Th. 3e cycle--Physiol. animale et cell.--Nice, 1986.

In recent years demand from production industry for high performance cutting tools, aero and automobile engine parts has prompted research into both existing and novel methods of laying down hard, low friction coatings . A key process for the production of such coatings has been Physical Vapour Deposition (PVD) which has proved to be a consistent and reliable tool for industry. For this technique to continue to be improved and more advanced coatings to be produced, research at the fundamental level is required. This thesis describes research investigating the behaviour of the steered arc cathode spot and methods of improving existing steered arc coating technology. The majority of existing steered arc systems use either permanent magnets or a combination of permanent and electromagnets to steer the arc. Described here is a novel system which employs a pair of electromagnetic coils of cylindrical geometry which enable the arc to be positioned on a circular orbit through a range of continuously variable radii. In addition to this the coils are capable of controlling the transverse and normal magnetic field profiles independently of the steering radius selected. This enables the behaviour of the arc spot to be investigated under a range of magnetic field conditions thus allowing the comparison of measured arc behaviour with a new model of arc motion. Care has described the motion of the arc spot as a biased random walk and has derived an analytical solution to describe the time dependent, probability density function for the arc position in two dimensions. Two distributions are proposed (one in each dimension); the first describes the probability density for the arc position in the direction of driven motion, the second the probability density in the direction of arc confinement. The shape of these distributions is dependent upon the transverse and normal components of the applied magnetic field. A series of experiments are described here that measure the shape of these distributions as a function of either magnetic field, cathode material or both. In the case of the distribution of the arc position in the direction of confinement the width of the distribution is measured as a function of normal and transverse field components. In the direction of steered motion, distributions of arc orbital transit times are measured for a number of cathode materials: analysis of these distributions allow the determination of a mean macroscopic spot velocity and the spot diffusion coefficient. In both cases comparison with Care's model reveals good agreement between experiment and theory to the limits of the experimental apparatus. A further experiment was also conducted to test the prediction that the diffusion coefficient (measured in the direction of steered motion) is independent of the applied field. In this case results are inconclusive and further work is recommended. The mean spot velocity and diffusion coefficients for four materials were measured; titanium, zirconium, aluminium and 316 stainless steel. The results for aluminium and stainless steel compared favourably with some measurements performed by other workers, whilst those for titanium and zirconium are new results with no data available for comparison.

Thesis (MTech (Mechanical Engineering))--Peninsula Technikon, Cape Town, 2001 / Since the discovery of arc welding at the beginning ofthe century, metal transfer has been a topic ofresearch interest. Metal transfer can, in fact be related to weld quality, because it affects the arc stability. Furthermore, it determines the weld spatter, penetration, deposition rate and welding position. Gas Metal Arc Welding (also known as Metal Inert Gas- or MIG welding) is the most co=on method for arc welding steels and aluminurn alloys. Approximately 40% of the production welding in the country is accomplished by this process in which the thermal phenomena and melting ofthe solid electrode are coupled to the plasma arc and the weld pool. Thus the therrno- fluid behaviour of the electrode and detaching drops can have significant effects on the subsequent weld quality and production rate. The knowledge of how metal transfer affects this arc welding process is important for welding control and process automation, as well as in the development of improved welding consumables. Gas metal arc welding has a distinct feature, indicated by the results of Lesnewich [24], [23], that for most gases, there is a discrete metal droplet formation change between low and high current operations. Naturally the droplet size will have a significant influence on the properties ofthe welds. In globular transfer which occurs at low current, the welding electrode melts and produces large droplets (usually larger in diameter than the electrode wire diameter). This mode of transfer is associated with high spatter levels and thus undesirable in terms of welding economics. An increase in welding current will, for most welding! shielding gases, produce metal transfer with smaller droplets, which is termed spray transfer. This mode oftransfer is associated with high voltage and amperage settings, thus producing high deposition rates limited to the flaUhorizontal position.

M.Ing. / \Vith electrical power networks expanding beyond their original design limits and the various non-linear loads on the network, power quality has become an important issue in electrical engineering. Some of the largest non-linear loads on the power network are arc furnaces used to melt various metal orrs and scrap metal. In this study the focus is on fliCker as a power quality issue and more specifically the measurement and analysis methods of flicker at arc furnace installations. By measuring flicker and 3 phase voltage and current waveforms the effect of changes in arc furnace system parameters on flicker is determined. Various analysis methods based on statistical theory are described in this study. The aim is to gain a better understanding of flicker and arc furnaces to contribute to the current flicker knowledge base.

Master of Science / Department of Geology / Matthew E. Brueseke / The Wrangell arc (WA) is a ~29 Ma magmatic belt, extending from south-central Alaska into the Yukon Territory, that lies above the edges and leading front of the Yakutat microplate, a buoyant oceanic plateau that is causing shallow subduction (11-16º) in the region. The WA occurs in a transition zone between “normal” Aleutian subduction to the west and dextral strike-slip tectonics to the east, accomplished by the Totschunda, Denali, and Duke River faults. This geologic setting offers a chance to study the interrelations between subduction, strike-slip motion, and slab-edge magmatic processes in a relatively well-exposed arc. We implemented a novel technique of applying geochemical and geochronologic analyses on volcanic cobbles collected from glacio-fluvial systems (rivers, streams, and glaciers) encircling/draining the WA. Our primary objective is to integrate our cobble datasets with the existing bedrock and detrital sand records to develop a comprehensive understanding of WA magmatism through time and space. Our secondary objective is to test the validity of this novel technique for reproducing what is documented from bedrock samples and its potential for utilization in other locations. This study provides new major element data from 215 samples and trace element data from 236 samples collected from 17 major rivers that drain from the modern western and central WA (this study excludes the eastern WA). This study also provides new age data from a total of 119 samples from 10 major rivers. New geochronology of modern detrital volcanic cobbles and sand/zircons reveal that the WA initiated at ~29 Ma and that magmatism migrated northwestward through time. Cobble ages and locations across the arc agree with the northwestward progression of magmatism previously identified by Richter et al. (1990). Forty-seven cobbles are dated <~1 Ma and only nine cobbles are dated 29 – ~20 Ma, whereas there are no cobbles from 17 – ~13 Ma. Geochemical data reveal similarities between our data and that of the <~5 Ma WA defined by Preece and Hart (2004): Trend 1 (transitional-tholeiitic), Trend 2a (calc-alkaline), Trend 2b (calc-alkaline, adakite-like). Therefore, we use the geochemical framework defined in Preece and Hart (2004) to contextualize spatio-temporal trends of magmatism and tectonic implications in the WA during its ~29 m.y. history. Trend 2a and 2b cobbles are spatially and temporally ubiquitous in the WA, indicating that subduction and partial slab melting have been the dominant tectonic processes throughout WA history. Trend 1 cobbles are not found in southwestern WA rivers and are temporally restricted to ~11 – ~6 Ma and <1 Ma, suggesting intra-arc extension has occurred in discrete periods during WA history. These conclusions are confirmed by the existing (Richter et al., 1990; Skulski et al., 1991; 1992; Preece and Hart, 2004; Trop et al., 2012) and new (Berkelhammer, 2017; Weber et al., 2017) bedrock records. Finally, this study shows that the sampled cobble lithologies largely reproduce the known bedrock record in geochemical, temporal, and spatial contexts, which suggests the novel methodology applied here can be used in other locations where field conditions limit access to bedrock.

An investigation of a pulsed Hg arc has been undertaken to test the relation V α I°⁴ between the voltage and current of an arc discharge. Although the experimental V-I characteristic exhibited this tendency, further considerations strongly indicated that the agreement between theory and experiment was probably a coincidence. The Paschen breakdown curve was determined, and together with simultaneous measurements of the current and voltage, it was possible to measure the average column field strength as a function of current. As a result of an unexpected feature of the apparatus, namely an inherent motion of the electrodes, the voltage waveform could be used to obtain an approximate measurement of the combined thickness of the anode and cathode falls df, and also the average field strength in these regions Ef. The measurements df = 3x10⁻⁶ cm, and Ef = 2.4x10⁶ volt/om appear to be in agreement with the field emission theory. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

Continuous current control, so familiar in the operation of high-vacuum tubes, has not been possible, except under special circumstances, for gas tubes. Even current interruption has been awkward, except for low currents, for the usual manner of interrupting the current is to decrease the anode potential to zero. The time to switch off the gas tube has been of the order of the deionization time for the gas employed. Now a method is developed for switching off mercury-pool arcs using a third electrode. There is no interference with the main power circuit and, in fact, the potential on the anode causing the electric field aids the dispersal of the charge carriers when the arc has been interrupted. The switching-off time is much decreased because this anode-to-cathode voltage sweeps all the charge carriers out of the tube. Switching off is effected by passing a reverse current of equal or greater magnitude than the arc cathode current through the tube for a time long enough to interrupt the cathode spot. A technical difficulty arises in that the third electrode introducing the reverse current has to have an already formed or an easily formed cathode spot since this third electrode is a cold cathode. Many methods for forming the cathode spot are discussed. The method finally used is probably not the best one but it has the virtue of being easily effected. There appears to be no limit as to the current that can be interrupted if the spot-forming mechanism is altered. Energy used in not an important factor. The amount varies with the time to switch off and does not influence the actual switching process. / Applied Science, Faculty of / Electrical and Computer Engineering, Department of / Graduate

Vortex stabilized argon arcs are of interest as sources of high intensity light. Previous workers in the field have found that the heat transported to the wall of the arc vessel greatly exceeded that predicted by theory. Two modifications to the theory have been proposed to account for the observed values of heat transport. This thesis describes a specialized arc vessel which has been constructed to allow the measurement of the axial profile of heat transported to the wall. The axial profile of radiation produced by the arc has also been measured. These experiments elucidate the mechanisms responsible for the large values of heat transported to the wall. In this work a 225 A d.c. arc was used. It was stabilized by an argon vortex at a pressure of 5.5 atm. It is found that the electrode regions of the arc contribute significantly to the total heat transported to the wall. Midway between the electrodes however, the profile is found to be quite flat. The experiments also indicate that some heat is transported upstream from the arc, and it has been shown that this is due to a reverse axial flow core in the gas vortex. The measured profile of radiation produced by the arc is found to be very uniform in the arc column. In the region midway between the electrodes the dependence of the radiation and heat transported to the wall on the gas flow rate in the vortex were examined. The measured radiation is found to be 30-35% less than predicted by theory. It is shown that this may be due to the axial transport of energy in the arc. The scaling of the amount of heat transported to the wall is found to be in good agreement with the predictions of a model for the arc which includes turbulent heat transport by using a mixing length model. The d.c. power supply used in this work produces a waveform with considerable ripple. Time dependent measurements were therefore made of the radiation produced by the arc and the electrical power input to the arc column. These results are compared with a time dependent theory of the arc column. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

A comparative study of high intensity DC and AC vortex stabilized arcs operating in argon (at pressures of one to five atmospheres) has been conducted. The energy balance for both the AC and DC arcs has been determined calorimetrically. From these measurements the radiative efficiency (radiation losses/input power) has been calculated. It was found over the current range examined (150-450 amperes) that the radiative efficiency of the AC vortex stabilized arc was comparable to the DC arc. Since DC vortex stabilized arcs have been used as a high intensity radiation source, these results indicate that the AC vortex stabilized arc shows promise for use as a high intensity radiation source. From the energy balance results the heat transfer to the wall was surprisingly found to scale linearly with the radiation losses. The wall loading is not due to absorption of radiation and is much larger than that expected from laminar radial heat transfer. To investigate this further a simple channel model was developed for the luminous DC arc core. From this model the radius and temperature of the luminous arc core was determined as a function of current. The predicted radii were in good agreement with time integrated photographs of the luminous core of the arc. At high current (I>350 amperes) the DC arc radius was essentially constant. The wall heat transfer continued to increase when the arc radius was essentially constant, so that highly efficient heat transfer processes must be taking place outside the central luminous arc core. It is believed that turbulent mixing might be present in this region and be responsible for the large wall heat transfer. The heat transfer processes to the arc electrodes have been measured calorimetrically and the electrode surface temperature has been measured spectroscopically. For both AC and DC electrodes the heat transfer scaled linearly with the arc current. The electrode voltage drop is strongly dependent on gas flow direction with the voltage drop always larger for flow towards the electrode than for flow away from the electrode. These results are not due to convective heat transfer effects. The geometry of the electrode arc attachment region changes when the flow direction is reversed. It is believed that both the anode and cathode fall potentials are altered when the flow direction is reversed, and this is responsible for the difference in electrode voltage drop when the flow direction is reversed. From the electrode surface temperature measurements the heat transfer to the arc electrodes was shown to be essentially one-dimensional in nature. A model of the AC electrode heat transfer was developed using the DC heat transfer results which predicts results for the electrode voltage drop that are in good agreement with the experimental results. The AC electrode heat transfer was found to be <50% of the anode heat transfer in a DC arc at the same current. In the DC arc the anode heat transfer is much larger than the cathode heat transfer. For a practical DC vortex stabilized arc radiation source anode failure is a serious problem, so that the results for the AC electrode heat transfer is of considerable practical importance. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

Although arc flash has been a concern amongst the electrical industry for many years, it is only relatively recently that standards by the IEEE have been established on calculating the amount of energy behind an arc flash event. However, these standards only apply to AC systems, where extensive testing and research have been performed. Although the NFPA has provided recommendations on how to calculate the incident energy for DC arc flash events, these have not become the defining standard like those seen for AC. One equation outlined in the NFPA70E, the Maximum Power Method, does provide engineers with a formula to calculate DC arc flash incident energy but as the NFPA states this can be quite conservative. However, the NFPA70E also mentions a Detailed Arcing Current and Energy Calculations Method which contains formulas proposed by various researchers who conducted their own DC arc flash testing but there is scarce info on how these methods compare to the Maximum Power Method. This paper will investigate the relative power of two of the formulas proposed in the alternate method, the results from Stokes/Oppenlander and the results from Paukert, over a variety of parameters that affect DC arcing power. These will then be compared to relative power of the Maximum Power Method, as well as the relative power of the AC equations formed from measurements. Although the results in this paper are not aiming to be a defining standard, the aim is to provide engineers with information on when one methodology is more suitable to use for a given set of certain parameters.