Lecture 6 : Properties of vacuum, Related ionization Process
PREBREAKDOWN ELECTRON EMISSION IN VACUUM
When the voltage across a very small gap (a few mm) is sufficiently increased, a relatively steady current begins to flow.
A general observation made for small gaps is, that the prebreakdown current flow has been found to consist mainly of electrons.
For longer gap spacings (> 1 cm), small pulse currents of millisecond durations and charges of the order of micro-Coulombs (micro-discharges) are measured.
On raising the voltage further, the micro-discharge eventually give rise to a steady current .
Mechanisms of Electron Emission from Metallic Surfaces
There are a large number of mechanisms described in the literature that can produce electron emission from metallic surfaces under different conditions, for example, thermionic emission, field assisted thermionic, also known as Schottky or T-F emission, field or cold emission, photoelectric emission, secondary emission caused by electron bombardment of an electrode and emissions caused by positive ion bombardment and metastable atoms [2.2]. Out of these, the most important mechanisms of electron emission in vacuum are due to field, field assisted thermionic and thermionic emission.
Non-Metallic Electron Emission Mechanisms
Prebreakdown conduction currents between vacuum insulated high voltage electrodes frequently originate from nonmetallic mechanisms. These are associated with some form of insulating/semiconducting oxide layer on the surfaces or impurity concentrations. The microinclusions present on electrode surfaces can stimulate strong electron emission and significantly reduce the breakdown strength of the gap.
Fig 6.1 A schematic representation of insulating microinclusion emission regime with conducting channels and the associated microscopic field enhancement, Latham, [2.3, 2.4]
Fig 6.2 A band structure representation of an insulating microparticle in 'switched on state' of a conducting channel yielding electron emission. F.L.-Fermi level, c.b.-conduction band, v.b.-valence band, Latham
