Table of contents

Atmospheric-pressure diffuse discharge plasmas are susceptible to instabilities and, in particular, to arcing (the glow-to-arc transition). Some of the most promising approaches to 'stabilizing' atmospheric-pressure plasmas are based on the recognition that arc formation can be avoided when the plasmas are generated and maintained in spatially constricted geometries with dimensions from tens to hundreds of microns. Known as microplasmas or microdischarges, these weakly-ionized discharges represent a new and fascinating realm of plasma science in which several scientific issues, such as the potential breakdown of pd scaling and the role of boundary-based phenomena, come to the fore. In contrast to 'macroplasmas' typically produced at pressures well below one atmosphere, high-pressure microdischarges offer the formation of transient molecular species, such as the rare gas excimers, that are generated by three-body collisions. When excited with sub-microsecond pulses, microplasmas can exhibit significant shifts in electron and ion temperatures and in the electron and ion energy distribution functions, which can be exploited in applications such as intense UV/VUV light sources and for the selective production of chemically reactive radicals. The technological applications of these stable high-pressure microplasmas have outpaced the detailed understanding of the plasma physics and the key processes in these plasmas on a microscopic level.

In February 2003, Professor K Tachibana organized a workshop in Japan entitled 'The New World of Microplasmas', which, for the first time, devoted an entire conference to the exciting new scientific challenges and emerging technological opportunities for microplasmas and microdischarges. The 'Second International Workshop on Microplasmas (IWM-2004)', organized by K H Becker, J G Eden and K H Schoenbach, was held on 6–8 October 2004 at the Stevens Institute of Technology in Hoboken, NJ, USA. IWM-2004 attempted to (i) summarize recent scientific achievements in the area of microplasmas and microdischarges, (ii) highlight current applications, and (iii) identify future scientific challenges and technological opportunities in this field.

Three invited plenary talks, 15 invited progress reports, and 32 contributed papers addressed basic plasma physics aspects relevant to microplasmas and microdischarges, such as experimental, theoretical, and computational studies devoted to the determination of electron and ion energy distribution functions and temperatures, species concentrations, and I–V characteristics, as well as to a wide range of technological applications of microplasmas and microdischarges. In addition, one afternoon was devoted to a symposium entitled 'Environmental, Biological, and Medical Applications of Microplasmas', which was organized and sponsored by the Center for Environmental Systems (CES) at Stevens Institute of Technology and featured five invited talks and five contributed papers.

All participants at IWM-2004 were invited to submit original research papers based on their presentations to this cluster issue of Journal of Physics D: Applied Physics. Devoted entirely to microplasmas, this cluster issue is intended to provide the reader with a sense of the scientific energy and anticipation that characterized IWM-2004. It contains an excellent cross-section of papers describing advances in experimental, theoretical, and computational research efforts to furthering our understanding of microplasmas and the processes and mechanisms that determine their properties and highlight their impact on a growing number of applications in materials processing, displays, and medical diagnostics, to name just a few.

Microdischarge devices (MDs) share many properties with their macroscopic counterparts while having unique features resulting from their ability to sustain large current densities and power depositions on a continuous basis at pressures approaching atmospheric. The dynamics of cylindrical, metal–dielectric–metal sandwich MDs sustained in Ar having characteristic sizes of a few hundred micrometres have been computationally investigated using a plasma-transport model which includes gas dynamics. We found that these devices closely resemble negative glow discharges, as they are sustained by, and particularly sensitive to, ionization resulting from secondary electron emission from the cathode. Since these MDs operate on a cw basis with large current densities and power deposition, gas heating and flow dynamics are important considerations in optimizing their electrical and kinetic properties. For example, the formation of excimer species is particularly sensitive to gas heating and rarefaction due to their dependence on three-body formation processes. Scalings of MDs with pressure, current and secondary emission coefficient are discussed.

Selected highlights in the recent development of microplasma devices are reviewed with emphasis on large arrays of Si-based hybrid plasma/semiconductor pixels. Arrays of 40 000 (200 × 200) pixels, excited by sinusoidal ac waveforms at frequencies of 5–20 kHz, have now been realized. The fabrication of these arrays and their electrical and optical performance with rare gases and Ar/N₂ mixtures are briefly described. Metal/dielectric/metal devices having a piezoelectric dielectric (BaTiO₃), a cylindrical microcavity 50 µm in diameter, and a total thickness of ∼ 110 µm are also discussed. Finally, the introduction of multiwall carbon nanotubes into microdischarge devices as an auxiliary source of current is presented as being exemplary of the opportunities afforded by the integration of nanotechnology into microcavity plasma structures.

Microdischarges (MDs) of filamentary barrier discharges (BDs) in air and N₂/O₂ gas mixtures at atmospheric pressure were investigated using the techniques of spatially resolved cross-correlation spectroscopy (CCS) and short exposure time photography (ICCD-camera). The BDs were generated in symmetric discharge cells (configuration of the type 'glass–glass') with two semi-spherical electrodes in order to localize the repetitive MDs at a fixed position. In the case of CCS measurements, the MD development was imaged through spatio-temporal distributions of the radiation intensity of the (0–0) transition of the 2nd positive system of molecular nitrogen (λ = 337 nm). Two-dimensional optical scanning of the MD channel (in the axial and radial directions of the MD) was carried out for a BD operated in a gas mixture consisting of 6 vol% O₂ and 94 vol% N₂. This gas composition had been found to provide the extremely high stability of the discharge that was necessary for the time-consuming scanning procedure. In the middle of the gap, the MD channel diameter was found to be about 0.3 mm and to expand towards both electrodes. With dielectrics, outward propagating discharges were observed. Short exposure time photos of the MDs taken using an ICCD-camera under the same experimental conditions as for the CCS measurements revealed a branched structure of these discharges on the surfaces, not only on the cathode but on the anode as well. The influence of voltage amplitude on the spatio-temporal distribution of individual MDs of a BD in air was investigated using the CCS-instrument operated in a so-called 'direct start–stop' mode which enabled statistical analysis of the MD sequences within a time range of about 30 µs.

The characterization of plasma by microwaves has been performed for decades. Generally, all known microwave approaches can be classified as analysis of either the wave interference within a quasi-transparent plasma or the absorption near the so-called critical point. These approaches can be generalized for the study of media containing absorbing space-localized irregularities. Essentially, the problem is to identify the power transmittance dependence on the shape, size and refraction or absorption of these irregularities. Moreover, the probing field should have a duration sufficiently short compared to the time scale of the irregularity evolution in order to clearly image the medium. The generalized analysis involves a diffraction theory development for typical plasma formations. The Khizhnyak method of integral equations is exploited here for this purpose. Solutions are obtained in a robust analytical form that allows constructive inversion to determine the particular plasma properties that produce the observed diffraction pattern. The specific cases of a flat plasma layer, plasma cylinder and plasma sphere are described in this manner. The proposed analysis can be implemented experimentally with a compact, tunable, pulsed terahertz source. This development is discussed as well.

The generation of a high energy microelectron bunch in vacuum by an intense short pulse laser in the TEM(1, 0) + TEM(0, 1) mode is investigated in this paper numerically and analytically. A focused short pulse laser in the TEM(1, 0) + TEM(0, 1) mode has a confinement effect on electrons in the transverse direction due to the transverse ponderomotive force, and at the same time the electrons are accelerated and compressed longitudinally by a longitudinal electric field. In our three-dimensional particle simulations, the maximum kinetic energy of electrons reaches 455 MeV, the maximum density is 3.87 × 10¹⁰ cm⁻³, and the normalized transverse and longitudinal rms emittances of accelerated electrons are of the order of 10⁻⁶ m rad at the following parameter values: a₀ = eE₀/(m_eω c) = 10 (where a₀ is the dimensionless parameter of the laser amplitude, e and m_e are the electron charge and rest mass, respectively, E₀ is the laser amplitude, ω the angular frequency of the laser and c the speed of light in vacuum), a laser wavelength λ = 0.8 µm, laser spot size 20λ, laser pulse length 5λ and initial electron velocity 0.99c. Moreover, the transverse and longitudinal sizes of the compressed electron bunch are about 600λ and 10λ, respectively. In this paper, we also present a scaling law of the maximum electron energy. The estimated results of the maximum electron energy coincide well with the simulation results.

The vacuum ultraviolet (VUV) emissions from a cylindrical dielectric barrier discharge (C-DBD) excited by radio frequency (rf) power at 13.56 MHz in both pure Ar and mixtures of Ar with N₂ and air were analysed. Measurements of the relative emission intensity of the excimer second continuum in pure argon were carried out as a function of pressure, rf power and gas flow rate. In Ar–N₂ and Ar–air mixtures, we studied the emission of the N I and O I atomic lines as a function of N₂/air concentration and total pressure. Channels of energy transfer from the argon metastables and excimers to atomic nitrogen and oxygen in excited Ar–N₂ (Ar–air) mixtures were analysed based on the dependence of the intensity of the VUV emissions on the discharge operating parameters and the gas mixture.

An atmospheric microplasma is generated by direct-current (dc) discharge in air with a miniature gas flow through a nozzle, which limits plasma volume. Two discharge modes appear in a nozzle-to-mesh electrode system with helium or argon. One is a repetitive pulsed discharge with a current of 10–30 mA and a short pulse width. The fast pulsed current is powered by electric charges stored in the parasitic capacitance, which depends on the spatial arrangement of the electrodes and the power leads. The pulsed discharge makes it possible to develop a discharge scheme for microplasma generation without a high-voltage pulse generator. The other is a sustained dc discharge, which develops with increasing applied voltage. In the case of helium, a glow discharge configuration is observed with a positive column and a layered structure near the cathode. The length of the positive column is affected by electrode separation and gas flow rate.

Optical emission spectroscopy measurements were performed with added trace probe gases in an atmospheric pressure direct current helium microplasma. Spatially resolved measurements (resolution ∼6 µm) were taken across a 200 µm slot-type discharge. Gas temperature profiles were determined from N₂ emission rotational spectroscopy. Stark splitting of the hydrogen Balmer-β line was used to investigate the electric field distribution in the cathode sheath region. Electron densities were evaluated from the analysis of the spectral line broadening of H_β emission. The gas temperature was between 350 and 550 K, peaking nearer the cathode and increasing with power. The electron density in the bulk plasma was in the range (4–7) × 10¹³ cm⁻³. The electric field peaked at the cathode (∼60 kV cm⁻¹) and decayed to small values over a distance of ∼50 µm (sheath edge) from the cathode. These experimental data were generally in good agreement with a self-consistent one-dimensional model of the discharge.

An atmospheric-pressure air microplasma is ignited and sustained in a 25 µm wide discharge gap formed between two co-planar gold electrodes. These electrodes are the two ends of a microstrip transmission line that is microfabricated on an Al₂O₃ substrate in the shape of a split-ring resonator operating with a resonant frequency of 895 MHz. At resonance, the device creates a peak gap voltage of ∼390 V with an input power of 3 W, which is sufficient to initiate a plasma in atmospheric pressure air. Optical emission from the discharge is primarily in the ultraviolet region. In spite of an arc-like appearance, the discharge is not in thermal equilibrium as the N₂ rotational temperature is 500–700 K. The intrinsic heating of the Al₂O₃ substrate (to 100°C) causes a downward shift in the resonant frequency of the device due to thermal expansion. The temperature rise also results in a slight decrease in the quality factor (142 > Q > 134) of the resonator. By decreasing the power supply frequency or using a heat sink, the microplasma is sustained in air. Microscopic inspection of the discharge gap shows no plasma-induced erosion after 50 h of use.

A brief survey of collisionless plasma phenomenology and nanostructure plasma physics, replete with quantum effects, is presented in conjunction with the determination of the joint collective modes of double quantum wire plasmons interacting with semi-infinite host optical phonons.

This paper focuses on the diagnostics and applications of the microhollow cathode discharge operated in Ar and He at high pressures. The gas temperature and electron number density in Ar and He are deduced from absorption and emission spectroscopy employing line profile analysis. Additionally, a collisional radiative model is used for the estimation of the electron number density in He. The gas temperature increases in Ar from 500 K at 100 mbar to 2000 K at 1000 mbar. The electron density also increases from 2 × 10¹⁵ to 9 × 10¹⁵ cm⁻³ in the same pressure range. In He, the gas temperature reaches values up to 800 K and the electron density does not exceed 5 × 10¹⁴ cm⁻³ at atmospheric pressure. The discharge was coupled with emission and mass spectrometry for analytical applications. The detection of organometallic compounds (ferrocene) reveals good detection limits of about 500 ppb for Fe.

In this paper we present a parameter study on deactivation of Escherichia coli (E. coli) by means of a non-thermal plasma (plasma needle). The plasma needle is a small-sized (1 mm) atmospheric glow sustained by radio-frequency excitation. This plasma will be used to disinfect heat-sensitive objects; one of the intended applications is in vivo deactivation of dental bacteria: destruction of plaque and treatment of caries. We use E. coli films plated on agar dishes as a model system to optimize the conditions for bacterial destruction. Plasma power, treatment time and needle-to-sample distance are varied. Plasma treatment of E. coli films results in formation of a bacteria-free void with a size up to 12 mm. 10⁴–10⁵ colony forming units are already destroyed after 10 s of treatment. Prolongation of treatment time and usage of high powers do not significantly improve the destruction efficiency: short exposure at low plasma power is sufficient. Furthermore, we study the effects of temperature increase on the survival of E. coli and compare it with thermal effects of the plasma. The population of E. coli heated in a warm water bath starts to decrease at temperatures above 40°C. Sample temperature during plasma treatment has been monitored. The temperature can reach up to 60°C at high plasma powers and short needle-to-sample distances. However, thermal effects cannot account for bacterial destruction at low power conditions. For safe and efficient in vivo disinfection, the sample temperature should be kept low. Thus, plasma power and treatment time should not exceed 150 mW and 60 s, respectively.

The hydroxylation behaviour of benzene and toluene were studied using a micro-plasma reactor, where an atmospheric non-thermal plasma was generated by a dielectric barrier discharge (DBD). The results indicated that oxidation products primarily consisted of phenol and C₄-compounds for benzene hydroxylation, whereas cresol, benzaldehyde, benzylalcohol and C₄-compounds were detected for toluene hydroxylation. By taking into consideration the reaction mechanism in the plasma reactor, these products were classified into (1) oxidation of the aromatic ring and functional group on the ring and (2) cleavage of the aromatic ring or dissociation of the functional group on the ring.

Electrosurgical medical devices based on repetitively pulsed nonequilibrium micron-scale to millimetre-scale plasma discharges in saline solutions are described. The formation of vapour layers (bubbles) around active electrodes appears to be a common feature at moderate (<300 V rms) voltages, and dissociation, excitation and ionization of the vapour in these bubbles produces chemical conditions that are thought to be the source of beneficial tissue removal and treatment. Experimental data are discussed, as are the results of modelling efforts of the plasma chemistry. Hydroxyl radicals, hydrogen atoms and other species are observed spectroscopically and their interactions with collagen, a common component of tissue encountered in surgical situations, are considered. Several pathways by which hydroxyl radicals interacting with collagen can lead to tissue removal are discussed.

The performance of a new structure plasma display panel (PDP) cell with counter sustain electrodes was diagnosed by optical emission and laser absorption spectroscopic measurements of the spatiotemporal behaviours of the microplasma. Two different types of panels were prepared: one with sustain electrodes only and the other with additional data and auxiliary electrodes on the rear and front plates. By using cells of the former type, the basic performance was measured as the dependences on the Xe concentration in Ne, the total pressure and the applied sustain voltage. It was seen that a higher Xe concentration was advantageous for the luminous efficiency although the required minimum sustain voltage became larger. By using the latter type of cell, the effects of driving schemes were investigated by varying the applied potentials onto the auxiliary and data electrodes. It was found that the role of the auxiliary electrode is remarkable; the discharge expands largely in the cell when the voltage and the width of the pulses applied to the auxiliary electrode are optimized to be 200 V and 0.3 µs, respectively, while the data electrode is grounded. The production efficiency of Xe atoms in the metastable (1s₅) and resonance (1s₄) states was derived from the measured absolute densities normalized by the input power per cell and the decay rates of those densities. The vacuum ultraviolet emission efficiency estimated from the results was compared between the tested conditions, and a noticeable improvement was recognized in the optimized condition, about 40% larger than a conventional driving condition without additional pulses. It was also seen that this cell structure is potentially superior in luminous efficiency to the conventional coplanar structure currently used in commercial PDPs.

We report electrical properties of radio frequency (RF)-driven hollow slot microplasmas operating in open air but with uniform luminous discharges at RF current densities of the order of A cm⁻². We employ interelectrode separations of 100–600 µm to achieve this open-air operation but because the linear slot dimension of our electrode designs are of extended length, we can achieve, for example, open-air slot shaped plasmas 30 cm in length. This creates a linear plasma source for wide area plasma driven surface treatment applications. RF voltages at frequencies of 4–60 MHz are applied to an interior electrode to both ignite and sustain the plasma between electrodes. The outer slotted electrode is grounded. Illustrative absolute emission of optical spectra from this source is presented in the region from 100 to 400 nm as well as total oxygen radical fluxes from the source. We present both RF breakdown and sustaining voltage measurements as well as impedance values measured for the microplasmas, which use flowing rare gas in the interelectrode region exiting into open air. The requirement for rare gas flow is necessary to get uniform plasmas of dimensions over 30 cm, but is a practical disadvantage. In one mode of operation we create an out-flowing afterglow plasma plume, which extends 1–3 mm from the grounded open slot allowing for treatment of work pieces placed millimetres away from the grounded electrode. This afterglow configuration also allows for lower gas temperatures impinging on substrates, than the use of active plasmas. Work pieces are not required to be part of any electrical circuit, bringing additional practical advantages. We present a crude lumped parameter equivalent circuit model to analyse the effects of changing RF sheaths with frequency of excitation and applied RF current to better understand the relative roles of sheath and bulk plasma behaviour observed in electrical characteristics. Estimates of the bulk plasma densities are also provided. Finally, we present results of afterglow plasma based bacteria inactivation studies (Escherichia coli, Bacillus atrophaeus and B. atrophaeus spores) in which we employ the flowing afterglow plume from a hollow slot microplasma device rather than the active plasma itself, which is fully contained between electrodes.

The number, size, location and velocity of turbulent structures in an argon plasma jet were experimentally investigated using a fast-shutter CCD camera. The camera yielded sequences of plasma jet images with a time interval of 11 µs, and the turbulent structures in the jet were identified by subtraction of matrices representing the succeeding images in the series. This method reveals the regions characterized by increasing or decreasing luminosity that move along the plasma flow and reflect high energy and mass transport in these areas. The results show a high diversity of measured quantities characterizing these structures, and the evaluation thus required the involvement of statistical methods. The statistical distributions of the structure velocities change from non-Gaussian to Gaussian-like ones, depending on the gas flow rate, and so indicate the transitions from a turbulent flow characterized by a relatively regular structure generation to more developed turbulence.

Reactive magnetron sputtering of dielectrics using pulsed DC power in the frequency range between 5–350 kHz provides a deposition process without arcing. We studied the optical emission spectra of aluminium, argon and oxygen during the magnetron sputter deposition of Al in Ar and the reactive sputter deposition of Al₂O₃ in an Ar/O₂ gas mixture using a fast intensified CCD camera. Time-resolved as well as time-averaged optical emission spectroscopic studies were carried out. The time-resolved studies focused on the temporal behaviour of the various emissions during the decay of the plasma after the power is turned off. Decay times ranging from 1 to 4 µs were observed. A detailed analysis of the various processes that contribute to the emission of a particular emission line and its decay was carried out and an attempt was made to relate the various decay times to the dynamics of, respectively, the decay of the fast electrons, the Ar metastables, the Al atoms (metallic mode) and the O atoms (oxide mode).

An investigation of cathode erosion has been conducted for conditions similar to those encountered in a plasma cutting process. A hafnium insert in a water-cooled copper sleeve serves as the cathode. Modifications to the torch allowed the observation of the cathode surface during operation and measurement of material loss from the cathode during different phases of an operating cycle. Erosion has been found to be predominantly due to ejection of molten material droplets. Most ejection events are associated with changes in the conditions of the plasma, e.g. during start-up, change of gas flow and shutdown. The ejections can be explained by imbalances of the forces acting on the molten surface, those associated with the arc current, those due to surface tension, and those associated with the fluid dynamics within the torch.

The dielectric recovery of an axially blown arc in an experimental set-up based on a conventional HV circuit breaker was investigated both experimentally and theoretically. As a quenching gas, synthetic air was used. The investigated time range was from 10 µs to 10 ms after current zero (CZ). A fast rise in the dielectric strength during the first 100 µs, followed by a plateau and further rise later was observed. The dependences on the breaking current and pressure were determined. The measured dielectric recovery during the first 100 µs after CZ could be reproduced with good accuracy by computational fluid dynamics simulations. From that it could be deduced that the temperature decay in the axis does not depend sensitively on the pressure. The dielectric recovery during the first 100 µs scales therefore mainly with the filling pressure. The plateau in the breakdown characteristic is due to a hot vapour layer from the still evaporating PTFE nozzle surface.

In this work, large-scale molecular dynamics simulations are conducted to explore the thermal and mechanical phenomena in surface nanostructuring with a laser-assisted scanning tunnelling microscope. Employing a super parallel computer, more than 200 million atoms are modelled to provide substantial details about how the localized thermal and mechanical perturbations result in surface nanostructures. Extremely localized stress accumulation beneath the sample surface leads to an explosion of the melted/vaporized material, leaving a nanoscale hole in the sample surface. Normal and shear stress development are observed. Stress propagation in space is strongly influenced by the anisotropic nature of the crystal. The high pressure in the melted/vaporized region pushes the melt adjacent to the solid to move, thereby forming a protrusion at the edge of the hole. More importantly, visible sub-surface nanoscale structural damages are observed in a direction 45° with respect to the axial direction. Detailed study of the lattice structure reveals atomic dislocations in the damaged regions. Both temporary and permanent structural damages are observed in the material. The temporary structural damage is featured with a formation, propagation and disappearing procedure.

The electrical properties of CaCu₃Ti₄O₁₂ ceramic materials, showing an enormously large dielectric constant, were investigated. It was found that the grain boundary plays an important role in the giant dielectric behaviour of these ceramics. Measurement of the electrical current density (J) versus the electrical field (E) was carried out. A good linear relationship between lnJ and E^1/2 was found, which demonstrates that the Schottky barrier should exist at the grain boundary. A double Schottky barrier model composed of a depletion layer and a negative charge sheet was proposed, analogous to the barrier model for ZnO varistors. An activation energy value of about 0.6 eV was obtained from the data of the characteristic frequency corresponding to the peak of the imaginary part of the dielectric permittivity versus temperature, which may be attributed to the activation of to in the depletion layer.

A superconducting fragment separator (Super-FRS) is being designed for the production and separation of radioactive isotopes at the future FAIR (Facility for Antiprotons and Ion Research) facility at Darmstadt. This paper discusses various aspects and requirements for the high-power production target that will be used in the Super-FRS experiments. The production target must survive over an extended period of time as it will be used during the course of many experiments. The specific power deposited by the high intensity beam that will be generated at the future FAIR facility will be high enough to destroy the target in most of the cases as a result of a single shot from the new heavy ion synchrotrons SIS100/300. By using an appropriate beam intensity and focal spot parameters, the target would survive after being irradiated once. However, the heat should be dissipated efficiently before the same target area is irradiated again. We have considered a wheel shaped solid carbon target that rotates around its axis so that different areas of the target are irradiated successively. This allows for cooling of the beam heated region by thermal conduction before the same part of the target is irradiated a second time. Another attractive option is to use a liquid jet target at the Super-FRS. First calculations of a possible liquid lithium target are also presented in this paper. One of the advantages of using lithium as a target is that it will survive even if one uses a smaller focal spot, which has half the area of that used for a solid carbon target. This will significantly improve the isotope resolution.

A similar problem associated with these experiments will be safe deposition of the beam energy in a beamdump after its interaction with the production target. We also present calculations to study the suitability of a proposed beamdump.

Electro-rheological (ER) fluids are becoming popular in modern industrial applications. The advantage of employing ER devices is due to the ease of energizing the ER fluids at fast speeds of response. One innovation in ER applications could be in the positioning control of the robotic arm using an ER clutch. In order to actuate the manipulator, the ER output torque response is required. However, the behaviour of this ER torque response at different input conditions is not clearly understood. Therefore, in this paper, a sample study of the ER output torque is conducted. The ER output torque responses at different input parameters are studied carefully for the establishment of an appropriate ER transfer function in shear mode. This transfer function will serve as an important feature in future ER-actuated robot arm's control process.