Table of contents

Atmospheric pressure air plasmas are often thought to be in local thermodynamic equilibrium owing to fast interspecies collisional exchange at high pressure. This assumption cannot be relied upon, particularly with respect to optical diagnostics. Velocity gradients in flowing plasmas and/or elevated electron temperatures created by electrical discharges can result in large departures from chemical and thermal equilibrium. This paper reviews diagnostic techniques based on optical emission spectroscopy and cavity ring-down spectroscopy that we have found useful for making temperature and concentration measurements in atmospheric pressure plasmas under conditions ranging from thermal and chemical equilibrium to thermochemical nonequilibrium.

We describe an example of the use of a high-speed camera in the study of transient phenomena in planar magnetron discharges. Low-current, spontaneous arcs are used to inject a stream of electrons into a magnetron plasma. Fast imaging of the arcs shows how the perturbation produced by the injected electrons travels in the E×B direction with minimum average speeds of approximately 7.4×10⁵ m s⁻¹.

A high density of metastable helium atoms has been created using a Penning-type discharge. Experimental data indicate that the metastable density increases with the increase in the helium gas pressures in the range from 0.1 mTorr to 0.5 Torr. The highest metastable density of 3.5×10¹⁶ m⁻³ is observed at a steady-state gas pressure of 0.5 Torr. The relationships between the densities of the metastable helium atoms and the plasma discharge current are also obtained.

Based on the results obtained using a spectral absorption technique and a theoretical collisional–radiative model, the data of the helium plasma spectroscopic emissions have been used in diagnosing plasma electron density and electron temperature. The results suggest the possibility of generating high-density metastable atoms with a high discharge current.

A two-dimensional array of planar Langmuir probes on a 200 mm diameter silicon wafer was used in an inductively coupled plasma reactor to follow the spatial and temporal variation of ion flux impinging on the wafer in the presence of an instability. Small amplitude low frequency (∼2 Hz) oscillations superimposed on a steady state ion flux distribution were observed in SF₆ plasmas. The magnitude and phase of the oscillations depend on position on the wafer and analysis of these variations reveals that these low frequency oscillations correspond to waves that move across the wafer.

Attachment-induced ionization instability has been experimentally observed in O₂ and CF₄ capacitive RF discharges using time-resolved voltage probe, Langmuir probe, optical emission and mass spectrometry measurements. This instability occurs under specific conditions of power and pressure, and produces synchronized oscillations in the kilohertz range on potentials, emission intensity and positive ion fluxes. In contrast, the SF₆ plasma was observed to remain stable under all experimental conditions. This can be understood by considering attachment and ionization cross sections of these gases and applying the theoretical criterion of instability. Contrary to O₂ and CF₄, the attachment rate coefficient of SF₆ is very high at low energy and has a negative dependence on the electronic temperature. The application of the criterion shows clearly that O₂ and CF₄ plasmas are unstable at low electronic temperature, and that the SF₆ plasma is stable due to its particular low-energy attachment cross section.

This paper discusses a large area multifilamentary plasma source used in the large volume plasma device. This source, based on directly heated filaments, is simple in design and produces quiescent (δn/n≈1%) plasmas of high density (⩽10¹⁸ m⁻³), low temperature (∼1–2 eV), over a large area (≈1.1 m²) and a large volume (≈1.6 m³). With the investment of ≈40 kW (1350 A, 30 V) power, the filaments are heated to ≈2000 K to yield emission current density ∼1 A cm⁻² at the filament surface. Experiments demonstrate that this source is suitable for carrying out electromagnetic wave excitation studies in the electron magnetohydrodynamics regime. There are certain inherent difficulties associated with direct heating which sets a maximum limit to the filament length and with the requirement of field tailoring. As far as the present study is concerned, these difficulties are acceptable in comparison with the distinct advantages the source possesses, in terms of its low cost and technical features, making it user-friendly.

In this paper we present the experimental study of a glow plasma jet (GPJ) obtained from a transferred atmospheric pressure glow discharge (APGD) operating at 60 Hz. The characterization of the emission spectra for both electrical discharges is presented and the electrical circuit features for APGD generation are discussed. The potentiality of GPJ as a source of active species for depletion of contaminants in liquid hydrocarbon fractions is also established.

A self-consistent one-dimensional model was developed to study the effects of non-local electron conductivity on power absorption and plasma density profiles in a planar inductively coupled argon discharge at low pressures (⩽10 mTorr). The model consisted of three modules: (1) an electron energy distribution function (EEDF) module to compute the non-Maxwellian EEDF, (2) a non-local electron kinetics module to predict the non-local electron conductivity, radio frequency (RF) current, electric field and power deposition profiles in the non-uniform plasma, and (3) a heavy species transport module to solve for the ion density and velocity profiles as well as the metastable density. Results using the non-local electron conductivity model were compared with predictions of a local theory (Ohm's law), under otherwise identical conditions. The RF current, electric field, and power deposition profiles were very different, especially at 1 mTorr for which the effective electron mean free path was larger than the skin depth. However, the plasma density profiles were almost identical (within 10%) for the same total power deposition in the plasma. This result suggests that, for computing plasma density profiles, a local conductivity model (Ohm's law), with much reduced computational expense, may be employed even in the non-local regime.

A model of solitary current filaments formation in a high pressure electronegative gas discharge is proposed. An explicit analytical solution of the balance equation for electrons including stepwise ionization, electron attachment and diffusional losses is obtained. The contracted distribution of electrons in the positive column is found to have a soliton form. The conditions favouring non-thermal plasma contraction are considered. Analytical results are compared successfully with simulation and experimental data.

Characteristics of laser produced Cu plasma were investigated using spectroscopy, a CCD camera, and a Langmuir single probe. A pulsed Nd : YAG laser of 52 mJ, 335 nm, and pulse duration 7 ns was used for generating high density plasma in vacuum and argon buffer gas. Spectroscopic measurements were devoted to determine plasma lifetime, electron temperature T_e, plasma velocity V_p, and electron density N_e. T_e was determined using a Boltzmann plot and N_e was determined using Stark line broadening. Langmuir single probe was located at 3.5 mm from Cu target to measure T_e and N_e. The T_e values of the probe were coincident with the spatial profile of determined by spectroscopic measurements. Plasma lifetime and the CCD camera image were dependent on the Ar pressure. These plasma parameters improve the laser plasma deposition thin films.

X-ray emission in different energy windows, from a low energy Mather-type plasma focus by employing high Z inserts at the anode tip is investigated. Quantrad Si pin diodes with differential filtering are employed as time-resolved x-ray detectors, whereas a pinhole camera is used for time-integrated analysis. The x-ray flux from the focus region is found to be measurable within the pressure range 0.25–3.5 mbar of hydrogen. The maximum emission in 4π-geometry is found to be 29.4±0.2 J, 3.43±0.05 J and 4.00±0.02 J with Pb, W and Mo inserted anodes, respectively, and corresponding wall plug efficiencies for x-ray generation are 1.28%, 0.15% and 0.2%. The x-ray emission is found predominantly as a result of electron beam activity on the anode tip, which is confirmed by the images recorded by the pinhole camera.

Carbon nanotubes (CNTs), due to their unique electronic and extraordinary mechanical properties, have been receiving much attention for a wide variety of applications. Recently, plasma enhanced chemical vapour deposition (PECVD) has emerged as a key growth technique to produce vertically-aligned nanotubes. This paper reviews various plasma sources currently used in CNT growth, catalyst preparation and growth results. Since the technology is in its early stages, there is a general lack of understanding of growth mechanisms, the role of the plasma itself, and the identity of key species responsible for growth. This review is aimed at the low temperature plasma research community that has successfully addressed such issues, through plasma and surface diagnostics and modelling, in semiconductor processing and diamond thin film growth.

The spectra of oscillations are examined in a short helicon source excited at a frequency of 13.56 MHz. The parametric interaction of an rf field with plasma was found to give rise to turbulence in the low-frequency (LF) range about 1 MHz. The excitation of LF oscillations is shown to have thresholds on the rf antenna current (input power) and the dc magnetic field. The direction of propagation, phase velocities and correlation characteristics are measured for the components of the LF spectrum. Turbulent pulsations are attributed to the ion-acoustic waves; their amplitudes and spatial profiles are determined. Special measurements are performed and theoretical estimations are made to ascertain the physical origin for turbulence. Computations show that under experimental conditions both strong parametric instability and supersonic dc electron flow induced by the ponderomotive force can give rise to the excitation of ion-acoustic waves.

A global uncertainty and sensitivity analysis is performed for a detailed gas-phase reaction set in a CHF₃ plasma. The goal of this paper is to ascertain the uncertainties in plasma reactor model results (plasma and radical densities) that originate from the uncertainties in the gas-phase chemistry database. We discuss the rates of reactions and their uncertainties. Comparisons with experimental data show that gas-phase rate uncertainties do not explain the disagreements at higher pressures (>30 mTorr). We also find that electron impact dissociation reactions of dominant neutrals are the largest sources of uncertainties. HF kinetics are also found to be critical in determining radical and feedstock gas densities. Relative ion densities are uncertain due to poor understanding of charge transfer mechanisms.

This paper presents some fundamental characteristics and performance of a secondary emission electron gun (SEEG) using a pulsed glow discharge wire ion plasma source (WIPS). The positive helium ions extracted from WIPS are accelerated in vacuum toward a negatively biased cold cathode surface, which is set oblique to the ion loci in order to inject the secondary electrons emitted perpendicular to the foil window. The physical mechanisms governing the characteristics such as space charge, charge exchange and secondary electron emission have been reviewed. The dependence of such characteristics on the accelerating voltage of the incident ion and on the ion incidence position has been experimentally investigated. The experimental results are discussed together with available theoretical models of each characteristic to establish the relative understanding of such phenomena in a side-extraction-type SEEG. The experimental results are further discussed in the light of a self-developed numerical simulation using a finite element method, which presents a good understanding of particle trajectories as well as potential distribution inside the gun geometry.

Large area radio frequency (rf) capacitive discharges have attracted recent interest for materials etching and deposition on large area substrates. A distinguishing feature is that the radial distribution of the absorbed rf power in these discharges depends on the rf voltage across the plates, independent of the radial variation of the plasma density n(r). A reduced set of steady-state fluid equations has been used to investigate the radial variation of n and electron temperature T_e. The derived equations are shown to be invariant with respect to pL and pR, where p is the pressure, L is the plate separation and R is the discharge radius, and can be further reduced to the equations of the usual global balance model when R≪λ_ε, the energy relaxation length. In this limit, the ionization frequency and T_e are essentially independent of radius and n can be approximately described by the usual radial profile of a zeroth-order Bessel function. When R⩾λ_ε, n and T_e are predominantly determined by local particle and power balance, and the n and T_e radial profiles are flat over most of the volume except near the radial boundary, where n falls and T_e rises to account for the increased losses at the boundary. The scale length of the edge density variation in the local balance regime is shown to be proportional to the energy relaxation length.

The operational features and thermal plasma characteristics of a plasma torch with hollow electrodes are investigated based on their dependence on input current, gas flow rate and electrode diameter when air is used as a plasma gas. A plasma torch with a hollow cathode and anode has been designed and fabricated, and the arc voltages and thermal efficiencies are measured from its discharge. The newly modified similarity criteria are derived from the measured data related to torch performances. From the fact that these criteria successfully describe both the arc voltage and thermal efficiency behaviour of the torch, depending on its operating and geometrical parameters, it is proved that they can be usefully applied to the design and operation of high power torches. For the numerical modelling of the interior region of the torch, a cold flow analysis is employed along with a simplified balance equation of the Lorentz and gas dynamic drag forces in order to determine a cathode spot position on the cathode surface. The validity of this method is confirmed by comparison of the calculated and measured net powers. As a practically useful result of this analysis, carried out through this numerical and experimental work, it is suggested that low input current, high gas flow rate and relatively large electrode diameter are more favourable as appropriate operating conditions of the torch for the efficient treatment of hazardous organic wastes.

A reproducible instability, which appears similar to those reported previously, has been observed and studied in a low-pressure 13.56 MHz inductively coupled gaseous electronics conference rf cell operating in oxygen. The instability has been observed in the form of periodic modulations in the light output, floating potential, electron and positive and negative ion densities. The magnitude and frequency of the modulations is sensitive to the plasma operating conditions and the modulation amplitude has been observed to be as high as 40%. The instability is observed in a pressure and power regime where both the capacitive and inductive modes can exist. The frequency of the oscillations increases with increase in gas pressure from 3 to 21 kHz. This pressure window coincides with the pressure regime where there exists a significant fraction of negative ions in both modes. Time-resolved measurements of the electron energy distribution functions and charged particle densities indicate that at all phases of the instability, the plasma parameters remain close to those of the inductive mode. A global model has been modified for an oxygen discharge and this provides a qualitative description of the instability. The global model predicts a smaller power and pressure window for the instability but it can provide a framework for the discussion of instabilities in weakly electronegative discharges.

The electromagnetic fields and plasma parameters have been studied in an azimuthally symmetric surface wave-excited plasma (SWP) source, by using a two-dimensional numerical analysis based on the finite-difference time-domain (FDTD) approximation to Maxwell's equations self-consistently coupled with a fluid model for plasma evolution. The FDTD/fluid hybrid simulation was performed for different gas pressures in Ar and different microwave powers at 2.45 GHz, showing that the surface waves (SWs) occur along the plasma–dielectric interfaces to sustain overdense plasmas. The numerical results indicated that the electromagnetic SWs consist of two different waves, Wave-1 and Wave-2, having relatively shorter and longer wavelengths. The Wave-1 was seen to fade away with increasing pressure and increasing power, while the Wave-2 remained relatively unchanged over the range of pressure and power investigated. The numerical results revealed that the Wave-1 propagates as backward SWs whose phase velocity and group velocity point in the opposite directions. In contrast, the Wave-2 appeared to form standing waves, being ascribed to a superposition of forward SWs whose phase and group velocities point in the same direction. The fadeaway of the Wave-1 or backward SWs at increased pressures and increased powers was seen with the damping rate increasing in the axial direction, being related to the increased plasma electron densities. A comparison with the conventional FDTD simulation indicated that such fine structure of the electromagnetic fields of SWs is not observed in the FDTD simulation with spatially uniform and time-independent plasma distributions; thus, the FDTD/fluid hybrid model should be employed in simulating the electromagnetic fields and plasma parameters in SWPs with high accuracy.

Plasma densities produced by half- and full-wavelength (HW and FW) helical antennae in helicon discharges are compared. It is found that HW antennae are more efficient than FW ones in producing plasma downstream from the antenna. The measured wave amplitudes and the apparent importance of downstream ionization do not agree with computations.