Table of contents

A gas–liquid gliding arc (glidarc) discharge reactor is used to degrade high concentration phenol solution. Phenol solution with 1878 mg L⁻¹ initial chemical oxygen demand (COD) is treated by spraying directly into the plasma zone formed between two electrodes through a gas–liquid two phase atomizing nozzle. A number of parameters such as voltage waveform, solution concentration, electrode material, nature of the carrier gases and the gas–liquid ratio are examined. It is found that the voltage waveform of gas–liquid gliding arc discharge is more irregular than that of gas gliding arc discharge, and the breakdown voltage of gas–liquid gliding arc discharge is lower than that of gas gliding arc discharge. The COD abatement of phenol solution with stainless steel as the electrode material is higher than that with brass and aluminium. The increasing electrode thickness and the increasing gas–liquid ratio and carrier gas such as oxygen can increase the degradation of phenol. The final COD of the solution is 173 mg L⁻¹; either air or oxygen is used as the carrier gas, and the solution treated is acidic. The variation of pH and conductivity and the formation of hydrogen peroxide and ozone are measured. The occurrence of CO₂ is detected during the plasma treatment: the maximum concentration is 18 000 ppm. While H₂ and NO_x are also detected during the plasma phase, p-nitrosophenol (C₆H₅NO₂) and p-nitrophenol (C₆H₅NO₃) are detected in the solution treated.

A critical analysis of photoionization as the major process for seed electron production ahead of the cathode-directed streamer has been made. The accumulation of ions between pulses and fast electron detachment in an electric field could be an effective source of seed electrons for repetitively pulsed discharges in electronegative gases. Measurements and two-dimensional calculations in the hydrodynamic approximation of streamer parameters in air (anode current, propagation velocity and radiative channel diameter) can only agree if one assumes uniform background pre-ionization and not by taking into account only the photoionization process generally adopted for pulsed discharges in air. The results of analytical estimates and full three-dimensional simulations confirm that the seed charge distribution in the undisturbed gap has an effect on the streamer head formation and leads to the streamer branching phenomena.

Polycrystalline diamond and carbon nanotubes (CNTs) exhibit excellent vacuum field emission properties, characterized by low turn-on voltage and high current density. Their atmospheric field emission and ionization capabilities are reported in this paper. Highly graphitic polycrystalline diamond (HGPD) film was grown in a plasma-enhanced chemical vapour deposition process, and its ability to ionize atmospheric air was characterized and compared against CNTs. The HGPD sample was activated by applying a moderate voltage bias (340 V) for an extended period across a 10 µm electrode gap. After activation, a turn-on voltage of 20 V and a sustainable current of 10 µA were observed with the same gap. Results also indicate that field emission helps to create a moderate ionization effect without catastrophic air breakdown. A hydrogen plasma treatment is shown to restore emission current back to or even exceeding the original level, which suggests an important role of surface termination in the electron emission process. CNTs were grown and tested but did not perform as well under similar conditions.

A hydrodynamic model of the vacuum arc thruster and its plume is described. Primarily an effect of the magnetic field on the plume expansion and plasma generation is considered. Two particular examples are investigated, namely the magnetically enhanced co-axial vacuum arc thruster (MVAT) and the vacuum arc thruster with ring electrodes (RVAT). It is found that the magnetic field significantly decreases the plasma plume radial expansion under typical conditions. Predicted plasma density profiles in the plume of the MVAT are compared with experimental profiles, and generally a good agreement is found. In the case of the RVAT the influence of the magnetic field leads to plasma jet deceleration, which explains the non-monotonic dependence of the ion current density, on an axial magnetic field observed experimentally.

In this paper, a plasma source discharging at atmospheric pressure and its characterization diagnosed by a Langmuir probe and a digital camera are presented. As an application the dielectric barrier discharge (DBD) gun modifying an ultraviolet cured resin surface for ink printability is reported. The results from the digital camera indicate the uniformity and homogeneity of the plasma generated from the gun in the downstream but depending on the input power, diameter of electrodes, gas flow rates and the distance between the substrates and the nozzle. The contact angle measurement proves the efficiency of gun during the surface modification. The ink printability following DBD gun processing described here was significantly improved. The performed surface analysis using Fourier transform infrared indicates that the polarity of surface grafted in plasma is responsible for the film printability.

A pure inductive regime of pulsed RF discharge in N₂–He mixtures at 0.2 Torr has been investigated by time resolved Langmuir probe and optical emission spectroscopy. A planar coil ICP with a Faraday shield has been used. Mixture compositions of 5–100% N₂ and duty cycles, T_ON = 1–15 ms and T_OFF = 15 ms, have been explored. The Langmuir probe analysis evidences a Maxwell electron energy distribution in the discharge and post-discharge, characterized by electron density and temperature that depend on both duty cycle and composition. The vibrational excitation of N₂(C) state is inferred by the N₂ second positive emissions. Its kinetic analysis reveals the formation of a high vibrational excitation of ground state nitrogen. A vibrational temperature higher than 10 000 K can be reached at the end of the 5 ms discharge pulse on the levels v = 0–3 of the X state. Such a high vibrational temperature slows the post-discharge relaxation of the electron temperature.

A new type of ac plasma display panel (PDP) with shadow mask (SM-PDP) is presented for its effective structures with lower cost. A two-dimensional self-consistent fluid model is used to analyse the discharge characteristics of the SM-PDP cell in a simplified driving scheme. The effects of the addressing time, the sustaining frequency, the duty ratio of the sustain voltage and the rise time of the voltage pulse on the discharge efficiency are investigated in this paper. The simulation results can help us optimize the driving scheme of SM-PDP.

X-ray measurements of the plasma of a compact 2.45 GHz electron cyclotron resonance ion source (ECRIS) are performed to determine the temperature and density of the electrons heated resonantly in the ECRIS. The x-ray detector used to investigate the plasma consists of a small silicon (Si-PIN) photodiode to detect photons in the energy range of 1–100 keV. The detector has an energy resolution of 180 eV at 5.9 keV that allows us to record detailed x-ray spectra. Assuming two temperature electron populations, both Maxwellian distributed, the analysis of the x-ray spectra shows a temperature of about 2 keV for the hot electron fraction in addition to the population of cold electrons at less than 2 eV. The fraction of the hot electrons amounts to 1–10%. We present a description of the x-ray detector set-up as well as x-ray spectra and calculations for the temperature and density of the electrons in the ECRIS plasma.

Atmospheric pressure dc glow discharges were generated between a thin cylindrical anode and a flat cathode. Voltage–current characteristics, visualization of the discharge and estimations of the current density indicate that the discharge is operating in the normal glow regime. Emission spectroscopy and gas temperature measurements using the 2nd positive band of N₂ indicate that the discharge forms a non-equilibirum plasma. Rotational temperatures are 700 K and 1550 K and vibrational temperatures are 5000 K and 4500 K for a 0.4 mA and 10 mA discharge, respectively. The discharge was studied for inter-electrode gap spacing in the range of 20 µm–1.5 cm. It is possible to distinguish a negative glow, Faraday dark space and positive column regions of the discharge. The radius of the primary column is about 50 µm and is relatively constant with changes in electrode spacing and discharge current. Estimations show that this radial size is important in balancing heat generation and diffusion and in preventing thermal instabilities and the transition to an arc.

The Coaxial Plasma Source-1 facility (Mayo R M et al1995 Plasma Sources Sci. Technol.4 47) was modified from a short pulse, high current (SPHC) pulse forming network (PFN) with very low inductance (∼200 nH) to a large inductance ladder circuit. This modification allows for a longer, flat top gun current pulse that eliminates the under-damped, sinusoidal behaviour of the gun current with consequent interruptions in plasma parameters. The new PFN was designed to produce a current waveform for a much longer period (∼1 ms). As a consequence of increasing the pulse length, the magnitude of the gun current was reduced as no additional energy storage was added to the PFN. The characterization of the electrical and plasma behaviour of the experiment operated with the long pulse, low current (LPLC) PFN is presented. The gun currents produced by the LPLC PFN are approximately one-fifth in magnitude of the gun currents produced by the SPHC PFN. Axial plasma parameters were measured near the muzzle of the plasma source, and electron densities were found to range from 1 × 10¹⁹ m⁻³ to 7 × 10¹⁹ m⁻³ depending upon the axial location. These values are approximately 1–2 orders of magnitude less than the electron densities produced by the SPHC PFN at the same locations. Electron temperatures range from 30 to 60 eV at these locations and are very similar to those produced by the SPHC PFN. A resistive MHD model was applied as an order estimate of the plasma resistivity and demonstrates reasonable agreement with measured values of the magnetized coaxial gun resistance.

Fluid models of gas discharges require the input of transport coefficients and rate coefficients that depend on the electron energy distribution function. Such coefficients are usually calculated from collision cross-section data by solving the electron Boltzmann equation (BE). In this paper we present a new user-friendly BE solver developed especially for this purpose, freely available under the name BOLSIG+, which is more general and easier to use than most other BE solvers available. The solver provides steady-state solutions of the BE for electrons in a uniform electric field, using the classical two-term expansion, and is able to account for different growth models, quasi-stationary and oscillating fields, electron–neutral collisions and electron–electron collisions. We show that for the approximations we use, the BE takes the form of a convection-diffusion continuity-equation with a non-local source term in energy space. To solve this equation we use an exponential scheme commonly used for convection-diffusion problems. The calculated electron transport coefficients and rate coefficients are defined so as to ensure maximum consistency with the fluid equations. We discuss how these coefficients are best used in fluid models and illustrate the influence of some essential parameters and approximations.

This work investigates the use of hairpin probes in plasma where RF plasma potential is present. The microwave resonance of the hairpin is used to determine electron density. Two types of hairpin probe were used. One type was dc coupled: its dc potential could be varied while monitoring its resonance frequency and collected current. The other probe was designed to be fully floating, being (dc) isolated from ground and able to float with RF variations in the plasma potential. Additional measurements of the RF plasma potential and its effect on the dc floating potential of the former probe were made using a wire loop probe. The resonant frequency of the dc coupled probe at zero current (nominal floating potential) was less than that determined from the fully floating probe. This is attributed to the wider sheath around the former caused by RF plasma potential across it. The presence of the electron-free sheath around the wires of the hairpin is included in the analysis that links the resonant frequency to the electron density in the bulk plasma. When the dc coupled probe was biased at the true floating potential (determined from independent loop probe measurements) its resonant frequency was closer to, though still consistently higher than, that of the floating probe. This work shows that RF potential across the probe sheath affects the resonance of a hairpin probe and should be accounted for when using hairpin probes in discharges where RF plasma potential variations are even as low as a few times the electron temperature (in volts).

We treat the kinetics of the three dominant species in a hydrogen plasma expansion, which was studied using various laser spectroscopic techniques. Whereas the ground-state H₂ molecules expand like a normal gas, the behaviour of H atoms and ro-vibrationally excited molecules is considerably altered as a consequence of their reactivity. The hydrogen atoms are primarily lost via surface association. In the source, H atoms are lost on the surface of the nozzle, reducing the atomic flux, and in the expansion, the H atoms diffuse out due to large H atom concentration gradients between the plasma and the background that exists as a result of surface association on the vessel walls. The loss process of H atoms in the nozzle, surface association, is the main production process of H₂ molecules with high rotational and vibrational excitation. The behaviour of these ro-vibrationally excited H₂ molecules in the expansion shows the relaxation process from the high temperature in the upstream region to the low temperature of the downstream region. The lower rotational levels (J < 5) adapt to the local temperature as expected, but the higher levels (J > 7) do not because of the large energy spacing between the hydrogen rotational levels. The consequence of this high rotational excitation is demonstrated in the dissociative attachment of electrons to H₂.

A pulsed microwave discharge burning in pure nitrogen was studied theoretically. The time-dependent Boltzmann equation for electrons was solved numerically in multi-term approximation. It was assumed that the discharge was ignited by a 100 kW microwave (f = 9.4 GHz) pulses with 2.5 µs duration; the repetition frequency was 400 Hz. It was shown that the electron distribution function approaches very quickly the steady state distribution function after a change of the amplitude of electric field intensity. The steady state time averaged values of electron mean energy, diffusion and rate coefficients and drift velocity were calculated for different values of electric field intensity. With these values the actual values of electric field intensity from a previous experiment were determined from the measured time dependence of electron concentration. The calculated values were compared with previous experimental results.

The influence of nitrogen additions to helium flow on the non-equilibrium plasma parameters and the discharge onset voltage of the self-sustained normal dc glow discharge at atmospheric pressure is studied. The concentrations of the low-excited helium atoms in states 2¹S, 2¹P, 2³S and 2³P are determined in glow discharge in helium (99.98%He) and in helium with a nitrogen admixture using the absorption spectroscopy technique. It is shown that the addition of a small amount of nitrogen into helium (less than 5%) leads to the increase of both interelectrode gap voltage and gas temperature. The drastic reduction of concentration of the low-excited helium atoms (n = 2) in the cathode region even at a nitrogen admixture of 0.5% occurs due to their quenching by the nitrogen excited species. At the same time, concentrations of higher excited helium atoms (n = 3) are essentially unchanged.

The temporal evolution of parallel and perpendicular ion velocity distribution functions (ivdf) in a pulsed, helicon-generated, expanding, argon plasma is presented. The ivdf's temporal evolution during the pulse was determined with time resolved (1 ms resolution), laser induced fluorescence. The parallel ivdf measurements indicate that, in the expansion region of the plasma and for certain operational parameters, two ion populations exist: a population moving at supersonic speeds (1.1 Mach) resulting from acceleration in an electric double layer (EDL) and a slow moving population (0.7 Mach) generated by local ionization. After 100 ms, although present, the EDL is not fully developed and has not reached a steady-state. Measurements of the perpendicular ivdf indicate constant radial expansion, with ion speeds of ≈400 m s⁻¹, in the expansion region.

A technique for the measurement of the absolute electron density in low-pressure plasmas using microwaves is described. It is based on observing the propagation of electromagnetic surface waves (SW) at a plasma-sheath boundary, guided by a dielectric cylinder immersed in the plasma. The transmission spectrum is measured between two antennas situated at either end of the dielectric cylinder and connected to a network analyser. Analytical theory based on the Trivelpiece–Gould work (Trivelpiece and Gould 1959 J. Appl. Phys.30 1784, Trivelpiece 1967 Slow-Wave Propagation in Plasma Waveguides) indicates that the lowest frequency at which the SW can propagate is equal to of the plasma frequency, which is directly related to the electron number density at the plasma-sheath boundary. We call this probe the plasma transmission probe (PTP) in contrast to the plasma absorption probe proposed by Sugai and co-workers (Kokura et al 1999 Japan. J. Appl. Phys.38 5262). The PTP is promising for the measurement of low densities (⩾10⁹ cm⁻³) at relatively high gas pressure (⩽1 Torr). An axi-symmetric finite element model of the probe is presented and used to calculate transmission spectra. Experimental spectra measured in a radio-frequency capacitively coupled discharge in argon at various plasma densities and pressures (40–750 mTorr) are presented and compared with the calculated ones. Plasma densities derived from the transmission spectra were compared with those obtained with a Langmuir probe. The PTP was also compared with a microwave 1/4-wave resonator ('hairpin probe') at low pressure (5–45 mTorr) in an ICP discharge in argon. The densities determined by the PTP were found to be lower by a factor of 0.5–0.7 compared with those obtained with a Langmuir and a hairpin probe. We believe this can be attributed to the pre-sheath plasma density gradient, as the PTP determines the sheath edge electron density, not the bulk value.

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