectively create a strong presumption that the code is correct.   

Predicting ion flux uniformity at the ion extraction plate in a 3D ICP reactor

In order to achieve better control in processing the wafer surface, the ion fluxes in a remote plasma system are often focused through one or more ion extraction plates between the main plasma chamber and the downstream wafer plane. The ion extraction plates are typically of showerhead pattern with multiple holes. The focus of this particular study is to predict the ion flux uniformity over the ion extraction plate for a full 3D inductively coupled discharge reactor model using Argon chemistry. We will use the commercial modeling tool, CFD-ACE$+$, which can address such a process involving gas flow, heat transfer, plasma physics, reaction chemistry and electromagnetics in a coupled fashion. The plasma characteristics in the chamber and uniformity of the ion fluxes at ion extraction plate are discussed. Parametric studies varying the geometrical dimensions and process conditions to determine the effect on ion flux uniformity are presented. The showerhead-like ion extraction plate will be modeled as a porous media with a specified porosity. Further, a spatially varying porosity of the ion extraction plate is used to simulate ion recombination in order to reduce the ion flux non-uniformity. The goal is to optimize the system maximizing the ion flux while maintaining the uniformity.   

Fast multipole and space adaptive multiresolution methods for the solution of the Poisson equation

This work focuses on the conjunction of the fast multipole method (FMM) with the space adaptive multiresolution (MR) technique for grid adaptation. Since both methods, MR and FMM provide a priori error estimates, both achieve O($N$) computational complexity, and both operate on the same hierarchical space division, their conjunction represents a natural choice when designing a numerically efficient and robust strategy for time dependent problems. Special attention is given to the use of these methods in the simulation of streamer discharges in air. We have designed a FMM Poisson solver on multiresolution adapted grid in 2D. The accuracy and the computation complexity of the solver has been verified for a set of manufactured solutions. We confirmed that the developed solver attains desired accuracy and this accuracy is controlled only by the number of terms in the multipole expansion in combination with the multiresolution accuracy tolerance. The implementation has a linear computation complexity O($N$).   

Evaluation of the Aleph PIC Code on Benchmark Simulations.

Aleph is a massively parallel, 3D unstructured mesh, Particle-in-Cell (PIC) code, developed to model low temperature plasma applications. In order to verify and validate performance, Aleph is benchmarked against a series of canonical problems to demonstrate statistical indistinguishability in the results. Here, a series of four problems is studied: Couette flows over a range of Knudsen number, sheath formation in an undriven plasma, the two-stream instability, and a capacitive discharge. These problems respectively exercise collisional processes, particle motion in electrostatic fields, electrostatic field solves coupled to particle motion, and a fully coupled reacting plasma. Favorable comparison with accepted results establishes confidence in Aleph's capability and accuracy as a general purpose PIC code. Finally, Aleph is used to investigate the sensitivity of a triggered vacuum gap switch to the particle injection conditions associated with arc breakdown at the trigger. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.   

An Upwarming Effect in Rarefied RF Plasma Stream at Low Pressure

A mathematical model of the RF plasma flow at 13.3-133 Pa in transition regime at Knudsen $8 \times 10^{-3} \leq Kn \leq 7 \times 10^{-2}$ and the nozzle pressure ratio $n=10$ for the carrier gas is described. The model based on both the statistical approach to the neutral component of the RF plasma and the continuum model for electron and ion components. The results of plasma flow calculations performed both for the free flowing and for the sample overflowing at a prescribed electric field are described. The effect of a warming up of a stream in a mixture zone confirmed by comparison of numerical results with experimental ones is found.   

Afivo, a framework for the 2D/3D simulation of streamers and other discharges

Over the last couple of years we have been developing Afivo, a framework for the simulation of streamers and other discharges in 2D and 3D. The main features of Afivo are: adaptively refined quadtree/octree grids, a geometric multigrid solver, OpenMP parallelism, VTK and Silo output, and an open source license. We have recently focused on improving Afivo's documentation and we have added new examples, which are demonstrated on our poster.   

Particle charge distribution in dusty plasmas

Distribution of charge carried by a nano-sized particle in plasma is calculated using zero dimensional Monte Carlo simulations. In this work, several phenomena are taken into account, which place a limit on the negative charge that a nanoparticle can acquire. Electron depletion is seen in plasmas due to electronegativity or due to electron attachment to nanoparticles, and causes particle charge reduction. Secondary electron emission, photo emission place additional limitations on particle charge. Increased positive ion current to particle at higher pressures causes charge reduction [1]. Further, particle size and material dependent charge limits exist -- placing a limit on the amount of charge a particle can hold [2]. The effect of all these factors is studied in the context of nanoparticle charge distributions in conditions typically seen in deposition process plasmas. Charge distributions are obtained for a wide range of conditions. The relative importance of each phenomenon in giving rise to significant fractions of non-negative nanoparticles is demonstrated. [1] Gatti, M., Kortshagen, U., ``Analytical model of particle charging in plasmas over a wide range of collisionality,'' Physical Review E 78 (2008) 046402. [2] Le Picard, R., Girshick, S., ``The effect of single-particle charge limits on charge distributions in dusty plasmas,'' J. Phys. D: Appl. Phys. 49 (2016) 095201 (9pp).   

PIC-DSMC simulation of a triggered vacuum switch with a copper/beryllium cathode

Typical vacuum discharge simulations rely on the injection of neutral or ionized metal vapor from the cathode into an electrically stressed anode-cathode gap. Simultaneous electron emission, also from the cathode, allows for electron-impact ionization of the emitted metal vapor allowing for plasma formation and subsequent closing mechanism to begin. This work looks to analyze the effect of photoemission from the cathode and/or photoionization of metal vapor on the switch closing process through kinetic simulation techniques. A 500 micron anode-cathode gap is chosen with a variable voltage applied to the anode and a grounded half copper, half beryllium cathode. Injection of the metal vapor for both cathode materials is modeled as a linearly ramped flux with a temperature of 1500 K and a bulk velocity (13.2 km/s for Cu and 22 km/s for Be) away from the cathode. Electron-impact excitation of the emitted metal vapor allows for subsequent spontaneous emission of photons which can then photoionize the metal vapor or cause photoemission from the cathode.   

Parametric investigations of plasma characteristics in a remote inductively coupled plasma system

Designing a remote plasma system involves source chamber sizing, selection of coils and/or electrodes to power the plasma, designing the downstream tubes, selection of materials used in the source and downstream regions, locations of inlets and outlets and finally optimizing the process parameter space of pressure, gas flow rates and power delivery. Simulations can aid in spatial and temporal plasma characterization in what are often inaccessible locations for experimental probes in the source chamber. In this paper, we report on simulations of a remote inductively coupled Argon plasma system using the modeling platform CFD-ACE$+$. The coupled multiphysics model description successfully address flow, chemistry, electromagnetics, heat transfer and plasma transport in the remote plasma system. The SimManager tool enables easy setup of parametric simulations to investigate the effect of varying the pressure, power, frequency, flow rates and downstream tube lengths. It can also enable the automatic solution of the varied parameters to optimize a user-defined objective function, which may be the integral ion and radical fluxes at the wafer. The fast run time coupled with the parametric and optimization capabilities can add significant insight and value in design and optimization.   

Coulomb Collision for Plasma Simulations: Modelling and Numerical Methods

We are motivated to model weakly ionized Plasma applications. The modeling problem is based on an incorporated explicit velocity-dependent small-angle Coulomb collision terms into a Fokker-Planck equation. Such a collision is done with so called test and field particles, which are scattered stochastically based on a Langevin equation. Based on such different model approaches, means the transport part is done with kinetic equations, while the collision part is done via the Langevin equations, we present a splitting of these models. Such a splitting allow us to combine different modeling parts. For the transport part, we can apply particle models and solve them with particle methods, e.g., PIC, while for the collision part, we can apply the explicit Coulomb collision model, e.g., with fast stochastic differential equation solvers. Additional, we also apply multiscale approaches for the different parts of the transport part, e.g., different time-scales of an explicit electric field, and model-order reduction approaches. We present first numerical results for particle simulations with the deterministic-stochastic splitting schemes. Such ideas can be applied to sputtering problems or plasma applications with dominant Coulomb collisions.   

Electrical and Optical Structural Analysis of Pure Nitrogen RF-CCP}

In this work, 13.56 MHz pure (99.995 {\%}) nitrogen discharges were generated in a stainless steel cylindrical reactor (R$\sim $500 mm, H$\sim $400 mm). Two identical aluminum electrodes with R$\sim $200 mm were placed in the reactor at a 4 cm gap distance. A High-Resolution HR2000 fiber optic spectrometer (200-1100 nm) was connected to the system to do parametrical analyses. The RF power was in the range of 50-200 W and the pressure was in the range of 0.2-0.7 Torr. I detected many nitrogen atomic lines of N, N$^{\mathrm{+\thinspace }}$and N$^{\mathrm{++\thinspace }}$in the UV-Vis-NIR spectral regions. Strong N$^{\mathrm{++}}$ atomic lines (336.3, 379.45 nm) are mainly dominated the spectrum. Two atomic lines (677.28, 773.26 nm) of the N are \textasciitilde four times weaker than that of N$^{\mathrm{++}}$. The atomic lines of the N$^{\mathrm{+\thinspace }}$are \textasciitilde 10 times weaker than that of N$^{\mathrm{++}}$. Also many molecular nitrogen bands, which are the first positive N$_{\mathrm{2}}$ (B-A) system (530-970 nm), the second positive N$_{\mathrm{2}}$ (C-B) system (290-531 nm) and the first negative N$_{\mathrm{2}}^{\mathrm{+}}$ (B-X) system (410-530 nm) are observed. The excitation temperature (T$_{\mathrm{exc}})$ and the electron density (n$_{\mathrm{e}})$ of the N$^{\mathrm{+}}$ and N$^{\mathrm{++}}$ atomic ions were calculated for each discharge condition.   

Two-dimensional PIC/MCC simulation for magnetized capacitively coupled plasmas.

Magnetized capacitively coupled plasma (MCCP) has been used in microelectronic industry. In MCP, external static magnetic field is applied on the discharging plasma to improve the plasma confine and adjust the electron energy spectra. Typical about 100's Gauss magnetic field can work well. In these cases, the magnetic confine increase the plasma density and (often) decrease the electron temperature. In this work, we have studied MCCP with two-dimensional Particle-in-cell/Monte Carlo collision (PIC/MCC) model under different gas pressure and magnetic field. All the simulating cases apply external magnetic field in the r and z direction. Electron cooling and the E¨wB drifting motions are observed. And the effects of magnetic field on the plasma properties, such as the plasma density, electron temperature and energy distribution functions are discussed.   

Etching of Niobium in an Argon-Chlorine Capacitively Coupled Plasma

Ion assisted etching of the inner surfaces of Nb superconducting radio frequency (SRF) cavities requires control of incident ion energies and fluxes to achieve the desired etch rate and smooth surfaces. In this paper, we combine numerical simulation and experiment to investigate Ar /Cl$_{\mathrm{2}}$ capacitively coupled plasma (CCP) in cylindrical reactor geometry. Plasma simulations were done in the CRTRS 2D/3D code that self-consistently solves for CCP power deposition and electrostatic potential. The experimental results are used in combination with simulation predictions to understand the dependence of plasma parameters on the operating conditions. Using the model we were able to determine the ion current and flux at the Nb substrate. Our simulations indicate the relative importance of the current voltage phase shift and displacement current at different pressures and powers. For simulation and the experiment we have used a test structure with a pillbox cavity filled with niobium ring-type samples. The etch rate of these samples was measured. The probe measurements were combined with optical emission spectroscopy in pure Ar for validation of the model. The authors acknowledge Dr Shahid Rauf for developing the CRTRS code.   

Effect of weak static magnetic field on electron and ion dynamics in low pressure capacitive discharges

We investigated effect of weak static magnetic field applied parallel to electrodes on capacitive discharge in helium making use of the fully self-consistent EDIPIC code. It is observed that without magnetic field both the sheaths are symmetric, but with increase magnetic field, sheath at the grounded electrode gets thinner and the sheath next to the powered electrode become broader and discharge becomes very asymmetric, similar to experimental observations of Ref.[1]. The study of ion velocity distribution functions at the electrode showed that IVDFs can be controlled by a weak magnetic field of order 20G. Reference: [1] S. J. You et al., \textit{Thin Solid Films} 519 (2011) 6981-6989.   

Effect of dual frequency rf power in an inductively coupled plasma

Dual frequency inductively coupled plasma discharge is investigated. Dual RF power is applied independently to each antenna (inner and outer coil), and the electron energy distribution functions (EEDFs) are measured using a RF compensated Langmuir probe. As the ratio of low frequency power (P$_{\mathrm{low}})$ and high frequency power (P$_{\mathrm{high}})$ is changed, the variation of EEDF is observed. When P$_{\mathrm{low}}$ is higher than P$_{\mathrm{high}}$, the low energy electrons effectively heated compared to the case when P$_{\mathrm{low}}$ is comparable to P$_{\mathrm{high}}$. This difference in the shape of the EEDF can be understood by correlation between the driving frequency and the collision frequency.   

Plasma characteristics in inductively and capacitively coupled hybrid source using single RF power

Parallel combined inductively coupled plasma (ICP) and capacitively coupled plasma (CCP) using single RF generator was proposed to linear control of the plasma density with RF power. In the case of ICP, linear control of the plasma density is difficult because there is a density jump up due to E to H transition. Although the plasma density of CCP changes linearly with power, the density is lower than that of ICP due to high ion energy loss at the substrate. In our hybrid source, the single RF power generator was connected to electrode and antenna, and the variable capacitor was installed between the antenna and the power generator to control the current flowing through the antenna and the electrode. By adjusting the current ratio between the antenna and the electrode, linear characteristic of plasma density with RF power is achieved.   

On the optimal chamber length and electron heating mechanism in low pressure inductive discharges

Plasma resistance with the chamber length was measured at different plasma densities in low pressure inductively coupled plasmas. It was found that the plasma resistance has a maximum at specific chamber length, $L_{\mathrm{opt}}$, and the $L_{\mathrm{opt}}$ is changed with the plasma density. It is related to the maximum collisionless electron heating, which simultaneously satisfies the conditions of both the bounce resonance and the transit time resonance. Therefore, $L_{\mathrm{opt}}$ is an optimal chamber size for the power transfer to the plasma.   

Effect of complex permeability of ferrite material to the plasma discharge in a large size ferrite inductively coupled plasma

For the investigation in effect of the complex permeability to the ferrite inductively coupled plasma (Ferrite ICP), three different types of the ferrite material were adopted. Although the strength of the inductive coupling with the plasma can be improved the higher permeability, the plasma density can be reduced by higher permeability when the antenna resistance is higher enough to offset positive effect of it.   

Model reduction for streamer coronas

Pulsed gas discharges in nature and technology can consist of hundreds to ten thousands of streamers. Such streamer coronas up to now are modeled in a phenomenological manner; a model setup to include more microscopic information was provided by Luque and Ebert [New J. Phys. 16, 013039 (2014)]. To implement the proper microphysics, we here analyze the interior dynamics of propagating streamer heads, we review, unify and extend earlier analytical approximations, and we compare them with fluid simulation results derived with the Afivo computational framework http://www.cwimd.nl/doku.php?id=codes:afivo developed by Teunissen.   

N2(A) vibrational kinetics in streamer discharges: effect of oxygen on formation of low vibrational levels

In the present study we report on the N2(A) vibrational kinetics in nitrogen-oxygen mixtures revealed by LIF technique under DBD streamer discharge conditions at low pressures. In pure nitrogen, the observed evolution of the N2(A) LIF signal during the decaying streamer channel period evidences fast initial relaxation of high vibrational levels towards the v $=$ 2 and 3 levels, followed by a delayed increase of terminal v $=$ 0 and 1 levels. In nitrogen-oxygen mixtures however, the efficient quenching of higher N2(A) levels by oxygen significantly inhibits vibrational relaxation towards the lower and terminal levels, causing much lower populations of the v $=$ 0--3 levels. This is already clearly visible in the N2 $+$ 0.8{\%} O2 mixture with all the kinetics limited to the first 10 microseconds. In synthetic air, the kinetics is limited to few microseconds in the post discharge. Furthermore, much more effective quenching of fluorescence makes the measurements extremely challenging. Obtained results show that with the addition of oxygen the evolution of the N2(A) vibrational distribution is effectively terminated during the collisional-radiative cascade inhibiting energy pooling mechanism which is effective in pure nitrogen   

Discharge modes of a DC operated atmospheric pressure air plasma jet

By flowing air or nitrogen through a microhollow cathode discharge geometry an afterglow plasma jet can be generated at atmospheric pressure in air. The plasma jet has been successfully used for the inactivation of bacteria and yeast. The responsible reaction chemistry is based on the production of high concentrations of nitric oxide. Production yields depend in particular on gas flow rate and energy dissipated in the plasma. The same parameters also determine different modes of operation for the jet. A true DC operation is achieved for low to moderate gas flow rate of about 1 slm and discharge currents on the order of 10 mA. When increasing the gas flow rate to 10 slm the operation is changing to a self-pulsing mode with characteristics similar to the ones observed for a transient spark. By increasing the current a DC operation can be achieved again also at higher gas flow rates. The parameter regimes for different modes of operation can be described by the reduced electric field E/N.   

Characteristics of the Plasma Environment and Discharge Process in a High-Pressure Pulsed Arc Discharge.

The characteristics and properties of a plasma generated in a pulsed arc discharge are investigated. Arc discharge plasmas are prevalent in the production and treatment of materials. Photodetectors and optical emission spectroscopy (OES) are used to probe the plasmas and characterize their spectral responses. OES allows for species identification and provides information about the state of the plasma, such as the electron temperature. Discharges generated with inert gas such as argon, as well as with nitrogen and air, are studied and compared. In the case of reactive gases, OES also provides information on the possible reactions that took place. Microwave interferometry is used to measure the electron density to provide spatial information on the discharges. In addition, the measurement is synchronized with the discharge pulse to obtain temporal information, for instance, during the pulse initialization phase to investigate the arc discharge process prior to plasma generation, where optical information is absent. Together, this allows for the characterization of the pre-, during, and post-discharge processes.   

High speed deposition of SiO$_{\mathrm{2}}$ film by slot-type microwave CVD system

High density microwave plasma is attractive because of its ability for high-throughput processing. So far, we have successfully produced large-area surface wave excited plasma (SWP) and have applied it to plasma-enhanced chemical vapor deposition (PE-CVD) of silicon films. However, the SWP requires a dielectric plate for the surface wave propagation, and high density plasma sometimes erodes the dielectric plate to produce oxygen contamination. To avoid such problem, we propose the PE-CVD using the microwave plasma produced inside slots of a waveguide without using the dielectric plate. A 2.45 GHz pulsed microwave (repetition: 20 kHz, duty ratio: 20{\%}, average power: 40 W) is introduced to a rectangular waveguide through an isolator, a tuner, and a vacuum window. A slot of 4 mm in length and 0.2 mm in width is placed at the end of the waveguide, and is connected to a vacuum chamber. Both the waveguide and the chamber are evacuated by a turbomolecular pump. Oxygen and tetraethyl~orthosilicate (TEOS) gases are introduced from the waveguide and from the outside of the waveguide, respectively, to deposit SiO$_{\mathrm{2}}$ film on Si substrates at a pressure of 15 Torr and a slot-substrate distance of 1.1 cm. Deposition rate as high as 80 nm/s is observed at a TEOS flow rate of 0.8 sccm. The result suggests that the present PE-CVD system is promising as a new high-speed film deposition technique.   

Investigation of neutral and ion dynamics in a HiPIMS plasma by tunable laser diode absorption spectroscopy (TDLAS)

High power impulse magnetron sputtering (HiPIMS) discharges are known for complex plasma interactions, and complex temporal and spatial dynamics. Spatial and temporal dynamic of argon metastable (Ar$^m$), Ti atom (Ti$^0$) and Ti ion (Ti$^+$) density and temperature is studied by an extended tunable diode laser absorption spectroscopy setup (TDLAS) during a HiPIMS pulse. The TDLAS setup used a beam expander in combination with a 6 photo diode array to simultaneously measure spatial (resolution 5 mm) and time resolved absorption profiles of an Ar$^m$, Ti$^0$ and Ti$^+$ transition. This in combination with moving the magnetron in axial direction gives a complete 2D map of the density evolution. Temporal resolution of 400 ns was achieved by recording the photo diode signal on the National Instruments card. Final results allowed to investigate temporal evolution of the observed species in the volume between the target and the substrate.   

Lateral epitaxial overgrowth of silicon thick films during nanocluster assisted mesoplasma CVD

Mesoplasma epitaxy has been applied to the deposition on a SiO$_{\mathrm{2}}$ masked Si wafer to identify the feasibility of lift-off and layer transfer of the thick epitaxial films. Under a certain deposition condition, the Si epitaxial film was deposited over the patterned mask with 4 µm width. The surface topography on patterned mask has revealed that the epitaxial film grows laterally over the pattern from the Si window and its lateral epitaxial overgrowth (LEO) rate is 4-5 times faster than the vertical growth rate and reaches 2500 nm/sec at the trichlorosilane flow rate of 100 sccm. Growth model was developed, assuming the surface diffusion of the nanoclusters-constituent Si atoms on the mask surface and and also that the Cl etching effect of both SiO2 and Si. The model reproduces reasonably the LEO tendency and identified the shorter diffusion length of 127 nm than that of the conventional CVD, as the unique LEO mode with cluster assisted epitaxy. Furthermore, as predicted by the model, the deposition at greater TCS rates successfully produces LEO on the pattern with wider 8µm mask width as a result of LEO coverage before completion of the mask etching.   

Nanoparticle formation and thin film deposition in aniline containing plasmas

This contribution deals with plasma based polymerization processes in mixtures of argon and aniline. The investigations are performed in a capacitively coupled RF discharge (in pulsed and continuous mode) and concern both the observed formation of nanoparticles in the plasma volume and the deposition of films. The latter process was used for the deposition of ultra-thin layers on different kind of nanocarbon materials (nanotubes and free standing graphene). The analysis of the plasma and the plasma chemistry (by means of mass spectroscopy and in-situ FTIR spectroscopy) is accompanied by several ex-situ diagnostics of the obtained materials which include NEXAFS and XPS measurements as well as Raman spectroscopy and electron microscopy. The decisive point of the investigations concern the preservation of the original monomer structure during the plasma polymerization processes and the stability of the thin films on the different substrates.   

Underwater plasma discharge and its water treatment applications.

In recent, the quality of water has been exacerbated by the influx of wastewater and water pollutants. There have been frequent occurrences of water blooms due to the eutrophication of river. Therefore, the needs for water treatment are increased through effective and environment-friendly method. In this work, we propose the plasma system to overcome the problems mentioned above using underwater discharge plasma. The underwater discharges are generated by capillary electrode, and have the advantages of low cost, high efficiency and eco-friendly processing. The proposed technologies can be suitable for eliminating cyanobacteria, decreasing the concentration of oil dissolved in water, and purifying wastewater. Cyanobacteria is killed directly by the underwater discharge and water-dissolved oil and heavy-metal wastewater are purified by coagulation effect, which may result from the chemical reactions of underwater plasma. Consequently, these technologies using underwater discharge can be alternative methods to replace the existing technologies.   

Design of experiment analysis of CO$_2$ dielectric barrier discharge conditions on CO production

Dielectric barrier discharges (DBD) are frequently used for the generation of $\mathrm{CO}$ from $\mathrm{CO_2}$ which is of particular interest for syngas production. It has been found by means of fluid modelling in [1] that the $\mathrm{CO_2}$ conversion frequency in a $\mathrm{CO_2}$ DBD depends linearly on the specific energy input (SEI) while the energy efficiency of $\mathrm{CO}$ production is only weakly dependent on the SEI. Here, the same numerical model as in [1] is applied to study systematically the influence of gas pressure, applied voltage amplitude and frequency on the $\mathrm{CO_2}$ conversion frequency and the energy efficiency of CO production based on a 2-level 3-factor full factorial experimental design. It is found that the operating conditions of the $\mathrm{CO_2}$ DBD for $\mathrm{CO}$ production can be chosen to either have an optimal throughput or a better energy efficiency.\\ $[$1$]$ S. Ponduri, et al., \textit{J. Appl. Phys.} \textbf{119} (2016) 093301   

Carbon dioxide reduction in low-pressure ICP

This work experimentally investigates the efficiency of carbon dioxide dissociation in inductively coupled plasma (ICP) at low gas pressure. The plasma source operates at 13.56 MHz in the RF power range of 100-500 W. Pure CO$_{\mathrm{2}}$ is fed into the plasma while the output gas composition is measured by a mass spectrometer. The pressure range inside the source was changed in the range of 1-200 mTorr. Excitation processes in the plasma are studied by means of optical emission spectroscopy, and the plasma density along with the electron temperature are monitored using a Langmuir probe. Experimental results have shown that the conversion efficiency of CO$_{\mathrm{2}}$ to CO and O$_{\mathrm{2}}$ increases with the RF and reaches the values more than 50{\%}. A theoretical treatment of the dissociation pathway is also given allowing estimation of the mean dissociation length of the carbon dioxide molecule in plasma. The plasma parameters necessary for efficient CO$_{\mathrm{2}}$ reduction are discussed.   

Valorization of biogas into liquid hydrocarbons in plasma-catalyst reactor

Biogas represents an important source of renewable energy issued from biological degradation of biomass. It is planned to produce in Europe the amount of biogas equivalent to 6400 kWh electricity and 4500 kteo (kilo tons equivalent oil) in 2020. Currently the biogas is used in cogeneration engines to produce heat and electricity directly in farms or it is injected in gas networks after purification and odorisation. The aim of this work is to propose a third option that consists of valorization of biogas by transformation into liquid hydrocarbons like acetone, methanol, ethanol, acetic acid etc. These chemicals, among the most important feed materials for chemical industries, retain CO$_{\mathrm{2}}$ molecules participating to reduce the greenhouse gas emissions and have high storage energy capacity. We developed a low temperature atmospheric plasma-catalyst reactor (surface dielectric barrier discharge) to transform biogas into chemicals. The conversion rates of CH$_{\mathrm{4}}$ and CO$_{\mathrm{2}}$ are respectively about 50{\%} and 30{\%} depending on operational conditions. The energetic cost is 25 eV/molecule. The yields of liquid hydrocarbon reaches currently 10{\%} wt. More the 11 liquid chemicals are observed in the liquid fraction.   

Understanding the vibrational distribution in CO$_{\mathrm{2}}$ microwave plasma for production of carbon neutral fuels, using time resolved in-situ spectroscopy

A microwave plasma could prove to be a cost effective way of converting CO$_{\mathrm{2}}$ to CO. The efficiencies of such a reactor have been shown to be very high, up to 90{\%}. It is currently understood that the preferable vibrational excitation by plasma electrons plays a key role in the efficient CO$_{\mathrm{2}}$ conversion. In the case that Vibrational-Vibrational (VV) relaxation times are much shorter than Vibrational-Translational (VT) relaxation times, molecules are vibrationally excited via intermolecular collisions until the dissociation energy is reached. As the VT-relaxation rate increases with temperature, a low temperature is needed to promote an overpopulation of high vibrational levels. To reduce the temperature, The microwave power was pulsed. Raman-scattering was employed to measure the temperature in the radial center and sides of the plasma, over an axial distance of a few centimeter. The infrared absorption spectrum of the CO$_{\mathrm{2}}$-plasma is recorded using an in-situ step-scan FTIR spectrometer. The absorption bands of higher vibrational levels are visible lower wavenumbers, down to 2000 cm$^{\mathrm{-1}}$. This enables us to look at the evolution of the densities of the vibrational levels. It was found that the vibrational temperature increased during plasma ON-time.   

Development of atmospheric pressure large area plasma jet for sterilisation and investigation of molecule and plasma interaction

The intention of the project is the development and improvement of an atmospheric plasma jet based on various discharge forms (e.g. DBD, RF, micro-array) for sterilisation of biomedical equipment and investigation of biomolecules under the influence of plasma stress. The major objective is to design a plasma jet with a large area and an extended length. Due to the success on small scale plasma sterilisation the request of large area plasma has increased. Many applications of chemical disinfection in environmental and medical cleaning could thereby be complemented. Subsequently, the interaction between plasma and biomolecules should be investigated to improve plasma strerilisation. Special interest will be on non equilibrium plasma electrons affecting the chemical bindings of organic molecules.   

Comparative study of non-thermal atmospheric pressure discharge plasmas for life science applications

We are comparing several non-thermal atmospheric pressure discharge plasmas for life science applications. Here we measured discharge period dependence of pH value and 750 nm absorbance of KI-starch solution of deionized water after plasma irradiation with two discharge devices; a dielectric barrier discharge (DBD) jet device and a scalable DBD device [1]. The pH and the absorbance of KI-starch solution are useful indicator of their oxidizability [2, 3]. We have obtained a map of the absorbance and proton concentration [H$^{\mathrm{+}}$] which is deduced from pH value. For the scalable DBD, the range of the absorbance is between 0.7 and 1.3 and that of [H$^{\mathrm{+}}$] is between 10$^{\mathrm{-7}}$ and 10$^{\mathrm{-5}}$ mol/L. For the DBD jet, the range of the absorbance and [H$^{\mathrm{+}}$] are 2.0-3.2 and 10$^{\mathrm{-4}}$-10$^{\mathrm{-3}}$ mol/L, respectively. Measured data for both devices shows same tendency in the map, while the range of values for the scalable DBD is smaller than that for the DBD jet. The results indicate the oxidazability for the scalable DBD is much weaker than that for the DBD jet. [1] S. Kitazaki, et al., Curr. Appl. Phys. \textbf{14} (2014) S149. [2] T. Sarinont, et al., JPS Conf. Proc. \textbf{1} (2014) 015078. [3] T. Kawasaki, et al., J. Appl. Phys. \textbf{119 }(2016) 173301.   

Trial of Growth Control of Farm-raised Fish by Plasma-generated Reactive Species

As one of the biological applications of plasmas, growth control of agricultural products attracts attentions. There are many papers on growth enhancement of crops by plasma treatment. However, there are few published papers concerning growth enhancement of fishery products excepting reports of goldfish growth enhancement in 1980s. In this study, growth characteristics of edible fish (tilapia) under the plasma treatment has been investigated. An arc discharge reactor was employed and plasma treated air was introduced to two aquariums with a flow rate of 2.5~L/min. Measured concentrations of main reactive species were 43~ppm for NO, 23~ppm for NO$_2$ and 7.5~ppm for O$_3$. Each aquarium had 60~L capacity and contained 15 tilapia fish. The plasma treated air was supplied to an aquarium once a day and to the other aquarium twice a day with total duration of 10~min. Compared to no plasma treatment case, the growth rate decreased by 18\% by once a day plasma treatment, whereas almost same growth rate was observed by twice a day plasma treatment. A possible reason of growth suppression is excess concentrations of nitrite and nitrate in water. The relationship between their concentrations and growth characteristics under several treatment conditions will be shown at the conference.   

Investigation of relationship between plasma gas temperature and reactive species

In recent years, atmospheric non-thermal plasmas have attracted attention as a new sterilization device. In conventional plasma source, since the plasma gas temperature depends on the discharge power, influence of the plasma gas temperature on bactericidal ability by constant power has not been investigated. Therefore, we developed a new plasma source that can control the plasma gas temperature independently of the power, and it was shown that the bactericidal ability is increased with the plasma gas temperature. However, this factor has not been revealed. In this study, we investigated relationship between the bactericidal ability and the concentration of reactive species at each plasma gas temperature. Because reactive species generated by plasma are thought to affect sterilization. So, to investigate lifetime of the sterilizing factor bactericidal ability of Plasma Treated Water made by each gas temperature plasma was investigated. In both experiments, the correlation (R$^{\mathrm{2}}=$0.999) was observed between the concentration of singlet oxygen ($^{\mathrm{1}}$O$_{\mathrm{2}})_{\mathrm{\thinspace }}$and the bactericidal ability. These results show long-lifetime reactive species generated by $^{\mathrm{1}}$O$_{\mathrm{2}}$ affects the bactericidal ability.   

Impact of electric field from a plasma jet on biological targets

Atmospheric pressure plasma jets have demonstrated their ability in biomedical applications thanks to their low gas temperature and their capacity to produce radicals, ions, electrons, UV radiation and electric fields. However the understanding of the interactions between the plasma and living cells and tissues is still far from being completely understood. Recently, Robert \textit{et al} characterized two components of the electric field from a plasma jet and showed that the latter can propagate deeply in tissues on several mm [1]. In this work, we focus on the study of the electric field induced by the plasma and its influence on the cell membrane. Propidium iodide, dextran sulfate and plasmid DNA are used to measure the permeability of the membrane, while an electro-optic probe is used to measure the longitudinal and the radial components of the electric field. The two components are both spatially and temporally resolved. To investigate the contribution of the electric field on the cell membrane, a dielectric barrier is used between the plasma and the biological target. A comparison with and without the barrier will be presented for both biological and agriculture applications. [1] E. Robert \textit{et al}, 2015, Phys. Plasmas 22 122007   

Anti-tumor Effects of Plasma Activated Media and Correlation with Hydrogen Peroxide Concentration.

Plasma activated media (PAM) can induce death in cancer cells. In our research, PAM is produced by exposing liquid culture medium to a helium plasma pencil. Reactive oxygen and nitrogen species in the aqueous state are known factors in anti-tumor effects of PAM. The duration of plasma exposure determines the concentrations of reactive species produced in PAM. Stability of the plasma generated reactive species and their lifetime depend on parameters such as the chemical composition of the medium. Here, a complete cell culture medium was employed to make PAM. Later, PAM was used to treat SCaBER cancer cells either as an immediate PAM (right after exposure) or as an aged-PAM (after storage). SCaBER (ATCC\textregistered HTB­3\texttrademark ) is an epithelial cell line from a human bladder with the squamous carcinoma disease. A normal epithelial cell line from a kidney tissue of a dog - MDCK (ATCC\textregistered CCL-34\texttrademark ) - was used to analyze the selective effect of PAM. Correspondingly, we measured the concentration of hydrogen peroxide- as a stable species with biological impact on cell viability- in both immediate PAM and aged-PAM. In addition, we report on the effect of serum supplemented in PAM on the H$_{\mathrm{2}}$O$_{\mathrm{2}}$ concentration measured by Amplex red assay kit. Finally, we evaluate the effects of PAM on growth and morphological changes in MDCK cells using fluorescence microscopy.   

Optical Characteristics Investigation of the Cold Argon Plasma Jet for the Medical Applications

The medical setup was designed for the treatment of wounds, disinfection of inflammation, for the destruction of damaged cells. The results of experimental determination of the optical characteristics of Argon cold plasma at atmospheric pressure are presented in the paper. The main components of the experimental setup are plasma torch, spectrometer, photo-electron multiplier, oscilloscope, gas consumption $Q_{\mathrm{Ar\thinspace }}=$ 1---20 l/min. Spectrum of the plasma jet is obtained using the grating spectrometer Spectra with radiometric calibration, operating in the visible range $\lambda \quad =$ 380---760 nm. The sun-blind photodetector was used for determination of the intensity of radiation in the UV range $\lambda \quad =$ 190---380 nm. The emission spectrum consists of a continuous radiation and the emissions of atoms and ions ArI and ArII. The analysis of spectral lines was carried out.   

DSMC simulations of leading edge flat-plate boundary layer flows at high Mach number

The flow over a 2D leading-edge flat plate is studied at Mach number \textit{Ma }$= (U_{inf}/ \backslash $\textit{sqrt\textbraceleft k}$_{B}T_{inf}$\textit{/ m\textbraceright ) }in the range \textit{\textless Ma \textless 10}, and at Reynolds number number \textit{Re }$= (L_{T} U_{inf}$\textit{ rho}$_{inf\thinspace }$\textit{)/ mu}$_{inf\thinspace }$ equal to 10$^{\mathrm{\thinspace \thinspace }}$using two-dimensional (2D) direct simulation Monte Carlo (DSMC) simulations to understand the flow phenomena of the leading-edge flat plate boundary layer at high Mach number. Here, $L_{T}$is the characteristic dimension, $U_{inf}$and $T_{inf}$are the free stream velocity and temperature, \textit{rho}$_{inf}$ is the free stream density, $m$is the molecular mass, \textit{mu}$_{inf\thinspace }$is the molecular viscosity based on the free stream temperature $T_{inf},$and $k_{B}$is the Boltzmann constant. The variation of streamwise velocity, temperature, number-density, and mean free path along the wall normal direction away from the plate surface is studied. The qualitative nature of the streamwise velocity at high Mach number is similar to those in the incompressible limit (parabolic profile). However, there are important differences. The amplitudes of the streamwise velocity increase as the Mach number increases and turned into a more flatter profile near the wall. There is significant velocity and temperature slip ((Pradhan and Kumaran, J. Fluid Mech-2011); (Kumaran and Pradhan, J. Fluid Mech-2014)) at the surface of the plate, and the slip increases as the Mach number is increased. It is interesting to note that for the highest Mach numbers considered here, the streamwise velocity at the wall exceeds the sound speed, and the flow is supersonic throughout the flow domain.   

Performance optimization of an EHD thruster: the influence of secondary emission and the electrodes gap.

We have developed a numerical model to study the performance (thrust, maximum output velocity of the fluid and thrust-to-power ratio) of a single-stage electrohydrodynamic (EHD) thruster with a rod anode and a funnel-like cathode configuration. The electrohydrodynamic processes embody interlocking aspects of non-compressible gas dynamics (Navier-Stokes equations), ionized gas physics, self-consistent accelerating electric field adequately described by Poisson equation and migration of charged particles in an electric field in the drift-diffusion approximation. We considered the following neutral and ionized nitrogen species as the working media: N, N$^{\mathrm{+}}$, N$_{\mathrm{2}}$, N$_{\mathrm{2}}^{\mathrm{+}}$, and N$_{\mathrm{4}}^{\mathrm{+}}$. In order to optimize the thruster performance, we present two studies: i) a sweeping of the gap between electrodes in order to detect the optimal distance for the proposed model; and ii) a study of the influence of the secondary electron emission coefficient, $\gamma_{\mathrm{i}}$, on the discharge mechanism, as $\gamma_{\mathrm{i}}$ relies on the material used to build the cathode. The working pressure employed in the simulations is 10 Torr (1.3 kPa) and the gas temperature is 300 K.   

Spatially resolved optical-emission spectroscopy of a radio-frequency driven iodine plasma source

Iodine is of interest for potential use as a propellant for spacecraft propulsion, and has become attractive as a replacement to xenon due to its similar mass and ionisation potential. Optical emission spectroscopy has been undertaken to characterise the emission from a low-pressure, radio-frequency driven inductively coupled plasma source operating in iodine with respect to axial distance across its transverse magnetic filter. The results are compared with axial profiles of the electron temperature and density for identical source conditions, and the spatial distribution of the emission intensity is observed to be closely correlated with the electron temperature.   

The effect of tailored voltage waveforms on neutral gas heating in a radio-frequency driven electrothermal microthruster

Over the past few decades there has been a growing interest in the development compact sources of electric propulsion. In this study the effect of driving the ‘Pocket Rocket’ radio-frequency electrothermal microthruster with non-sinusoidal voltage waveforms, consisting of multiple harmonics of 13.56 MHz, is investigated using the Hybrid Plasma Equipment Model (HPEM). The results are compared to previous experiments and simulation results using CFD-ACE+ to investigate the potential to generate an increased neutral gas temperature and density in the source. The authors gratefully acknowledge M. Kushner of the University of Michigan for the use of the Hybrid Plasma Equipment Model (HPEM).   

Neutral-depletion-induced asymmetric plasma density profile and momentum transport in a helicon thruster

Axial momentum lost to a lateral wall of a helicon source is directly measured by using a pendulum force balance, where only the lateral wall is attached to the balance immersed in 60-cm-diam and 1.4-m-long vacuum tank (pumping speed of 300-400 L/s). When operating the source with highly ionized krypton and xenon, the strong density decay along the axis is observed inside the source tube, which seems to be due to the neutral depletion. Under such a condition, a non-negligible loss of the axial momentum to the lateral wall is detected. The presently detected loss of the axial momentum indicates the presence of the ions which are axially accelerated by the electric field in the plasma core and then lost to the lateral wall. Furthermore, the helicon thruster immersed in 1-m-diam and 2-m-long vacuum tank (pumping speed of 4000-5000 L/s) is operated at high rf power up to 5 kW in argon, to demonstrate the neutral-depletion-induced axially asymmetric density profile. Combination between the Langmuir probe and the optical diagnosis indicates that the neutral density at the axial center of the source is reduced to 20 % of the initial neutral density.   

Effect of High Z material on the performance of an air-breathing laser ablation thruster

A Laser propulsion, such as a Lightcraft, is a candidate for the low cost transportation system between the ground to space instead of the chemical rocket. Using the shock wave induced by focusing laser beam on the ablator in air, the huge fuel is unnecessary to generate the thrust. In this study, the high-Z material was doped into the polystyrene to emphasize the ionization effect in air. We evaluate the intensity of the bremsstrahlung radiation, the plasma parameter, and the thrust performance.   

Generation of composite Au/TiO2 nanoparticles by pulsed laser ablation in aqueous media

Composite nanoparticles have been known for their potential applications in photocatalysis, medical and optical limiters. In particular, Au/TiO2 composite nanoparticles have attracted attention because of its remarkable properties. However, commonly Au/TiO2 composite nanoparticles are synthesized by chemical method using toxic precursor and reducing agents, and problems by their residue arised. Here, we examined a new synthesis method of composite nanoparticles by pulsed laser ablation (PLA) without any chemical agents, but only with distilled water. Au/TiO2 composite nanoparticles were obtained by PLA of Ti plate covered with Au and TiO2 nanoparticles, which were preliminarily synthesized by PLA in distilled water. The synthesized nanoparticles were characterized by using TEM, UV-vis absorption spectroscopy, dynamic light scattering and XRD. The TEM images showed that composite nanoparticles including Au-TiO2 core-shell nanoparticles were successfully generated with diameter around 100 nm.   

Spectroscopic studies of plasma in a carbon arc discharge for synthesis of nanomaterials

An atmospheric pressure arc discharge with graphite electrodes is commonly used for synthesis of carbon nanomaterials such as buckyballs, nanotubes and graphene. In operation, the graphite anode ablates providing a feedstock material for synthesis these carbon nanostructures. Existing models [1] predict that nucleation and growth of these nanomaterials in an arc discharge are governed by spatial distributions of density and temperature of plasma species. Control of these distributions can potentially enable optimization of nanosynthesis processes, to achieve the best combination of synthesis selectivity at the synthesis yield. In this work, we report first detail measurements of spatial distribution of arc plasma parameters obtained with a set of in-situ diagnostics, including optical emission spectroscopy and fast framing imaging. These parameters were measured in low- and high- anode ablation modes [2]. Results of these measurements demonstrate a strong correlation between arc plasma and synthesis processes. [1] M. Keidar, A. Shashurin, O. Volotskova, Y. Raitses, and I. I. Beilis, Phys Plasmas 17, 057101 (2010). [2[ J. Ng and Y. Raitses, J Appl Phys 117 (2015).   

Synthesis of Copper and Nickel Nanoparticles from a Plasma Discharge in Liquid

Nanoscale metal particles have been attracting much attention because of their unique size- and dimensionality dependent physical and chemical properties. In order to fabricate metal nano-particles, many methods are ported such as chemical vapor deposition, thermal decomposition, micro-emulsion, UV-irradiation, the polyol process, and so on. However, previous methods may cause secondary environment pollution. Moreover, most of the synthetic methods are not economically feasible due to low throughput and poor scalability. In this work, we propose the synthesis methods of metal nano-particles by underwater discharge to overcome the shortcomings of reported methods. The copper and nickel nano-particles are synthesized by underwater discharge, and they have the diameter less than 100 nm. Also, we confirmed Cu and Ni nanoparticles were not oxidized through XRD analysis. We expect that the metal nano-particles synthesized by underwater discharge can be applied to electronic industry such as printed electronics and multi-layer ceramic capacitors (MLCCs).   

Surface diffusion of a carbon adatom on charged SWCNT

Diffusion of a carbon adatom on SWCNT could be a mechanism for a CNT growth in a volume plasma, supplementing its growth from a transition metal catalyst nanoparticle. However, being embedded in plasma, the nanotube can charge by the plasma particles irradiation, in particular by electrons. Using Density Functional Theory, Nudged Elastic Band and Kinetic Monte Carlo methods we find (1) equilibrium sites, (2) adsorption energies, (3) potential barriers, (4) vibrational frequencies and (5) most probable pathways for diffusion of the adatom on external surfaces of SWCNTs of (5,5), (10,0) and (10,5) chirality, as function of its charge. The metal (5,5) SWCNT can support a fast diffusion of the carbon adatom, which is accelerated by the presence of the SWCNT negative charge. Reduced model of SWCNT growth is proposed.   

Synthesis of epitaxially grown core/shell nanocrystals with nonthermal plasmas

Nonthermal plasmas have gained increasing adoption as capable sources of nanocrystal materials that are challenging to grow in solution due to the high synthesis temperatures required. To date, little progress has been made to grow core/shell nanocrystals with nonthermal plasmas. In colloidal synthesis, core/shell structures have proven to be indispensable to improve the optical properties of nanocrystal materials. The epitaxially grown shells terminate surface states on the nanocrystal cores and can be selected to form heterojunctions that confine charge carriers in the core region. Here, we present the nonthermal plasma synthesis of germanium (Ge) nanocrystals with epitaxially grown silicon (Si) shells. Core/shell growth is achieved in a single flow-through plasma reactor by first injecting the core precursor and, after its depletion, injecting the shell precursor further downstream. Electron microscopy studies confirm epitaxial shell growth with minimal intermixing of core and shell material. Due to the lattice mismatch between core and shell, we find that Ge cores are compressively strained, which enables tuning of the Ge band structure via shell thickness. This demonstration of core/shell nanocrystals can be extended to an exciting array of heterostructures.   

Application of bipolar gas discharge for water sterilization from {\it S.aureus} and {\it E-coli}

Recently, water treatment by gas discharge above the surface of the liquid has attracted a lot of attention. In most cases, however, the unipolar power source is used. Bipolar pulses of high voltage and current can increases degree of water sterilization from organic compounds, both chemical and bacterial since non equilibrium atmospheric plasma contains not only electrons but also positive and negative ions as well as an excited molecules or atoms and active radicals. Heavy charged particles of both signs, accelerated by bipolar electric field, can easily destroy chemical and biological contaminants in water. To evaluate this phenomenon, high voltage bipolar pulse generator was used. The amplitude of the pulse voltage was approaching value of 200 kV at the discharge ignition. The repetition time was varied from 1 to 14 milliseconds. Current pulse had a shape of a superposition of bipolar pulses with decaying amplitude. Liquid surface was used as a cathode or anode.Two types of contaminants, {\it S.aureus} and {\it E.coli}, with was $1.5\times 10^8$~CFU/mL were treated by bipolar high voltage pulse discharge. After 30 minutes of exposition, no contaminants were observed within the water.   

Dynamics of turbulent front at the correlation between atmospheric pressure plasma jet {\&} gas flow field.

Among variety of plasma applications, there is significant interest recently in the use of plasma as an actuator in flow control for aerodynamic applications in which the correlation between atmospheric pressure plasma jet (APPJ) and gas flow field is a crucial role. In this contribution, dynamic characterizations of the turbulent flow field in APPj are investigated by focusing on the effect of different parameters of APPJ, such as applied voltage, pulse repetition frequency, gas flow rate, and time duration of the pulse We utilized Schlieren photography and photomultiplier tubes (PMT) as a signal triggering of an intensified charge coupled device (ICCD) and also a high speed camera to examine the formation of the turbulent front and its dynamics. The results reveal that the turbulent front will appear earlier and closer to the tube nozzle by increasing the gas flow rate and applied voltage amplitude. It is found that the pulse time duration and repetition frequency cannot change the dynamics and formation of the turbulent front. Further investigation demonstrated that every pulse can excite one turbulent front which is created in a specific position in a laminar region and propagates downstream and the effect of increasing frequency results in the increasing of the number of turbulent front and expansion of their region of formation.   

Measurements of energetic electrons in a Current-Free Double layer.

In inductively coupled helicon sources, current-free double layers (CFDL) can be formed self-consistently without external current forcing. The CFDLs are evidenced by an ion beam formed as a result of a potential drop between the source and the diffusion chamber. The electrons in the double layer play an important role in balancing the ion beam current, but apart from some observations of electron energy probability functions (EEPFs) by means of Langmuir probes, little information has up to now been obtained about the electron population. By means of an inverted retarding field energy analyzer (RFEA) we have measured for the first time the high-energy part of the electron distribution along the radial direction in the diffusion chamber. In this configuration, the RFEA repeller grid is set to a large positive potential, repelling ions and collecting electrons through the discriminator grid. We find a prominent peak of high-energy electrons up to 60 eV at the footprint of the magnetic field lines emerging from the layer near the wall of the source. This coincides with increased electron temperatures and ion densities at this position. Another small but significant distribution of electrons at energies more than 100 eV are observed within the region of the ion beam itself.   

Experimental study of a very high frequency, 162 MHz, segmented electrode, capacitively coupled plasma discharge

Radio-frequency capacitively coupled plasma (CCP) discharge operating at a very high frequency, 30-300 MHz, offers many advantages over standard 13.56 MHz CCP. However, there is a limited flexibility on the choice of driving frequency and substrate size due to plasma non-uniformity caused by the standing wave effect and edge effect. To overcome this issue segmented electrode CCP's are proposed and researched. Despite its numerous advantages the power coupling mechanism and plasma chemistry in this type of discharge are not fully understood due to lack of experimental data. In this paper, we present the experimental study of a segmented electrode, 3x4 tile array (10x10 cm square tile with 1 cm tile-to-tile separation), CCP discharge driven at 162 MHz. We measured plasma uniformity and gas temperature using hairpin probe and optical emission spectroscopy respectively. A homemade RF compensated Langmuir probe is employed to measure the Electron Energy Distribution Function (EEDF) by second harmonic technique. Energy resolved quadrupole mass spectrometer is utilized to measure the ion energy distribution. Discharge/plasma properties are investigated for several operating conditions and for power coupling mode in both washer board and checker board configuration. The experimental results show that the uniform plasma density can be maintained over a large area along with highly non-equilibrium condition to produce unique gas phase plasma chemistry.   

Electron and negative ion densities in a CW and pulsed 100 MHz capacitively coupled plasma discharge

Capacitively coupled plasma (CCP) discharges operating at a very high frequency, 30 -300 MHz, are becoming very popular now a days due to enhanced plasma processing rates and lower damage to the substrate. This is mainly achieved due to higher plasma densities and lower electron temperature produced at higher driving frequencies. Moreover, pulsing of the discharge system is known to deliver charging-free plasma processes which is highly desirable for high-aspect-ratio plasma etching. In this study, we present electron and negative ion densities in a CW and pulsed 100 MHz CCP discharge produced in O$_{\mathrm{2}}$ and Ar/O$_{\mathrm{2}}$/C$_{\mathrm{4}}$F$_{\mathrm{8}}$ gas mixture. Electron density is determined by the Hairpin probe and negative ion density is determined by the pulse laser photo-detachment combined with Hairpin probe. Photo-detachment is performed at 532, 355 and 266 nm laser wavelengths in order to selectively photo-detach different negative ions present in the discharge. Experimental results are presented for several power (100-500 W), pressure (1-10 Pa) conditions and for several duty ratios (25 -- 75{\%}) for 1 KHz pulse repetition frequency. In CW O$_{\mathrm{2}}$ plasma, we observed a similar trend in electron and negative ion density vs power, whereas, in Ar/O$_{\mathrm{2}}$/C$_{\mathrm{4}}$F$_{\mathrm{8}}$ gas mixture an opposite trend is observed in electron and negative ion density.   

Electron Energy Distribution function in a weakly magnetized expanding helicon plasma discharge

Helicon wave heating is well known to produce high-density plasma source for application in plasma thrusters, plasma processing and many more. Our previous study \textit{(B Ellingboe et al APS Gaseous Electronics Conference 2015, abstract {\#}KW2.005}) has shown observation of helicon wave in a weakly magnetized inductively coupled plasma source excited by m$=$0 antenna at 13.56 MHz. In this paper, we investigated the Electron Energy Distribution Function (EEDF) in the same setup by using an RF compensated Langmuir probe. The ac signal superimposition technique (second harmonic technique) is used to determine EEDF. The EEDF is measured for 5-100 mTorr gas pressure, 100 W -- 1.5 kW rf power and at different locations in the source chamber, boundary and diffusion chamber. This paper will discuss the change in the shape of EEDF for various heating mode transitions.   

Magnetic Field Tailored Annular Hall Thruster with Anode Layer

Plasma propulsion system is one of the key components for advanced missions of satellites as well as deep space exploration. A typical plasma propulsion system is Hall effect thruster that uses crossed electric and magnetic fields to ionize a propellant gas and to accelerate the ionized gas to generate momentum. In Hall thruster plasmas, magnetic field configuration is important due to the fact that electron confinement in the electromagnetic fields affects both plasma and ion beam characteristics as well as thruster performance parameters including thrust, specific impulse, power efficiency, and life time. In this work, development of an anode layer Hall thruster (TAL) with magnetic field tailoring has been attempted. The TAL is possible to keep discharge in 1 to 2 kilovolts of anode voltage, which is useful to obtain high specific impulse. The magnetic field tailoring is used to minimize undesirable heat dissipation and secondary electron emission from the wall surrounding the plasma. We will report 3 W and 200 W thrusters performances measured by a pendulum thrust stand according to the magnetic field configuration. Also, the measured result will be compared with the plasma diagnostics conducted by an angular Faraday probe, a retarding potential analyzer, and a ExB probe.   

Influence of wake potential on the propagation of the space-charge wave in a waveguide dusty plasma

The wake potential effects on the propagation of the space-charge dust ion-acoustic wave are investigated in a cylindrically bounded dusty plasma with the ion flow. The results show that the wake potential would generate the double frequency modes in a cylindrically bounded dusty plasma. It is found that the positive mode of the wave frequency with the root of higher-order is smaller than that with the root of lower-order in intermediate wave number domains. However, the negative mode of the scaled wave frequency with the root of higher-order is found to be greater than that with the root of lower-order. It is found that the influence of order of the root of the Bessel function on the wave frequency of the space-charge dust-ion-acoustic wave in a cylindrically confined dusty plasma decreases with an increase of the propagation wave number. It is also found that the double frequency modes increase with increasing Mach number due to the ion flow in a cylindrical dusty plasma. In addition, it is found that the positive mode of the group velocity decreases with an increase of the scaled radius of the plasma cylinder. However, the negative mode group velocity increases with an increase of the radius of the plasma cylinder.   

Synthesis of N-graphene using microwave plasma-based methods

In this work a microwave atmospheric plasma driven by surface waves is used to produce free-standing graphene sheets (FSG). Carbonaceous precursors are injected into a microwave plasma environment, where decomposition processes take place. The transport of plasma generated gas-phase carbon atoms and molecules into colder zones of plasma reactor results in carbon nuclei formation. The main part of the solid carbon is gradually carried from the ``hot'' plasma zone into the outlet plasma stream where carbon nanostructures assemble and grow. Subsequently, the graphene sheets have been N-doped using a N2-Ar large-scale remote plasma treatment, which consists on placing the FSG on a substrate in a remote zone of the N2-Ar plasma. The samples were treated with different compositions of N2-Ar gas mixtures, while maintaining 1 mbar pressure in the chamber and a power applied of 600 W. The N-doped graphene sheets were characterized by scanning and by high-resolution transmission electron microscopy, X-ray photoelectron spectroscopy and Raman spectroscopy. Plasma characterization was also performed by optical emission spectroscopy.   

Magnetic field effect on spoke behaviour

The investigations of the non-reactive high power impulse magnetron sputtering (HiPIMS) discharge using high-speed camera imaging, optical emission spectroscopy and electrical probes showed that plasma is not homogeneously distributed over the target surface, but it is concentrated in regions of higher local plasma density called spokes rotating above the erosion racetrack. Magnetic field effect on spoke behaviour was studied by high-speed camera imaging in HiPIMS discharge using 3 inch titanium target. An employed camera enabled us to record two successive images in the same pulse with time delay of 3\,$\mu$s between them, which allowed us to determine the number of spokes, spoke rotation velocity and spoke rotation frequency. The experimental conditions covered pressure range from 0.15 to 5 Pa, discharge current up to 350 A and magnetic fields of 37, 72 and 91 mT. Increase of the magnetic field influenced the number of spokes observed at the same pressure and at the same discharge current. Moreover, the investigation revealed different characteristic spoke shapes depending on the magnetic field strength - both diffusive and triangular shapes were observed for the same target material. The spoke rotation velocity was independent on the magnetic field strength.   

Quantum Cause of Gravity Waves and Dark Matter

Per Einstein's theory mass tells space how to curve and space tells mass how to move. How do they tell"? The question boils down to information created by quantum particles blinking ON and OFF analogous to 'Ying and Yang' or some more complex ways that may include dark matter. If not, what creates curvature of space-time? Consciousness, dark matter, quantum physics, uncertainty principle, constants of nature like strong coupling, fine structure constant, cosmological constant introduced by Einstein, information, gravitation etc. are fundamentally consequences of that ONE TOE. Vedic philosophers, who impressed Schrodinger so much, called it ATMA split in the categories of AnuAtma (particle soul), JivAtma (life soul) and ParamAtma (Omnipresent soul) which we relate to quantum physics, biology and cosmology. There is no separate TOE for any one thing. The long range relativistic propagations of the strong and weak couplings of the microscopic black holes in [1] are just gravity waves. What else could they be? [1]Dark Matter from Our Probabilis- tic Gravity, Journal of Physical Science and Application 5 (5) (2015) 373-376 (doi: 10.17265/2159-5348/2015.008).   

Magnetic field effect on spoke behaviour

The investigations of the non-reactive high power impulse magnetron sputtering (HiPIMS) discharge using high-speed camera imaging, optical emission spectroscopy and electrical probes showed that plasma is not homogeneously distributed over the target surface, but it is concentrated in regions of higher local plasma density called spokes rotating above the erosion racetrack. Magnetic field effect on spoke behaviour was studied by high-speed camera imaging in HiPIMS discharge using 3 inch titanium target. An employed camera enabled us to record two successive images in the same pulse with time delay of 3 $\mu$s between them, which allowed us to determine the number of spokes, spoke rotation velocity and spoke rotation frequency. The experimental conditions covered pressure range from 0.15 to 5 Pa, discharge current up to 350 A and magnetic fields of 37, 72 and 91 mT. Increase of the magnetic field influenced the number of spokes observed at the same pressure and at the same discharge current. Moreover, the investigation revealed different characteristic spoke shapes depending on the magnetic field strength - both diffusive and triangular shapes were observed for the same target material. The spoke rotation velocity was independent on the magnetic field strength.   

Secondary electron emission in the limit of low incident electron energies

A detailed review of experimental and theoretical studies of secondary electron emission (SEE) at low incident electron energies has been recently given in paper$^{\mathrm{1}}$. In particularly, discussion of some authors' statement$^{\mathrm{2,3}}$ on increase of the SEE yield up to unity if the primary electron energy tends to zero was reviewed. Present paper considers a technique for measurements of SEE yield near a sample surface$^{\mathrm{4}}$ making use of a magnetic field parallel to the surface. Using this technique it was shown that the SEE yield can approach unity for a polycrystalline, but not for a monocrystalline sample. This result was explained by additional reflection of primary electrons from a potential barrier near the sample surface. Therefore for suppression of the deleterious effects of SEE, e.g, for better performance of accelerators, it is important to monitor and control micro electric-fields arising near a polycrystalline surface. [1] A.N. Andronov. St. Petersburg State Polytechnic. Univ. J. Phys. and Math. Sc. V. 1. P. 67. 2014. [2] R.Cimino et.al., Proceedings of IPAC Dresden, Germany.2014. [3] J. Cazaux, J. Appl. Phys$.$ V.111, Ð. 064903. 2012. [4] A. Mustafaev, I. Kaganovich, et.al. Bull. of the APS. V. 60. No 9. Ð. 40. 2015.   

An analytical model of multi-component single frequency capacitively coupled plasma and experimental validation

An analytical model describing the hydrogen added argon capacitively coupled plasma (CPP) is pesented and its predictions are tested with the experimental results. In the analytical model, it is found that the radio frequency (rf) current density, electron temperature and density, as well as the density of ion in multi-component plasma collectively influence the normalized sheath potential and thickness. As for low pressure rf plasma, the sheath potential is the qualitative measure of the DC self bias, the trend of variation of DC self bias with hydrogen addition is predicted in this model. The behavior of single frequency multi-component CPP is experimentally studied by a homogeneous discharge model using discharge parameters. In the experiment with hydrogen added argon plasma, the rf power as well as the working pressures are varied. The addition of hydrogen to the argon discharge leads to a decrease of electron density and DC self bias. It also results an increase of electron temperature. Agreements of the experimental results with theoretical predictions are obtained at different experimental conditions.   

2D probe diagnostics of anisotropic plasmas with no velocity space symmetry

This paper is devoted to the development of the probe method used for analysis of non-equilibrium anisotropic plasmas. The probe method for determination of full electron and ion velocity distribution functions (EVDF) in axially symmetric plasmas was theoretically and experimentally approved$^{\mathrm{1,2}}$. To recover the full EVDF to the Legendre polynomials of order N it is necessary to measure the second derivative of probe current I$^{\mathrm{\mbox{''}}}_{\mathrm{U}}$ in probe's N different orientations. In$^{\mathrm{3}}$ there are the theoretical principles of the EVDF recovering method for plasmas with no velocity space symmetry. To determine the full EVFD with the same degree of accuracy it is necessary to measure I$^{\mathrm{\mbox{''}}}_{\mathrm{U}}$ in flat probe's N$^{\mathrm{2}}$ orientations. This paper gives further development of probe method. While restoring the full EVDF in plasma objects with bilateral symmetry it became possible to reduce the number of the probe's angular orientations by two. It opens up new possibilities to obtain new information about Langmuir paradox in plasma$^{\mathrm{4}}$. [1] Mustafaev A.S. et al. London: Plenum Press. V.367. P.531. 1998. [2] Mustafaev A.S., Soukhomlinov V.S., et al. Techn. Phys. V.60. P. 1778. 2015. [3]. R. Woods C., Sudit I. Phys.Rew. V.50.¹3. P.222. 1994. [4]. Godyak V. et.al. Plasma Sour. Sci. Techn. 24.052001 (5pp). 2015.   

Photoionization sensors for non-invasive medical diagnostics

The analysis of biomarkers can help to identify the significant number of diseases: lung cancer, tuberculosis, diabetes, high levels of stress, psychosomatic disorders etc. To implement continuous monitoring of the state of human health, compact VUV photoionization detector with current-voltage measurement is designed by Saint-Petersburg Mining University Plasma Research Group. This sensor is based on the patented method of stabilization of electric parameters -- CES (Collisional Electron Spectroscopy). During the operation at atmospheric pressure VUV photoionization sensor measures the energy of electrons, produced in the ionization with the resonance photons, whose wavelength situated in the vacuum ultraviolet (VUV). A special software was developed to obtain the second-order derivative of the I--U characteristics, taken by the VUV sensor, to construct the energy spectra of the characteristic electrons. VUV photoionization detector has an unique set of parameters: small size (10*10*1 mm), low cost, wide range of recognizable molecules, as well as accuracy, sufficient for using this instrument for the medical purposes. This device can be used for non-invasive medical diagnostics without compromising the quality of life, for control of environment and human life.   

Modeling of plasma in a hybrid electric propulsion for small satellites.

As space flight becomes more available and reliable, space-based technology is allowing for smaller and more cost-effective satellites to be produced. Working in large swarms, many small satellites can provide additional capabilities while reducing risk. These satellites require efficient, long term propulsion for manoeuvres, orbit maintenance and de-orbiting. The high exhaust velocity and propellant efficiency of electric propulsion makes it ideally suited for low thrust missions. The two dominant types of electric propulsion, namely ion thrusters and Hall thrusters, excel in different mission types. In this work, a novel electric hybrid propulsion design is modelled to enhance understanding of key phenomena and evaluate performance. Specifically, the modelled hybrid thruster seeks to overcome issues with existing Ion and Hall thruster designs. Scaling issues and optimization of the design will be discussed and will investigate a conceptual design of a hybrid spacecraft plasma engine.   

Multiply charged ion generation according to magnetic field configurations in Hall thruster plasmas

Plasma propulsion is the most promising techniques to operate satellites for low earth orbit as well as deep space exploration. A typical plasma propulsion system is Hall thruster (HT) that uses crossed electromagnetic fields to ionize a propellant gas and to accelerate the ionized gas. In HT the tailoring of magnetic fields is significant due to that the electron confinement in the electromagnetic fields affects thruster performances such as thrust force, specific impulse, power efficiency, and life time. We designed an anode layer HT (TAL) with the magnetic field tailoring. The TAL is possible to keep discharge in 1\textasciitilde 2 kilovolts, which voltage is useful to obtain high specific impulse The magnetic field tailoring is adapted to minimize undesirable heat dissipations and secondary electron emissions at a wall surrounding plasma In presentation, we will report TAL performances including thrust force, specific impulse, and anode efficiency measured by a pendulum thrust stand. This mechanical measurement will be compared to the plasma diagnostics conducted by angular Faraday probe, retarding potential analyzer, and ExB probe   

Magnetic Field Tailored Annular Hall Thruster with Anode Layer

Plasma propulsion system is one of the key components for advanced missions of satellites as well as deep space exploration. A typical plasma propulsion system is Hall effect thruster that uses crossed electric and magnetic fields to ionize a propellant gas and to accelerate the ionized gas to generate momentum. In Hall thruster plasmas, magnetic field configuration is important due to the fact that electron confinement in the electromagnetic fields affects both plasma and ion beam characteristics as well as thruster performance parameters including thrust, specific impulse, power efficiency, and life time. In this work, development of an anode layer Hall thruster (TAL) with magnetic field tailoring has been attempted. The TAL is possible to keep discharge in 1 to 2 kilovolts of anode voltage, which is useful to obtain high specific impulse. The magnetic field tailoring is used to minimize undesirable heat dissipation and secondary electron emission from the wall surrounding the plasma. We will report 3 W and 200 W thrusters performances measured by a pendulum thrust stand according to the magnetic field configuration. Also, the measured result will be compared with the plasma diagnostics conducted by an angular Faraday probe, a retarding potential analyzer, and a ExB probe.   

Affect of an electrostatic wave on optical pumping

Extensive information can be obtained on wave-particle interactions and wave fields by direct measurement of perturbed ion distribution functions using laser-induced fluorescence (LIF). For practical purposes, LIF is normally performed on metastable states -- here we consider singly ionized Argon in an inductively coupled plasma. Wave detection is best performed using phase-coherent detection, but power spectra can be obtained through correlation functions. If laser intensity is increased to obtain a better LIF signal, then the effects of optical pumping will produce systematic effects depending on the collision rates which control metastable population and lifetime. We simulate the wave-detection process using a Lagrangian model for the LIF signal. This approach separates the classical dynamics of the ion orbits from the quantum-state transitions produced by optical pumping. The two dynamics nevertheless become coupled in the presence of an electrostatic wave. The numerical simulation is compared with experimental data from a CW magnetized plasma discharge with externally launched ion acoustic waves.