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.

Experimental results of the development of a negative discharge in a 16.7 m rod-plane air gap are presented. Similarities with the cloud-to-earth discharge are found and some characteristic parameters are compared. Use of an image converter linked to two still cameras allows quite accurate analysis of the discharge propagation. Information relative to the space leader inception is provided. The current pulse shapes are related to the mechanisms of propagation and the thermal channel diameter of the discharge in the vicinity of the rod, which is measured with the help of schlieren records.

Experimental results concerning the propagation of a spark in a 16.7 m rod-plane gap are presented. Positive switching impulses are used. The parameters of the discharge are measured and the trajectory of the leader-leader corona system is studied as a function of time (withstand and breakdown events). Three main types of propagation are found: continuous, oscillatory and restrike. For each type, the propagation mechanisms of the leader-leader corona system are discussed from a physical point of view.

Measurements have been carried out with a fast photomultiplier on a unique streamer in a point-plane gap in atmospheric air (positive direct tension applied to the point). Three different kinds of experiments have been done: streamer heads statistics as a function of applied voltage; streamer propagation in weak applied field; and streamer head characteristic dimension for which the value obtained is about 170 mu m.