This paper reports a study of the origin and propagation of the space stem when a bi-exponential voltage impulse is applied to a 1.3 m point-plane air gap. The tests are carried out by using impulse shapes ranging from 0.25/2000 to 120/2000 mu s. Among these impulse shapes, the optimum with respect to the observation of the space stem formation is the 0.3/2000 mu s one. Some detailed photographs obtained with an image converter allow one to identify the following phenomena: negative streamers, a Trichel zone, a negative corona, a cathodic zone (cathodic stem), positive streamers and space stems. The current measurements allow one to quantify some energetic thresholds. The 5/2000 mu s impulse shape leads to a quasi-continuous space stem propagation in the gap. Thus it is possible to study the role of the positive and negative streamers in the space stem's behaviour, which is defined as their zone of origin. A stem replica mechanism is explained and some associated energetic criteria are discussed.

This paper first presents an experimental electrical and optical study of the development of an electrical discharge in water. The point–plane water gap is subjected to a 0.02 µs/350 µs impulse voltage. A Schlieren device associated with an image converter or a photomultiplier demonstrates that the discharge phenomenon requires heating of the water located around the extremity of the point. This thermal process leads to the formation of gas bubbles in which an electrical discharge propagates. In the experimental conditions a threshold value of 80 J is necessary to create bubbles. No UV or IR light emission is recorded before the presence of bubbles is detected. When the energy conditions are sufficient (⩾200 J), the volume of bubbles grows until the whole inter-electrode space is filled; then a breakdown of the gap occurs. When this happens, a high amplitude pressure shock wave is generated. In the second phase of this work the shock wave created by the gap breakdown was studied for energy levels up to 100 kJ. It is clearly pointed out that the pressure shock wave peak value depends on the energy remaining at breakdown time. For a constant remaining energy, the peak pressure value increases with increasing gap length.

Several theories have been proposed to explain the current pulse of Trichel, at low pressure, in accordance with experimental results. Nevertheless, these theories failed to explain the very fast rise time (a few nanoseconds) observed at high pressure. The aim of this study is to propose a numerical simulation of the Trichel pulse which explains the typical current shape observed in air at atmospheric pressure in terms of field-effect emission. This theory explains the principal mechanisms responsible for the formation of Trichel pulses and takes into account the cathode material and its surface state. The effects of the field magnification factor on the pulse shape resulting from cathode microprotrusions are discussed. In the same way, the value of the discharge channel's radius is also discussed and compared with the experimental measurements.

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