Table of contents

In the analysis of X-ray emission spectra from laser compressed plasmas it is usually assumed that electron density and temperature are spatially uniform and time independent. A full calculation is reported solving the radiation transport equation for hydrogen-like and helium-like lines. One conclusion from the results is that differences in 'effective' electron density and temperature for different spectral lines are within the experimental error. The angular distribution of the radiation is also discussed.

In a 1 mm² area at the thoriated tungsten cathode tip of a stabilised arc discharge, very high temperatures and temperature gradients have shown up under experimental conditions which are explained.

The role of metastables in electrical breakdown processes in a nitrogen-filled (13.33 mbar) diode has been investigated. This investigation was done by measurement of electrical breakdown time delay as a function of passive time tau , electrode gap d and E/p ratio in the diode with the bulk copper electrodes of which one is gold-plated. The results show that the gold film cathode is more sensitive to the secondary electron emission from its surface on metastable impact than that of bulk copper.

The results of the theoretical study of the hole formation process in thin films, following the end of the laser action, are presented. It is shown that the final form and dimensions of the hole depend on the physical and thermophysical properties of the film and the substrate. Expressions are obtained which may be used to evaluate the final dimension of the hole in every case.

The importance of diode edge effects in practical semiconductor devices and in test structures for electrophysical investigations, is discussed. Included in the review are power devices, devices utilising avalanche effects, and radiation detectors. The main effects are on reverse characteristics, introducing premature breakdown or excess currents. The discussion starts with experiments clarifying the role of field distribution and surface treatment of the edge, and the application of simple methods of improvement, such as using overlay electrodes or surface protection.

In optics, Voigt functions are useful approximations for the intensity distribution in spectral lines, and the parameters are roughly estimated by means of diagrams. The authors give a very accurate analytical method to compute these parameters by using momenta of the distribution in the Fourier space, as in the theory of probability. The extent to which results conform to theory ( chi ²) can be used to verify the Voigt profile hypothesis. They extend this method to a linear combination of Voigt profiles, and give the mathematical expansions and study some experimental examples. Where there is superposition of several profiles, a discussion is developed on the limitations of this method due to the numerical instabilities.

The authors present here a method of finding the bulk two-body correlation functions for an isotropic ensemble of hard spheres by examining a single section through the ensemble. The authors derive an integral equation which gives the probability of seeing a disc of radius sigma ₁ a distance rho from a disc of radius sigma ₂, in the cross-section. The authors solve this explicitly for the case where the distribution of sphere radii in the bulk is a discrete distribution and hence obtain a discrete set of partial two-body correlation functions for the bulk ensemble.

A double beam laser method for measuring attenuation coefficients of sea-water throughout the 414-662 nm wavelength region is presented. The radiant source is a pulsed wavelength-tunable dye laser, with a spectral width of 0.01 nm. A test with de-ionised filtered distilled water is described.

Results of experiments are presented which show that single hardened steel spheres (8 mm in diameter) travelling at velocities in the range 50-400 m s^-1 can remove material when they strike mild steel targets at normal impingement. Impacts were photographed using a high-speed camera at microsecond framing rates. The results are interesting since some erosion theories predict zero material removal at normal angles of impingement, whilst others require cooperative effects between impacts at normal impingement which result in incubation times. It is shown that there is an initial regime where the contact circle expands faster than any wave speed (elastic or plastic) in the target. Material loss eventually occurs by the rapid radial expansion (jetting) of material from under the sphere when the velocity of the plastic wave exceeds that of the expanding contact circle.

Two experimental methods are described which explore the contact time as a shear strength variable. The available data suggest that polymethyl methacrylate, and to a lesser extent polyvinyl alcohol, are unable to respond to the rapid transient pressure pulses generated in simple sliding experiments. One experiment simply involves changing the contact diameter whilst maintaining the constancy of other variables. The second more versatile experiment involves the application of a sinusoidal load to the contact in addition to the constant load. The magnitude of the oscillatory component of the frictional force may be detected as a function of sine wave frequency. The application of high frequencies reveals that high density polyethylene shows a response attenuation. The results from the two experiments are similar and are rationalised in terms of the presence of relaxations which cause a 'slowing down' of the rate of mechanical response.

Spectroscopic and electronic diagnostics are applied to determine the temporal and spatial evolution of excited species created in He or Ne-N₂ plasmas produced by a microwave surfaguide at a pressure of a few Torr. The excited-states population has a maximum near the wall. Electron density is maximum on the axis (10¹³ cm^-3 at 10 Torr). Its radial distribution is well fitted by a Bessel J₀(r) function. As a consequence the metastable population is also maximum along the axis.

Shows how electron temperature and density in low pressure cold plasma (T_e<10 eV) can be measured directly from cyclotron resonance absorption measurements in a strong homogeneous magnetic field ( approximately=20 T). For pressures below 0.5 Torr, the cyclotron line width is Doppler broadened and the electron temperature is directly derived from this parameter. At high pressures the line width becomes collision broadened and the electron temperature is indirectly derived using the known energy dependence of the collision frequency.

The properties of a new spectroscopic light source are analysed. It is demonstrated that the gas-liner pinch emits resonance lines from a homogeneous central layer without self-absorption effects. For investigations of the Lyman- alpha line, the electron density is adjusted to N_e approximately 2*10¹⁸ cm^-3 and the electron temperature to kT_e approximately 7 eV.

Hydrogen plasma parameters have been calculated for a glow discharge driven by 0.1 A current in a 0.4 cm tube at 270 Pa and 530 Pa initial gas pressures. The set of differential equations that includes levels population, electron density and electron temperature rate equations has been solved simultaneously and the stationary plasma parameters have been evaluated.

For pt.I see ibid., vol.15, p.621 (1982). For the particular case of a positive column glow discharge hydrogen plasma perturbed by a nanosecond pulse, the system of kinetic equations that includes also equations for electronic concentration and temperature has been numerically solved. One of the remarkable results is that the electron temperature, after initial rapid rise due to the perturbation, drops below the stationary value during the relaxation. The drop is more marked with stronger perturbation, and occurs earlier at higher initial gas pressures.

Corona light in coaxial cylindrical electrodes is measured photoelectrically and photographically. Calculations are made in the space charge field. Calculated intensity of the light emission within the corona sheath decreases steeply as the distance from the surface of the central electrode increases. The logarithmic gradient near the central electrode and near the outer boundary of the sheath is larger than that in the intermediate portion. The thickness of the corona sheath is independent of applied voltage. The thickness is 0.62, 1.01, 1.57 and 2.00 mm for a central electrode 0.25, 0.5, 1.0 and 1.5 mm in radius, respectively. These values can be expressed as 1.56r₀^0.65, where r₀ is the radius of the central electrode in mm. The peak level of the measured brightness distribution curve is more than 10³ times higher than the skirt level.

In the first part (DC voltages) some experimental results, for predischarge regimes, in air under pressure and in SF₆ are discussed. A comparison of the results obtained indicate a similar behaviour in air and SF₆ if one allows for a shift in pressure. In the second part some image converter recordings obtained in air and SF₆ are presented (impulse work). They show details of the streamer-to-spark transition with positive impulses. The physical mechanism responsible for that transition are discussed. These studies were effected in point-to-plane geometry with small gaps (1-2 cm).

An optical interferometer has been used to measure the increase in gas temperature in a point-to-plane corona discharge in air as a function of radial and axial position. For a corona current of 150 mu A, a 10 mm gap and a point radius of 0.35 mm, a maximum change in temperature of approximately 6K was observed. The authors' results suggest that under these conditions, where the corona voltage is approaching the spark breakdown voltage, the predominant ionisation process is two-step ionisation.

Performance characteristics of Schottky-barrier detectors are investigated in terms of their physical parameters. It is shown that the detector cut-off frequency and responsivity-bandwidth product can be maximised while the level of minimum detectable power can be minimised by the proper choice of detector parameters. For zero-bias detectors, the optimum barrier heights are found to range between 0.2 and 0.4 eV. For higher barrier heights, the optimised performance can be obtained by forward-biasing the detector.

The variation of electrical conduction with applied voltage and temperature has been studied in solution-grown pure and iodine-doped (PS) films 2500 AA thick. The study of the conductivity variation with the polymer thickness and with its temperature shows that only the Poole-Frenkel effect can explain the experimental results. Also, hysteresis and polarisation effects were observed in the I-V characteristics. The resistivity of the PS films was about 10¹⁵ Omega cm at room temperature and at a field of 10⁵ V cm^-1. An increase of about 10²-10³ orders of magnitude in conductivity was observed due to the doping of iodine in PS at room temperature. This is interpreted on the basis of the formation of charge transfer complexes in the film. The activation energy for conduction decreased from 0.79 to 0.67 eV at temperatures below T_g and from 1.0 to 0.89 eV above T_g on doping PS films with iodine (0.2 g of iodine per 100 ml PS solution).

For pt.I see ibid., vol.15, p.677 (1982). The dielectric loss behaviour of solution-grown thin films (thickness approximately 2500 AA) of pure and iodine-doped polystyrene (PS) has been studied as a function of temperature in the frequency range from 100 Hz to 300 kHz. The loss behaviour exhibits two high-temperature loss peaks- beta at approximately 70 degrees C and alpha at approximately 100 degrees C at a frequency of 100 Hz. Iodine-doping is found to affect both the magnitude as well as the position of the alpha -relaxation peak.

The critical state stability in non-uniform superconducting composites is investigated, taking into account the plastic yield of the material. It is shown that the critical state instability in the non-uniform region may occur considerably earlier than in the uniform part of the sample. The instability criteria have been obtained for a number of particular cases. The training phenomenon in superconducting composites is discussed as a sequence of local magnetic flux jumps and plastic strain jerks developing simultaneously.

Good electroluminescent diodes have been fabricated from neodymium- implanted or erbium-implanted zinc selenide crystals. These diodes have an external power efficiency of about 5 × 10^-5 and can be operated at very low voltages and at low temperatures (e.g. 77 K). They show the characteristic emission spectra of Nd³⁺ or Er³⁺.

The one-dimensional Lagrangian fluid code MEDUSA has been used to study the process of laser-driven ablation for a variety of irradiances and laser wavelengths. Values are given for physically important parameters such as ablation pressure and coronal temperature as a function of wavelength and irradiance. The effect of changing the physical assumptions in MEDUSA is discussed and appropriate comparison is made with analytic models. Within the limitations of the numerical model an attempt is made to deduce optimal values of laser wavelength and irradiance for inertially confined fusion targets.