Table of contents

The X-ray emission from CO₂-laser-produced plasmas has been studied for target elements covering the atomic number range Z=6.92. For irradiations of approximately 7*10^1C W cm^-2 the X-ray yield was less than approximately 3*10^-6. Plasma temperature has been measured as a function of incident power over the range (2-8)*10¹⁰ W cm^-2 for carbon and titanium and is typically 100-150 eV.

Measurements of breakdown voltage in hydrogen between concentric hemispheres are shown to be in general agreement with the similarity theorem. Since the breakdown field in the geometry used closely approximates to that between concentric spheres, it is concluded that the similarity theorem may be applied to breakdown in spherical geometry.

The decay of neutral copper vapour density which follows the extinction of a high current (2-11 kA) vacuum arc between copper electrodes has been measured. The results show that during the period 1-8 ms from current zero the vapour density decays at least an order of magnitude slower than expected on the assumption that vapour atoms generated by the arc move freely to the electrode and vacuum chamber surfaces where they condense. The results can be explained, however, if it is assumed that the vapour is produced by evaporation from molten droplets which are emitted from the cathode spots.

Of the existing theories for estimating the tensile strength of a liquid, that proposed by Temperley (1947) is the one which agrees best with the experimental results in the case of water. Temperley's theory is used to estimate the tensile strength of liquid argon, oxygen and nitrogen and the results are compared with those of other theories and also with the experimental values obtained for these liquids.

Viscoelastic and dielectric measurements are discussed that prove a difference in structure between recovery under shear and recovery at rest. The data include both systems with intermolecular and with interparticulate structures. The difference in structure can be explained on the basis of a three-dimensional network which develops after cessation of flow, whereas under shear the structure consists usually of isolated elements of varying size and shape. This picture is in agreement with the measured spectra. Finally a simple experiment is suggested which allows one to estimate the necessity for a multi-parameter description of a given thixotropic system.

It is pointed out that new nematogenic liquids with much larger Kerr constants than nitrobenzene can be used in Kerr cells. Some properties of 4'-n-heptyl- to -cyanobiphenyl are succinctly described. The response time of the Kerr effect is shorter than 0.1 mu s, and the driving voltage and power of a light modulator using this liquid instead of nitrobenzene may be reduced by factors of 5 and 100 respectively.

A knowledge of test time in shock tubes is important. Calculated values are unreliable because of the large role played by non-ideal effects such as turbulent flow through the opening diaphragm and by boundary layer phenomena. Moreover, in the case of combustion-driven and arc-driven shock tubes the driver gas state is usually only poorly known. For these reasons the test time must be evaluated experimentally. The author has developed a technique which employs a smear camera to measure the test time. The technique also provides a method for determining that regime of shock tube operation where the rarefaction wave reflected from the driver section plays a dominant role.

A new method is presented for the evaluation of the edge-capacitance correction for electrodes of finite thickness. The corrections are divided into three separate components: electrode thickness, the lower edge of the electrodes, and the back of the electrodes. Simple correction formulae are derived for the first two components and a set of curves is provided which enables the direct evaluation of the third component from an equally simple formula. Numerical results show excellent agreement with those obtained by more tedious graphical methods.

The determination of the image forming wavefunction is considered for normal operation of the conventional transmission electron microscope. No assumption is made about the relative strengths of unscattered and scattered radiation. Spatial coherence and monochromaticity are assumed. For objects which are represented by a finite number of Fourier components, an algorithm for phase determination is given. Only the locations and not the intensities of the reflections in the diffraction pattern are needed. The main information used is contained in two or three images with different defocus. Combinations of defocus values which should be avoided since they yield insufficient information, can be determined beforehand from the diffraction pattern.

A mass spectrometer was used to determine the effect of spark discharges on sealed CO₂ TEA laser mixtures. The carbon dioxide in the spark channel was completely dissociated, but the overall dissociation coefficient of the CO₂ was less than that in a comparable distributed discharge. No oxides of nitrogen were found in concentrations which would lead to instability in the main discharge.

Suggests practical methods for stabilizing the frequency of a laser to the center of a weak inverted Lamb dip in a saturable absorber contained between the mirrors of an optical resonator. In one method the resonator forms part of a three-mirror laser cavity, and in two others the resonator is outside the laser. In each case, a servo system is suggested for controlling both the frequency of the laser and the tuning of the resonator, the limiting frequency stability is estimated, and causes of displacement of the controlled frequency from the absorption frequency are discussed. The results are compared with those for an internal cell laser with gas discharge fluctuations, derived using the theory of Greenstein. The practical examples make use of the estimated parameters of the He-²⁰Ne/¹²⁹I₂ combination.

A study of plasma and neutral gas flow dynamics has been carried out in pure helium flowing afterglow. The measured plasma flow velocity nu _p is observed in effect to be radially independent, in contrast to the instantaneous neutral gas flow velocity profile nu _g(r) which is approximately parabolic with some slip at the walls. This observation has led to a new and simplified approach to the flow dynamics based on the concept of a radially independent plasma flow velocity from which, in conjunction with the instantaneous charged particle velocity radial profile (assumed to be equal to nu _g(r) and a fundamental mode Bessel radial distribution of charged particle (plasma) density, an averaged plasma flow velocity nu _p has been calculated which is shown to be accurately equal to nu _p in the approximate pressure range 0.3-0.9 Torr. nu _p (hence nu _p) is shown to be related to the mean neutral gas flow velocity nu ₀ according to nu _p approximately ⁴/₃ nu ₀.

A direct current arc source between graphite electrodes was investigated with regard to its application as radiation and temperature standard. The arc plasma discharge created between the central positive electrode and the annular negative electrode was stabilized in rotation by a weak coaxial magnetic field. The true temperature of the central electrode was estimated close to 3810+or-40K. Time stability and reproducibility of the radiance of the continuum gave satisfaction in the wavelength range used (2400-5000 AA).

The effects of insulating materials in the auxiliary gap, auxiliary gap current, main gap current and electrode separation on the delay have been investigated. The presence of insulating material in the auxiliary gap having low auxiliary gap resistance appears to exhibit large delay. Delay decreases considerably with increase of current in the auxiliary and the main gaps, but it increases with increase of electrode separation. The scatter in the delay is less than 25 mu s and 500 mu s with supramica (Mycalex Corporation of America) and silicon carbide respectively at lower values of auxiliary gap current and it becomes negligible for supramica at auxiliary gap currents greater than 6A. This investigation appears to indicate that the simple device can be used as a fast switch.

For pt.IV see abstr. A67969 of 1973. Gas-phase fluxes, with and without additional surface flow, have been calculated for random, isotropic, two-dimensional and three-dimensional networks comprising a wide range of connectivities. A set of distribution functions which have previously been studied by the authors in parallel (P) and serial (S) models was used. Evidence for the importance of connectivity is augmented by these results. The intermediate status of the network model between the P and S models is confirmed. However when surface flow is present the surface-flow structure factor k_s, but not the total flux, can be anomalous in this respect.

A new reflection mode twisted nematic liquid crystal display scheme is described that uses only one-sheet-type polarizing filter. In this scheme the reflector and the second polarizer of the conventional twisted nematic display are replaced by a combination of a quarter-wave plate and a cholesteric liquid crystal layer. This combination is equivalent to a combination consisting of a linear polarizer, a colour filter and a diffuse reflector. The display scheme shows either black characters against an unusually brilliant iridescent background or the converse situation of iridescent characters against a black field. The performance of this new display scheme is compared to the conventional twisted nematic reflective mode display.