Table of contents

A differential equation for the arc shape is proposed which accounts for three factors which influence the column motion: (i) magnetic fields, (ii) gas flows and (iii) asymmetrical Joule heating. The physical conditions required for the validity of the equation are outlined. The equation is given in a vector form suitable for treating cases with arbitrary field and discharge geometries.

The thickness shear mode forms and frequency spectrum of AT-cut plano-convex and biconvex quartz crystal resonators are obtained by an approximate analytic method. The results show good agreement with the mode forms observed by x-ray topography and the measured frequencies.

The behaviour of electrical discharges confined by metal rather than dielectric walls, in a gas mixture suitable for CO₂ infrared laser action, has been examined. These discharges have positive differential characteristics and require very much lower axial fields and breakdown potentials than those in similar dielectric tubes, but the operating currents are higher.

There are two distinct modes of discharge operation, each of which can lead to laser oscillation at 10·6 μm. The first mode at higher pressures (greater, similar 10 Torr for a 100 mA discharge in a 2 cm diameter tube more than 10 cm long) has a non-uniform axial current distribution, and cannot be expected to produce the most efficient excitation conditions for laser action. The second mode at lower pressures (less, similar 5 Torr for a 100 mA discharge in a 2 cm diameter tube 10 cm long) has a uniform current distribution, and leads to more efficient conversion of electrical energy to infrared output than the first. The performance of such devices is comparable with conventional dielectric confined lasers, but long, and hence high-power, lasers can only be made with multi-segment construction.

In a sealed CO₂-N₂-He system with a clean discharge tube the degree of dissociation of the CO₂ is greater than 80% (with no hydrogen present) and laser action cannot be obtained. If platinum is distributed along the discharge tube walls as a discontinuous film it catalyses back-reactions reforming CO₂. The degree of dissociation is then less than 40% and efficient laser action at 10·6 μm is obtained. Using such distributed heterogeneous catalysis a CO₂-N₂-He-Xe laser has operated for more than 3000 h. In this system both hydrogen and deuterium are undesirable additives because they decrease the excitation rate of the upper laser level.

In this communication the growth of a perturbation in the intensity distribution of a two-dimensional Gaussian beam in a plasma has been studied. When the beam travels in the self-made uniform waveguide (ie when the power of the beam is equal to the critical power for self-focusing) the intensity distribution is stable. For beams converging on account of self-focusing the amplitude of perturbation increases much faster with distance than the axial amplitude in the absence of perturbations. In a medium with saturating nonlinear dielectric constant, the maximum growth rate of perturbation occurs at an optimum value of the beam power. The optimum power depends on the nature of electron scattering, when the nonlinearity of the dielectric constant is caused by the heating of carriers by the beam. The growth of a two-dimensional Gaussian perturbation in a plane beam of uniform intensity has also been analysed.

The total internal reflection electro-optic deflector/modulator has so far been limited by the subdiffraction patterns that appear in the far field when light is transmitted through the device with zero applied voltage. This paper isolates the causes of these subdiffraction patterns and describes a simple means of removing them from the far field without affecting the performance of the device. The results of measurements taken at zero applied voltage are included and give an indication of the order of this effect.

The optical transmission through oil-dispersed colloidal molybdenum disulphide has been investigated in a magnetic field at two different wavelengths. The experimental results show that the optical transmission strongly depends on the state of polarization of incident light. The results are interpreted in terms of the contribution due to magnetic dipole radiation in addition to that due to electric dipole.

This paper presents the first systematic graphical analysis for determining the input coupling efficiency of the prism-coupler method in a planar, lossless optical waveguide. Two common prism couplers and film materials on glass substrates are used as illustrative examples. The method shows that we can obtain optimum coupling efficiency of 81·45% with relative ease by varying any of the three main parameters - the coupling air gaps d₂, the thickness of the film W, and the coupling length l.

Degassing rate and infrared reflectivity measurements upon proprietary optical blacks are reported. After bake-out to 250 °C, degassing rates range from 3×10⁻⁷ Torr litre s⁻¹ cm⁻² at 230 °C to 1×10⁻¹⁰ Torr litre s⁻¹ cm⁻² at 25 °C. The hemispherical diffuse and angular specular reflectivities have been measured at discrete wavelengths near 28 μm, 118 μm and 337 μm. For wavelengths less than 30 μm the blacks are shown to be essentially diffuse reflectors, with reflectivities of the order of 5%. At longer wavelengths specular reflection dominates, reaching values of up to 50% at normal incidence for 337 μm radiation. Substrate reflection is believed to be significant in this long-wavelength region.

Calculations are presented of the ²³⁷Np, ²³⁵U and ²³⁹Pu fundamental mode fission rates in a thorium assembly based on a diffusion theory code with a DB² leakage term. Three different Th cross section sets derived from the ABBN set, the UKNDL68 file and the ENDF/B-II file were used in the calculations and the results compared with each other and with experimental results reported previously.

The sensitivity of the instantaneous decay constant of the fundamental mode to changes in partial cross sections is investigated.

Two techniques are used to investigate time-dependent neutron behaviour in a large iron assembly. The first, covering the energy range from 1·4 to 132 eV, uses resonance absorbers in conjunction with either a Ge(Li) detector or a small BF₃ counter whilst the second, from 110 to 846 keV, uses inelastic scattering indicators with a Ge(Li) detector. The measurements in the eV region are compared with values calculated using a multigroup transport code. For those in the keV region corrections are applied for level lifetime and source burst width and comparison is made with the result of a preliminary calculation.

Resonant effects upon the ion current to electrical probes in a magnetic field are presented. When the ion cyclotron radius becomes equal to an even fraction of the distance between the double probes, or between the probe and the wall, a sudden decrease or increase in the ion current collection by the probes takes place as a result of resonant plasma depletion or supply between the interacting parts (ion cyclotron radius resonance).

Experimental observations on the motion of s prime variety ionization waves in neon are reported. The results are explained in terms of mode-jumping and forced modulation due to nonlinear effects.

The ratio of the radial diffusion coefficient to mobility D_r/μ has been measured for electron swarms moving through uniform electric fields in carbon monoxide and carbon dioxide for the range 3 less-than-or-eq, slant E/p less-than-or-eq, slant 500 and 10 less-than-or-eq, slant E/p less-than-or-eq, slant 550 V cm⁻¹ Torr⁻¹ at 0 °C respectively using the analytical methods described by Kontoleon et al (1972). The values of D_r/μ in the range E/p>50 are the first results in both the gases. The ionization coefficient α/p has been calculated from the current-growth experiments in carbon monoxide and carbon dioxide in the range 50 less-than-or-eq, slant E/p less-than-or-eq, slant 500, and the values of α in CO, in the range E/p>90, have been measured for the first time.

The present experimental data obtained for carbon dioxide is used to calculate the electron energy distribution and the mean energy using the techniques described by Lucas (1969).

Previous work has shown that a low-current DC arc in an axial gas flow is analogous to a heated solid rod surrounded by a gas stream which removes heat from the surface. The heat transfer from the arc core is then described by the well-known Nusselt heat-transfer relationship for this situation. The present work tests the effect of different gases and electrode configurations. The relationship is valid for the non-reacting gases examined, and the different electrical characteristics obtained for each gas are accounted for simply by the different thermal/physical properties of the gases. The model enables such an arc to be characterized by measurements of electric field, current and gas-flow velocity. The arc characteristics for reacting gases such as propane/air mixtures are strongly affected by other factors such as electrode arrangement.

Inferences about the gas flow in the arc system drawn from this model have led to the construction of a small double-annulus device that enables a gas fed to the inner annulus to be heated by the arc while being shielded from the surrounding atmosphere by an inert gas flow. It is suggested that this device may find applications in low-current-plasma chemistry.

A method of temperature measurement behind reflected shock waves in argon is investigated, which utilizes the integrated emission over a large number of rotational lines in the ²Σ->²II band system of OH. In the majority of experiments the OH emission is generated by the addition of 0·5% (H₂+O₂) to the argon, and the intensity, from two spectral regions, is monitored by two monochromators. From the ratio of the observed intensities the gas temperature can be calculated from the known spectroscopic data for OH. Theoretically derived intensity ratios for several bands, compared with that of the (0,0) band, are given as a function of temperature.

Values of spectroscopic temperatures, over the range 3000-6000 K, agree with the gas dynamic values, derived from incident shock wave velocities with Mach numbers varying from 3·5 to 5·2, to within the predicted experimental error. The method is shown to be inapplicable at initial pressures greater than about 80 Torr because of self-absorption and at pressures lower than 20 Torr radiative disequilibrium may equally limit its use. Moreover, the pre-association reaction of oxygen and hydrogen atoms is found to enhance the intensity of most bands except the (0,0) and (1,1); only the latter bands may therefore be used to obtain the time variation of temperature behind the shock front.

The dusty-gas model is employed to obtain expressions for the flow of a dissociating gas (in local chemical equilibrium) through a porous medium, or a capillary tube. The resulting equations can be expressed in a manner that is formally similar to those for the flow of a single nonreacting gas, but with a viscous term involving a dependence of viscosity on pressure, and a diffusive term which involves not only Knudsen diffusion coefficients but also the binary mutual diffusion coefficient D_1v for interdiffusion of dissociated and associated species.

For an isomerizing gas, however, these complications virtually disappear. The equilibrium composition, and therefore the viscosity, no longer varies with pressure; and the dependence on the binary mutual diffusion coefficient (to a good approximation) is removed by a cancellation of terms.

The effect of flow in an adsorbed layer is considered.

Light from coronas at tungsten point electrodes in a number of insulating liquids has been photographed and the dimensions of the luminous region measured. It was found that the radius of curvature of the tip of the corona region was proportional to the applied voltage. The ratio of the change in the applied voltage to the change in the corona radius which resulted is taken to be a measure of the local electric field at the boundary of the corona region. The effects of viscosity on the corona in hexachlorodiphenyl are also reported.