Streamer and leader formation in SF₆ and SF₆ mixtures under positive impulse conditions. II. Streamer to leader transition

For pt.I see ibid., vol.12, p.2351 (1986). This second paper on the breakdown mechanisms in electronegative gases under non-uniform field conditions deals with the development of the leader type of discharge, the emphasis being laid on details of the streamer to leader transition, which is found to be very much different from the corresponding process in air. In a strongly electronegative gas, the streamer to leader transition involves a 'leader precursor' that has been observed for the first time. A hypothesis relating precursor inception to an ion drift phenomenon is shown to explain the observed parameter dependences satisfactorily. Numerical simulations have been performed, revealing details of precursor formation. Some experimental results concerning the further development of the leader channel are also presented.