Table of contents

Explicit formulae are given for the energy reflectance and phase change on reflection of electromagnetic waves at the boundary of an absorbing medium.

This letter discusses the criticism - based on the fact that real fluids are multistructured - that has been made of theoretical studies on thixotropy that employ only one structural parameter. The kinds of experimental observations that could be obtained on multistructured fluids are suggested, but a review of recent results indicates that there is no evidence that multistructured behaviour is significant. There are, of course, many experimental difficulties associated with the study of thixotropy, resulting in much scatter in data. It is concluded that at the present state of knowledge the first approximation in the description of thixotropy in the form of the single-structured theory is adequate, particularly for engineering and technological purposes. In case multistructured behaviour should become apparent, the way in which a second approximation can be set up is summarized, with reference being made to the report in which details are given.

A straightforward NMR technique which determines the spatial profile of the velocity in laminar fluid flow is described. An improved method is also presented for directly measuring the velocity distribution function. The methods employ a linear magnetic field gradient, which introduces a spatial variation to the nuclear Larmor frequency. Experimental results for the velocity profile of water flow in circular and rectangular pipes are presented, as well as for the velocity distribution function in a circular pipe.

The paper describes a method for determining the density of solids, which is based on a well-known buoyancy formula and which uses analytical and microanalytical balances.

Previous treatments of the electron optics of image converters utilizing homogeneous axial electric and magnetic fields have ignored relativistic effects. It is shown that the inclusion of relativistic terms leads to a small correction (about ½% at an accelerating voltage of 30 kV) to the classical focus condition. In addition, the chromatic aberration is found to be reduced by several percent compared with nonrelativistic estimates.

Signals near 1 GHz were multiplied in frequency by up to 825 times and mixed with the hydrogen cyanide laser frequency at 891 GHz in point-contact Josephson junctions. The 1 GHz signals were generated by direct multiplication from a 120 MHz quartz-crystal oscillator, and no microwave transfer oscillator was needed. The maximum IF output powers obtained in these and other harmonic mixing experiments (using the same laser) were found to agree quite well with a simple theory which predicts a limit proportional to the square of the frequency being multiplied in the Josephson junction. This provides a criterion for determining the lowest frequency at which to begin multiplication in a Josephson device.

The performance of Nb-Nb point-contact Josephson junction mixers was investigated experimentally with signals from a 891 GHz laser. The local oscillator (LO) was provided by the appropriate junction-generated harmonic of either a 74 or 99 GHz klystron. The intermediate frequency (IF) power varied linearly with signal power for signals less than 10⁻⁵ to 10⁻⁶ W. The optimum conversion loss (from 891 GHz to the IF) was not significantly different for the two klystron frequencies used: the lowest useful conversion loss for the whole mixing system including the signal input matching loss, was 42 (±3) dB. This is 10 dB greater than that observed with a 891 GHz laser LO and the same junction apparatus. Conversion loss of the mixer element alone was independent of IF with the latter varied up to 1·2 GHz. The noise at the output of the IF system was usually dominated by that of the transistor preamplifier with a noise temperature of about 500 K. Thus the input signal necessary to give a signal-to-noise ratio of one at the IF was equivalent to 10⁻¹⁶ W Hz⁻¹ of IF bandwidth. This mixing system, which can be significantly improved, forms the basis of a fully tunable heterodyne receiver for the submillimetre region.

Velocity measurements in an under-expanded supersonic free jet have been made using the Fabry-Perot laser-Doppler technique. The jet was produced by a nozzle of exit diameter 2·7 mm operating at a pressure ratio P₀/P_infinity=6·6. Measurement of the complex structure, characteristic of this type of jet, on such a small scale proved to be an excellent test of the capability of the Fabry-Perot laser-Doppler technique for making air velocity measurements.Sufficient scattering particles were present in the flow to make artificial seeding unnecessary. The particles were found to respond to the velocity change across the Mach disc (a normal shock) in a distance of similar 0·1 mm. Laser-Doppler velocity measurements were compared with velocity measurements deduced from Pitot pressure measurements. The difference in results between the two methods was < 1%. Sufficient laser-Doppler velocity measurements were obtained to construct the axial and four transverse velocity profiles which clearly showed the positions and magnitudes of the major features characteristic of an under-expanded free jet.

An analysis of the role of primary spherical aberration ϕ in governing the intensity distribution of a focused laser beam has been confirmed by recording the fluorescence caused by the passage through a dye solution of radiation from both an Nd³⁺ laser and an He-Ne laser.

The agreement between these recorded distributions and those computed using the Fresnel-Kirchhoff integral leads to the specification of the limit ϕ less-than-or-eq, slant 1λ for the reliable use of the geometric-optics approximations d=fθ and l=2(radical 2-1)f²θ/D for the focused beam diameter d and effective length l of the focal volume in terms of the beam divergence θ, unfocused beam diameter D and lens focal length f. Use of these relations under conditions where ϕ>1λ can lead to grossly inaccurate evaluation of the focal dimensions and of the focused beam intensity.

The role of spherical aberration in laser-beam-induced gas breakdown has been investigated experimentally using single 20 ps Nd³⁺ laser pulses and combinations of beam diameters and lenses which changed ϕ from 0·41λ to 164λ. The results confirm the importance of aberration in controlling the focused beam intensity. The breakdown threshold intensity for argon at atmospheric pressure was found to be 3 × 10¹³ W cm⁻².

A complete solution of the neutron slowing-down problem is developed for isotropic elastic scattering in an infinite homogeneous medium. An exact formula for obtaining all the complex roots of the transcendental equation is given. This, together with the exact nontrivial real root determined elsewhere, constitutes the exact solution of the problem. A general misconception about the application of these roots in the first collision interval is pointed out. Numerical results obtained for different magnitudes of absorption rates show agreement with more conventional solutions, like that of Teichmann. The number of terms in the infinite sum necessary for convergence depends on the relative magnitude of lethargy and the pre-assigned permissible error. Good results are obtained with a modest number of terms for small values of relative lethargy, but only with a few for larger values of u/.

Exact expressions are derived for the time, space and angle dependence of neutron flux discontinuities due to inelastic and elastic scattering. It is demonstrated that these discontinuities can exist for periods of the order of nanoseconds after the initial neutron source pulse.

A study is made, based on the two-component MHD equations, of the effect of a low-frequency averaged force on the DC component of an ion-resonance probe's potential. The final formula for the change of the potential shows a resonance near the ion plasma frequency ω_Li. In the frequency range ω double less-than sign ω_Li, ion-acoustic resonances are found, which makes it possible for the electron temperature T_e to be determined from the experiment.

Measurements are described of the temporal and spatial variations of the space charge fields in afterglow plasmas together with parallel measurements of electron density (n_e), positive ion density (n₊), negative ion density (n_-), electron temperature (T_e) and mass-analysed wall currents of both positive and negative ions. In the case of the plasmas containing electrons and positive ions only, it has been shown that the spatial variation of plasma potential (Δ V_a) is related to gradients in the charged particle densities and to the electron temperature via diffusion theory, and no complications in the sampling of positive ion wall currents with an orifice probe arise. For plasmas in which significant concentrations of negative ions exist, the previously observed `trapping' of negative ions within the plasma occurs, as manifest by a lack of negative ion wall current at early afterglow times followed later by a sudden onset. The probe measurements indicate a continuous diffusive loss of electrons and positive ions during the afterglow, whereas the negative ions remain within the plasma until n_e has fallen to less, similar 10⁻²n_- at which point an avalanche loss process of the remaining electrons begins which rapidly leads to the establishment of a negative-ion/positive-ion plasma. During the afterglow period, a progressive collapse of the wall potential and the ambipolar field occurs from those characteristic of an electron-dominated plasma to those of a negative-ion-dominated plasma. The sudden appearance of negative ion wall current is closely correlated with the avalanche collapse of the ambipolar field. The significance of these observations to mass-spectrometric sampling from afterglow plasma is discussed.

The equations for a cylindrical plasma column with radial variations of the discharge parameters are examined for solutions corresponding to ion waves and ionization waves. Comparison with other less complete treatments shows that allowing for such radial variations introduces no new physical processes and that the numerical corrections involved are usually less than a factor of two in the dispersion relations for such waves.

Theoretical estimations of the voltage gradient are reported for cylindrical, wall-stabilized arcs established in highly pressurized SF₆ gas. The calculations are performed numerically without using a heat flux potential or any other analytical approximations. Numerical data of the electrical and thermal conductivities are stored in a computer memory, and those values are processed through lagrangian interpolation. The SF₆ gas pressures are 1, 4 and 16 atm; the axial temperatures of the arc columns are in the range 7000-40 000 K; and the stabilized bore diameters are 0·2, 0·4, 0·5 and 0·8 cm. The calculated results are compared with experimental measurements and other theoretical results, and the proposed calculations are found to be closer to the experimental results for the wide range of magnitude of the arc current.

Inelastic light scattering from the free surface of the liquid crystal ethyl p-[(p-methoxybenzylidene)amino] cinnamate (EAC) has been used to determine its surface tension and viscosities in both the nematic and smectic-A mesophases. In the nematic mesophase (117-138 °C) the molecules are parallel to the surface and are aligned by the 2 kG magnetic field. At 133 °C the surface tension (dyn cm⁻¹) and viscosities (P) (σ; η₁, η₂, η₃) are (47·5±2·5; 0·09±0·02, 0·14±0·02, 0·30±0·10), and at 120 °C they are (50·0±2·5; 0·17±0·04, 0·18±0·02, 0·30±0·15). For the smectic-A mesophase (107-117 °C) the molecules are perpendicular to the free surface and to the smectic-A layers. The scattering spectrum is independent of η₂ in this case. Thus at 109 °C the three parameters (σ; η₁, η₃) were obtained: (52·5±2·5; 0·19±0·04, 0·70±0·30).

Acoustic waves in water may be produced by the absorption of infrared energy from a pulsed laser. The temperature distribution in water for a kilowatt pulse with a duration of the order of a millisecond can be described in terms of the beam parameters and the optical absorption coefficient of water. The expression for the temperature distribution has been solved numerically. The acoustic waves were studied by the use of hydrophones in an anechoic tank filled with tap water. Cylindrical waves with exponentially decreasing amplitudes with depth were produced by the absorption of laser radiation. The velocity of these waves was in close agreement with the velocity of acoustic waves in water.

Dark currents in benzene and pyridine have been studied through a range of temperatures including the melting point for a variety of electrode materials, electrode spacings and applied potentials. Photocurrents have also been studied, the parameters specified above and the wavelength range of the incident photons all being varied. It is deduced that in the liquid state the current is carried by ions, and in the solid state by electrons. The results obtained can be interpreted in terms of a combination of bulk and electrode phenomena, the former being dominant for large electrode spacings and the latter for small electrode spacings. In pyridine for small electrode spacing an apparent negative photoconductivity is observed.

A closed-form equation is given for the normal freezing of ideal ternary liquid solutions of A, B, and C atoms into ideal solid solutions of mixed AB-AC compounds. Sample computations with the use of this equation were made and are given for the Ga-Al-As system.

It is shown that the absorption correction f(χ) is dependent on two parameters: the mean depth which essentially determines the correction for f(χ)>0·8 and which accounts for variations with incident beam energy, and a shape function that is a function of overvoltage ratio and the mean atomic number of the specimen. The commonly used modified Philibert correction represents the mean depth reasonably well but has a very inadequate shape function.

The general effect of the shape function can be demonstrated conveniently by the behaviour of the ratio f(χ)/f_sq(χ) where f_sq(χ) is the absorption correction predicted by a square model for the depth distribution of ionization. For many practical purposes the overvoltage variation of the shape function can be neglected, but there is always a substantial atomic number dependence in the range 0·05<f(χ)<0·5.

Although there are not sufficient consistent experimental data available on which to base an accurate absorption correction covering the full range of f(χ), possible forms for such a correction procedure are discussed. A simple expression for use in the case of high absorption, f(χ)<0·2, is proposed.