Showing papers in "Plasma Sources Science and Technology in 2009"

SDBD plasma actuator with nanosecond pulse-periodic discharge

Abstract: This paper presents a detailed explanation of the physical mechanism of the nanosecond pulsed surface dielectric barrier discharge (SDBD) effect on the flow. Actuator-induced gas velocities show near-zero values for nanosecond pulses. The measurements performed show overheating in the discharge region on fast (? 1??s) thermalization of the plasma input energy. The mean values of such heating of the plasma layer can reach 70?K, 200?K and even 400?K for 7?ns, 12?ns and 50?ns pulse durations, respectively. The emerging shock wave together with the secondary vortex flows disturbs the main flow. The resulting pulsed-periodic disturbance causes an efficient transversal momentum transfer into the boundary layer and further flow attachment to the airfoil surface. Thus, for periodic pulsed nanosecond dielectric barrier discharge, the main mechanism of impact is the energy transfer and heating of the near-surface gas layer. The following pulse-periodic vortex movement stimulates redistribution of the main flow momentum.

Nanosecond repetitively pulsed discharges in air at atmospheric pressure—the glow regime

Abstract: In atmospheric-pressure air preheated to 1000 K, nanosecond repetitively pulsed (NRP) discharges are shown to generate three plasma discharge regimes. In addition to the well-known corona and spark regimes, there exists a glow-like regime that develops through an initial cathode-directed streamer, followed by a return wave of potential redistribution. The applied electric field is then switched off before the formation of the cathode fall, resulting in an 'imminent' glow discharge. Previously, this regime had been observed only at 2000 K in air at atmospheric pressure. Measurements of the plasma dynamics, current–voltage characteristics, gas temperature and plasma chemistry of the excited species N2(B), N2(C), , NO(A) and O(3p 5P) in the pulsed glow regime are presented. Using 10 ns pulses applied repetitively at 30 kHz, we find that this glow regime generates an estimated electron number density of 1013 cm−3, while consuming only 1–10 µJ per pulse and heating the gas by less than 200 K.

Plasma assisted ignition and high-speed flow control: non-thermal and thermal effects

Abstract: The paper reviews recent progress in two rapidly developing engineering applications of plasmas, plasma assisted combustion and plasma assisted high-speed flow control. Experimental and kinetic modeling results demonstrate the key role of non-thermal plasma chemistry in hydrocarbon ignition by uniform, repetitively pulsed, nanosecond pulse duration, low-temperature plasmas. Ignition delay time in premixed ethylene‐air flows excited by the plasma has been measured in a wide range of pulse repetition rates and equivalence ratios and compared with kinetic modeling calculations, showing good agreement. Comparing ignition delay time predicted by the model for plasma assisted ignition and for ignition by equilibrium heating demonstrated that chain reactions of radicals generated by the plasma reduce ignition time by up to two orders of magnitude and ignition temperature by up to 300K. These results provide additional evidence of the non-thermal nature of low-temperature plasma assisted ignition. Experiments and flow modeling show that the dominant mechanism of high-speed plasma flow control is thermal, due to heating of the flow by the plasma. Development and characterization of pulsed dc and pulsed RF localized arc filament plasma actuator arrays for control of high-speed atmospheric pressure jet flows are discussed. Actuator power is quite low, ∼10W at 10% duty cycle. Plasma emission spectra show that a greater fraction of the pulsed RF discharge power goes to heat the flow (up to 2500 ◦ C), while a significant fraction of the pulsed dc discharge power is spent on electrode and wall heating, resulting in their erosion. Rapid localized heating of the flow by the pulsed arc filaments, at a rate of ∼1000K/10 µs, results in the formation of strong compression/shock waves, detected by schlieren imaging. Effect of flow forcing by repetitively pulsed RF actuators is demonstrated in a M = 1.3 axisymmetric jet. These two case studies provide illustrative examples of isolating non-thermal (non-equilibrium plasma chemistry) and thermal (Joule heating) effects in plasmas and adapting them to develop efficient large-volume plasma igniters and high-speed flow actuators. (Some figures in this article are in colour only in the electronic version)

Characterization of a direct dc-excited discharge in water by optical emission spectroscopy

Abstract: Dc-excited discharges generated in water at the tip of a tungsten wire which is located at the orifice of a quartz capillary are investigated by time-averaged optical emission spectroscopyTwo distinctive discharge modes are observed For small conductivities of the liquid the discharge is a streamer-like discharge in the liquid itself (liquid mode) For conductivities above typically 45 µS cm−1 a large vapour bubble is formed and a streamer discharge in this vapour bubble is observed (bubble mode)Plasma temperatures and electron densities are investigated for both modes The gas temperature is estimated from the rotational temperature of N2(C–B) and is 1600 ± 200 K for the bubble mode and 1900 ± 200 K for the liquid mode The rotational temperature of OH(A–X) is up to 2 times larger and cannot be used as an estimate for the gas temperature The rotational population distribution of OH(A), ν = 0 is also non-Boltzmann with a large overpopulation of high rotational states This discrepancy in rotational temperatures is discussed in detailElectron densities are obtained from the Stark broadening of the hydrogen Balmer beta line The electron densities in the liquid mode are of the order of 1021 m−3 In the bubble mode electron densities are significantly smaller: (3–4) × 1020 m−3 These values are compared with the Stark broadening of the hydrogen alpha and gamma lines and with electron densities obtained from current density measurements The chemical reactivities of the bubble and liquid modes are compared by means of the hydrogen peroxide production rate

Generation of atomic oxygen in the effluent of an atmospheric pressure plasma jet

Abstract: The planar 13.56 MHz RF-excited low temperature atmospheric pressure plasma jet (APPJ) investigated in this study is operated with helium feed gas and a small molecular oxygen admixture. The effluent leaving the discharge through the jet's nozzle contains very few charged particles and a high reactive oxygen species' density. As its main reactive radical, essential for numerous applications, the ground state atomic oxygen density in the APPJ's effluent is measured spatially resolved with two-photon absorption laser induced fluorescence spectroscopy. The atomic oxygen density at the nozzle reaches a value of ~1016 cm−3. Even at several centimetres distance still 1% of this initial atomic oxygen density can be detected. Optical emission spectroscopy (OES) reveals the presence of short living excited oxygen atoms up to 10 cm distance from the jet's nozzle. The measured high ground state atomic oxygen density and the unaccounted for presence of excited atomic oxygen require further investigations on a possible energy transfer from the APPJ's discharge region into the effluent: energetic vacuum ultraviolet radiation, measured by OES down to 110 nm, reaches far into the effluent where it is presumed to be responsible for the generation of atomic oxygen.

Microwave-excited atmospheric-pressure microplasmas based on a coaxial transmission line resonator

Abstract: We report the design, fabrication and characterization of two microwave-excited microplasma sources based on coaxial transmission line resonators (CTLR). The sources are capable of generating electric fields of ~106 V m−1 at 900 MHz and 2.45 GHz. These devices can self-ignite helium or argon discharges in a wide pressure range including atmospheric pressure. The gas temperature in an argon discharge open to atmospheric air is ~400 K. Using air as a dielectric, the working gases can be passed through the CTLR, resulting in the formation of plasma jets suitable for surface treatments. The device efficiency on transferring the input power into the plasma is 50–85% depending on the gas used. No thermal damage or electrode erosion has been observed in the devices.

A Langmuir probe system for high power RF-driven negative ion sources on high potential

Abstract: A fully automated Langmuir probe system capable of operating simultaneously with beam extraction has been developed and commissioned for the negative hydrogen ion source testbeds at IPP Garching. It allows the measurement of temporal and spatial distributions of the plasma parameters within a single plasma pulse ( 1018 m−3) and hot (Te > 10 eV) plasma with bi-Maxwellian electron energy distribution at low pressures. The plasma found near the plasma grid is very different being of low density (≤1017 m−3) and very cold (Te < 2 eV). This plasma is also strongly influenced by the presence of caesium, the potential of the plasma grid, and if an ion beam is extracted from the source. Caesium strongly reduces the plasma potential of the source and enhances the negative ion density near the plasma grid. Extracting an ion beam is observed to reduce the electron density and increase the potential near the plasma grid. Applying a potential greater than the plasma potential to the plasma grid is found to significantly decrease the electron density near the plasma grid.

Time dependent optical emission spectroscopy of sub-microsecond pulsed plasmas in air with water cathode

Abstract: This paper reports an experimental study of sub-microsecond pulsed discharges between a metal pin electrode and a tap water cathode in atmospheric pressure air. The dynamics of excited molecular and atomic species produced in the discharge is investigated by time resolved optical emission spectroscopy. The initial diffuse plasma constricts around 300 ns after the start of the voltage pulse. This constriction is correlated with an increase in gas temperature from 1000 to 5000 K and a strong increase in emission from O (777 nm), Hα and NH(A–X). The formation of OH(A–X) is discussed in the framework of OH (and thus H2O2 production) in plasmas in and in contact with liquids. It is argued that electron dissociative recombination of the water ion plays an important role in the production of OH(A) and that the relative intensity of the OH(A) emission may not provide a good estimate of the OH concentration without correction for electronic quenching of OH(A).

Measurement of metastable and resonance level densities in rare-gas plasmas by optical emission spectroscopy

Abstract: Excitation and ionization of atoms out of the 4 energy levels of the excited np5(n + 1)s configuration of rare gases play an important role in many low temperature rare-gas plasmas. We compare two optical methods for measuring the number densities of atoms in these excited levels in an inductively coupled plasma under a variety of operating conditions (600 W, 1–25 mTorr). The first method is a standard white light absorption technique, whereas the second method exploits changes in the effective branching fractions of np5(n + 1)p → np5(n + 1)s emissions brought about by radiation trapping of atoms in np5(n + 1)s levels. The branching fraction method was found to produce results that agree well with the direct white light absorption method for both argon and neon plasmas using little more than a low-resolution spectrum of the plasma glow.

Neutral gas flow and ring-shaped emission profile in non-thermal RF-excited plasma needle discharge at atmospheric pressure

Abstract: We demonstrate that a balance between convective helium flow and ambient air diffusion creates a unique ring-shaped light emission profile by means of finite element analysis of the atmospheric pressure RF-excited plasma needle. The plasma needle has a point-to-plane geometry with a radius of 30 µm at the tip and an inter-electrode gap of 1 mm. We employ a coupled model between time-dependent plasma dynamics based on a fluid model and steady state neutral gas flow in two-dimensional cylindrical coordinates. When the mean inlet gas velocity is 1.5 m s−1 and the discharge is in high-power glow mode at 200 mW, the concentration of air drastically increases near a treated surface being away from the needle tip. As a result, Penning ionization by helium metastables and air (nitrogen) peaks at an off-axis position, corresponding to the ring-shaped emission profile in cylindrical coordinates. The off-axis ionization peak leads to an off-axis flux peak of nitrogen ions onto the treated surface. The 'ion wind' and gas heating have only minor effects on the discharge structure under the conditions considered here.

A global (volume averaged) model of a nitrogen discharge: I. Steady state

Abstract: A global (volume averaged) model is developed for a nitrogen discharge in the steady state for the pressure range 1?100?mTorr. The electron energy distribution function is allowed to vary from a Maxwellian to a Druyvesteyn distribution. Varying the electron energy distribution function from a Maxwellian-like to a Druyvesteyn-like influences mainly the density of excited species, ground state species being more important when the distribution is Druyvesteyn-like. We find that the nitrogen discharge is essentially atomic when the pressure is around 1?mTorr and is highly molecular when the pressure is 100?mTorr. The relative reaction rates for the creation and destruction of nitrogen atoms and atomic ions are explored over the pressure range of interest. The model calculations are compared with measurements found in the literature. There is excellent agreement between the model and the measurements for the electron and ion densities as well as the electron temperature. However, a large discrepancy between the model predictions and the measurements of the nitrogen atom density remains unexplained.

Langmuir probes in RF plasma: surprising validity of OML theory

Abstract: Low-temperature, partially ionized plasmas are commonly used in industry for materials processing, and many of these are created by radiofrequency (RF) power. For the characterization of plasma sources, electrostatic probes are the easiest to use, but interpretation of the current–voltage (I–V) curves is not straightforward. The presence of strong RF pickup and neutral collisions further complicates the problem. Langmuir's orbital-motion-limited (OML) theory provides a simple formula for ion current, but this is not expected to be valid in high-density plasmas. With carefully designed probes, it is found experimentally that OML can be used successfully under adverse conditions. Careful examination of the OML theory shows that its validity is fortuitous but can give correct results in commonly encountered conditions. The probe design, results, caveats, and methodology are given for use of probes in RF plasmas.

PIC code for the plasma sheath in large caesiated RF sources for negative hydrogen ions

Abstract: Powerful negative hydrogen ion sources are required for heating and current drive at ITER. The physics of the production and extraction of high negative ion currents is much more complex than that for positive ions. One of the most relevant parameters is the shape of the plasma sheath, which determines the velocity of surface produced negative ions and thus the probability of the ions to reach the extraction system. In order to investigate the influence of hydrogen atoms, positive and negative hydrogen ions and positive caesium ions on the plasma sheath, a 1d3v particle in cell code (PIC) code for the plasma close to the extraction system has been developed. For typical plasma parameters of such ion sources, surface conversion of impinging atoms is the main negative ion production channel, while conversion of positive ions plays a minor role. Due to the formation of a potential minimum close to the surface, the emission of negative ions into the plasma is space charge limited. As a consequence, the flux of negative ions can be increased only by increasing the density of positive hydrogen ions. At identical plasma parameters, an isotope effect is determined by the mass of the particles only, resulting in lower fluxes of negative deuterium ions compared with hydrogen. A small amount of positive Cs does not change the plasma sheath and the H− flux significantly.

The Child-Langmuir law and analytical theory of collisionless to collision- dominated sheaths

Abstract: This paper is concerned with summarizing simple analytical models of space-charge sheaths and tracing their relation to the Child–Langmuir model of an ion sheath. The topics discussed include the Child–Langmuir law and model of a collisionless ion sheath, the Mott–Gurney law and model of a collision-dominated ion sheath, the Bohm model of a collisionless ion–electron sheath, the Su–Lam–Cohen model of a collision-dominated ion–electron sheath, ion sheaths with arbitrary collisionality, high-accuracy boundary conditions for the Child–Langmuir and Mott–Gurney models of an ion sheath and the mathematical sense of Child–Langmuir type models of an ion sheath from the point of view of modern theoretical physics.

Equilibrium states of anodic double layers

Abstract: Equilibrium states of anodic double layers that form near a positively biased disk-shaped electrode immersed in a partially ionized plasma are studied experimentally using electrostatic probes. When the potential drop from the electrode to the plasma is less than a critical value, Δc, an anode glow is observed where a double layer potential drop is within a few millimeters of the electrode surface. For larger biases an anode spot forms where the double layer potential drop is centimeters from the electrode and the intervening region is a plasma more luminous than the bulk plasma. A theoretical model is developed which predicts that Δc ∝ 1/P + C, where P is the neutral pressure and C is a constant, and it predicts hysteresis in the current–voltage characteristic at the electrode; both effects are observed experimentally. The model also provides an estimate for the distance between the electrode and double layer potential drop that agrees with the measurements. Near small electrodes, anode spots are observed to be 'fireballs,' which are spherical in shape. Near larger electrodes 'firerods' are found instead, which have a cylindrical shape. It is shown that firerods are required by global current balance because they have a smaller effective electron collecting area than fireballs. Experiments also confirm that global nonambipolar flow (Baalrud S D et al 2007 Phys. Plasmas 14 042109) accompanies firerods. In this case all electrons are lost through the firerod to the electrode, while all positive ions are lost to the other plasma boundaries.

Transition between the discharge regimes of high power impulse magnetron sputtering and conventional direct current magnetron sputtering

Abstract: Current and voltage have been measured in a pulsed high power impulse magnetron sputtering (HiPIMS) system for discharge pulses longer than 100 mu s. Two different current regimes could clearly be ...

Electron energy distributions and plasma parameters in high-power pulsed magnetron sputtering discharges

Abstract: We report on the results obtained using time-resolved Langmuir probe measurements in high-power pulsed dc magnetron sputtering discharges. Time evolutions of the electron energy distribution and the local plasma parameters were investigated at a substrate position of 100 mm from a planar target of 100 mm diameter during a high-rate deposition of copper films. The average target power density in a pulse was 500 W cm−2 at a repetition frequency of 1 kHz, a voltage pulse duration of 200 µs and an argon pressure of 1 Pa. The electron energy distributions with two energy groups and sharply truncated high-energy tails were observed during a pulse. After a fast rise in a 50 µs initial stage of the pulse, the kinetic temperature of electrons, defined using the mean electron energy, remained in the range from 10 500 to 12 200 K till the pulse termination. The temperature of weakly populated hot electrons decreased rapidly in the initial stage of the pulse approaching the kinetic temperature approximately 100 µs after a pulse initiation. High plasma densities, being in the range 1 × 1012–2 × 1012 cm−3 for 100 µs, were achieved at the substrate position with a 50 µs delay after establishing the 125 µs steady-state discharge regime with the target power density of 650–680 W cm−2 during a pulse. The plasma potential slowly increased from 0.5 to 3.5 V during the pulse and 25 µs after its termination.

The plasma–sheath boundary: its history and Langmuir's definition of the sheath edge

Abstract: The introduction of the terms sheath in 1923 and plasma in 1928 by Langmuir is described, followed by their use in the Tonks and Langmuir theory of the positive column at low pressures in 1929. Attention is drawn to the development of Langmuir's ideas during the period from 1923 to 1929. The well-known Bohm criterion for sheath formation, published in 1949, is shown to be closely related to the earlier work of Tonks and Langmuir. The much-used version of the Bohm criterion with the equality sign is obtained by employing the two-scale theory of the plasma and sheath, for the case where λD/L → 0.A generalized Bohm criterion is obtained by introducing the ion velocity distribution; the resulting expression can be understood by considering the propagation of ion-acoustic waves. The plasma–sheath boundary is found to be a sonic surface. Other generalizations of the Bohm criterion are given, including a mixture of positive ions, the presence of negative ions and a non-Maxwellian electron velocity distribution.

Effect of inhomogeneities on streamer propagation: I. Intersection with isolated bubbles and particles

Abstract: The branching of streamers in high pressure gas discharges and discharges in liquids is an almost universal occurrence having many causes. In this paper, we discuss results of an investigation of one possible cause—inhomogeneities in the media through which the streamer propagates. These inhomogeneities produce corresponding enhancements or decreases in ionization and excitation as the avalanche front encounters them, some of which may produce branching. Three types of inhomogeneities were investigated—negative bubbles (regions having a lower density than ambient), positive bubbles (having a higher density) and solid bubbles (particles). Depending on the size and density of the bubble, the streamer can be focused into the bubble (negative small bubble), deflected and split (positive bubbles and particles) or refracted (large negative bubble). In the case of gaseous bubbles, this behavior is partly explained by the larger E/N (electric field/gas number density) in the negative bubble, producing more ionization by electron avalanche, and smaller E/N in the positive bubble, producing less ionization. A streamer may diverge into a negative bubble located off axis due to seeding of electrons in the bubble by photoionization and subsequent avalanching in the large E/N. (Some figures in this article are in colour only in the electronic version)

Different modes of electron heating in dual-frequency capacitively coupled radio frequency discharges

Abstract: A systematic study of different modes of electron heating in dual-frequency capacitively coupled radio frequency (CCRF) discharges is performed using a particle-in-cell simulation. Spatio-temporal distributions of the total excitation/ionization rates under variation of gas pressure, applied frequencies and gas species are discussed. Some results are compared qualitatively with an experiment (phase resolved optical emission spectroscopy) operated under conditions similar to a parameter set used in the simulation. Different modes of electron heating are identified and compared with α- and γ-mode operation of single-frequency CCRF discharges. In this context the frequency coupling and its relation to the ion density profile in the sheath are discussed and quantified. In light gases the ion density in the sheath is time modulated. This temporal modulation is well described by an analytical model and is found to affect the excitation dynamics via the frequency coupling. It is shown that the frequency coupling strongly affects the generation of beams of highly energetic electrons by the expanding sheath and field reversals caused by the collapsing sheath. The role of secondary electrons at intermediate and high pressures is clarified and the transition from α- to γ-mode operation is discussed. Depending on the gas and the corresponding cross sections for excitation/ionization the excitation does not generally probe the ionization as is usually assumed.

Cavity ring-down spectroscopy on a high power rf driven source for negative hydrogen ions

Abstract: Cavity ring-down spectroscopy (CRDS) is a very sensitive diagnostic technique for absorption measurements. It is capable of measuring the absolute line-of-sight (LOS) integrated density of negative hydrogen ions (H−, D−) which induce a weak absorption (α 10−6 cm−1) along a LOS in plasmas containing negative hydrogen ions. CRDS has been applied to a high power rf driven negative ion source which is now the reference source for the ITER neutral beam injection system. The rf source operates at low pressure (typically 0.3 Pa). Negative hydrogen ions are produced mainly by the conversion of hydrogen particles at a caesium coated surface achieving negative ion densities comparable to the electron density near the surface. It is shown that CRDS very reliably measures the absolute volume density of negative hydrogen ions in these sources. The densities range from 1016 m−3 in volume operation to 1017 m−3 in caesium seeded operation. The measured volume density close to the extraction system and the extracted current density change consistently while varying different source parameters, such as the total pressure or the input power applied to the source. Results are shown for measurements in hydrogen and deuterium discharges with caesium seeding. An additional absorption is measured in the afterglow of the discharge and is attributed to the caesium dimer Cs2.

Nanofocus: an ultra-miniature dense pinch plasma focus device with submillimetric anode operating at 0.1 J

Abstract: As a method for investigating the minimum energy to produce a pinch plasma focus (PF) discharge, an ultra-miniature device for pinch discharges has been designed, constructed and characterized (nanofocus (NF): 5 nF, 5–10 kV, 5–10 kA, 60–250 mJ, 16 ns time to peak current). Submillimetric anode radii (0.8 and 0.21 mm) covered by coaxial insulators were used for experiments in hydrogen. Evidence of pinch was observed in electrical signals in discharges operating at 3 mbar and ~100 mJ. A single-frame image converter camera (4 ns exposure) was used to obtain plasma images in the visible range. The dynamics observed from the photographs is consistent with (a) the formation of a plasma sheath close to the insulator surface, (b) the plasma covering the anode, (c) radial compression over the anode; (d) finally the plasma is detached from the anode in the axial direction. The total time from stages (a) to (d) was observed in ~30 ns. This ultra-miniature device has a value for the 'plasma energy density parameter' and for the 'drive parameter' of the same order or greater than PF devices operating at energies several orders of magnitude higher.

Ion diagnostics of a discharge in crossed electric and magnetic fields for electric propulsion

Abstract: The velocity distribution function (VDF) of metastable Xe+ ions was measured along the channel centerline of the high-power PPS?X000 Hall effect thruster by means of laser induced fluorescence (LIF) spectroscopy at 834.72?nm for various discharge voltages (300?700?V) and propellant mass flow rates (6?15?mg?s?1). The development of the on-axis profile of the velocity dispersion reveals the interrelation between ionization and acceleration layers. The ion velocity profiles are in accordance with outcomes of a hybrid numerical model in which the electron mobility is assessed from particle-in-cell simulations. The axial distribution of the effective electric field is inferred from the mean ion velocity profile, despite the parasitic effect due to ions created in the acceleration region. Most of the acceleration process takes place outside the thruster channel. The electric field augments and it moves upstream when the applied voltage is ramped up. The impact of the xenon mass flow rates is found to depend upon the voltage. A novel approach based on the moments of the experimental VDFs in combination with the Boltzmann's equation is introduced in order to determine the real electric field distribution. The method also provides the ionization frequency profile. The LIF diagnostics reveals the existence at the end of the acceleration region of fast ions of which the kinetic energy is above the supplied energy. The fraction of these supra-sped up ions grows when the voltage increases. The ion VDFs were also recorded in the plasma plume far field by way of a retarding potential analyzer (RPA). The shape of the RPA traces as well as their evolution with operating conditions are in agreement with trends observed by means of LIF spectroscopy. Finally, physical mechanisms at the origin of supra-sped up ions are discussed in light of numerical simulation outcomes and a set of new experimental results.

On the formation mechanisms of the diffuse atmospheric pressure dielectric barrier discharge in CVD processes of thin silica-like films

Abstract: Pathways of formation and temporal evolution of the diffuse dielectric barrier discharge at atmospheric pressure were experimentally studied in this work by means of optical (fast imaging camera) and electrical diagnostics. The chosen model system is relevant for applications of plasma-enhanced chemical vapor deposition of thin silica-like film on the polymeric substrate, from cost-efficient gas mixtures of Ar/N2/O2/hexamethyldisiloxane. It was found that the discharge can gradually experience the phases of homogeneous low current Townsend-like mode, local Townsend to glow transition and expanding high current density (?0.7Acm?2) glow-like mode. While the glow-like current spot occupies momentarily only a small part of the electrode area, its expanding behavior provides uniform treatment of the whole substrate surface. Alternatively, it was observed that a visually uniform discharge can be formed by the numerous microdischarges overlapping over the large electrode area.

Dust formation in Ar/CH4 and Ar/C2H2 plasmas

Abstract: This paper discusses the growth of nanoparticles in capacitively and inductively coupled radio-frequency plasmas operated in hydrocarbon gases and the back-reaction of particles on the plasma properties. The focus is on the growth mechanism in CH4- and C2H2-containing plasmas, on the role of atomic hydrogen and on the dynamic charging and decharging of particles in pulsed plasmas.

Development of dielectric barrier discharge-type ozone generator constructed with piezoelectric transformers: effect of dielectric electrode materials on ozone generation

Abstract: The dependence of ozone generation on the types of dielectric electrode material has been investigated using an ozone generator constructed with the piezoelectric transformer developed in our laboratory. The ozone generator is based on the excitation of the dielectric barrier discharge (DBD), which has the advantage of a compact configuration for generating ozone. Four kinds of dielectric materials are prepared for dielectric barrier electrodes. Electrical properties of the DBD and the ozone generation characteristics are investigated for the different dielectric materials. Differences in the discharge mode among the barrier electrode materials are recognized and discussed on the basis of the results of the Lissajous figures and voltage–current waveforms. During the continuous running of the generator, a temporal decrease in ozone concentration is observed owing to the temperature increase inside the reactor. Although the ozone generation characteristics are influenced by many properties of dielectrics, two important factors for achieving high-efficiency ozone generation are identified in this study. One is the use of a high-thermal conductivity material for the dielectric electrode, which functions well as a heat sink for transferring the generated heat to the outside through the material. The other factor is the control of the discharge mode. Our results show that the discharge mode that is considered as Townsend-like DBD is suitable for high-efficiency ozone generation.

Global model and diagnostic of a low-pressure SF6/Ar inductively coupled plasma

Abstract: A global model has been developed for low-pressure (3–20 mTorr), radio-frequency (rf) (13.56 MHz) inductively coupled plasmas (ICPs), produced in SF6/Ar mixtures. The model is based on a set of mass balance equations for all the species considered, coupled to the discharge power balance equation and the charge neutrality condition. Simulations are used to show the impact of operating conditions, such as the rf power, the pressure and the percentage of argon in the mixture, on the evolution of charged and neutral species. Langmuir probe and optical emission spectroscopy measurements are used to determine the electron temperature and the densities of electrons, ions and atomic fluorine in the SF6/Ar ICPs under study. These data are compared with simulation results obtained from the global model. A satisfactory agreement is found between the simulation results and the measured values of the electron density and temperature, for rf powers in the range 900–1700 W, regardless of the percentage of argon in the mixture. Predictions for the atomic fluorine density (~1014 cm−3) are in good agreement with experiment, for various rf powers.

On the use of non-hydrogenic spectral lines for low electron density and high pressure plasma diagnostics

Abstract: Spectral line shapes and shifts of non-hydrogenic spectral lines of neutral atoms are considered for diagnostics of low electron density high pressure plasmas. Difficulties in application and limitations of this spectroscopic diagnostic technique are discussed in detail. The simultaneous presence of comparable Stark, Van der Waals and sometimes resonance broadening is discussed and a procedure to deduce the electron number density from the measured line width and/or shift is described. The importance and requirement of high accuracy theoretical and experimental data for plasma diagnostics are also discussed.

Laser photodetachment on a high power, low pressure rf-driven negative hydrogen ion source

Abstract: Powerful, low pressure negative hydrogen ion sources are a basic component of future neutral beam heating systems for fusion devices. The required high ion currents (>40?A) are obtained via the surface production process, which requires negative ion densities in the range of in the plasma region close to the extraction system. For spatially resolved diagnostics of the negative hydrogen ion densities, the laser photodetachment method has been applied to a high power, low pressure, rf-driven ion source (150?kW, 0.3?Pa) for the first time. The diagnostic setup and the data evaluation had to cope with the rf field (1?MHz), the high source potential during extraction (?25?kV) and the presence of magnetic fields (<10?mT). Horizontal profiles of negative ion densities and electron densities along 15?cm with a typical step length of 1?cm and a probe tip of 5?mm length show a broad maximum in the centre of the extraction region. The variation of a bias voltage applied to the plasma grid with respect to the source body yields a correlation between the detachment signals for the negative ion density and the electron density with the extracted ion and electron currents, respectively. The density ratio of negative hydrogen ions to electrons is in the range of , demonstrating that the negative ions are the dominant negatively charged species in these types of ion sources. Absolute negative ion densities are in good agreement with line-of-sight integrated results of cavity ring-down spectroscopy and optical emission spectroscopy.