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Effect of corona stabilisation in atmospheric air in DC and impulse conditions

Mermigkas, Athanasios and Timoshkin, Igor and Given, M and Macgregor, Scott and Wilson, Mark (2011) Effect of corona stabilisation in atmospheric air in DC and impulse conditions. In: 8th Technological Plasma Workshop, 2011-01-06 - 2011-01-07. (Unpublished)

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Corona discharges in electro-negative gases can generate space charge which re-distributes the electric field in the inter-electrode gap. In the case of negative energisation of the sharp high-voltage (HV) electrode in atmospheric air, negative space charge reduces the ideal Laplacian electric field in the vicinity of the cathode, which may result in an increase in the breakdown voltage (corona stabilisation effect). Such stabilisation can be used in repetitive plasma closing switches to maintain the same voltage level between HV impulses generated by pulsed power systems. Positive energisation of the HV electrode in air demonstrates lower breakdown voltage due to less pronounced space charge effects, resulting in a reduced field-grading effect. The present paper reports on an investigation into breakdown characteristics in point-plane electrode topologies stressed with positive and negative DC voltages, and HV impulses produced by a Marx generator with maximal voltage of 100 kV and rise time of ~800 ns. It has been shown that the corona stabilisation effect strongly depends upon the polarity of HV energisation and electrode topology: radius of curvature of the point electrode and inter-electrode distance. In the present paper stainless steel electrodes with three different curvatures have been used: 0.115 mm, 1.0 mm and 2.5 mm. The gap width has been varied from 2 to 20 mm. At shorter distances the stressed air gap demonstrated the same values of positive and negative breakdown voltages. However, the critical gap could be identified at which the negative breakdown voltage becomes higher than the positive one, and with an increase in the inter-electrode distance this difference in the breakdown voltages continues to grow. This identification of the critical gap width and corresponding critical breakdown voltage allows the development of switch topologies with well pronounced corona stabilisation effect. In the case of impulse stresses, the critical breakdown time has been obtained as a maximum pre-breakdown time, which is identical for impulses of both polarities. With further increase in the gas spacing, the pre-breakdown time for negative impulses increases significantly as compared with the time to breakdown for positive impulses (~3 fold increase has been observed for the point electrode with the radius of 2.5 mm, from ~500 ns to ~1500 ns). This systematic study provides valuable information on corona stabilisation effect as a function of the electrode curvature, inter-electrode distance and HV polarity. Based on the obtained results, an algorithm for the development of corona stabilised electrode topologies has been established. This may help in the design of advanced plasma closing switches for pulse power systems which require a high level of operational stability.