Table of contents

The rotational temperature evaluation method based on only the partially resolved (0,0) emission band of B² Sigma _u⁺-X² Sigma _g⁺ of the N₂⁺ molecule at 391.44 nm is proposed when a low- or medium-resolution power (R=4000 to 12000) transportable spectral instrument has to be used, i.e. to diagnose an industrial plasma source. The band was simulated for large temperatures and apparatus width ranges, and six local peaks on the simulated spectrum were found to be quite sensitive to the temperature evolution; they are composed of the line P26 or complex lines P26+R1, P30+R3, P32+R5, P34+R7, and P36+R9. These local intensity maxima are compared to the band head peak and a practical formula which takes into account the band head shape is proposed for the temperature range of 2000-7000 K. The accuracy on temperature evaluation should be about 6% near 7000 K when the relative peak intensities are known within 3% error. The presented method can be easily used by a spectroscopist not involved in molecule structure.

The authors have investigated the contact charging of polymers by metals, paying particular attention to the variation of charge transfer from place to place on the sample surface, and from one sample to another. For most polymers they find that charge transfer is unaffected by attempts at purification and is much the same for samples prepared in quite different ways, and they conclude that charge transfer is an intrinsic property of polymers, not normally dominated by the presence of accidental impurities etc. There is one exception however: charge transfer to PTFE appears to be determined by physical damage. For some polymers the charge transfer tends to increase as the work function of the contacting metal increases but this tendency is not universal; charge transfer to some polymers is insensitive to the metal work function. (They show that this is true for multiple contacts and for sliding as well as for single non-sliding contacts.) The apparent charge density shows remarkably little variation from one polymer to another, and this implies that the density of the electron states responsible for contact electrification is much the same for a wide range of polymers. However, the density of states required to account for the observed charge densities is much smaller than the density of intrinsic states, such as the orbitals of 'pendant' chemical groups, and too small to allow significant back-tunnelling during separation. The observations cannot be satisfactorily reconciled with current models of charge transfer which assume electron exchange between the metal and localised states in the polymer unless intrinsic states of sufficiently low density can be plausibly identified.

The purpose of the paper is the determination of the main mechanisms populating the upper level BL of the 441.6 nm laser line in optimum experimental conditions in the positive column of a He-Cd discharge. The problem needing to be clarified is whether the stepwise electron collision or the Penning process is the dominant excitation process for the BL state. For this spectroscopic study, the experimental cell the authors use is similar to the Nagoya group one in order to have an available comparison. The n=2 He excited state concentrations are measured as functions of cadmium density by self-absorption, laser absorption and laser induced fluorescence methods in a He-Cd discharge. The decrease of the He(n=2) concentrations with cadmium density is dominated by the simultaneous decrease of electron energy. Two kinds of model are used to estimate the electron temperature, the Cd II ion concentration and the Cd II excited states population densities. By combining experimental results with modelling, the population rates by electron collisions and Penning ionisation of the upper level (BL) of the 441.6 nm laser line are determined. Finally, in the conditions of maximum emission of the 441.6 line, BL appears to be excited by 75% electron collisions and 25% Penning collisions. But if the electron collision quenching rate of the BL level is taken into account, these percentages becomes 60% and 40%. Thus the two processes, electron collision and Penning ionisation, are similarly important in the balance equation. It is necessary to consider the electron collision quenching of the BL level, because this elementary process redistributes the population between the neighbouring sub-levels and limits the population inversion. All the experimental and modelling results are compared with the works of the Nagoya group and a fairly good agreement is obtained.

The performance of a Sr⁺ laser reported earlier has been substantially improved and the effects of buffer gas mixtures on laser output investigated. In addition, the operation of a longitudinal, discharge heated Ca⁺ laser at a wavelength of 373.7 nm corresponding to the 5²S_1/2 to 4²P_3/2 transition is reported. Average powers up to 0.3 W at this wavelength were obtained from a helium buffer gas pressure of 200 mbar. For 50% output coupling, light pulses with 200 ns FWHM were observed and calculated to have peak powers of around 0.35 kW. At a pulse repetition frequency of 4 kHz, 1.2 kW of power was delivered to the discharge tube and the calculated laser efficiency was 0.03%. The effects of buffer gas mixtures on the temporal variations of stimulated and spontaneous emission were also used to gain some discrimination between two proposed mechanisms for laser emission from Sr⁺ and Ca⁺.

A theoretical model of laser cutting of glass tubing through melting is presented. The thinning down of the wall of the glass tube has been taken into account. The dependence of the duration of cutting upon absorbed laser power has been derived and compared with experimental data. The model gives a good physical comprehension of the laser cutting of glass tubing through melting.

The study of a new breaking method has enabled Merlin Gerin to produce a medium-voltage SF₆ circuit breaker at atmospheric pressure combining the effects of interruption by the expansion gases and of electrical arc rotation due to the action of an external magnetic field. Optical diagnostics methods (high-speed camera, Schlieren effect, differential interferometry) contributed to optimising the operating parameters of this switchgear unit. The circuit-breaker model studied and the diagnostics methods used are presented, followed by important conclusions relating to the mechanisms of electrical arc rotation and of its interruption by the expanding gases, drawn from the results of experiments.

An experimental analysis of the structure of a Wood metal invasion front into a crushed glass sample is described. The correlation function of the invading fluid repartition is determined in horizontal plane cuts over the whole vertical extent of the invasion front. A fractal-like geometry is observed at length scales between the individual grain size R and a correlation length of up to 20 R. At large invading fluid saturations, the fractal dimension D_f is close to the value 2.5 predicted for 3D infinite percolation clusters; at lower saturations, it decreases to D_f approximately=2. At high and medium saturations, the mean Wood metal saturation S_MW in a horizontal plane decreases linearly with height, due to finite size effects and at low saturations, S_MW decreases slowly corresponding to an 'invasion tail'. The interpretation of these experimental results as a percolation process with a spatial percolation parameter gradient is then discussed.

The authors have measured the recombination rate for positive and negative ions in sulphur hexafluoride and in mixtures of methane and sulphur hexafluoride in an electron beam sustained discharge. Measurements were made as a function of gas composition, gas pressure in the range 100 to 600 Torr, and reduced electric field strength in the range 12 to 150 Td. When plotted as a function of an effective ion temperature, defined by the drift velocities of the ions in the applied electric field, the rates in each of the gas mixtures follow the same T_eff^-1.6 power law. From comparisons with theories appropriate to the recombination of positive ions with electrons it is suggested that the index of the power law might indicate that the recombination process follows an indirect path, with the approaching ions being initially trapped in stable orbits before final stabilisation of the collision.

Results of experiments on a partially ionised argon plasma centrifuge are presented and interpreted. Centrifuge operation is quasi-continuous with a driving current pulse lasting for several seconds. An optical cross-correlation technique has been used to study plasma non-uniformities. Two distinct types of non-uniformity have been identified: (i) slow-moving contra-rotating spokes associated with cathode arc spots, and (ii) rapidly moving co-rotating striations. The striations lie at an angle of about 45 degrees to the electric field and are likely to be caused by the ionisation instability. From the observations it can be concluded that the heavy particles in the centrifuge are not rotating at the full E_r/B_z velocity.

A criterion is developed for calculating the discharge inception voltage in three-electrode systems, i.e. inception voltage from free particles existing in a uniform field gap in air. The criterion is a formulation of the conditions necessary for a self-sustained corona discharge. These conditions are identical to those of the uniform field. As the individual processes and events involved in this formulation are field dependent, the electric field is calculated using the charge simulation technique and the method of images to account for the distortion of the applied uniform field by the particle as well as the space charge developed in the gap. The calculated field values normalised to the applied-field value agreed very well with the analytical solution for spherical particles. Also, the calculated inception voltages agreed well with those measured experimentally for spherical and spheroidal particles.

By solving the Elenbaas-Heller equation, the influence of copper vapours originating from the electrodes on the arc plasma characteristics in air are shown. The axial temperature and electric field-current characteristics are strongly modified.

The formative time lag of an electric discharge for an overvoltage of 1-8%, below the minimum of the Paschen curve in nitrogen is investigated. The Laue distribution of the breakdown time delays is found to apply. The formative time rises with decreasing pd below pd_min from values of 10^-3 s, up to the order of seconds. The suggested explanation is that avalanches in nitrogen are not independent. This poses the question of the definition of the discharge formative time.

It has been independently suggested by Ashworth and Moulavi-Kakhki and by Giles and Gibbons that two-dimensional ion-implantation profiles can be constructed from one-dimensional projections (marginal distribution functions). However, the types of projections used by the two sets of workers are different and in the present paper the two methods are compared and their limitations investigated.

Pure metals were used to prepare Cu(S) and Ag(S) solid solutions at high temperatures; resistivity was measured at 4.2 K. The effects of possible contamination and evolution of the alloys during cooling were examined. the intrinsic resistivities of S in Cu (8.7+or-0.1 mu Omega cm per at%) and in Ag (8.9+or-0.9 mu Omega cm per at%) confirm the resemblance between Cu and Ag and the existence of a large electronic disturbance close to S atoms.

Localised levels inducing high photosensitivity in CdGaInS₄ crystals are investigated. An electrical conductivity increase of about 10⁸ and 10⁵ in magnitude was measured at 110 K and 300 K respectively, under an illumination power of about 1 mW cm^-2. A photoconductivity spectral response displaying a maximum at 2.64 eV and 300 K with a thermal shift of about 5*10^-4 eV K^-1 in the range 77-300 K has been observed. The high photosensitivity and a super-linear region in the lux-ampere characteristics (LAC) have been explained by assuming the presence of sensitising and recombination centres following the Rose-Bube model. By studying the thermal quenching of photoconductivity, a concentration N_s>10¹⁵ cm^-3, an ionisation energy E_s=0.24 eV and a ratio of the electron to hole capture cross section of sensitising centres S_n/S_p=(2-5)*10^-4 were calculated. The origin of these centres is hypothesised to be due to anti-structural defects caused by a partial redistribution of the Cd-Ga atoms within the cationic sub-lattice.

Presents some experimental results that could render doubtful a recent proposal about the presence of the peak IV in the TL and PTTL glow curves of TLD-100 annealed at 150 degrees C for 23 h. A possible explanation is given for the origin of the glow peaks created by this thermal treatment.

It has been observed experimentally that the sheet resistivity increases by a constant fraction on laying the next-higher-decade ink over its predecessor. SEM/WDX line mapping analysis reveals the effect of inter-diffusion of the conducting phase between two resistor layers. Based on these results, design formulae for segmented resistors have been presented.

A heterojunction CuS/CuCNS photocathode where the outer film of CuCNS is sensitised with rhodamine B is found to have higher quantum efficiency than CuCNS or CuS surface-sensitised with the same dye. The enhanced photocurrent quantum efficiency is attributed to the presence of a strong space charge electric field that assists charge separation.

Zinc oxide films, both intrinsic and doped with TbCl₃, have been prepared by spray pyrolysis at atmospheric pressure, using air as the carrier gas. Photoluminescence (PL) studies show differing spectra from intrinsic films and from doped films with 10 atomic per cent of TbCl₃ in the spraying solution. The PL spectra from intrinsic films show a peak at approximately 510 nm, while those from the doped films show a peak at approximately 550 nm.