AN EXPERIMENTAL-STUDY OF NEGATIVE DISCHARGE IN A 1.3 M POINT-PLANE AIR-GAP - THE FUNCTION OF THE SPACE STEM IN THE PROPAGATION MECHANISM

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 rang ing from 0.25/2000 to 120/2000 mu s. Among these impulse shapes, the o ptimum with respect to the observation of the space stem formation is the 0.3/2000 mu s one. Some detailed photographs obtained with an imag e converter allow one to identify the following phenomena: negative st reamers, a Trichel zone, a negative corona, a cathodic zone (cathodic stem), positive streamers and space stems. The current measurements al low 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 str eamers in the space stem's behaviour, which is defined as their zone o f origin. A stem replica mechanism is explained and some associated en ergetic criteria are discussed.