Table of contents

Double axis X-ray rocking curves from epitaxial layers of GaInAsP on InP have been recorded using synchrotron radiation at glancing incidence angle down to 1.1 degrees . For 0.2 and 0.3 mu m thick layers, excellent agreement is found over the whole range between experiment and simulations based on the Takagi-Taupin equations.

Simple dimensional arguments are employed to obtain the functional form of energy broadening expressions which often arise in the analysis of charged particle beam interactions. It is thereby shown that some conclusions of a recent theoretical model for energy broadening in beams from metal ion sources (LMIS) do not result from the physics of the model but arise only from the requirement for dimensional consistency.

Switching results are presented for bistable chiral smectic liquid crystal cells aligned using a rubbed polymer. These include the effect on light transmission of an AC field in the range 0.1 Hz-1000 kHz when accompanied by positive or negative DC bias. This has a possible application as a macroscopic grey scale.

Optical fibres possess many advantages for measurement sensing. An especially important feature is the one-dimensional nature of the medium, since this offers unique advantages for spatially-distributed measurement. This Review outlines the principles on which distributed optical-fibre sensors are based, and looks at recent progress which has been made towards their implementation.

A novel spectroscopic cell has been devised which allows measurement of the changes of non-polarised infrared absorption of a liquid in the range of wavenumbers from 330 to 5000 cm^-1 due to the application of an electric field applied parallel to the infrared beam. The cell is therefore radically different from conventional Kerr cells. The technique has potential for the spectroscopic study of organic liquids and solutions generally and for electrically insulating liquids in particular, allowing the dielectric response to be related to the chemical structure. In this case the application of the cell is illustrated using nitrobenzene.

The paper is concerned with identifying the dominant electromagnetic modes of a tape helix loaded in a conducting waveguide. Two distinct regions of operation of the helix are found. For k_nu r_h<1(k_nu :free space wavenumber; r_h:helix radius), the helix supports slow as well as fast waves, while for k_nu r_h>1, it supports only fast waves. For each region separate asymptotic formulae have been derived. These have also been verified experimentally. Since the exact dispersion relation for a helix is extremely complex these formulae serve to eliminate the associated computational problems. Moreover, a helix in a waveguide supports a large number of fast wave modes. This makes the simplified formulae for fast waves extremely important since these isolate the dominant fast wave directly.

Continuous laser operation has been achieved on the 546.1 nm line in Hg(I) using optical pumping with a UHF excited resonance lamp at 2.45 GHz. The mercury laser with UHF excitation in comparison with discharge pumping has potential for higher efficiency, high stability output, compactness and long laser lifetime.

Using a general expression for the spatial dependence of the temperature determined by a steady-state heat flow equation for a cylindrical X-ray target anode with dimensions much larger than the incident electron beam radius, the radius of the melt region on the anode surface has been calculated as a function of the total incident electron beam power. These calculations are shown to be in excellent agreement with experimentally measured values of the melt radius for tungsten, copper, and molybdenum target anode materials which indicates that electron beam melting is well described using the steady-state heat flow equation. A novel method is also presented for the unambiguous determination of the incident electron beam radius, and therefore the X-ray source size, which makes use of the dependence of the onset of target anode melting on the electron beam radius. This approach is shown to yield consistent values for the electron beam radius on tungsten, copper, and molybdenum target anodes.

Observations have been made under several different circumstances of the increased development of streaming potential with cavitating flow of high purity water. Two separate mechanism for this increase are proposed and shown to be consistence with the observations. The magnitude of these electrical effects and previous work with non-cavitating flow suggests that these effects could be important to erosion-corrosion problems with cavitating flow.

For pt.I see ibid., vol.17, p.699 (1984). An investigation of the deep structure of the electron diffusion tensor D in an isolated swarm has been developed from the Boltzmann equation as applied to the electron conservative case including inelastic collisions. It is shown that D consists of three components. They are the weighted ensemble average in the velocity space, and the two-additional terms which represent both the curvature effect due to the electric field and the scattering effect at collision. Numerical calculations in Ar have been executed to emphasise the importance of the last two terms. A difference of over 5% appears between the transverse diffusion coefficient D_T and the conventional expression D_V with E/N subject purely to elastic collisions due to the curvature effect. As is expected, the discrepancy between D_T and D_V grows with E/N where the recycling effect due to the excitation-energy barrier is included.

The electron distribution function is calculated in argon by solving the Boltzmann equation in the classical two-term approximation for reduced electric fields in the range 10^-19<or=E/N<or=2*10^-15 V Cm². A detailed set of collision cross sections including about 30 excitation levels is used. This set of inelastic cross sections is obtained directly from a microscopic approach without any adjustment to fit the calculated and measured electron swarm parameters. The predicted values of these parameters are in excellent agreement with the most recent experimental data. For each level, the excitation rate coefficients are reported for the direct excitation from the ground state and for the cascades from upper states. They agree with the experimental results when the latter are available, but for most levels the authors' results are the first ones.

A technique of determining the density distribution of an electron plasma column confined by non-uniform magnetic fields is presented. The non-uniform confining fields cause the electron density distribution to be non-uniform not only radially, but also axially. Calculations of the plasma shapes show that for a stable plasma mode the plasma surface follows a flux tube in some range of the z axis and gradually shrinks towards the ends. For the plasma whose surface follows a flux tube, the central density profile can be deduced from the measured axially projected density profile. Analysis of the l=0 plasma wave dispersion relation further supports the diagnostics in the electron density distribution.

Deviations to Paschen's law for SF₆ at high pressures are commonly explained by field enhancement due to protrusions supposed to exist at the surface of the electrodes. New experiments for positive polarity allow the authors to draw a different conclusion, i.e. to ascribe these deviations to the impurities of the gas, especially to the combined effect of O₂ and H₂O. For negative polarity, impurities play a minor role and the threshold is very sensitive to the surface state of the stressed electrode.

The probability of electrical breakdown in neon is investigated under a pressure of 13.3 mbar, using the breakdown time-delay measurement method. It is found, by applying a laser spectroscopy technique, that the 1S₅ metastable states of neon atoms dominantly contribute to the breakdown probability in the moment when more than 1 s has passed after the previous discharge. With this method the presence of 1S₅ states of Ne atoms is detected in the gas, under the pressure mentioned, up to about 500 s after the cessation of the breakdown.

Streamer propagation and the streamer to leader transition have been thoroughly investigated for SF₆ and air/SF₆ mixtures under positive impulse conditions. This paper, being the first of two related papers on the matter, concentrates on the streamer corona. Streamer propagation in strongly electronegative gases is found to require the average applied field along the streamer path to be close to critical rather than being characterised by a lower 'guiding field'. A theoretical explanation for this behaviour is given. The improved understanding of streamer propagation allows the prediction of corona extensions and corona changes.

For pt.I see ibid., vol.12, p.2351 (1986). This second paper on the breakdown mechanisms in electronegative gases under non-uniform field conditions deals with the development of the leader type of discharge, the emphasis being laid on details of the streamer to leader transition, which is found to be very much different from the corresponding process in air. In a strongly electronegative gas, the streamer to leader transition involves a 'leader precursor' that has been observed for the first time. A hypothesis relating precursor inception to an ion drift phenomenon is shown to explain the observed parameter dependences satisfactorily. Numerical simulations have been performed, revealing details of precursor formation. Some experimental results concerning the further development of the leader channel are also presented.

Results of optical investigations of SF₆ high-current arc plasmas are presented. The experimental arrangement allowed measurements at arc currents up to 2.6 kA and at discharge pressures up to 100 kPa. The dependence of the electron density of the arc plasma on these parameters and the laser wavelength were determined using a Michelson interferometer and a Mach-Zehnder interferometer. Besides these measurements, the temperature in the arc core was determined using spectroscopic methods and the diameter of the arc was measured using an image converter camera. The results of the measurements were compared with calculated data supposing local thermodynamic equilibrium. It can be shown that there is a good agreement between the experimental and theoretical results and those of other authors.

The Townsend first ionisation coefficient alpha defined in pulsed Townsend (PT) experiments in Ar/Ne mixtures are analysed using a Boltzmann equation in which the influence of generation of secondary electrons through Penning ionisation in a discharge is taken properly into consideration. It is found that the values of alpha /E, where E is the electric field strength, for PT experiments differ considerably from those for steady-state Townsend (SST) experiments especially in the E/N range, where N is the gas atom concentration, in which Penning ionisation affects the SST values of alpha /E considerably (Sakai et al. 1986). This difference of the alpha /E is shown to be a peculiar phenomenon in Penning gases and to occur due to difference between the observation method for the metastable Ne and the electron concentrations in a swarm of PT and SST experiments.

It is shown that a conducting shield, as a third electrode in a vacuum arc plasma, can temporarily act as an auxiliary anode. This is due to an inductive shift of arc plasma potential caused by a sufficiently rapid decrease of arc current. Using current injection experiments, the existence of cathode spot activity after circuit current zero is demonstrated and explained both qualitatively and quantitatively. Further, vacuum gap recovery during the first microsecond is studied. Under certain conditions, a delayed recovery due to postzero cathode spot activity is observed. The recovery measurements suggest a space charge dominated recovery during the first microsecond. Finally, it is observed (and explained) that spontaneous instabilities in arc current can also lead to electron fluxes to the shield.

Ionisation currents in atmospheric air at various gas pressures have been measured between the saturation current regime and the region approaching sparking voltage. The gas was uniformly irradiated with gamma rays emitted by a ¹³⁷Cs radioisotope of 15 mCi activity. Some of the resulting i-V curves thus represent the characteristics of the transition from the Townsend to the self-sustaining dark discharge. Stagnant dry air at pressure 500-1000 hPa (376-760 Torr) fills the volume of a cylindrical chamber of 15 cm active length and 6.05 or 1.70 mm inner radius. The anode is a thin central wire of radius 0.05, 0.027 or 0.016 mm. A model is developed in which the theory of ionisation avalanches in proportional counters is extended by including negative ions O₂^-.

Conducting Langmuir-Blodgett (LB) films made of a ternary compound N-docosylpyridinium-tetracyanoquinodimethane-iodine have been recently obtained by Ruaudel and co-workers (1985), after an in-situ oxidation of a precursor LB film. This paper describes a new well controlled method used to perform the iodine 'doping' in the precursor LB film. The chemical reactions involved in this process are followed by UV-visible spectroscopy. Direct current and microwave conductivities of samples obtained in that way have both been measured in the range 300-120 K and are correctly fitted by the relation sigma = sigma ₀ exp - Delta E/kT with two different activation energies: 0.15 eV and about 0.08 eV respectively. Thermoelectric power measurements have also been performed in the range 298-176 K and the Seebeck coefficient, the value of which is 26 mu V K^-1 at room temperature, exhibits a linear-like behavior against 1/T with a slope dS/d(1/T) approximately=11 mV. All these measurements suggest a semiconducting behavior and are tentatively analysed in terms of an intrinsic semiconductor model. However, as an important discrepancy about the mobility value appears, the authors suggest a presumable interpretation in terms of hopping model between localised states. But they conclude that more precise information about the inplane molecular arrangement and the carrier density is needed to provide a more coherent model for the transport properties in these conducting films.

By means of permittivity, spontaneous polarisation and polarisation reversal measurements the authors studied the influence of the substitution of deuterium atoms for hydrogen on the ferroelectric properties of tris-sarcosine calcium chloride (TSCC). Although most of the protons in the hydrogen bonds were substituted by deuterium (as checked by means of multi-pulse NMR techniques for solids and by infrared spectroscopic measurements) there is no change of the phase transition temperature. By combining density and NMR measurements it could also be shown that the volume of the unit cell did not alter for the various deuterated crystals. The absence of the deuteration effect on T_c and on the parameters of the Landau expansion of P_s suggests that a proton ordering in the hydrogen bond of the type N-H. . .Cl does not occur in practice. This behavior can be understood because the hydrogen bond is rather asymmetric so that the protons occupy predominantly equilibrium positions near the nitrogen atoms. For an interpretation of the results presented the theory of Chaudhury (1982) and co-workers may be applied.

Multiple-beam fringes of the Fizeau-type transmission and on reflection were utilised for the studies of the optical properties of gamma -irradiated Dralon fibres. Irradiation was carried out over a period of 24-1948 h. The dose rate was maintained at 14.7+or-0.2 rad s^-1. Empirical formulae were suggested to represent the variation of refractive indices and birefringence with the dose. Variation of the mean refractive index with the wavelength of light has been studied, and the constants A and B of Cauchy's dispersion formula were calculated. This multiple-beam interferometric method was also applied to determine the thermal coefficient of refractive index of the fibres. Micro-interferograms are given for illustration.