ELECTRIC FIELD MODELLING FOR POINT-PLANE GAP V. Repän, M. Laan a and T. Plank Institute of Experimental Physics and Technology, University of Tartu, Tähe 4, 51010 Tartu, ESTONIA Abstract. The electric field distribution for point-plane gap is modelled both for stressed point and stressed plane electrodes. In simulations, the influence of the discharge chamber walls is taken into account. The size of an avalanche and the corresponding current pulse are calculated. The results are compared with those got using other field distribution approximations. 1. INTRODUCTION For gas discharges, the distribution of the electric field is the most important parameter, which determines the spatio-temporal development of discharges as well as the waveform of the recorded current. In the case of the modelling of corona discharges in point-plane gaps, the simplest Laplacian field approximation is the spherical one and it is widely used [1, 2]. On the basis of numerical modelling, Abou-Seada [3] presented for the ionisation zone of the gap an analytical expression of the field distribution E(x) along the gap axis. The approximation does not satisfy the normalization condition ∫ = d dx x E U 0 ) ( , where U is the applied voltage and d is the gap length. Coelho and Debeau [4] derived a formula for field distribution in hyperbolic approximation. The latter approximation satisfies the normalization condition. It should be pointed out that the approximations mentioned do not take into account the influence of the grounded walls of the discharge chamber on the field distribution. Our experience of the modelling of non-self-sustained negative DC corona pulses confirms that the using of above-described approximations does not assure a satisfactory coincidence of simulated and recorded current waveforms. For this reason we undertook the numerical simulation of the field distribution which considers also the role of the discharge chamber walls. This paper presents the main results of the modelling and compares waveforms of avalanches calculated using different electric field E(x) approximations. 2. EXPERIMENTAL We studied [5] the regularities of negative corona discharge at atmospheric pressure in air initiated by UV light pulses. UV pulses of 3.5 ms duration were formed using a deuterium lamp and a chopper and focused to the point tip. The discharge was ignited in a 4-cm point-plane gap. The point electrode was a 1-mm wire with hemispherical tip and the diameter of the plane electrode was 15 cm. The diameter of the metal walls surrounding the discharge chamber was 22 cm. Under these conditions, the onset potentials of the current pulses of negative corona (Trichel pulses) depend on which electrode is stressed. The onset potential for stressed point is ≈ 8 kV and for stressed plane it is ≈ 13 kV. Below the onset potential, the discharge was non-self-sustained, i.e. the current arises only due to the action of UV light. For a certain voltage, the maximum current recorded depends on properties of the point coating material and the intensity of UV light but in any case the current waveform has a strong dependence on which electrode is stressed. Figure 1 presents current pulses induced by UV light pulses at ≈ 20% below the corresponding onset of Trichel pulses (6.5 and 12 kV for stressed point and a Electronic address: laan@physic.ut.ee