Research Activities

Electron Emission

back to Experimental Beam-Wave Instability page

An integral part of beam formation is the emission of electrons from bulk material, here we review the main techniques which as described in section 3 we use in our experimental work

Thermionic Emission

In a conducting material, the electrons are governed by Fermi-Dirac statistics. The baseline electron energy is the Fermi Energy*, and at low temperatures the electrons all exist at or below this level. As temperature is increased, so the distribution function for the electrons develops a high energy 'tail'. Some of these electrons have sufficient energy to pass over the surface potential barrier between the material and the vacuum. This process of increasing the temperature of a bulk material to increase the number of electrons which can leave the material is called thermionic emission. This is invariably aided by the Schottky effect, here an electric field is applied to the surface of the emitter, reducing the surface potential barrier in both magnitude and width, increasing the number of electrons able to escape from the surface. The thermionic emission current density is determined by the 'work function' of the material, which is basically the magnitude of the surface potential. Good emitters have low work functions. Unfortunately this also means they are chemically prone to damage by reacting with contaminants.

*At this energy, independent of temperature, the statistics require that half of the available 'states' will be occupied by an electron. At 0K, the distribution function has a step from 1 to 0 at the Fermi Energy. This transition smooths out as the temperature is increased.

Field Enhanced Emission

The current density in Thermionic-Schottky emission is weakly dependent on the magnitude of the applied electric field. Experimentally it is known that if the field is increased then a point is reached when the emission current starts to exceed the predictions of this theory. There are two models which may be used to explain this behaviour. First though we should discuss the Enhanced Electron Emission concept. If the field is increased above some finite level, then the width of the surface potential barrier becomes small enough that a considerable fraction of the electrons in the bulk material can 'quantum tunnel' through it. The number of electrons at or close to the Fermi energy (for cool metals (T less than T(Fermi)) in particular the bulk of the electrons exist just below the Fermi energy) is very much larger than at the high energies corresponding to the height of the surface potential. Therefore when the width of the barrier becomes such that these electrons can tunnel out, the emission current becomes very sensitive to the electric field. This field emission current quickly becomes much larger than the thermionic current. However the experiments indicate that this happens when the electric field strength is a few orders of magnitude less than theoretical considerations would indicate.

At this point we meet the two competing models: both are based on the concept of surface discontinuities

i) The difference between experiment and theory can be explained if one assumes that on the surface of a cathode, micron sized discontinuities having the form of an outward spike exist. If the length of the spike is>10 times its base dimension then it is relatively easy to expect the local electric field to be enhanced by a considerable amount (> x1000). Then the theoretical model discussed above applies to the surface of the discontinuity. It is interesting that the enhanced emission current from a handful of such micron sized sites can exceed (by many orders of magnitude) the total thermionic current from the bulk cathode at sufficiently high electric fields. Such discontinuities have been observed on real cathode surfaces, and in some experiments at the University of Strathclyde specially manufactured Field Emission Arrays consisting of millions of such individual tips machined in a semiconductor surface have been used as a source of a high quality, highly controllable, electron beam.

ii) The other model is based on the assumption that the onset of Field Enhanced Emission at such a low electric field can be attributed to insulating inclusions (micron sized) in the cathode surface. Electrons emitted from the bulk cathode into the conduction band of the insulator are accelerated towards the vacuum, collisions with electrons in the valance band of the insulator cause an 'avalanche' of electrons. In this model it is assumed that electrons are emitted from the insulator by the thermionic process and the temperature is described in terms of the electric field that has accelerated them through the insulator. This yields a close match to many experimental measurements of Field Enhanced Emission.**

**R.V. Latham, 1983, "Prebreakdown Electron Emission", IEEE Trans. on electrical Insulation, EI-18, pp194-203

Explosive Electron Emission

If the electric field is increased beyond the onset of Field Enhanced Emission, then the emission current density increases very rapidly. This causes intense localised heating of the surface discontinuities which then typically sublimate. The gas, primarily consisting of cathode material, expands into the vacuum where it is ionised by bombardment from the thermionic emission due to the very hot underlying cathode. The ions of course are accelerated back towards the cathode where they place pressure on the molten, cooling cathode material, resulting in a hollow crater with sharp edges. These sharp edges can then act as a further site of Field Enhanced Emission, thus repeating the cycle. In the meantime, the plasma formed by the ionised gas is emitting electrons into the vacuum gap. Plasmas typically have minimal work functions, so the emission current is usually limited by the Child-Langmuir law (the space charge limit). Another term used to describe this type of electron emission is 'vacuum spark'. The cathode ejecta propagates into the vacuum gap with a velocity strongly dependent on the thermal and electrical properties of the bulk cathode material, typically a few centimetres per microsecond. This form of emission results in electron beams with a very high current, but only for a short duration (usually hundreds of nanoseconds). Because of the randomness of the emission process, one has to be prepared to accept a lower beam quality than thermionic or field emission

Pseudospark

Information to follow...