Electron Beam Generation-1 Thermionic Emission

Hi Steemians, today i will give some information about electron beam generation.

There are many technological areas for using electron beam for example, material fabrication, sterilization, etc. The electron beam is also used in the high energy and high energy fields (LHC), which are more exotic areas than the mentioned technological fields. Certain sources which are thermionic emission and photoelectric emission, used for electron beam generation.

------Thermionic Emission------

Figure-1: A simple setup for thermionic emission.

The energy distribution of the electrons in the metals is indicated by the Fermi-Dirac distribution. This distribution proves to have only one conduction electron according to the wave function and spin polarity for the metal [1]. n(E), electron density, in the dE energy range can be found as,

m_e - electron mass, k - Boltzmann constant, T - temperature of the metal, E_Fermi - maximum energy level @T=0K.

As the metal temperature increases, the electrons move to higher energy positions, some of which may be above the work function of the metal. The density of the electrons above this metal work function is collected. Thus, the electron density is extracte(d from the metal (Richardson Dushman equation) [2].

T - temperature of the metal, Φ_is - work function, A_G - constant in terms of type of cathode.

It is observed that the current is directly proportional to the temperature of the material. At the same time, the current is also inversely proportional to the work function. So the lower the work function, the more current will flow. Table 1.1 contains information on some of the materials.

Table-1: Some types of materials according to their current (A), work function and temperature (T).
(C.J. Smithells, Metals Reference Book, Vol III Butterworths, London, 1967.)

Examining Table 1, it is observed that the metal tungsten and tantalum are at the highest temperature. Despite the low melting temperature of cesium, the work function is much lower than the others. However, the effect of temperature is more prevalent in tungsten and tantal electron beam production sources, as it is higher than the work function.

It is possible to reduce the work function by covering the surfaces of the metals and thus the best cathodes are obtained. Oxides such as thorium or barium can be used as coatings on W (tungsten) Ta (tantalum), or Mo (molybdenum) surfaces. However, these coatings shorten the lifetime of the cathodes so this is disadvantageous of the coatings.

Next time, we will examine the photoelectric emission [3] and make a comparison between photoelectric and thermionic emission.

Have a curious day...