Streamer Theory of Breakdown in Gases
According to the Townsend theory
- firstly, current growth occurs as a result of the ionization process only. But in practice, breakdown voltages were found to depend on the gas pressure and the geometry of the gap.
- Second chances time lags of the order of 10-5 s, but practically it was observed to occur at a very short time of 10-8 s.
- Also, the Townsend mechanism predicts a very diffused form of discharge, that actually discharges were found to be filamentary and irregular.
- Townsend mechanism failed to explain all these observed phenomena and as a result, The Streamer theory was proposed.
- The theory predicts the development of a spark discharge directly from a single avalanche in which the space charge develop by the avalanche itself is said to transform the avalanche into a plasma steamer.
- In Fig 1.7, a single electron starting at the cathode by ionization builds up an avalanche that crosses the gap.
- The electrons in the avalanche move very fast compared with the positive ions.
- By the time the electrons reach the anode the positive ions are in their original positions and form a positive space charge at the anode.
- This enhances the field, and the secondary avalanches are formed from a few electrons produced due to the photo-ionization in the space charge region.
- This occurs first near the anode where the space charge is maximum and a further increase in the space charge.
- This process is very fast and the positive space charge extends to the cathode very rapidly resulting in the formation of a streamer.
- Comparatively narrow luminous tracks occurring at the breakdown at pressures are called streamers.
- As soon as the streamer tip approaches to the cathode, a cathode spot is formed and a stream of electrons rush from the cathode to neutralize the positive space charge in the streamer the result is a spark and the spark breakdown has occurred.
- A simple quantitative criterion to estimate the electric field Er which is produced by the space charge, at the radius r and that transforms an avalanche into streamer is Townsend’s first ionization coefficient, p is the gas pressure in torr and x is the distance to which the streamer has extended in the gap.
- The breakdown voltage is given by the corresponding product Ed.
- It is generally assumed that for pPDvalues below 1000 torr-cm and gas pressures varying from 0.01 to 300 Torr.
- The Townsend mechanism operates, while at higher pressures and PD values the streamer mechanism plays the dominant role in explaining the breakdown phenomena.
- However, controversies still exist in these statements.