J. Lucas

Electron swarm parameters in argon and krypton

Experimental measurements of the drift velocity and the ratio of longitudinal diffusion coefficient to mobility for electrons have been obtained for the range of 2.8<or=E/N<or=566 Td for argon and 14<or=E/N<or=849 Td for krypton (1 Td=10-17 V cm2). The results obtained from Monte Carlo simulation have been compared with the present and previously obtained data and a good fit has been achieved for the entire range of E/N. The simulation also produced results for the ionisation coefficient and the ratio of radial diffusion coefficient to mobility and these values have also been compared with other theoretical and experimental values, where available. The mean energy has also been computed and compared with the experimental breakdown voltage. This comparison has shown that equilibrium conditions exist in the region in which the present measurements have been made. As a result of this investigation, two sets of cross-sections are given which accurately simulate electron swarm parameters in argon and krypton respectively.

A comparison of a Monte Carlo simulation and the Boltzmann solution for electron swarm motion in gases

Electron warm motion across a uniform electric field in the presence of elastic, excitation, ionisation and attachment collisions with the gas has been simulated. The motion has been analyzed to give swarm parameters for drift velocity (v), mean energy, radial (Dr) and longitudinal (DL) diffusion coefficient, ionisation and attachment coefficients. The results have been compared with solutions of Boltzmanns equations. In the presence of ionisation and for a fixed mean energy, the Monte Carlo results for v and DL increased as the proportion of energy going into ionisation increased. In the presence of attachment decreasing values of both v and DL are observed.

Infrared sensor for top face monitoring of weld pools

The development of a novel sensor for the measurement of bead size in tungsten inert gas (TIG) welds is described. A passive vision-based sensor combines infrared imaging of the weld pool and selective optical filtering, both to reduce excessive arc glare and improve contrast between weld pool and heat affected zone (HAZ). The arc interruption and external illumination techniques, used by others, are not required. Image quality is shown to be comparable to that obtained with external illumination and superior to that of arc interruption methods. The technique has been successfully applied to both pulsed and continuous TIG welding of stainless steel in order to maintain the bead size during a welding run.

Design and construction of a 2.45 GHz waveguide-based microwave plasma jet at atmospheric pressure for material processing

We have designed a low-cost and reliable 2.45 GHz waveguide-based applicator to generate a microwave plasma jet (MPJ) at atmospheric pressure. The MPJ system consists of a 1-6 kW magnetron power supply, a circulator, a water-cooled matched load and the applicator. The applicator includes a tuning section, which is required to reduce the reflected power, and the nozzle section. The plasma is formed by the interaction of the high electrical field, generated by the microwave power, between the waveguide aperture and the gas nozzle. A variety of gases have been used to produce the plasma including argon, helium and nitrogen. A 2 kW, 2.45 GHz MPJ, constructed using a rectangular waveguide WG9A (WR340), has been investigated. An MPJ has been used for material processing applications including cutting, welding, glass vitrification and quartz/ceramic processing. This paper discusses the design parameters and the potential of the MPJ for industrial applications and how the jet can be tailored to suit different tasks, by adjusting the various parameters such as the type of gas, the flow rate, the input power and the nozzle design.

The ratio of radial diffusion coefficient to mobility for electrons in helium, argon, air, methane and nitric oxide

The ratio of the radial diffusion coefficient to mobility Dr/ mu exp has been measured for the first time in five gases at high E/N (ratio of electric field to gas number density). The range of E/N was as follows: helium 3<or=E/N<or=847 Td; argon 8<or=E/N<or=1271 Td; air 14<or=E/N<or=1412 Td; methane 14<or=E/N<or=5650 Td; nitric oxide 56<or=E/N<or=1412 Td. The Townsend ionization coefficient alpha T/N has also been measured in each gas for the same E/N range.

Simulation of electron swarm motion in hydrogen and carbon monoxide for high E/N

Electron swarm motion has been simulated by using a Monte Carlo technique and swarm parameters have been evaluated for electrons in both hydrogen and carbon monoxide gases for a wide range of E/N varying from 30 to 3000 Td. Comparison has been made with the experimental results for the ionization coefficient, the drift velocity and both the radial and longitudinal diffusion coefficients. A set of electron/molecule collision cross-sections has been assembled for each gas which gave a good fit between the simulated and experimental values over the entire E/N range.

Low-pressure microwave plasma ultraviolet lamp for water purification and ozone applications

Low-pressure mercury lamps are commonly used for germicidal applications. The germicidal effect is due to the emission of light at 254 nm, which leads to the destruction of the most waterborne bacteria and viruses. The microwave plasma ultraviolet (UV) lamp (MPUVL) is a new technology for generating a high-intensity UV light and that can be also controlled to operate at 185 nm; irradiation is in air at this wavelength produces ozone. The microwave power is injected into a resonant cavity and the surface wave excitation takes place within the cavity through that part of the discharge tube (fused silica) protruding inside it. The MPUVL has many advantages over conventional lamps, which are limited to an output power in the region of 30 W m-1, while MPUVL can deliver any amount of power per unit length and the tube can be of any shape, length or diameter. This paper describes the design of the MPUVL and compares its efficiency with that of conventional lamps through spectral analysis. Other results, which include the effects of temperature and different power inputs, are also discussed.

Simulation of electron swarm parameters in carbon dioxide and nitrogen for high E/N

The Monte Carlo technique has been used to simulate the electron swarm motion in carbon dioxide and nitrogen, and electron swarm parameters have been evaluated in the range of E/N varying from 14 to 3000 Td (1 Td=10-17 V cm2). These parameters, namely ionisation coefficient, drift velocity, ratios of both radial and longitudinal diffusion coefficients, to mobility (also attachment and dissociation coefficients in the case of carbon dioxide) have been compared with the available experimental data. A set of elastic and inelastic cross-sections has been collected for each gas such that the computed and experimental values gave good agreement for each swarm parameter over the entire E/N range. In addition the percentage of energy lost by different types of inelastic collisions and also the mean swarm energy have been given as a function of E/N for each gas.

A vision-based seam tracker for butt-plate TIG welding

In the automation of the TIG (tungsten inert gas) welding process, robots and dedicated machines have been used to place the welding torch at the correct orientation above the seam to be welded. Sensors are required to determine whether the workpiece is located correctly with respect to the welding torch, or whether distortion of the workpiece takes place due to thermal expansion during welding. The authors describe a seam tracker for following the small gap between closed butt plates. The system uses a CCD monochrome video camera with a coherent fibre-optic bundle to view the workpiece. Illumination is provided by an infrared laser diode mounted on the tracker head. The use of these components enables the tracker head to be kept extremely compact. The video signal from the camera is digitised and stored in a microcomputer which analyses the image to obtain the position and width of the gap. This information is then transferred, via a serial line, to the robot controller which then modifies the torch path to keep the welding torch over the seam throughout the welding process.

Ionization in oxygen-hydrogen mixtures

Current-growth measurements have been made in mixtures of oxygen and hydrogen for a wide range of experimental parameters and mixture concentrations. These mixtures were chosen because hydrogen removes the primary O− negative ions formed in oxygen, through a rapid associative detachment reaction, making the mixture behave as a non-attaching gas. This simplifies the analysis required to obtain ionization coefficients and should give more accurate values of the ionization coefficient for oxygen than can be obtained from measurement in pure oxygen. To check that at any value of E/N the mean energies of the O2-H2 mixtures were not significantly different from those of pure oxygen, the Boltzmann equation for the gas mixtures was solved and the transport properties of the mixture calculated. It was found that for O2-H2 mixtures containing less than 10% hydrogen, the mean energies and the other transport properties were not altered in any significant way. The values of the ionization coefficients obtained in these experiments are in general 30% below earlier published values but are in agreement with some recent current-growth analysis by Wagner (1971) which incorporates detachment and ion-molecule effects.