Direct current breakdown in gases for complex geometries from high vacuum to atmospheric pressure

The demand for improved performances of power transmission devices requires ever smaller dimensions and higher working voltages which lead to increased risk of breakdown, for example, in satellite slip rings. Previous works are mostly limited to breakdown in simple geometries such as parallel plates or pin to plate. Here we discuss the effect of more complex geometries for dc breakdown in gases over a large pressure range (2 x 10(-5) to 10(3) mbar). Experimental measurements of dc gas discharge breakdown in a ring assembly geometry are compared with a numerical simulation model for gas breakdown using a fluid model. Starting with parallel plates (1 and 100 mm gap width representing approximately the shortest and longest electric field path lengths in the ring assembly geometry) and extending to double gap and multi-gap geometries, an understanding of the overall shape of the breakdown voltage versus pressure curve is established. The high (low) pressure thresholds of gas discharge are determined by the shortest (longest) electric field path length in a complex geometry. Moreover, the availability of multiple path lengths leads to a breakdown voltage minimum over a wide range of intermediate pressure because breakdown can occur in the most favourable gap. Finally, the numerical simulation in the ring assembly shows the importance of parameters such as the secondary electron emission coefficient which play a major role in determining the breakdown voltage value.

DC Breakdown in gases for complex geometries from high vacuum to atmospheric pressure

Ralph Schnyder

This thesis presents an experimental investigation and a numerical simulation of breakdown in a ring assembly. Previous works are mostly limited to breakdown in simple geometries such as parallel plates or pin-to-plate. Here we discuss the effect of more complex geometries for DC breakdown in gases over a large pressure range from high vacuum to atmospheric pressure. The breakdown voltage versus pressure curves shows a similar shape as Paschen curves but with a wide flat plateau between the low and high pressure thresholds. The low pressure threshold determines the limit between gas and vacuum discharges. Additional optical emission spectroscopy confirms the presence of two different kinds of discharges: Gas and vacuum dis- charges. Moreover the global shape of the gas breakdown voltage curve in the ring assembly has been fully understood by a complementary numerical simulation. Fur- ther current-voltage study showed that voltage only is the most significant factor for breakdown and that current determines the kind of discharge after breakdown. As the breakdown voltages are lower for gas discharges than for vacuum discharges, a numerical simulation model for gas breakdown using a fluid model was developed in order to support the experimental conclusions. Starting as simple as possible with par- allel plates (1 mm and 100 mm gap width representing approximatively the shortest and longest electric field path length in the ring assembly geometry) and extending to double gap and multi-gap geometries, an understanding of the overall shape of the breakdown voltage versus pressure curve is established: The high (low) pressure thresholds of gas discharge are determined by the shortest (longest) electric field path length in a complex geometry. Moreover, the availability of multiple path lengths leads to a breakdown voltage minimum over a wide range of intermediate pressure because breakdown can occur in the most favorable gap. Finally, the numerical simulation in the ring assembly shows the importance of parameters such as the secondary electron emission coefficient which play a major role in determining the breakdown voltage value.

EPFL2013

Arcing in Very Large Area Plasma-Enhanced Chemical Vapour Deposition Reactors

Boris Legradic

This thesis presents an investigation into arcing and parasitic plasmas in large area plasma enhanced chemical vapour deposition reactors. Two types were investigated: RF breakdown in millimetric gaps in absence of plasma (e.g. dark space shielding), in a millibar pressure range, and RF hollow cathodes in glow discharges. RF breakdown curves (voltage vs. pressure) for parallel plate electrodes generally show a steep left-hand branch at low pressures and a flatter right hand branch at higher pressures. Introducing protrusions or holes in parallel plate electrodes will lower the breakdown voltage in certain conditions. This is, however, not due to the increased electric field at sharp edges or ridges. Instead, both experiments and simulation show that breakdown at high pressure will occur at the protrusion providing the smallest gap, while breakdown at low pressure will occur in the aperture providing the largest gap. This holds true as long as the feature in question is wide enough: Features that are too narrow will lose too many electrons due to diffusion, either to the walls of the apertures or to the surroundings of the protrusion, which negates the effect on the breakdown voltage. An analytical approximation of breakdown in parallel plates with cylindrical protrusions supports this argument. The simulation developed to study breakdown in structured parallel plate electrodes also presents a tool to aid the design of complex RF parts for dark-space shielding. A method was developed to measure the pressure-limits of ignition for RF hollow cathodes, and it could be shown that these limits not only depend on gas type, diameter and depth of the hollow cathode, but also on the presence and/or absence of other hollow cathodes in the vicinity. It could also be conclusively shown that hollow cathodes damage the electrode by sputtering and/or evaporation.

EPFL2011

DC Breakdown in gases for complex geometries from high vacuum to atmospheric pressure

Ralph Schnyder

EPFL2013