Table of contents

Temperature anomalies noted in ozone generators using DC high-voltage supplies and low air-flow rates led the authors to the study of energy transfer in a wire-to-cylinder corona discharge and to propose a simple explanation for the anomalies noted.

Breakdown threshold intensity measurements have been made in oxygen and nitrogen using single picosecond pulses from one of three laser systems operating at 1.06, 0.69 and 0.53 mu m. The results are compared with those made previously to confirm that in high radiation fields ( gamma <or=1), a breakdown threshold regime exists which is only weakly dependent on pressure, indicative of direct multiphoton ionisation.

Townsend and Huxley's technique has been used in measurements of the ratio of lateral diffusion coefficient to mobility D/ mu in hydrogen and nitrogen at ambient temperature for the ranges of reduced electric field E/N of 0.15<or=E/N127.09 Td and 0.61<or=E/N<or=233.2 Td respectively. The present results have been compared with those obtained by other authors.

The reaction rate in a highly compressed plasma is enhanced by the presence of suprathermal ions. The authors have computed this effect for a large range of densities (10²⁵-10²⁸ cm^-3), temperatures (0-15 keV) and D/T isotropic ratios. The results are presented graphically for a plasma seeded with a small amount of ⁶Li.

The spatiotemporal development of electron densities within homogeneous overvolted discharges in N₂ is investigated by optical means during the early build-up phases. The use of photomultiplier techniques rather than image devices enables a quantitative comparison between experiment and one-dimensional model calculations. The influences of space-charge fields and photosuccessors on discharge development are investigated. Considerable electron densities are also detected in front of the primary avalanche even during the early, field-distortion-free stages of the development.

A collisional radiative model is used to obtain the number densities of the ground level and of the excited levels of neutral argon as a function of the local n_e and T_e values. This model is first applied to a homogeneous and stationary medium for 5000K<T_e<12000K. The local values of the density of atoms in the ground level and of the heavy particles (neutrals and ions) temperature T_g in a non-LTE discharge are determined. The C-R model is then applied to the positive column of a stationary argon arc. The ambipolar flux of electrons is evaluated and compared to the net rate of electron production.

Measurements are reported of the decay of ionisation in positive-ion/negative-ion flowing afterglow plasmas. When one or both of the ion species is molecular, conditions of pressure and ionisation density can be readily established under which mutual neutralisation (ionic recombination) dominates over ambipolar diffusion and hence the recombination coefficient alpha _i can readily be determined. When both ionic species are atomic the recombination rate is so slow that ambipolar diffusion is seen to dominate over ionic recombination under the conditions of the experiment, and it has only been possible to determine upper limits for alpha _i.

The spatial and temporal variations of the line profiles are investigated for three high-pressure discharges operated on a 50 Hz supply. Direct measurements of shift and broadening are performed in the periphery of one of the discharges. Results show a shift to lower frequency and give, for broadening (2 delta _n) and shift ( Delta _n) produced by neutral atoms collisions, the following relations: 2 delta _n=2.1*10^-18 mAA cm^-3 K^-0.3, 1.5<2 delta _n/ Delta _n<2. Experimental profiles are compared with those obtained from a theoretical model for the radiation transfer. Broadening by charged particles is obtained from Sahal-Brechot's model; for collisions with neutral atoms two types of interaction potential are used: a Van der Waals potential characterised by a C₆ constant and a Lennard-Jones potential characterised by constants C₆ and C₁₂.

Studies of the dependence of the anisotropic properties of rare-gas afterglow plasmas on the external parameters are presented. The anisotropic properties exhibit themselves in a uniform magnetic field B; there is a rapid increase in the afterglow loss rate as the angle theta between the direction of B and the glass discharge tube long axis is increased from 0 to 5 degrees . The variable parameters are pressure in the range 0.1-1.0 Torr, the gas, the magnetic field strength in the range 0.01-0.1 T, and the input power to the pulsed discharge. It is found that the heavier rare gases show a more pronounced anisotropy than the lighter, at given pressure, magnetic field strength, and input power. An important new effect is the oscillation in loss rate which is observed in many cases for the higher magnetic field strengths when theta is increased in the range approximately 0.5-3 degrees .

Positive DC breakdown voltages of various N₂-O₂ gas mixtures have been measured to point-to-plane gaps over a temperature range 113 to 293K, the gas density being kept constant at any temperature. The breakdown voltage in N₂ increased at low temperatures. The increase in breakdown voltages became small, when a small amount of O₂ (less than 1%) was mixed into N₂. The breakdown voltage in synthetic air decreased at low temperatures. When small pulses were superposed on DC voltages, the breakdown voltage changes only slightly with temperature in both N₂ and synthetic air.

The conductivity of a weekly ionised flowing plasma subjected to weak static electric and magnetic fields has been investigated using the Boltzmann transfer equation (BTE). The velocity dependence of collisions and the effect of flow velocity have been taken into account by using an appropriate collision integral. The dependence of the transport coefficients on Mach number scattering and Hall parameters has been studied in the linear case. The transport coefficients are almost independent of the Mach number in the case of velocity-independent collisions. In a neutral particle collision-dominated plasma the conductivity decreases with increasing Mach number; the converse is however true for an ionic collision-dominated plasma.

Presents experimental results which show that a wall in contact with a dense hot metal vapour injects a very-high-density current. In the same way, at the root of a cathode spot the vapour density is very high and a calculation is made of the enhancement of the electron emission due to the lowering of the work-function caused by the polarisation energy of the excess electrons in the vapour. A relation is established which allows the density of the vapour to be determined and thus the rate of evaporation of a cathode made out of any material.

Three kinds of moving striations, called p, r and s waves, are known to be excited at low discharge current in positive columns of glow discharges in neon gas. The dispersion relation for r and p waves numerically calculated from the basic equations, which are modified from Wojaczek's (1971) to include the metastable atoms, agree fairly well with the experimental results. The frequency of the r wave decreases with increase in the discharge current, while that of the p increases. The frequency dependence of the gas pressure for both waves is inverse. The calculated gas temperature dependence of the frequency for both waves also agrees well with the experimental values for gas temperatures from 80 to 350K.

A single-dimensional model, based on an integral formulation of the conservation equations, has been developed which makes it possible to predict the behaviour of these arcs. The radial distribution of axial plasma velocity v₂(r,z) is represented by a function v_z(0,z)(1-(r/r_a)²)ⁿ where r_a is the arc radius and r and z are the radial and axial coordinates. The solution for n by by the use of radial integral of the axial momentum equation makes it possible to develop a simple but detailed picture of the axial velocity distribution in the arc column. A comparison of the axial distribution of plasma velocities with published experimental results shows that turbulent exchange processes are dominant for currents greater than about 500A.

The three-body attachment coefficient in oxygen is calculated by a Boltzmann equation method using the electron collision cross section for this process which is proportional to the gas number density. The calculated values of the coefficient agree well with previous experimental values and represent well the gas pressure dependency of this coefficient, suggesting that the present treatment is basically appropriate.

The energy absorption of 1-1.6 kJ gain-switched CO₂ laser pulses focused by an approximately f/22 mirror to a spot of innermost diameter 300 mu m (FWHM) on 0.5 and 1 mm suspended cubes of polyethylene has been measured to be 75+or-13%. For plane targets the absorption was also encouragingly high, 71+or-10%. The relative insensitivity of refraction and other losses to pellet size (vis-a-vis previous short-pulse Nd laser experiments) are fully consistent with the ruby laser shadowgram and interferogram observations made towards the end of the 1.5 mu s CO₂ pulse, which reveal dense cold material expanding almost randomly at velocities of approximately 0.5 mm mu s^-1.

The method used to obtain the transport parameters of a Townsend discharge reported by Blevin et al. in 1976 from the observation of the photon flux produced in the discharge has been extended to enable measurements of the transverse diffusion coefficient and Townsend's first ionisation to be made. The results obtained in molecular hydrogen are now reported.

Electron densities in laser-produced plasmas have been deduced from space-resolved spectra of oxygen-like ions Ca XIII, Ti XV, Fe XIX, Ni XXI and Cu XXII in the XUV (80-170 AA). The transition 2p⁶¹S₀ to 2s2p⁵¹P₁ has been identified for Cu XXII, the wavelength of the same transition for Ni XXI has been defined more precisely. Plasma images in the light of the different spectral lines have yielded the dimensions of the spatial regions in which Ca XIII, Fe XIX and Ni XXI exist.