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## Mechanism of electrical discharges

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The Lightning Flash — Recommend this title to your library

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The experiments performed by researchers in different countries, notably South Africa, England, Switzerland and the United States, during the last 60 years have greatly advanced our knowledge concerning the mechanism of lightning flashes. However, many pieces of the puzzle pertinent to the mechanism of lightning flashes are still missing and many of the theories put forth as explanation of its mechanism are mainly of qualitative nature. The reason for this slow progress is the impossibility of studying lightning flashes under controlled laboratory conditions. On the other hand, the mechanism of the electric spark, which could be studied under controlled conditions, may guide the researchers in their quest for understanding the mechanism of lightning flashes and creating more advanced theories of the phenomena. After all, it is the observed similarities between the small laboratory sparks and lightning discharges that forced Benjamin Franklin to conclude that lightning flash is a manifestation of electricity. This chapter is devoted to a description of the mechanism of laboratory sparks.

Chapter Contents:

• 3.1 Introduction
• 3.2 Basic definitions
• 3.2.1 Mean free path and cross section
• 3.2.2 Drift velocity and mobility
• 3.2.3 Thermal equilibrium and local thermal equilibrium
• 3.3 Ionisation processes
• 3.3.1 Ionisation due to electron impact
• 3.3.2 Photoionisation
• 3.3.3 Thermal ionisation
• 3.3.4 Ionisation caused by meta-stable excited atoms
• 3.3.5 Ionisation due to positive ions
• 3.4 De-ionisation processes
• 3.4.1 Electron-ion recombination
• 3.5 Other processes that can influence the process of ionisation
• 3.5.1 Electron attachment and detachment
• 3.5.2 Excitation of molecular vibrations
• 3.5.3 Diffusion
• 3.6 Cathode processes
• 3.6.1 Photoelectric emission
• 3.6.2 Thermionic emission
• 3.6.3 Schottky effect
• 3.6.4 Field emission
• 3.6.5 Incidence of positive ions
• 3.7 Electrical breakdown
• 3.7.1 Electron avalanche
• 3.7.2 The space charge electric field due to an avalanche
• 3.7.3 Formation of a streamer
• 3.7.4 Characteristics of the streamers
• 3.7.5 Streamer to spark transition and thermalisation
• 3.7.6 Electrical breakdown criterion in the presence of streamer discharges
• 3.8 Electrical breakdown in very small gaps: Townsend's breakdown mechanism
• 3.8.1 Townsend's experiment
• 3.8.2 Townsend's theory of electrical breakdown
• 3.9 Paschen's law
• 3.9.1 Physical interpretation of the shape of the Paschen curve
• 3.9.2 Validity of Paschen's law
• 3.10 Voltage and current (V-I) characteristics and the post breakdown stage (low pressures)
• 3.10.1 The glow discharge
• 3.10.2 Abnormal glow
• 3.10.3 The glow to arc transition
• 3.11 Resistance of spark channels
• 3.12 Corona discharges
• 3.12.1 Negative corona modes
• 3.12.2 Positive corona modes
• 3.12.3 Electrical breakdown and corona
• 3.13 Dependence of electrical breakdown conditions on atmospheric conditions
• 3.14 Statistical nature of electrical breakdown
• 3.14.1 Electrical breakdown under the application of impulse voltages
• 3.14.2 Statistical nature of the electrical breakdown
• 3.15 The long spark
• 3.15.1 Streamer to leader transition and the initiation of the leader
• 3.15.2 General characteristics of impulse breakdown in rod-plane gaps
• 3.15.3 Some features of mathematical modelling of positive leader discharges
• 3.16 Humidity effects
• 3.16.1 Critical electric field necessary for streamer propagation
• 3.16.2 Influence on the corona development at the initiation of long sparks
• 3.16.3 Influence on leader propagation
• References

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