Spark Breakdown Mechanism Streamer Concept

A strong primary avalanche is able to amplify the external electric field and form a streamer. When the streamer chaimel cotmects the electrodes, the current may be significantly increased to form the spark. The avalanche-to-streamer transformation takes place when the internal field of an avalanche becomes comparable with the external one. If the gap is short, the transformation occurs only when the avalanche reaches the anode. Such a streamer that grows from anode to cathode and called the cathode-directed or positive streamer. If the gap and overvoltage are large, the avalanche-to-streamer transformation can take place far from the anode, and the anode-directed or negative streamer grows toward both electrodes. 

The diameter of the streamer channel is 0.01-0.1 cm and corresponds to the maximum size of a primary avalanche head, 1/a. The plasma density in the streamer also corresponds to the maximum electron density in the head of the primary avalanche 10 -i- 10 cm. The specific energy input in a streamer channel is small during the short period ( 30 ns) of streamer growth between electrodes. In molecular gases it is about 10 eV/mol, which corresponds to heating of 10 K. 

The anode-directed streamer occurs if the primary avalanche becomes strong before reaching the anode. Such a streamer growing in two directions is illustrated in Fig. 4-7. The mechanism of streamer propagation in the direction of the cathode is the same as for cathode-directed streamers. The mechanism of streamer growth in direction of the anode is similar, but in this case the electrons from the primary avalanche head neutralize the ionic 

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