Table of contents

A new and very simple experimental technique for acoustically-detected (AD) EPR between room and liquid helium temperatures is described. Some experimental results are also presented. AD-EPR can be particularly sensitive at low temperatures.

The thermal conduction of a plasma between two boundaries at which the temperature is fixed is investigated using a simple model. The distribution function is taken to be the sum of two half maxwellian distributions and a Krook collision term is used. Approximate solution of the transfer equation yields the heat flow as a function of the temperature gradient and the average collision mean free path.

Particle emission from spherical targets uniformly irradiated in the ablative mode with lambda =1.05 mu m laser radiation has been studied using arrays of ion Faraday cups and differential calorimeters. The spherical geometry avoids the problems of angular integration and edge effects that are associated with plane target experiments. Plasma M/Z values and target mass ablation rates are deduced by combining the data from both types of ion detector. Ablation pressures are calculated from these mass ablation rates.

Angular dependent emission of plasma after irradiation of a solid tantalum target by a Nd-YAG Q-switched laser (I approximately=10¹⁰ W cm^-2) has been measured for different angles of incidence of the laser beam relative to the target normal. It is observed that the particles are preferentially emitted in the direction of the target normal, both for normal and oblique incidence. With increasing number of charge, the low-energy parts of the spectra tend to disappear, which the authors interpret as being due to recombination. For oblique incidence the emission spectra are no longer symmetrical relative to the target normal. This can be explained by the unsymmetrical heating of the expanding plasma by the laser pulse. In addition, this gradient in temperature seems to lead to a certain discrimination of three-body and radiative recombination.

The effect of polymorphous transitions on the regularities of the scattering of Ar ions by polycrystalline cobalt has been studied. The temperature dependence of the number of ions scattered in a given direction has been established to be non-monotonic in the temperature range near the phase transition temperature.

A decrease in carrier mobility on oxidation of polyethylene owing to increased trapping efficiency has apparently been observed by carrier transit time measurement (Mizutani et al. 1980). This is contrary to earlier observations, and further, the model presented by these investigators to account for the effect seems self-contradictory. The experiments are reviewed and further investigations suggested.

Optimum criteria are derived for the feasibility of winding L-fold symmetric magnetic coils whose angular densities contain a finite number of parameters and have, at worst, jump discontinuities but are not prescribed beyond their initial harmonic coefficients. This is the first of three theoretical papers addressing specifically constraints: (1) that specify the angular domains where current reversals are forbidden; (2) that state whether the winding is continuous, discontinuous, or discrete there; and (3) that prescribe whether the winding density is monotonic and/or has stated lower or upper bounds. It is shown that the curve fitting problem for arbitrary L, under any of the above constraints, can be transformed into the corresponding problem of a generic density that is non-negative, two-fold symmetric, and cosinusoidal (i.e. a horizontal deflection winding), the essential ingredient being a one-to-one correspondence between the harmonic coefficients for the original density and those of the generic density. The criteria for feasibility of constructing this generic density are derived from first principles, and those for the primitive densities under study are then inferred by use of the above correspondences. Each such set of criteria consists of inequalities among the harmonic coefficients, the different sets being thus related to each other in a fundamental way. The derivation takes advantage of the theory of power moments by exploiting the linear connection between the harmonic coefficients and the power moments of a certain transform of the generic density.

For pt.I see ibid., vol.14, p.523 (1981). Optimum criteria are derived for the feasibility of winding magnetic coils of L-fold symmetry whose angular densities are not prescribed beyond their initial harmonic coefficients, and moreover are discrete. This paper complements a previous theoretical paper dealing with densities that contain a finite number of parameters and have at worst jump discontinuities, as well as a theoretical paper to follow that treats densities with an infinite number of parameters. The inequalities derived for the harmonic coefficients result from the requirement that no current reversals occur in the fundamental angular domain. It is shown that, when a solution is feasible for a given choice of harmonic coefficients, there exist certain sums of delta functions whose weights are positive, where all the pole positions and associated weights are determined fully, but that only in special circumstances will these weights be integers. Although such densities are seldom directly useful as winding densities, they are important because their cumulative distributions serve as extremal distributions for the solution space generated by the given harmonic coefficients.

For pt.II see ibid., vol.14, p.543 (1981). Optimum criteria are derived for the feasibility of winding magnetic coils of L-fold symmetry whose angular densities contain an infinite number of parameters, but are not prescribed beyond their initial harmonic coefficients. The derivation complements the results of two previous theoretical papers dealing, respectively, with densities having a finite number of parameters and at worse jump discontinuities, and with densities defined as sums of delta functions. The inequalities written here for the harmonic coefficients refer only to the design requirement that precludes current reversals in the fundamental angular domain. The variations to treat monotonic and/or bounded densities are not explicitly stated, but readily follow from an analysis similar to that carried out in paper I (Vessel 1981a). Instead, attention is centred around the properties of the inequalities, and their implications for densities defined as finite Fourier sums.

It is shown that radial profiles of the Cd(II) 441.6 nm line measured by McKenzie (1977) cannot be satisfactorily interpreted as resulting from the Penning excitation process with an additional smoothing-out effect, but can be interpreted in terms of the stepwise excitation and electron de-excitation processes.

Population densities of He excited states of n>or=3 in a positive column He-Cd⁺ laser were measured accurately using end light from a discharge tube. It is shown that the sum of these populations is certainly smaller by about one order of magnitude than the population density of the 2³S metastable state. It is further shown that the monotonic increase of the intensity of the Cd(II) 441.6 nm line with discharge current cannot be explained with the Penning collision process between He excited state atoms and Cd ground state atoms even if all the He excited states of n>or=3 and 2³,¹P states are taken into account.

The population densities of He and Cd states and electron density were measured at low discharge currents (2 to 30 mA) in the positive column He-Cd⁺ laser discharge. It is shown that the Penning excitation rate to the upper state of the Cd(II) 441.6 nm laser line becomes saturated for currents well below 10 mA, and that although the Penning excitation process is dominant in the region of a few mA, the stepwise excitation process from Cd(II) ground state must become the dominant one under lasing conditions.

The design and production of an f/1 aspheric focusing lens for a high-power laser system operating at 1054 nm is described. The relevant lens design parameters and the degree of flexibility in their choice are described and these, when carefully chosen, result in an extremely versatile lens with a large wavelength cover. The manufacture of the lens is also simplified. A comparison is made between the production of an aspheric surface by high quality conventional optical manufacturing processes and one generated and polished by a CNC jig grinding machine.

The indentation of PMMA is shown to be highly dependent on indenter-PMMA friction and a correction for this effect is proposed. For penetrations greater than half a radius the pressure varies with penetration rate and ball size as predicted by the expanding cavity model of Puttick et al. (1977) provided the indentation diameter is determined under load and the friction correction is applied. This allows assignment of a value to the parameter plastic-zone radius/cavity radius, c/a. Observations of the growth and position of the region of anomalous refractive index beneath the indenter and the fracture surfaces shows the creation of a core of material beneath the indenter which can be identified with the cavity of the plastic-elastic model. This allows an alternative estimate of c/a and both methods give a value of 1.9.

A plane bimaterial model is introduced which allows the determination of the stress intensity factors K_I and K_II, as well as the fracture angle theta ₀. The model is composed of a long orthogonal strip penetrating into an elastic orthogonal soft matrix. The strip is cemented along three of its edges with the matrix and the composite is submitted to either symmetric tension, or compression. Simple tension of the model creates slowly progressing cracks in the matrix parallel to the transverse edge of the strip, whereas compression develops slowly growing oblique cracks at different angles to the transverse edge of the strip. For the evaluation of the components of the complex stress intensity factor, the optical method of reflected caustics is used. The orientation of both external and internal caustics yield also the fracture angle theta ₀. A significant advantage of the model is that the developed cracks in the matrix may grow slowly at any desired velocity. So it is possible to study the stress field near the propagating crack-tip without using expensive optical and electronic equipment. The results derived by testing different composites are examined according to the two main fracture criteria and useful conclusions are obtained.

A laser-Doppler velocimetry study of the features of a duct flow around the laminar-turbulent transition under the influence of a finite perturbation is reported. Through the spectral analysis of the light scattered by particles crossing interference fringes, the time-average velocity and the velocity fluctuation profiles are studied. It is shown that the ratio between the magnitude of the velocity fluctuations and the axial time-average velocity exhibits a sharp peak in the transition region. It is pointed out how that feature is independent of the discontinuities inducing the perturbation at the entrance as well as of the position downstream.

Potential retardation measurements with a variable length drift tube have yielded the most probable energies and the energy distributions of CO⁺ ions in He, Ne and Ar at 300K for selected values of the reduced field E/n. For He E/n was in the range 35-200 Td and for Ar between 60 and 300 Td. The most probable energies obtained agree with the mean energies predicted by Wannier's equation for 100-200 Td assuming ion drift velocities calculated from the polarisabilities. The shape of the energy distribution function was found to depend significantly on the nature of the buffer gas atoms used.

Reports the measurement of the polarisation of the nonthermal bremsstrahlung emitted from a vacuum spark plasma. The results, found by Compton scattering indicate the presence of a beam-like anisotropy in the velocity distribution function of the electrons with beam energies not exceeding 30 keV.

The authors propose a macroscopic model for gas breakdown ionising waves based on a consistent hydrodynamic system of equations describing the spatio-temporal evolution of particle density, momentum and energy. They apply this model to investigate the properties of the electron shock zone situated at the head of a discharge channel, so that it can in particular predict the non-equilibrium electron temperature as distinguished from the static equilibrium electron temperature. The temperature dependence of macroscopic coefficients can be predicted from the available electron transport and ionisation data. Their previous analysis (1980) of the role of the photoionisation in a gas discharge is extended to the case of fast electron shock waves by taking into account the finite lifetime of the molecular excited states. This shows that the gas photoionisation process cannot have an important role in fast breakdown waves except at low gas pressures. Finally, this study analyses the different possible causes of a non-equilibrium electron energy.

For pt.I see ibid., vol.14, p.649 (1981). The authors present here the results of the numerical solution to the macroscopic model of a gas discharge described in part I (preceding paper). This model considers the non-equilibrium between the electron energy and field in the electron shock leading the ionising wave in the discharge. It is possible to analyse the structure of this shock wave. It is found to be mainly composed of two unequal parts separated by the position of the electron pressure maximum. It is also possible through this model to calculate the principal physical quantities (density of charge carriers, field and electron temperature) in a non-equilibrium electron field discharge. The authors have elucidated the role of the different physical processes, or the roles of the terms of the model equation, in determining the difference between the static equilibrium temperature and the electron temperature. It is also shown that a steady-state shock solution to the nonlinear hydrodynamic model of the gas discharge can exist in the sense that an initially strong relative electron density gradient is maintained in the first zone of the electron shock, whereas the shock upstream can possibly produce a precursor effect due to the combined effects of thermal diffusion and overmultiplication of these hot electrons.

Dielectric gas mixtures comprising mainly one electron-attaching component and one dipolar component have been investigated. It has been found that polar electron-slowing-down components effect a sharp increase in the breakdown voltage, V_s, with small percentages of electron-attaching additives. The effect of electron-dipole scattering on V_s for multicomponent gas dielectrics is assessed, especially in combination with indirect electron scattering via negative ion states. The results demonstrate further the beneficial effect of large electron scattering cross-sections at subexcitation energies on V_s and suggest that a careful combination of gases slowing down electrons via dipole scattering and via negative ion states can effect large V_s values. This and earlier studies suggest that a number of dielectric gas mixtures containing one or two electron-attaching components from c-C₄F₈, 2-C₄F₈, SF₆ and a dipolar component from CHF₃, CH₂F₂ or 1,1,1-CH₃CF₃ with or without N₂ are excellent candidates for large-scale testing for possible eventual industrial adoption.

The influence of electrode temperature on the time delay of electrical breakdown in an argon-filled (13.3 mbar) diode at electrode temperatures of 300, 523 and 673K has been investigated by measuring the electrical breakdown time delay (t_d) dependence on both the time interval between two successive measurements ( tau ) ('memory curves') and the applied voltages (U_w). The results obtained show that values of t_d at 673K are about one order of magnitude smaller than those at 300K, whether cathode or anode was heated. It was shown that the complete curve t_d=f(U_w) moved towards smaller values of t_d with increasing temperature. Conclusions on the change with temperature of the temperature equilibrium concentration of metastables are drawn.

A statistical theory of dislocations has been proposed with specific application to creep in LiF and materials like it. The velocity of a dislocation is chosen to be a random variable. Based on various established mechanisms contributing to glide-controlled plastic flow, a semi-empirical continuity equation is set up for the velocity distribution function. The solution is obtained in terms of a power-series expansion of two small parameters, and the first four cumulants have been calculated within a certain approximation in which the third and fourth turn out negative, resulting in a distribution function having sharp edges on both sides. The average velocity of dislocations is shown to reduce linearly with the average density of dislocations, leading to an internal stress which is linear in the average density. The equation of motion of the dislocations exhibiting the drag, an equation for population dynamics of dislocations during creep, and a creep law proposed by Webster (1966) follow. The theory is applied to creep in LiF with excellent agreement. It also explains the shift in the stress-velocity relation in prestrained samples.

The breakdown voltage in nitrogen and air was studied at temperatures between 1400 and 4500K at a pressure of a few atmospheres in a shock tube. The measurements were carried out in the physically clean situation behind the reflected shock wave. Below a certain temperature the character of breakdown is dielectric; above this limit the character changes into a thermal one (ohmic heating). The transition occurs in nitrogen at about 2500K and in air at 3100K. The influence of gas relaxation on breakdown was also considered.

The time and temperature dependence of the thermoelectric properties of phosphorus-doped Si-Ge alloys has been investigated over the temperature range 600-1400K. Changes in the Seebeck coefficient, electrical resistivity and carrier concentration which result from the precipitation of phosphorus from solid solution at temperatures below 1100K are reported. The precipitation process is found to be reversible and the precipitated phosphorus can be put back into solid solution in a very short time by annealing at temperatures above 1200K. The Lifshitz-Slyozov model has been used to determine the equilibrium carrier concentration of phosphorus in the temperature range 600-1400K.

The complex relative permittivity of 1-heptanol, 3-heptanol, 3-methyl-2-hexanol and 2-methyl-2-hexanol has been measured from -30 degrees C to 20 degrees C at pressures up to 350 MPa in the range of frequencies between 50 Hz and 6 MHz. The dispersion in 1-heptanol and 3-heptanol is well characterised by the Debye equation, whereas the other two materials exhibit more than one relaxation time. The activation parameters are calculated and are found to be strongly dependent on the position of the hydroxyl group, and in the case of the branched isomers, of the methyl group. These results are compared with those in the literature for similar octanol isomers, and are discussed in terms of their dependence on liquid structure. The existence of closely ordered and loosely ordered chains is proposed to explain the results.

An examination has been made of the way in which the type of environment and its partial pressure influence the transition to high friction and wear-dusting-of graphite sliding against carbon. It is shown that the onset of dusting is invariably associated with the attainment of a critical temperature at the contacting asperities. This temperature increases with partial pressure of the vapour and, for a series of n-paraffins and alcohols, also increases with molecular size (chain length). From the variations in critical temperature with partial pressure, adsorption energies can be derived and these are broadly consistent with a mechanism of physical adsorption on graphite basal planes. It is suggested that the adsorbed vapour on basal planes functions as a 'reservoir' from which molecules can migrate to neutralise edge sites being continuously exposed during the normal, low-wear regime.