Table of contents

Two models of the initiation of gas discharges in a transverse discharge lamp are compared and contrasted in this paper; the volume and near-surface breakdown models. Particular consideration is given to the breakdown voltage, the breakdown formation time interval and the extent of the initial gas ionization. Experimental measurements of the breakdown parameters for Xe both in the static and in the pulsed mode of operation are given and the predictions of the two models are compared against these results. It is concluded that the near-surface gas-breakdown model is appropriate for the discharge lamps under consideration.

High-energy ball milling of pre-alloyed ingots, rapid quenching and mechanical alloying of elemental powder mixtures are used to prepare amorphous Fe-Al-P-C-B-(Ga) alloys, which are known to have interesting soft magnetic properties combined with a good glass-forming ability, promising the formation of bulk soft magnetic materials. Pieces of the melt-spun ribbon with a coercivity of are ball milled to investigate the influence of the milling process on the thermal and magnetic properties. X-ray diffraction analysis reveals the amorphous nature of all samples after milling. The samples exhibit a supercooled liquid region varying in the range 44-53 K between the glass-transition temperature and the crystallization temperature . Annealing of the powders was performed in this temperature region to reduce the high stresses induced during milling while maintaining the amorphous structure. The lowest coercivities after annealing of the powders are found to be correlated to the oxygen content. Ball milling of the melt-spun ribbon leads to a slight decrease of the thermal stability, an oxygen content of 0.21 at% and a high of which drops to upon subsequent annealing above . Mechanically alloyed powders with and 1.2 at% oxygen exhibit a coercivity of and a saturation polarization of 1.02 T after heating above to 748 K. The coercivity of found for ball milling of crystalline ingots (0.16 at% oxygen, ) after heating to 703 K is of the same order of magnitude as that of cast amorphous material, which was reported to be 12.7 or . Nonmagnetic inclusions are supposed to lead to the difference in between the as-spun ribbon on the one hand and the ball-milled, mechanically alloyed or cast samples on the other.

A double-shutter drift-tube technique based on an analysis of arrival-time spectra has been applied to experimentally determine three electron-transport coefficients in , namely the ratio of the effective ionization coefficient to the gas density , the mean-arrival-time drift velocity and the product of the longitudinal electron-diffusion coefficient and the gas density over a wide range of 7-5000 Td . The result shows that the obtained is in excellent agreement with that of the steady-state Townsend experiment at of 500-5000 Td. In a lower range, however, some difference is observed. As a result of examination, it is revealed that the difference approximately gives the true attachment coefficient under the present experimental conditions. The determined takes an almost constant value at Td and, in the other ranges, monotonically increases with . Its profile differs from available experimental data at Td but is rather close to theoretically derived data. The ratio of the longitudinal diffusion coefficient to the electron mobility is also deduced. Furthermore, the result analysed by the conventional time-of-flight method is presented for comparison.

A method based on classical physics, utilizing the first two even moments of the depolarized collision-induced light-scattering spectrum at various temperatures to derive an empirical model for the pair-polarizability anisotropy of interacting molecules, with only one adjustable parameter, is described and applied to the spectra of Hg. Good agreement with ab initio results in the literature is obtained and profiles calculated with these models are in excellent agreement with experiment.

Self-focusing of low-power CW He-Ne laser radiation, at the wavelength of , has been observed in liquid using a simple and effective technique. New experimental results supporting the hypothesis that self-focusing of laser radiation occurs in liquids are presented in this paper.

The contribution of the radiation diffusion to the thermal conductivity in high-pressure discharge lamps is evaluated from operating-voltage and wall-temperature measurements for several cylindrical ceramic discharge tubes filled with mercury and argon as the rare gas. From calculations of using spectral line data of mercury, it is shown that the electrical conductivity can be well represented by a linear function of the heat-flux potential S. Under this assumption, the energy balance can be solved analytically and plasma parameters, i.e. radiation-diffusion and plasma-temperature profiles, can be calculated from the operating voltage and wall temperature without knowledge of spectral lines. The results are in good agreement with numerical calculations of the energy balance involving calculation of the radiation diffusion from spectral line data. The method may also be applied to other buffer gases for which there is a linear dependency of . This may be checked qualitatively by measurement of the temporal evolution of the operating voltage using the Cassie-Francis equation.

We use a two-dimensional hybrid simulation (particle ions and Boltzmann electrons) to study sheath and ion dynamics around a small round hole in a flat, conducting plate following the application of a large, negative voltage pulse, such as might be encountered during plasma-based ion implantation. Results for hole radii of an eighth and half the ion-matrix overlap length and for depths of one, two and four times the radius are reported. For all these cases, it is found that the hole represents a small perturbation to a planar sheath since the sheath width is always greater than the hole radius. Consequently, most of the ions that enter the hole impact on its bottom at near normal angles; only a small fraction impact on the sidewall obliquely.

Gliding arc discharges are the subject of renewed interest in applications to a variety of chemical reactions. A gliding arc creates a weakly ionized gas `string' between two horn-shaped electrodes. The reacting gas introduced at the electrode base blows the arc string upwards. These devices can be applied in industry mainly for decontamination and general gas treatment. We propose a simple model to describe the electrical properties of a bi-dimensional d.c. gliding arc discharge. This model can be used to estimate characteristics of the power source (internal resistance r and voltage ) necessary for a particular application.

Typical values for the electron density and temperature in the Philips QL lamp are obtained from the radially resolved measurements of the gas temperature profile and the density of the lowest argon metastable state, as presented in a previous paper (Jonkers J et al 1997 30 1928-33). The results agree very well with the electron density and temperature as found by solving the electron particle and energy balance equations. For increasing argon filling pressure a lower electron temperature and a higher electron density are found, which are due to an increased residence time of the charged particles. It is also shown that in the QL lamp serious depletion of mercury occurs.

In order to make clear the physical grounds of the potential drop in front of the anode, namely, the anode fall in atmospheric free-burning argon arcs, the results of experimental measurements of the laser-scattering method and Langmuir-probe method are presented. The experimental results show that the anode boundary layer at low arc currents such as 50 A remarkably deviates from local thermodynamic equilibrium (LTE), whereas the boundary layer at higher arc currents such as 150 A preserves a similar state to the LTE. The Langmuir-probe measurements also show that the anode fall for 50 A is positive, whereas that for 150 A is negative. From these results, an assumption regarding the physical state of the anode boundary layer in the free-burning argon arcs is presented synthetically and it is also concluded that the sign and magnitude of the anode fall in the arcs relate vary closely to the thermal state of the anode boundary layer and that the thermal state should be influenced strongly by the arc current density, namely, the electron number density.

Results concerning the dynamics of streamer propagation in air under a uniform electric field are presented and discussed. Experiments were performed in a plane-parallel electrode arrangement with positive streamers initiated at a sharp point in the earthed anode. The basic properties of streamers are described in terms of the electric field required for a stable propagation and the associated propagation velocity. Critical parameters are the ambient electric field, the voltage used for streamer initiation and the distance of traverse. The present experiments permit the separation between the effects of the above parameters upon streamer advancement and propagation over the whole path up to the cathode. It is shown that an intrinsic propagation field with an associated velocity can be defined, which determine the propagation of streamers of a limiting, minimum energy. The propagation velocity is a power function of the electric field and, with the aid of an empirical equation, values can be expressed accurately in terms of these intrinsic streamer properties.

The purpose of this article is to study experimentally and theoretically the spectral line ratios O 700.22 nm/N 746.87 nm with a sample gas mixture of and (notably for pure air) in an inductively coupled plasma (ICP) torch.

We study theoretically the influence of the thermal disequilibrium on the composition and on the volumic enthalpy in a plasma produced in an ICP torch, and we show that the lower temperature in the plasma is set by the power supplied by the inductive coil. If the temperature (or heavy species temperature for plasma out of thermal equilibrium) is sufficiently low (<5000 K), we show that the thermal disequilibrium has no influence on intensity spectral line ratios (O 700.22 nm/N 746.87 nm). We compare these results with those obtained experimentally in the case of - mixtures, and we show that the determination of monatomic spectral line ratios (O 700.22 nm/N 746.87 nm) solely depends on the initial sample gas composition introduced in the ICP torch.

The thermal constriction resistance of a strip contact spot on a layer of material is analysed for the heat-flux specified boundary condition on the contact zone. Using Green's function, the solution of heat-conduction problems is reduced to a new type of hypersingular integral equations with a hyperbolic function kernel. The hypersingular integral equations are solved analytically, which provides closed-form solutions for the thermal constriction resistance. For a thin film and isoflux conditions over the contact zone, the thermal constriction resistance is proportional to the ratio of the film thickness to the contact width when the other side of the film is considered isothermal, whereas it is inversely proportional to this ratio for an adiabatic back. Such a large variation and reversal in trend reveals the possibility of using this method for the measurement of film thicknesses by measuring the thermal constriction resistance.

We used highly sensitive absorption spectroscopy to quantify molecular radicals in the gas phase of an oxyacetylene torch diamond growth system. We measured column densities of , CH, CN and OH as a function of position in the flame for several different flame mixtures. We derived the flame gas kinetic temperature from the Swan band spectra. We searched for , singlet , and and established upper limits on the column densities of these radicals. The results were qualitatively correlated with diamond growth as judged by Raman spectra and scanning electron microscopy. The column densities of and CH correlate best with the growth of diamond.

A mathematical model for the laser drilling of metals is given for the cases of constant and pulsed laser sources. Attenuation of the laser beam within the vapour is considered through an averaged absorption coefficient . The experimentally observed logarithmic dependence of the hole depth on the laser energy is predicted theoretically. A singular perturbation technique is used in order to find solutions valid for different regimes of time, namely pre-vaporization and post-vaporization times. Uniformly valid solutions are found for the one-dimensional analysis of the drilling-front position and speed by matching the inner and outer solutions. First-order approximations for the time-dependent hole profile for the various laser source profiles considered are also found. The model is compared with experimental data in the literature for the drilling speed of copper. An additional set of experiments is specifically carried out to allow comparison with the theoretical hole profiles for titanium. The predictions of the model are found to agree well with the experiments.

We report a full characterization of the three-dimensional density profile of non-cylindrical and cylindrical high-density pulsed gas jets by using a double orthogonal Mach-Zehnder interferometer. A simultaneous algebraic reconstitution technique (SART) algorithm was fully implemented for data inversion, thus constituting a novel approach to the current methods of inversion employed in this area. Also characterized is the collision of two jets produced by rectangular nozzles never to our knowledge reported before in the literature. Results obtained point to densities eight to twelve times larger than the sum of that of each separate jet, rendering this configuration extremely useful in laser-plasma interaction experiments, where higher densities are required, with no need to increase backing pressure. A theoretical model describing the density profiles obtained for the non-cylindrical jets is presented, and the results obtained are discussed.