Table of contents

Radiation and temperature fields of the continuum field are determined by using different spectroscopic methods based on the spectral emission of an argon plasma jet. An interferential filter of bandwidth 2.714 nm centred at a wavelength of 633 nm is used to observe only the continuum emission and to eliminate the self-absorption phenomenon. An optical multichannel analyser (OMA) of an MOS detector is used to measure argon plasma jet volumetric emissivity under atmospheric pressure and high temperatures. An emission spectroscopic method is used to measure the Stark broadening of the hydrogen line H_β and to determine the electron density. The local thermodynamic equilibrium is established and its limit is stated. The local electron temperature is determined by two methods (the continuum emission relation and the LTE relations), and the total Biberman factor is measured. The results given by the OMA are compared with those given by the imagery method. At a given wavelength, the Biberman factor, which depends on the electron temperature and the electron density, may serve as an indicator to show where the LTE prevails along the argon plasma jet core length.

The properties of a pulsed radio frequency capacitive discharge are investigated at atmospheric pressure in argon. The discharge can operate in two different modes: a homogeneous glow discharge or turn into filaments. By pulsing the 13.56 MHz generator both the filamentary and the glow modes can be selected depending on the pulse width and period. For a 5 µs pulse width (∼70 RF cycles in the pulse), short pulse periods (less than 100 µs) result in a filamentary discharge while long pulse periods (greater than 1 ms) result in a glow discharge.

Optical emission spectroscopy and power measurements were performed to estimate the plasma temperature and density. Water vapour was introduced to the discharge as a source of hydrogen and the Stark broadening of the Balmer H_β line was measured to allow the plasma density to be estimated as 10¹⁵ cm⁻³ in the filamentary mode. The estimation of the glow mode density was based on power balance and yielded a density of 5 × 10¹¹ cm⁻³. Emission line ratio measurements coupled with the Saha equation resulted in an estimate of electron temperature of approximatively 1.3 eV for the glow mode and 1.7 eV for the filaments.

Using the glow mode at a duty cycle of 10% is effective in decreasing the hydrophobicity of polymer films while keeping the temperature low.

The recently developed corona above water technique is applied to water containing 10 mg l⁻¹ methylene blue (MB) or methyl orange (MO). The corona discharge pulses are created with a spark gap switched capacitor followed by a transmission line transformer. The pulse amplitude is 40 kV; its duration is 50 ns. At a pulse repetition rate of 10 Hz this leads to an average power of 0.6 W into the discharge. MB and MO are completely decolourized in ∼20 min. This corresponds to a yield of ∼4.5 gr kW⁻¹h⁻¹, which is much higher than obtained with other discharge techniques or sonoluminescence. The high yield is reflected in the observed temperature increase of only ∼1 K. Tests with additional chemicals show that the initial speed of the conversion can be influenced but the total time required for total decolourization is constant. Further, it follows that the main oxidation path of the dyes is by direct ozone attack and the conversion products are strong acids.

A helium–neon laser Mach–Zehnder quadrature interferometer is used to measure the electron number density in a propagating current sheet. In some cases the density rise rate was too high to be resolved by the system. Otherwise, the interferometer is used to measure time-resolved density at a particular location in the accelerator. The location of the beam is then moved between shots and, because of the repeatability of the discharge, multiple shots can be pieced together to create a temporally and spatially resolved view of the evolution of the sheet and of a wake of plasma that is left behind the sheet. The measurements confirm an emerging picture of the evolution of these structures that is based on high-speed photographs and current density measurements. They also reveal that the wake originates out of a bifurcation of the current sheet and that it is of comparable density to the sheet.

A process for the synthesis of organic layer-coated metal nanoparticles is presented. Metal nanoparticles are synthesized by the low-pressure pulsed arc evaporation of a metal cathode surface, followed by the in-flight deposition of a thin organic layer by capacitively coupled radio-frequency plasma polymerization from a gaseous hydrocarbon monomer. The stability of the pulsed arc system and the cathode erosion rate are discussed. The effects of operating conditions such as power, reactor pressure and inert gas flow rate on the average size of produced bare copper nanoparticles and the properties of the organic coating are studied. A 'characteristic map' for the in-flight plasma polymerization from the C₂H₆ monomer of the organic layer onto the Cu nanoparticles is developed. The produced copper nanoparticles are characterized by field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM) and x-ray photoelectron spectroscopy (XPS). The FE-SEM and TEM images reveal coated copper nanoparticles with the diameter of the metal core ranging from a few to 50 nm and the thickness of the organic layer ranging from 3 to 10 nm. The XPS results show that the plasma polymer film is chemically adsorbed onto the surface of the copper nanoparticle.

Electron beam emission characteristics from neon, argon, hydrogen and helium in an NX2 dense plasma focus (DPF) device were investigated in order to optimize the plasma focus device for deposition of thin films using energetic electron beams. A Rogowski coil and CCD based magnetic spectrometer were used to obtain temporal characteristics, total electron charge and energy distributions of electron emission from the NX2 DPF device. It is found that hydrogen should be the first choice for thin film deposition as it produces the highest electron beam charge and higher energy (from 50 to 200 keV) electrons. Neon is the next best choice as it gives the next highest electron beam charge with mid-energy (from 30 to 70 keV) electrons. The operation of NX2 with helium at voltages above 12 kV produces a mid-energy (from 30 to 70 keV) electron beam with low-electron beam charge, however, argon is not a good electron beam source for our NX2 DPF device. Preliminary results of the first ever thin film deposition using plasma focus assisted pulsed electron deposition using a hydrogen operated NX2 plasma focus device are presented.

The energy distribution of ions (IED) bombarding a substrate during plasma etching has demonstrated effects on etch selectivity for integrated circuit fabrication. Accurate control of the IED is desired to better understand the nature of plasma–surface interaction and to control process outcomes. IED control can be achieved by tailoring the wave form shape of an rf bias applied to the substrate, using a programmable wave form generator in combination with a power amplifier. Due to the frequency dependence of the amplifier gain and the impedance of the plasma in contact with the substrate, it is not practical to predict the shape of the input wave form needed to produce a desired result at the substrate. Introduced here is an iterative approach using feedback control in the frequency domain to produce arbitrary wave form shapes at the substrate. A fast Fourier transform (FFT) of the substrate wave form is compared, one frequency at a time, with the FFT of a desired target wave form, to determine adjustments needed at the generator. This iterative procedure, which is fully automated and tested for several target wave form shapes, is repeated until the substrate wave form converges to the targeted shape.

The chemistry of NH₃/Ar/He plasmas was investigated, using a combination of ultraviolet (UV) optical absorption spectroscopy (OAS) and optical emission spectroscopy (OES). Absolute NH₃ number densities in 1 Torr plasmas were measured by OAS as a function of inductively coupled plasma power and substrate heater temperature (T_h). OES and actinometry were used to determine semi-quantitative H-atom density. A 'self-actinometry' method was introduced to measure the absolute number density of N₂ that formed following the dissociation of NH₃ and secondary reactions. In this approach, small amounts of N₂ are added to the NH₃-containing plasma, leading to an increase in the N₂(C ³ Π_u → B ³ Π_g) emission intensity. This provides an accurate calibration factor for converting relative N₂ emission intensities into absolute number densities. The number densities of NH₃ were found to decrease with increasing power and T_h, reaching >90% dissociation at 400 W and 900 K. N₂ densities increased with power and T_h. The majority of dissociated NH₃ was converted to N₂ (i.e. the total nitrogen content was conserved in the sum of these two species). The major hydrogen-containing species appeared to be H₂; however, a substantial amount of H-atoms (comparable to H₂) was present at the highest powers.

The generation of hydroxyl (OH) radicals and hydrogen peroxide (H₂O₂) in underwater capillary discharges with different geometries is studied. It is found that the efficiency of the production of these active species depends on the input power and on the length of the capillary. A maximal rate of H₂O₂ generation of 3.6 × 10⁻³ mmol l s⁻¹ has been measured in a 1 mm capillary at a discharge power level of 77 W. The hydrogen peroxide yield is lower for the 1 mm capillary than for the 5 mm capillary . The kinetics of OH formation is investigated by recording emission spectra in the UV range (280–400 nm). It is shown that the rate of OH formation sharply decreases with increasing capillary length, for capillaries longer than 3 mm. A comparison of the efficiency of different plasma-solution reactors for the generation of active species has been carried out. In the investigated capillary discharge the hydrogen peroxide yield is higher than the values reported for a diaphragm discharge and for a glow discharge with the electrolyte cathode at atmospheric pressure.

Controlling the number of excited atoms into a reaction chamber was investigated in Ar pulse-modulated induction thermal plasma (PMITP) with molecular gases at atmospheric pressure. Such control is important in promoting the modification of substrate surfaces in thermal plasma processing. The coil current required for sustaining a PMITP was modulated by setting the firing angle of the metal-oxide-semiconductor field-effect transistor in the inverter power supply used as a high-frequency power source. The radiation intensities of the excited atomic lines were measured using the optical emission spectroscopy technique to estimate the number of excited atoms flowing into the reaction chamber. In particular, we found an increase in the number of excited Ar and H atoms flowing into the reaction chamber in Ar or Ar–H₂ PMITPs as a result of the pulse modulation of the coil current.

Radio-frequency (RF), atmospheric-pressure glow discharge (APGD) plasmas using bare metal electrodes have promising prospects in the fields of plasma-aided etching, deposition, surface treatment, disinfection, sterilization, etc. In this paper, the discharge characteristics, including the breakdown voltage and the discharge voltage for sustaining a stable and uniform α mode discharge of the RF APGD plasmas are presented. The experiments are conducted by placing the home-made planar-type plasma generator in ambient and in a vacuum chamber, respectively, with helium as the primary plasma-forming gas. When the discharge processes occur in ambient, particularly for the lower plasma-working gas flow rates, the experimental measurements show that it is the back-diffusion effect of air in atmosphere, instead of the flow rate of the gas, that results in the obvious decrease in the breakdown voltage with increasing plasma-working gas flow rate. Further studies on the discharge characteristics, e.g. the luminous structures, the concentrations and distributions of chemically active species in plasmas, with different plasma-working gases or gas mixtures need to be conducted in future work.

A two-dimensional model of stationary convection-stabilized low-current glow and arc discharge columns in atmospheric-pressure air is developed which accounts for deviation of the plasma state from the local thermodynamic equilibrium (LTE). In addition to equations of energy, continuity and momentum (analogous to those used in LTE arc models), the non-LTE model includes balance equations for plasma species and for the vibrational energy of nitrogen molecules. The kinetic scheme is used which was developed recently for the simulation of low-current wall-stabilized discharges in air. Results of calculation of discharge parameters over a wide current range are presented. It is shown that the non-equilibrium effects are substantial at currents lower than ∼ 100 mA. The calculated plasma parameters agree with available experimental data.

Characterization of the pulse plasma source through the determination of the local thermodynamic equilibrium (LTE) threshold is described. The maximum electron density measured at the peak in discharge current is determined by the width of the He II Paschen alpha spectral line, and the electron temperature is determined from the ratios of the relative intensities of spectral lines emitted from successive ionized stages of atoms. The electron density and temperature maximum values are measured to be 1.3 × 10¹⁷ cm⁻³ and 19 000 K, respectively. These are typical characteristics for low-pressure, pulsed plasma sources for input energy of 15.8 J at 130 Pa pressure in helium–argon mixture.

The use of LTE-based analysis of the emission spectra is justified by measurement of the local plasma electron density at four positions in the discharge tube using a floating hairpin resonance probe. The hairpin resonance probe data are collected during the creation and decay phases of the pulse. From the spatio-temporal profile of the plasma density a 60 µs time-window during which LTE exists throughout the entire plasma source is determined.

Long plasma columns generated by high-frequency (HF) fields and extending over distances longer than the free-space wavelength of the applied electromagnetic (EM) field are of interest in various applications. A commonly used method to achieve such long plasma columns calls for the propagation of EM surface waves that use the plasma as their propagating medium. In such a case, the HF field applicator, called a wave launcher, is much shorter than the actual length of the plasma column. Long plasma columns can also be sustained by using field applicators that run along the full length of the discharge tube. Most such linear applicators rely on waveguide components. However, it is possible to use transverse electric magnetic planar-transmission-lines based on stripline technology to design efficient linear field applicators. Using such an approach, we have developed a new type of HF linear field applicator that operates on a relatively wide frequency range (typically, 200–2450 MHz). Comparison of the discharge that it generates with a surface-wave discharge (SWD) sustained under similar operating conditions shows that the discharge volume is larger than that obtained with a SWD at the same power level, hence a lower gas temperature and a plasma column more axially uniform, two valuable features for some applications. The contraction of these plasma columns is shown to occur at higher gas pressures than with SWDs. All these measurements are carried out in argon as the discharge gas.

A multistring-type CF₄ plasma has been produced by setting a multihole-type metal obstacle plate in a magnetized electron cyclotron resonance plasma. Separative diffusion of negative ions (F⁻) has been realized in the region downstream of the obstacle plate around the string-type plasma columns. Langmuir probes are used to estimate negative-ion density using the modified Bohm model. The experiment reveals that F⁻ density is of the order of 10¹⁵–6 × 10¹⁵ m⁻³ around the multistring-type plasma. A column-type negative-ion source with a diameter of about 20 cm and a cross section of about 300 cm² is realized, which could be scaled up by introducing more string plasmas. The ratio of negative-ion density to electron density around the plasma column is in the range 10–150.

The spatial structure and temporal evolution of the electric fields in a sheath formed in a dual frequency, 300 mm capacitive argon discharge are measured as functions of relative mixing between a low frequency current and a high frequency current. It is found that the overall structure of the sheath (potential across the sheath and the thickness of the sheath) are dominated by the lower frequency component while (smaller) oscillations in these quantities are dictated by the higher frequency component. Comparisons of the measured spatial and temporal profiles are made for Lieberman's and Robiche et al sheath model and with a particle in a cell calculation.

This paper contains the results of the detailed simulation study of the role of ion-enhanced field emission on the breakdown voltage in argon, xenon and krypton at high frequencies. Calculations were performed by using a one-dimensional particle-in-cell/Monte Carlo collisions (PIC/MCC) code with the secondary emission model adjusted to include field emission effects in microgaps. The obtained simulation results clearly show that electrical breakdown across micron-size gaps may occur at voltages far below the minimum predicted by the conventional Paschen curve. The observed breakdown voltage reduction may be attributed to the onset of ion-enhanced field emission.

Basic electrical, optical and calorimetric characteristics of an ac (50 Hz) driven capillary discharge produced in a water solution were studied for initial water solution conductivity in the range 50–1000 µS cm⁻¹. Typical current and voltage waveforms and emission intensities produced by several electronically excited species were recorded with high time resolution. The evolution of the electrical current, power and capillary resistance was inspected during positive ac half-cycle for various operational regimes. A fast relaxation of the discharge following a breakdown event was observed. Optical measurements indicate that radiative species are mostly generated during the first few hundreds of nanoseconds of plasma generation and that the average duration of plasma emission induced by a discharge pulse is of the order of a few microseconds. Results of calorimetric measurements are in good agreement with average electrical measurements and support the assumption that the discharge is a constant source of heat delivered to the liquid. Assuming that only a fraction of the heat released inside the capillary can be transported by conduction through the capillary wall and via its orifices, the processes of bubble formation, expulsion and re-filling the capillary with 'fresh' water must play a key role in maintaining a thermal balance during long-time steady-state operation of the device. Furthermore, a simplified numerical model and a first order energy deposition calculation prove the plausibility of the bubble breakdown mechanism.

The sheath dynamics in the afterglow of a pulsed inductively coupled plasma, operated in hydrogen, is investigated. It is found that the sheath potential does not fully collapse in the early post-discharge. Time resolved measurements of the positive ion flux in a hydrogen plasma, using a mass resolved ion energy analyser, reveal that a constant 2 eV mean ion energy persists for several hundred micro-seconds in the afterglow. The presence of a finite sheath potential is explained by super-elastic collisions between vibrationally excited hydrogen molecules and electrons in the afterglow, leading to an electron temperature of about 0.5 eV. Plasma density decay times measured using both the mass resolved energy analyser and a Langmuir probe are in good agreement. Vibrational temperatures measured using optical emission spectroscopy support the theory of electron heating through super-elastic collisions with vibrationally excited hydrogen molecules. Measurements are also supported by numerical simulations and modelling results.

We discuss electron heating mechanisms in the sheath regions of dual-frequency capacitive discharges, with the twin aims of identifying the dominant mechanisms and supplying closed-form expressions from which the heating power can be estimated. We show that the heating effect produced by either Ohmic or collisionless heating is much larger when the discharge is excited by a superposition of currents at two frequencies than if either current had acted alone. This coupling effect occurs because the lower frequency current, while not directly heating the electrons to any great extent, strongly affects the spatial structure of the discharge in the sheath regions.

SiO_x films were deposited using a large area capacitively-coupled plasma produced by flowing oxygen and argon between two parallel plate electrodes that were supplied by 13.56 MHz radio frequency (RF) power at atmospheric pressure. Tetraethoxysilane (TEOS) was adopted as the precursor. The effects of the RF power and oxygen flow rate on deposition rate were studied. Both XPS and FT-IR results testified to the temperature effect on film deposition. Reactive gaseous species were obtained by optical emission spectroscopy to reveal the possible process of SiO_x film deposition. An interesting feature of the evolution of the 777 nm line of O on the O₂ flow rate is that the O atom could contribute to the deposition process.

The excitation of harmonics in the current of capacitive radio frequency (RF) discharges is a frequently observed phenomenon. The effect is of interest for several reasons. It forms, for instance, the basis of a successful diagnostic concept for technical plasmas, and it is intimately connected to the process of electron heating in capacitive discharges. Recently, mathematical models were proposed which interpret the phenomenon as the self-excitation of the plasma series resonance by the nonlinearity of the boundary sheath. These models are surprisingly successful but suffer from the limitation that they analyse the plasma dynamics in terms of global equations with concentrated parameters. They are unable to account for the complex multi-mode current waveforms seen in experiments and also cannot resolve the electromagnetic effects which dominate contemporary/large area, high frequency processing discharges. This paper aims to correct the deficiency by presenting a spatially resolved, fully electromagnetic model of the nonlinear RF dynamics of a bounded plasma. The model holds for arbitrary plasma reactor geometries and external RF excitations and makes no assumptions on the homogeneity of the plasma or the characteristics of the boundary sheath. A functional analytic (Hilbert space) formulation of the model is given which allows for an exact solution in terms of an infinite power/Fourier series. For the case of an idealized cylindrical reactor, the model is also explicitly evaluated. The calculated RF current wave forms exhibit the complex multi-mode structure of the currents observed in experiments and follow the same scaling laws. It is concluded that the presented model is capable of describing the nonlinear dynamics of capacitive RF discharges of all sizes and that it provides a significant improvement over both the established nonlinear global models and linear models with spatial resolution.

The pulsed corona offers real promise for degradation of pollutants in gas and water streams. This paper presents a study of NO_x removal from diesel exhaust. Special emphasis is laid on the investigation of the dependence of the NO removal rate and efficiency on the pulse repetition rate (PRR). A nanosecond solid state power supply (45 kV, 60 ns, up to 1 kHz) was used for driving the corona reactor. A Mitsubishi 10 kW 3-cylinder diesel-generator engine with a total volume of 1300 cm³ was used as a source of exhaust gas. At an NO removal rate of 35% the NO removal efficiency was 53 g kW⁻¹h⁻¹ for PRR = 500 Hz and the initial NO concentration was 375 ppm. A semi-empirical expression for the corona reactor removal efficiency related both to PRR and to the residence time is presented. The removal efficiency decreases with increasing PRR at constant flow rate or constant residence time. This expression demonstrates reasonable agreement between the calculation results and the experimental data.

A two-dimensional self-consistent numerical simulation model is used to analyse the effects of the ramp reset pulses on the address discharge in a shadow mask plasma display panel (SM-PDP). Some basic parameters such as the slope of the ramp pulse and the terminal voltage of the ramp reset period are varied to investigate their effects. The simulation results illustrate that the wall voltage is mainly decided by the terminal voltage and the firing voltage at the end of the ramp reset period. Moreover, the variation of the ramp slope will also bring a few modifications to the wall voltage. The priming particles in the beginning of the addressing period are related to the slope of the ramping down voltage pulse. The simulation results can help us optimize the driving scheme of the SM-PDP.

We use a global (volume averaged) model to study the dissociation processes and the presence of negative ions and metastable species in a low pressure high density O₂/Ar discharge in the pressure range 1–100 mTorr. The electron density and the fractional dissociation of the oxygen molecule increases with increased argon content in the discharge. We relate this increase in fractional dissociation to an increase in the reaction rate for electron impact dissociation of the oxygen molecule which is due to the increased electron temperature with increased argon content in the discharge. The electron temperature increases due to higher ionization potential of argon than for molecular and atomic oxygen. We find the contribution of dissociation by quenching of the argon metastable Ar^m by molecular oxygen (Penning dissociation) to the creation of atomic oxygen to be negligible. The negative oxygen ion O⁻ is found to be the dominant negative ion in the discharge. Dissociative attachment of the oxygen molecule in the ground state and in particular the metastable oxygen molecule O₂(a ¹Δ_g) are the dominating channels for creation of the negative oxygen ion O⁻.

The hybrid barrier discharge plasma process combined with ozone decomposition catalysts was studied experimentally for decomposing dilute trichloroethylene (TCE). Based on the fundamental experiment for catalytic activities on ozone decomposition, MnO₂ was selected for application in the main experiments for its higher catalytic abilities than other metal oxides. A lower initial TCE concentration existed in the working gas; the larger ozone concentration was generated from the barrier discharge plasma treatment. Near complete decomposition of dichloro-acetylchloride (DCAC) into Cl₂ and CO_x was observed for an initial TCE concentration of less than 250 ppm. C=C π bond cleavage in TCE gave a carbon single bond of DCAC through oxidation reaction during the barrier discharge plasma treatment. Those DCAC were easily broken in the subsequent catalytic reaction. While changing oxygen concentration in working gas, oxygen radicals in the plasma space strongly reacted with precursors of DCAC compared with those of trichloro-acetaldehyde. A chlorine radical chain reaction is considered as a plausible decomposition mechanism in the barrier discharge plasma treatment. The potential energy of oxygen radicals at the surface of the catalyst is considered as an important factor in causing reactive chemical reactions.

It is shown that basic mechanisms of glow discharge, which are drift and diffusion of the ions and the electrons, volume ionization and recombination and secondary electron emission, are in principle sufficient to explain patterns on glow cathodes observed in recently published works, so there is no need to introduce special mechanisms to this end.