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IOPscience: latest papers

Transport properties of F − in SF 6 –Xe, SF 6 –Ar and SF 6 –He mixtures

Posted: February 1st, 2012, 6:00pm MST by M Benhenni and M Yousfi

F − mobility was calculated in the gas mixtures SF 6 –Xe, SF 6 –Ar and SF 6 –He from a Monte Carlo code for ion transport simulation. The elastic momentum transfer cross sections were determined from the semi-classical JWKB approximation while the inelastic ones (detachment and charge transfer) were taken from the literature. The resulting collision cross section sets were validated from the good agreement between the calculated F − mobilities in the individual gases and the measured ones. Moreover, the longitudinal and transverse density normalized diffusion coefficients and the reaction rate coefficients were calculated in these mixtures for the case where the share of SF 6 is 50%. Finally, a comparison of our calculated F − mobilities in these mixtures obtained from the Monte Carlo code and those obtained from linear Blanc's law showed good agreement in the case of SF 6 –Xe at low field but strong disa...

Self-consistent modelling of atmospheric micro-plasmas produced by a microwave source

Posted: February 1st, 2012, 6:00pm MST by J Gregório, P Leprince, C Boisse-Laporte and L L Alves

This paper presents the self-consistent modelling of argon micro-plasmas, produced by a microwave source (2.45 GHz) at atmospheric pressure. The source is a microstrip-like transmission line, with a 50–200 µm final gap where the micro-plasmas are created. Simulations use a one-dimensional, stationary code that solves the fluid-type transport equations for electrons, positive ions Ar + and ##IMG## [ej.iop.org] {${\rm Ar}^+_2$} , and the electron mean energy; the rate balance equations for the main neutral species; Poisson's equation for the space-charge electrostatic field; Maxwell's equations for the electromagnetic excitation field; the gas energy balance equation for its temperature distribution; and the kinetic electron Boltzmann equation considering several direct and stepwize electron collision processes. The model uses a kinetic scheme that considers the atomic excited states Ar(4s) an...

Ion energy distributions in laser-produced plasmas with two collinear pulses

Posted: February 1st, 2012, 6:00pm MST by Jon I Apiñaniz, Francisco J Gordillo-Vázquez and Roberto Martínez

The formation and expansion dynamics of laser-produced plasmas was studied by means of the effect of a second delayed laser pulse upon the ion kinetic energy of the plasma created by the first one. Two types of measurements were carried out: ion kinetic energy distributions and overall ion time of flight (t.o.f.). Ion energy distributions were obtained with an electrostatic energy analyser, which allowed the observation of the energy distributions of each charge state separately while the ion t.o.f. signal was measured with an ion probe. Laser power densities ranged from 2 × 10 8 to 2 × 10 9 W cm −2 with 532 nm photons, and studies were extended to Al, Cu and Co targets. The effects of the second laser pulse on the plasma created by the first were very different depending on the interpulse delay. At low delays (from 0 to 10 ns) the second pulse produced an increase of the plasma average charge state and the maximum ion kinetic energy, while at higher...

The effect of small additives of argon on the parameters of a non-uniform microwave discharge in nitrogen at reduced pressures

Posted: February 1st, 2012, 6:00pm MST by Yu A Lebedev, T B Mavludov, I L Epstein, A V Chvyreva and A V Tatarinov

The influence of a small addition of argon (2–5%) on the parameters of a strongly non-uniform microwave discharge (with the electrode microwave discharge as an example) in nitrogen at reduced pressures was studied. Experiments showed that the small addition of Ar strongly affected the discharge: it increased in size, and the power absorbed in the plasma and the emission intensities of nitrogen bands reduced. A self-consistent 2D modeling of the discharge was carried out. The model included the Maxwell, Poisson and Boltzmann equations and a set of balance equations for neutral excited and charged plasma species. The processes involving vibrationally excited ground state molecular nitrogen were taken into account by the well-known analytic expression for the vibrational distribution of molecules in the diffusion approximation. The results of modeling and experiment were in qualitative agreement. Additional information about the discharge allowed us to explain the experimental resul...

A comparative study of dc biased RF impedance probes and Langmuir probes for the measurement of processing plasma parameters

Posted: January 30th, 2012, 6:00pm MST by D R Boris, R F Fernsler and S G Walton

In this work, we compare measurements of electron density ( n e ), plasma potential ( ##IMG## [ej.iop.org] {phiv} p ), electron temperature ( T e ), and electron energy distribution function (EEDF or f ( E )) made with a dc biased RF probe and a Langmuir probe. The measurements show good agreement between the two probes across an order of magnitude in plasma density.

On a dielectric-barrier and a microwave-cavity discharge in synchronized operation—the case of a helium/oxygen mixture

Posted: January 30th, 2012, 6:00pm MST by M Nikolic, S Popovic, J Upadhyay, L Vuskovic, R Leiweke and B Ganguly

We have investigated the performance of a microwave-cavity discharge (MWD) operating in tandem with a fast rise-time pulsed dielectric-barrier discharge (DBD). The tandem discharge operated in a helium/oxygen mixture, where metastable molecular oxygen could be produced efficiently using MWD in proportionally large quantities (order of 20% of total oxygen number density). In this new arrangement, a DBD operating at high E / N provided a metastables-rich mixture, thereby modifying the discharge kinetics of the MWD, which operated in the E / N range centered around 10 Td. Both discharges operated in synchronized pulse-repetitive mode, which was tailored to maximize the oxygen metastable production efficiency. The system operated at pressures up to 350 Torr with an average power between 3 and 20 W.

Laser scattering on an atmospheric pressure plasma jet: disentangling Rayleigh, Raman and Thomson scattering

Posted: January 30th, 2012, 6:00pm MST by A F H van Gessel, E A D Carbone, P J Bruggeman and J J A M van der Mullen

Laser scattering provides a very direct method for measuring the local densities and temperatures inside a plasma. We present new experimental results of laser scattering on an argon atmospheric pressure microwave plasma jet operating in an air environment. The plasma is very small so a high spatial resolution is required to study the effect of the penetration of air molecules into the plasma. The scattering signal has three overlapping contributions: Rayleigh scattering from heavy particles, Thomson scattering from free electrons and Raman scattering from molecules. The Rayleigh scattering signal is filtered out optically with a triple grating spectrometer. The disentanglement of the Thomson and Raman signals is done with a newly designed fitting method. With a single measurement we determine profiles of the electron temperature, electron density, gas temperature, partial air pressure and the N 2 /O 2 ratio, with a spatial resolution of 50 µm, and including ab...

Uniformity of a dielectric barrier glow discharge: experiments and two-dimensional modeling

Posted: January 30th, 2012, 6:00pm MST by Xiaoxi Duan, Feng He and Jiting Ouyang

The experimental and calculated results of uniformity in a glow dielectric barrier discharge (DBD) under sub-atmospheric pressures are reported. Driven by a square-wave power source, the discharge in a parallel-electrode DBD system shows uniform or various lateral structures under different conditions. There exists a critical frequency below which the DBD is uniform for almost all the applied voltages. Above the critical frequency, a non-uniform (patterned) discharge is observed and the patterned structures change with frequency and voltage. A two-dimensional fluid modeling is performed on this DBD system which shows similar results in agreement with the experiments. The simulations reveal that the distribution of the space electron density at the beginning of each voltage pulse plays an important role in achieving the uniformity. Uniform space charge results in a uniform DBD. The patterned DBD always evolves from the initial uniform state to the eventual non-uniform one. During ...

Permalink for 'Dual frequency capacitive plasmas in Fe and Ni sputter applications: correlation of discharge
properties on thin film properties'

Dual frequency capacitive plasmas in Fe and Ni sputter applications: correlation of discharge properties on thin film properties

Posted: January 30th, 2012, 6:00pm MST by S Bienholz, E Semmler, P Awakowicz, H Brunken and A Ludwig

Dual frequency capacitively coupled plasmas (CCPs) are widely used in (large area) etching and plasma enhanced chemical vapor deposition processes. However, applications in physical vapor deposition (PVD) are still sparse due to the well-established dc magnetron cathode discharges. Nevertheless, there exist critical applications such as ferromagnetic or ceramic thin film deposition which are difficult to handle even for dc magnetron systems. For these materials systems dual frequency CCPs pose a good alternative, because for insulators charging can be avoided and for ferromagnetic materials the target thickness becomes independent of the magnetron configuration at comparable deposition rates. In this work we investigate two separate subjects. First, in dual frequency capacitive discharges a complex coupling of the applied excitation frequencies can be observed, which from a plasma parameter point of view limits the separability of ion flux (usually controlled by frequencies...

Pulsed, unstable and magnetized fireballs

Posted: January 30th, 2012, 6:00pm MST by R L Stenzel, J Gruenwald, C Ionita and R Schrittwieser

Fireballs are luminous regions produced by double layers in front of positively biased electrodes in plasmas. Although fireballs have been investigated previously there are a great variety of unexplained nonlinear phenomena, some of which are addressed in this work. First, it is shown that a fireball is not an isolated local phenomenon but an integral part of the entire discharge plasma. Current closure and limits are discussed. Fireballs with currents from milliamperes to tens of amperes are created depending on whether the electron source is temperature limited, space-charge limited or limited by ion currents in afterglow plasmas. Fireballs are created with highly transparent grids which allow electron transmission through the electrodes and optimize the ionization efficiency. The physics of pulsating fireballs is investigated. Fireballs disrupt when density outflow exceeds production, leading to density collapse and current disruption when the electron drift exceeds the Bunema...

Plasma deposition in an ideal showerhead reactor: a two-dimensional analytical solution

Posted: January 30th, 2012, 6:00pm MST by A A Howling, B Legradic, M Chesaux and Ch Hollenstein

The interplay of gas flow and depletion by plasma dissociation determines the spatial distribution of species and the deposition uniformity in a plasma source. Many plasma reactors use a gas showerhead and the design of the flow dynamics is a critical aspect of the reactor performance. In this paper, plasma deposition is considered as chemically reacting gas flow in an ideal showerhead reactor. The gas fluid flow is described by finite-gap stagnation-point creeping flow. The distribution of neutral species across the electrode gap is determined by diffusion equations, whereas their lateral transport is purely convective. Parameters relevant to large-area radio-frequency plasma deposition are particularly suitable for a complete analytical solution of the multi-component transport. A representative reaction scheme for hydrogen/silane plasma deposition is used for an analytical example from first principles which shows good agreement with numerical simulation. For a laterally unifo...

Modelling chemical reactions in dc plasma inside oxygen bubbles in water

Posted: January 30th, 2012, 6:00pm MST by N Takeuchi, Y Ishii and K Yasuoka

Plasmas generated inside oxygen bubbles in water have been developed for water purification. Zero-dimensional numerical simulations were used to investigate the chemical reactions in plasmas driven by dc voltage. The numerical and experimental results of the concentrations of hydrogen peroxide and ozone in the solution were compared with a discharge current between 1 and 7 mA. Upon increasing the water vapour concentration inside bubbles, we saw from the numerical results that the concentration of hydrogen peroxide increased with discharge current, whereas the concentration of ozone decreased. This finding agreed with the experimental results. With an increase in the discharge current, the heat flux from the plasma to the solution increased, and a large amount of water was probably vaporized into the bubbles.

Plasmas as metamaterials: a review

Posted: January 30th, 2012, 6:00pm MST by Osamu Sakai and Kunihide Tachibana

When we form a structure of plasmas distributed in a certain space in which electromagnetic waves propagate, such a plasma structure serves as a different medium from a homogeneous bulk plasma. We can also enhance or generate novel functions of the plasmas when we add other structural materials such as functional components. That is to say, when we estimate such a medium from the material properties such as permittivity, permeability and conductivity, it shows extraordinary and/or functional effects that arise from the synthesis of the structure. We call such an artificial material a plasma metamaterial. In this review, starting from a fundamental understanding of electromagnetic wave propagation in and around plasmas, we review the new functions of plasmas as metamaterials, including a photonic-crystal-like behavior, a negative refractive index state and a nonlinear bifurcated electric response, by describing specific plasma structures. In addition, we survey some specific appli...

Characterization of a steam plasma jet at atmospheric pressure

Posted: January 30th, 2012, 6:00pm MST by Guohua Ni, Peng Zhao, Cheng Cheng, Ye Song, Hirotaka Toyoda and Yuedong Meng

An atmospheric steam plasma jet generated by an original dc water plasma torch is investigated using electrical and spectroscopic techniques. Because it directly uses the water used for cooling electrodes as the plasma-forming gas, the water plasma torch has high thermal efficiency and a compact structure. The operational features of the water plasma torch and the generation of the steam plasma jet are analyzed based on the temporal evolution of voltage, current and steam pressure in the arc chamber. The influence of the output characteristics of the power source, the fluctuation of the arc and current intensity on the unsteadiness of the steam plasma jet is studied. The restrike mode is identified as the fluctuation characteristic of the steam arc, which contributes significantly to the instabilities of the steam plasma jet. In addition, the emission spectroscopic technique is employed to diagnose the steam plasma. The axial distributions of plasma parameters in the steam plasma...

Hydrogen production from methane using an RF plasma source in total nonambipolar flow

Posted: January 30th, 2012, 6:00pm MST by Benjamin W Longmier, Alec D Gallimore and Noah Hershkowitz

A radio-frequency (RF) helicon plasma reaction chamber (HPRC) is developed and used to decompose methane gas into high-purity hydrogen gas and solid carbon in the form of graphite. A single-turn ( m = 0) helicon antenna, operated at 13.56 MHz, and a 100 G dipole magnetic field are used to excite a helicon mode in a nonthermal plasma, creating plasma densities exceeding 10 13 cm −3 using 8–20 SCCM methane gas at up to 1300 W of RF power. The HPRC device takes advantage of a uniform large amplitude electron sheath across the exit aperture. At this aperture, all of the incident electron flux from the plasma is extracted and all ions are reflected back into the source. In this way, only neutrals and electrons are allowed out of the reaction chamber, enhancing the breakdown of methane into deposited carbon and hydrogen gas that escapes. A methane decomposition percentage of 99.99 ± 0.06% is demonstrated using 1300 W of RF power and a methane gas flow rate of ...

More evidence for azimuthal ion spin in HiPIMS discharges

Posted: December 18th, 2011, 6:00pm MST by P Poolcharuansin, B Liebig and J W Bradley

The velocity and energy distribution functions of ions escaping radially from the magnetic trap region of a HiPIMS discharge have been measured using a retarding field analyzer (RFA). Spatially and angularly resolved measurements recorded at a representative time show more energetic ions detected along a line-of-sight coincident with an oncoming rotating ion fluid, which circulates above the racetrack in the same direction as the electron E × B drift. The difference in the mean ion energies between measurements made into and against the direction of rotation is ~5 eV. Numerical solutions of the equation of motion for the ions accounting for azimuthal acceleration (modified two-stream instability model used by Lundin et al ) have been found. The centripetal force caused by the radial electric field and a drag force term accounting for ion collisions revealed that only a small fraction (typically

Characterization of an asymmetric parallel plate radio-frequency discharge using a retarding field energy analyzer

Posted: December 18th, 2011, 6:00pm MST by D Gahan, S Daniels, C Hayden, D O' Sullivan and M B Hopkins

A retarding field energy analyzer is used to characterize an asymmetric, 13.56 MHz driven, capacitively coupled, parallel plate discharge operated at low pressure. The characterization is carried out in argon discharges at 10 and 20 mTorr where the sheaths are assumed to be collisionless. The analyzer is set in the powered electrode where the impacting ion and electron energy distributions are measured for a range of discharge powers. A circuit model of the discharge is used to infer important electrical parameters from the measured energy distributions, including electrode excitation voltages, plasma potential and sheath potentials. Analytical models of the ion energy distribution in a radio-frequency sheath are used to determine plasma parameters such as sheath width, ion transit time, electron temperature and ion flux. A radio-frequency compensated Langmuir probe is used for comparison with the retarding field analyzer measurements.

Author index with titles

Posted: November 29th, 2011, 6:00pm MST

The PDF file provided contains web links to all articles in this volume.

Double magnetron self-sputtering in HiPIMS discharges

Posted: November 28th, 2011, 6:00pm MST by O V Vozniy, D Duday, A Lejars and T Wirtz

There is an increasing concern that high power impulse magnetron sputtering (HiPIMS) systems are being disadvantaged by relatively low deposition rates compared with traditional dc magnetrons operating at the same average power input. Nevertheless, a minimization of the losses of ionized and neutral species is possible in HiPIMS discharges. In the described magnetron configuration, the majority of metal ions escaping the discharge volume can be either used for deposition or, if they fail to reach the substrate, for discharge maintenance when charge redistribution from one ionization area to the other takes place. The anode-to-cathode configuration allows the neutrals, which are not deposited on the substrate, to be guided to the other instantaneous cathode and be ionized or re-sputtered. For the same average power input, it becomes possible to increase the self-sputtering efficiency, deposition rate and obtain a versatile control over the film microstructure.

Measurement of plasma parameters in the far-field plume of a Hall effect thruster

Posted: November 28th, 2011, 6:00pm MST by K Dannenmayer, P Kudrna, M Tichý and S Mazouffre

The far-field plume of a 1.5 kW Hall effect thruster is mapped with a Langmuir probe and an emissive probe. Time-averaged measurements of the plasma potential, the electron temperature and the electron number density are performed for different operating conditions of the thruster. The influence of the discharge voltage, the cathode mass flow rate as well as the magnetic field strength is investigated. The plasma potential decreases from 30 V at 300 mm on the thruster axis to 5 V at 660 mm and at 60°, the electron temperature decreases from 5 to 1.5 eV. The electron number density drops from 3.5 × 10 16 to 1 × 10 15 m −3 in the far-field plume. The values of the plasma potential and electron temperature measured with the Langmuir probe and the emissive probe are in good agreement.

Langmuir probe characterization of low-frequency oscillations in a radio-frequency SF 6 plasma

Posted: November 28th, 2011, 6:00pm MST by A J Barlow, A Deslandes and J S Quinton

A low-frequency oscillation (

Power dissipation, gas temperatures and electron densities of cold atmospheric pressure helium and argon RF plasma jets

Posted: November 28th, 2011, 6:00pm MST by S Hofmann, A F H van Gessel, T Verreycken and P Bruggeman

A set of diagnostic methods to obtain the plasma parameters including power dissipation, gas temperature and electron density is evaluated for an atmospheric pressure helium or argon radio frequency (RF) plasma needle for biomedical applications operated in open air. The power density of the plasma is more or less constant and equal to 1.3 × 10 9 W m −3 . Different methods are investigated and evaluated to obtain the gas temperature. In this paper the gas temperatures obtained by rotational spectra of OH( A – X ) and ##IMG## [ej.iop.org] {{\rm N}_2^+} ( B – X ) are compared with Rayleigh scattering measurements and measurements of the line broadening of hydrogen and helium emission lines. The obtained gas temperature ranges from 300 to 650 K, depending on the gas. The electron densities are estimated from the Stark broadening of the hydrogen α and β lines which yi...

An atmospheric-pressure plasma brush driven by sub-microsecond voltage pulses

Posted: November 28th, 2011, 6:00pm MST by X Lu, S Wu, Paul K Chu, D Liu and Y Pan

An atmospheric-pressure room-temperature plasma brush, which can deliver uniform surface treatment effects, is reported. The plasma structure, which includes the negative glow, Faraday dark space and positive column, is clearly visible to the naked eye. The width of the Faraday dark space diminishes with decreasing gap distance and this phenomenon is different from that observed from low-pressure glow discharge plasmas. High-speed photographs taken at an exposure time of 2.5 ns show that the plasma propagates from the nozzle to the object in about 100 ns and 10 ns for gap distances of 6 mm and 2 mm, respectively, and the results are consistent with electric measurements. The emission spectra reveal N 2 ( B – A ) bands in addition to those of O, ##IMG## [ej.iop.org] {{\rm N}_2^+} , N 2 ( C – B ) and He, indicating that the plasma source is reactive and suitable for a...

Effects of anode temperature on the arc volt–ampere characteristics and ejected plume property of a low-power supersonic plasma

Posted: November 21st, 2011, 6:00pm MST by W X Pan, X Meng, H J Huang and C K Wu

Low-power plasma generators with two kinds of hot anode/nozzle structures, one with a natural radiation-cooled nozzle and the other with a regeneratively cooled nozzle, were designed to investigate the dependence of the volt–ampere characteristics on the anode temperature. Pure argon, nitrogen or hydrogen gas was used as the plasma working gas at input powers from 130 to 1200 W in a plenum chamber kept at a pressure of below 20 Pa. Variations of the arc voltage with changes in arc current, gas flow rate and firing time (anode temperature) were examined, and the effects of the arc volt–ampere characteristics on the properties of the ejected plasma flow from the nozzle exit are discussed with respect to the evaluation of the average plume temperature and flow velocity. Results show that there are definitely non-negligible effects of anode temperature on these characteristics.

An ionization region model for high-power impulse magnetron sputtering discharges

Posted: November 21st, 2011, 6:00pm MST by M A Raadu, I Axnäs, J T Gudmundsson, C Huo and N Brenning

A time-dependent plasma discharge model has been developed for the ionization region in a high-power impulse magnetron sputtering (HiPIMS) discharge. It provides a flexible modeling tool to explore, e.g., the temporal variations of the ionized fractions of the working gas and the sputtered vapor, the electron density and temperature, and the gas rarefaction and refill processes. A separation is made between aspects that can be followed with a certain precision, based on known data, such as excitation rates, sputtering and secondary emission yield, and aspects that need to be treated as uncertain and defined by assumptions. The input parameters in the model can be changed to fit different specific applications. Examples of such changes are the gas and target material, the electric pulse forms of current and voltage, and the device geometry. A basic version, ionization region model I, using a thermal electron population, singly charged ions, and ion losses by isotropic diffusion is...

Emissive probes

Posted: November 17th, 2011, 6:00pm MST by J P Sheehan and N Hershkowitz

For over 80 years emissive probes have been used to measure plasma potential and a wide variety of methods for interpreting probe data now exists. Constructions, heating methods and measurement techniques are reviewed in detail and their various strengths and limitations are compared. Additionally, several novel uses for emissive probes, such as measuring electron temperature are presented. This review also includes tables of recommendations for emissive probe design given the type of plasma and desired measurements.

A plasma diagnostic technique using a floating probe for the dielectric deposition process

Posted: November 7th, 2011, 6:00pm MST by Jin-Young Bang, Kyoung Yoo, Dong-Hwan Kim and Chin-Wook Chung

Low-temperature RF discharges have been widely used in processing applications. Plasma diagnostics provide useful information about the plasma state which is important for processing results. In the deposition process, as the dielectric material is coated onto the probe surface, electrical diagnostic techniques using a dc current cannot be applied. Instead, an ac voltage is applicable for measuring plasma parameters. In this paper, electron temperatures and plasma densities were measured with an anodized aluminum probe using the floating-type harmonic method and the self-bias method. The Al 2 O 3 layer on the probe surface and the sheath were modeled as a series connection of a capacitor and a resistor, respectively. The applied ac voltage was divided into the two parts depending on their impedances, and the voltage across the sheath was determined by the phase between the voltage and the current. According to experimental results, the conventional harmonic met...

Permalink for 'Structure and dynamics of a magnetic neutral loop discharge plasma described using electron motion
in a quadrupole magnetic field'

Structure and dynamics of a magnetic neutral loop discharge plasma described using electron motion in a quadrupole magnetic field

Posted: November 7th, 2011, 6:00pm MST by Tsuyoshi Osaga, Hirotake Sugawara and Yohei Sakurai

Electron motion in a CF 4 magnetic neutral loop discharge (NLD) plasma was simulated using a Monte Carlo method. The spatial distribution of electrons illustrated the fundamental structure of the NLD plasma, and its dynamics were depicted from the distributions of the mean electron energy, the electron energy gain and the azimuthal electron flux. The peak of mean electron energy appeared at the neutral loop (NL), which confirmed that electrons gain energy near this loop. High mean electron energy was observed not only near the NL but also along the separatrices of the quadrupole magnetic field. Energetic electrons were transported along the separatrices and induced ionization at those locations. However, the electron distribution had valleys along the separatrices, because electrons accelerated near the NL were likely to leave this region where the binding of the magnetic field is weak. The azimuthal electron flux representing the loop plasma current showed that the ele...

Permalink for 'The relationship between the distribution of anode striations and negative and positive charge
accumulation in a plasma display panel'

The relationship between the distribution of anode striations and negative and positive charge accumulation in a plasma display panel

Posted: November 7th, 2011, 6:00pm MST by P P Zhang, Y Tu and L L Yang

Anode striations and wall charges, common phenomena in the plasma display panel (PDP) discharge process, are investigated by simulations in both alternating current coplanar and novel shadow mask PDPs. The formation process of striations is presented and the formation mechanism is investigated. The results reveal that in both structures there is an obvious correspondence between the striation distribution and both the negative and the positive charge accumulation on the dielectric layer above the anode. Each of the two contributions to the total wall charge reaches its peak at the positions where the striations emerge. The total wall charge distribution on the other hand, which is the sum of the wall charge collected from electrons and ions, does not show any striation effect at all.

Permalink for 'Axial light emission and Ar metastable densities in a parallel plate dc microdischarge in the steady
state and transient regimes'

Axial light emission and Ar metastable densities in a parallel plate dc microdischarge in the steady state and transient regimes

Posted: November 7th, 2011, 6:00pm MST by T Kuschel, B Niermann, I Stefanović, M Böke, N Škoro, D Marić, Z Lj Petrović and J Winter

Axial emission profiles in a parallel plate dc microdischarge (feedgas: argon; discharge gap d = 1 mm; pressure p = 10 Torr) were studied by means of time-resolved imaging with a fast ICCD camera. Additionally, volt–ampere ( V – A ) characteristics were recorded and Ar* metastable densities were measured by tunable diode laser absorption spectroscopy (TDLAS). Axial emission profiles in the steady-state regime are similar to corresponding profiles in standard size discharges ( d ≈ 1 cm, p ≈ 1 Torr). For some discharge conditions relaxation oscillations are present when the microdischarge switches periodically between the low current Townsend-like mode and the normal glow. At the same time the axial emission profile shows transient behavior, starting with peak distribution at the anode, which gradually moves toward the cathode during the normal glow. The development of argon metastable densities highly correlates with the oscillating discharge c...

Electrical and spectroscopic analysis of mono- and multi-tip pulsed corona discharges in air at atmospheric pressure

Posted: November 7th, 2011, 6:00pm MST by A Mraihi, N Merbahi, M Yousfi, A Abahazem and O Eichwald

This work is devoted to the analysis of experimental results obtained in dry air at atmospheric pressure in a positive point-to-plane corona discharge under a pulsed applied voltage in the cases of anodic mono- and multi-tips. In the mono-tip case, the peak corona current is analysed as a function of several experimental parameters such as magnitude, frequency and duration of pulsed voltage and gap distance. The variation of the corona discharge current is correlated with the ozone production. Then in the multi-tip case, the electrical behaviour is analysed as a function of the distance between two contiguous tips and the tip number in order to highlight the region of creation active species for the lowest dissipated power. Intensified charge-coupled device pictures and electric field calculations as a function of inter-tip distance are performed to analyse the mutual effect between two contiguous tips. The optical emission spectra are measured in the UV–visible–NIR wavelength ra...

Discharge expansion in barrier discharge arrangements at low applied voltages

Posted: September 29th, 2011, 6:00pm MDT by P Reichen, A Sonnenfeld and Ph Rudolf von Rohr

This study evaluates the spatial expansion of barrier discharges (BDs) in oxygen, carbon dioxide, synthetic air, and helium at atmospheric pressure. Despite being confined in narrow gas channels, the BD plasma only partially covers the available discharge area at low applied voltages as was experimentally observed. This is important for homogeneous surface treatment. The appearance of local discharge zones in the plasma development results in a variation of the capacitive behaviour of the equivalent electric circuit as a function of the applied voltage. This transient behaviour of the apparent dielectric capacitance is therefore investigated for various discharge areas and different gas compositions. Furthermore, a semi-empirical model describing the expansion behaviour is proposed and validated by means of the obtained experimental data. The results clearly show that a three times higher voltage excess is required for oxygen-containing gases compared with helium in order to deve...

Time-resolved plasma temperature measurements in a laser-triggered hydrogen-filled capillary discharge waveguide

Posted: September 29th, 2011, 6:00pm MDT by C J Woolley, K O'Keeffe, H K Chung and S M Hooker

Temporally resolved, spatially integrated measurements of the temperature of the plasma channel formed by a hydrogen-filled discharge capillary waveguide are presented. Plasma temperatures of 4–7 eV are measured for peak discharge currents between 80 and 150 A. It is demonstrated that laser-triggering the capillary discharge enables capillary discharges with a peak current as low as 23 A to be driven, reducing the plasma temperature to approximately 3 eV. This plasma temperature meets the requirements of a recently proposed soft x-ray recombination laser.

On reabsorption and radiation trapping in homogeneous and inhomogeneous gas discharge plasmas

Posted: September 27th, 2011, 6:00pm MDT by Yu B Golubovskii, S Gorchakov, D Loffhagen, A Timofeev and D Uhrlandt

Previous descriptions of the methods of radiation transport are generalized for two cases: small (order of unity) absorption coefficient and large coefficient with inhomogeneous distribution of absorbing atoms. Computation algorithms for the matrices equivalent to the integral operator of the radiation transport are deduced. The methods described are planned for future description of excited and resonance atoms in problems of self-consistent modeling of non-equilibrium plasmas of inert gas. The solution of the transport equation is illustrated by model examples of atom density computation typical for constricted discharge conditions and 'skin-effect' conditions with gas heating taken into account.

Contraction of the positive column of discharges in noble gases

Posted: September 26th, 2011, 6:00pm MDT by Yu B Golubovskii, V Nekuchaev, S Gorchakov and D Uhrlandt

This review describes the experimental studies of contraction in neon, argon and helium, discussing the basic regularities of the phenomenon. These studies, extended over a long time, are still urgent. For pressures that are not too high a noticeable contraction of the plasma glow and a smooth non-monotonic dependence of the degree of contraction on the current are observed. Above a critical pressure the plasma immediately contracts into a bright thin cord, if the current reaches a critical value. A hysteresis phenomenon is observed during the transition from the diffuse state to the contracted state and vice versa. Experiments that show the secondary role of non-homogeneous gas heating for contraction in neon and argon, and the main role for contraction in helium, are described. Studies of the ionization waves (the strata), which propagate as pulses of the current cord area, are reviewed showing the close relationship between contraction and stratification. The roles of various ...

Analytical approaches to glow discharge problems

Posted: September 19th, 2011, 6:00pm MDT by L D Tsendin

Combined experimental, numerical and analytical approaches are presented to address actual problems of glow discharge physics. Such a synthesis is able to clarify relatively simple, qualitative, self-consistent phenomena, and should allow improvement of the existing paradigms and/or formulation of novel approaches. The following illustrative examples are discussed: * Linear and non-linear diffusion in multispecies, currentless and current-carrying low-temperature plasmas. Both ambipolar diffusion and ambipolar mobility are considered. * Non-local electron kinetics in radio frequency and direct current glows. * Structures of the cathode and anode regions in direct current glows.

$Permalink for 'Electron and gas temperature dependences of the dissociative recombination coefficient of molecular ions ##IMG## [http://ej.iop.org/images/0963-0252/20/5/055012/toc_psst394258in001.gif] {{\rm Ar}_2^+} with electrons'$

Electron and gas temperature dependences of the dissociative recombination coefficient of molecular ions ##IMG## [http://ej.iop.org/images/0963-0252/20/5/055012/toc_psst394258in001.gif] {{\rm Ar}_2^+} with electrons

Posted: September 19th, 2011, 6:00pm MDT by P Lukáč, O Mikuš, I Morva, Z Zábudlá, J Trnovec and M Morvová

This paper presents the measured values of the total dissociative recombination coefficient of molecular ##IMG## [ej.iop.org] {{\rm Ar}_2^+} ions with electrons ##IMG## [ej.iop.org] {\alpha({\rm Ar}_2^+)} as a function of both electron and gas temperatures. The measured recombination coefficient can be expressed as ##IMG## [ej.iop.org] {\[ \begin{equation*} \alpha _{{\rm Ar}_2^+ } (T_{\rm e} ,T_{{\rm gas}})=(8.1\pm 0.5)\times 10^{-7}(T_{\rm e} /300\,{\rm K})^{-0.64}(T_{{\rm gas}} /300\,{\rm K})^{-0.86}\,{\rm cm}^3\,{\rm s}^{-1} \end{equation*} \]} in the temperature ranges 300 K ≤ T e ≤ 10 400 K and 300 K ≤ T gas ≤ 500 K.

Multi-dimensional optical and laser-based diagnostics of low-temperature ionized plasma discharges

Posted: September 14th, 2011, 6:00pm MDT by E V Barnat

A review of work centered on the utilization of multi-dimensional optical diagnostics to study phenomena arising in radiofrequency plasma discharges is given. The diagnostics range from passive techniques such as optical emission to more active techniques utilizing nanosecond lasers capable of both high temporal and spatial resolution. In this review, emphasis is placed on observations that would have been more difficult, if not impossible, to make without the use of such diagnostic techniques. Examples include the sheath structure around an electrode consisting of two different metals, double layers that arise in magnetized hydrogen discharges, or a large region of depleted argon 1s 4 levels around a biased probe in an rf discharge.

Excitation dynamics of a kHz driven micro-structured plasma channel device operated in argon

Posted: September 12th, 2011, 6:00pm MDT by A Greb, H Boettner, J Winter and V Schulz-von der Gathen

The excitation dynamics of a micro-structured plasma channel device with an inverted equilateral triangular cross-section are investigated. This device is operated in argon close to atmospheric pressure with an ac voltage. It is characterized by means of electrical measurements and optical emission spectroscopy. One emission phase is observed in each half-cycle of the applied voltage waveform. Each shows self-pulsing of the device. The integrated intensities for both phases differ by a factor of about two. Phase-resolved wavelength-integrated optical emission spectroscopy which resolves the intensity profile of the plasma channel reveals clear differences between positive and negative half period. Emission features propagating along the discharge channel at velocities of a few km s −1 are observed. Discharge dynamics are compared with those reported for micro-structured atmospheric pressure plasma arrays with discharge cavities of similar triangular cross-section. The c...

Characterization of a surface dielectric barrier discharge plasma sustained by repetitive nanosecond pulses

Posted: September 5th, 2011, 6:00pm MDT by Keisuke Takashima (Udagawa), Yvette Zuzeek, Walter R Lempert and Igor V Adamovich

The paper discusses experimental characterization of a surface dielectric barrier discharge plasma sustained by repetitive, high-voltage, nanosecond duration pulses. The discharge pulse energy is controlled primarily by the pulse peak voltage and scales approximately linearly with the length of the electrodes. Images of the plasma generated during the discharge pulse, taken by a nanosecond gate ICCD camera, show that the plasma remains fairly uniform in the initial phase of the discharge and becomes filamentary at a later stage. The temperature rise in the discharge, operated in both continuous mode and in burst mode, is inferred from UV/visible emission spectra. Phase-locked schlieren images are used to measure the speed of the compression waves generated by the nanosecond pulse discharge and the density gradient in the wave. The density gradient is inferred from the schlieren images using absolute calibration by a pair of wedged windows. The results demonstrate that discharge f...

Current-free double layers in helicon sources

Posted: August 24th, 2011, 6:00pm MDT by Yu-Jeng Su and K C Shaing

A model for the formation of a stationary current-free double layer in collisionless plasmas expanding in a magnetic nozzle is presented. The model consists of the dynamics of cold ions, isothermal hot electrons and fere-isothermal, i.e. almost isothermal, cold electrons. It can determine the position and amplitude of the double layer including the jump in cold electron temperature across the layer. The magnitude of the jump is consistent with experimental observations. Plasmas are accelerated mainly by the magnetic nozzle and the contribution from the double layer is small. The important effects of the ion charge state Z on the flow speed at the nozzle throat and at the exit are also discussed. It is found that the Mach number at the magnetic nozzle throat is ##IMG## [ej.iop.org] {\sqrt Z} which can be tested in experiments. The exit velocity also scales as ##IMG## {\sqrt Z}

Studies of plasma breakdown and electron heating on a 14 GHz ECR ion source through measurement of plasma bremsstrahlung

Posted: August 18th, 2011, 6:00pm MDT by T Ropponen, O Tarvainen, I Izotov, J Noland, V Toivanen, G Machicoane, D Leitner, H Koivisto, T Kalvas, P Peura, P Jones, V Skalyga and V Zorin

Temporal evolution of plasma bremsstrahlung emitted by a 14 GHz electron cyclotron resonance ion source (ECRIS) operated in pulsed mode is presented in the energy range 1.5–400 keV with 100 µs resolution. Such a high temporal resolution together with this energy range has never been measured before with an ECRIS. Data are presented as a function of microwave power, neutral gas pressure, magnetic field configuration and seed electron density. The saturation time of the bremsstrahlung count rate is almost independent of the photon energy up to 100 keV and exhibits similar characteristics with the neutral gas balance. The average photon energy during the plasma breakdown is significantly higher than that during the steady state and depends strongly on the density of seed electrons. The results are consistent with a theoretical model describing the evolution of the electron energy distribution function during the preglow transient.

Permalink for 'The role of helium metastable states in radio-frequency driven helium–oxygen atmospheric pressure
plasma jets: measurement and numerical simulation'

The role of helium metastable states in radio-frequency driven helium–oxygen atmospheric pressure plasma jets: measurement and numerical simulation

Posted: August 15th, 2011, 6:00pm MDT by K Niemi, J Waskoenig, N Sadeghi, T Gans and D O'Connell

Absolute densities of metastable He(2 3 S 1 ) atoms were measured line-of-sight integrated along the discharge channel of a capacitively coupled radio-frequency driven atmospheric pressure plasma jet operated in technologically relevant helium–oxygen mixtures by tunable diode-laser absorption spectroscopy. The dependences of the He(2 3 S 1 ) density in the homogeneous-glow-like α-mode plasma with oxygen admixtures up to 1% were investigated. The results are compared with a one-dimensional numerical simulation, which includes a semi-kinetical treatment of the pronounced electron dynamics and the complex plasma chemistry (in total 20 species and 184 reactions). Very good agreement between measurement and simulation is found. The main formation mechanisms for metastable helium atoms are identified and analyzed, including their pronounced spatio-temporal dynamics. Penning ionization through helium metastables is found to be significant for plasma s...

Optical diagnostics for characterization of electron energy distributions: argon inductively coupled plasmas

Posted: August 15th, 2011, 6:00pm MDT by John B Boffard, R O Jung, Chun C Lin, L E Aneskavich and A E Wendt

The assumption of thermodynamic equilibrium in low-temperature plasmas can lead to errors in determination of electron energy distribution functions (EEDFs) using optical diagnostics. Significant improvements in the accuracy of EEDFs determined using optical diagnostics on argon-containing inductively coupled plasmas (ICPs) are possible by fitting an analytic representation of the EEDF with two adjustable parameters ( x and T x ) to recorded optical emission spectra. This so-called x -form of the EEDF captures the well-known suppression of the high-energy portion of the EEDF (compared with the Maxwellian form) observed in ICPs. A review of electron kinetics summarizes the physical mechanisms that lead to this form, and explains the weak dependence of x on operating pressure. For the 1–25 mTorr pressure range of argon-containing mixtures in the system examined, the fixed value x = 1.2 is found to represent the EEDF very well, follow...

Discharge resistance and power dissipation in the self-pulsing regime of micro-hollow cathode discharges

Posted: August 10th, 2011, 6:00pm MDT by C Lazzaroni and P Chabert

Micro-hollow cathode discharges (MHCDs), driven by a dc voltage source, may operate in a self-pulsing regime during which the voltage and the current across the discharge are pulsed with a frequency of several tens of kilohertz. A model for the self-pulsing regime of MHCDs is proposed based on an equivalent electrical circuit of the whole device. The discharge itself is modeled by a non-linear resistance in series with an inductance, and it is placed in parallel with the capacitance of the electrode–dielectric–electrode sandwich hosting the discharge micro-hole. The capacitance of the coaxial cable used to feed the device is also included. It is shown that a detailed comparison between theory and experiments allows the discharge non-linear resistance to be accurately determined. When the discharge current is maximum, this resistance is about 2500 Ω at 50 Torr and decreases to about 750 Ω at 150 Torr. The absorbed power is calculated throughout the self-pulsing cycle and may be ab...

Control of ion energy distributions using a pulsed plasma with synchronous bias on a boundary electrode

Posted: August 9th, 2011, 6:00pm MDT by Hyungjoo Shin, Weiye Zhu, Lin Xu, Vincent M Donnelly and Demetre J Economou

Ion energy distributions (IEDs) on a grounded substrate in a Faraday-shielded argon inductively coupled plasma were measured with a retarding field energy analyzer. A Langmuir probe was also used to measure space- and time-resolved plasma parameters. IEDs and plasma parameters were studied with continuous or pulsed positive dc bias voltage on a 'boundary electrode' in contact with the plasma. For continuous wave plasmas without applied bias, the IED exhibited a single broad peak at the plasma potential. Applying a continuous positive dc bias on the boundary electrode shifted the peak of the IED to higher energy. Application of a synchronous dc bias on the boundary electrode during the afterglow of a pulsed plasma resulted in a double-peaked IED. The mean energies of the two peaks, as well as the peak separation, were controlled by adjusting the applied dc bias and the discharge pressure. The full width at half maximum (FWHM) of the peak corresponding to the synchronous dc bias di...

Permalink for 'Rectification effect of gradient antiparallel magnetic fields on electron conduction in a gas under
a radio-frequency electric field'

Rectification effect of gradient antiparallel magnetic fields on electron conduction in a gas under a radio-frequency electric field

Posted: August 9th, 2011, 6:00pm MDT by Hirotake Sugawara, Tsuyoshi Osaga and Hiromori Yamamoto

Electron transport in CF 4 under linearly gradient antiparallel magnetic fields was analysed in order to investigate the fundamental properties of magnetic neutral loop discharge plasmas used for material processing. The electron motion was simulated by a Monte Carlo method under a radio-frequency (rf) electric field applied perpendicularly to the directions of both the magnetic field and its gradient. Two typical electron motions, meandering in a weak magnetic field and gyration in a strong magnetic field, were observed with particular directionalities. The meandering electrons drifted forward on average similarly to those under a dc electric field. The gyration induced an electron drift towards the inverse direction. The direction of electron flux was dependent not only on the rf phase but also on the distance from the magnetically neutral midplane between the antiparallel magnetic fields. The electron conduction path formed along the midplane had a structure consisti...

The temporal evolution in plasma potential during laser photo-detachment used to diagnose electronegative plasma

Posted: August 9th, 2011, 6:00pm MDT by N Sirse, S K Karkari, M A Mujawar, J Conway and M M Turner

A floating emissive probe is applied in conjunction with pulsed laser photo-detachment of O − ions to enable measurement of the dynamic evolution in a plasma potential resulting from the presence of photoelectrons in a 13.56 MHz inductive radio-frequency oxygen discharge. The emissive probe emits thermionic electrons, allowing it to reach a saturation potential which is characterized as the local space potential of the plasma. After the photo-detachment pulse, the local space plasma potential in the illuminated region shoots up to a higher positive value and then relaxes to equilibrium in microsecond time scales. Using the relaxation time of the space potential, the negative ion temperature of O − is estimated over a 10–50 mTorr range and is found to be in the 0.19–0.03 eV range. The negative ion temperature measured by this method is found to be lower than that calculated from the time evolution in electron density resulting from photo-detachment which is ind...

Stable linear plasma arrays at atmospheric pressure

Posted: August 2nd, 2011, 6:00pm MDT by Chen Wu, Alan R Hoskinson and Jeffrey Hopwood

A non-thermal microplasma has been intensively investigated because of its ability to generate high electron densities with low gas temperatures, even at atmospheric pressure. This work demonstrates linear arrays of microplasmas generated in atmospheric pressure argon driven by a series of microstrip resonators. Small arrays of such resonators were previously shown to sustain up to five microplasmas, but intrinsically weak coupling between resonators is shown to be insufficient to form wider uniform arrays. An electrical connection between each resonator is shown to enhance the coupling among resonators, allowing arrays composed of at least 88 elements that extend 11 cm in width. The application of coupled-mode theory to this system shows good agreement with measurements of microplasma emission intensity as well as with electromagnetic simulations of these devices. Operation of arrays at microwave frequencies allows a nearby ground electrode to participate in the resonance. These...

Optical studies of dielectric barrier plasma aerodynamic actuators

Posted: July 6th, 2011, 6:00pm MDT by A Gulec, L Oksuz and N Hershkowitz

The optical characteristics of surface dielectric barrier plasma actuators with a wire/planar electrode combination and 1 kHz triangular applied voltages were studied using an intensified charge-coupled device camera and a monochromator in atmospheric pressure air. The spatial and temporal images were recorded both parallel and perpendicular to the aerodynamic actuator surfaces on nanosecond time scales. Arc-shaped discharges jumping from above the powered wire across the dielectric above the grounded planar electrode were observed for the first time. Nitrogen molecular second and first positive band lines were observed and the discharge temperatures were calculated by matching the experimental spectra with SPECAIR lines (Laux et al 2003 Plasma Sources Sci. Technol. 12 [/0963-0252/12/2/301] 125–138 ). The electron temperatures were found to be 6800 ± 400 K from the SPECAIR code.

Space-resolved velocity and flux distributions of sputtered Ti atoms in a planar circular magnetron discharge

Posted: July 6th, 2011, 6:00pm MDT by C Vitelaru, L de Poucques, T M Minea and G Popa

The tuneable diode-laser-induced fluorescence (TD-LIF) technique was used to investigate both velocity and flux distributions of ground-state titanium atoms sputtered from a planar circular magnetron target working at a low pressure (0.4 Pa) and a high pressure (4 Pa). TD-LIF Doppler profiles were measured in front of the racetrack: (i) normal ( v z ) and parallel ( v r ) to the target at several distances ( z ), and (ii) for different angles (α) at 10 mm from the target, addressing the same local volume. The space variation of the velocity and flux distribution functions was deduced with high accuracy from the Doppler shift. Thus it was possible to determine the energetic over thermalized relative atom flux ratio going away from the cathode. The energy distribution function of Ti sputtered atoms in the normal direction is in good agreement with the extended Thompson's formula only for the collisionless sputtering regime, namely in...