Streamer breakdown: cathode spot formation, Trichel pulses and cathode-sheath instabilities

The review provides an up-to-date overview and discussion of phenomena related to positive streamer breakdowns in short uniform and non-uniform field (corona) gaps. The terminology used to specify different types of streamer phenomena is critically discussed in light of a unified theory of high-pressure gas discharges describing the sequence of ionization events from initial electron avalanches up to a partial or complete breakdown. The emphasis is given to the understanding of the formation of an active cathode spot by the streamer arrival to the cathode, which is the critical but still obscure phase of the breakdown development. Based on the analysis including a computer simulation model a hypothesis is advanced that also such widely studied and practically important gas discharge phenomena as negative corona Trichel pulses and fast ionization instabilities in cathode regions of high-pressure gas discharges are due to the formation of positive streamers in the immediate cathode vicinity. The proposed hypothesis offers attractive feature of the unification of a wide scale of high-pressure gas discharges within the general class of positive streamer initiated breakdown phenomena. Moreover, it provides indications for further study in the field both by experiment and computer simulations. (topical review)