A gas in its normal state is almost a perfect insulator. However, when a high voltage is applied between the two electrodes immersed in a gaseous medium, the gas becomes a conductor and an electrical breakdown occurs.
The processes that are primarily responsible for the breakdown of a gas are ionization by collision, photo-ionization, and the secondary ionization processes. In insulating gases (also called electron-attaching gases) the process of attachment also plays an important role.
1. Ionization by Collision
The process of liberating an electron from a gas molecule with the simultaneous production of a positive ion is called ionization. In the process of ionization by collision, a free electron collides with a neutral gas molecule and gives rise to a new electron and a positive ion. If we consider a low pressure gas column in which an electric field E is applied across two plane parallel electrodes, as shown in Fig. 1 then, any electron starting at the cathode will be accelerated more and more between collisions with other gas molecules during its travel towards the anode.
If he energy (ε) gained during this travel between collisions exceeds the ionization potential, Vi, which is the energy required to dislodge an electron from its atomic shell, then ionization takes place. This process can be represented as
Where, A is the atom, A+ is the positive ion and a is the electron.
Fig 1 Arrangment for study of a townsend discharge
A few of the electrons produced at the cathode by some external means, say by ultra-violet light falling on the cathode, ionize neutral gas particles producing positive ions and additional electrons. The additional electrons, then, themselves make `ionizing collisions' and thus the process repeats itself. This represents an increase in the electron current, since the number of electrons reaching the anode
per unit time is greater than those liberated at the cathode. In addition, the positive ions also reach the cathode and on bombardment on the cathode give rise to secondary electrons.
2. Secondary Ionization Processes
Secondary ionization processes by which secondary electrons are produced are the one which sustain a discharge after it is established due to ionization by collision and photo-ionization.
They are briefly described below.
(i) Electron Emission due, to Positive Ion Impact
Positive ions are formed due to ionization by collision or by photo-ionization, and being positively charged, they travel towards the cathode.
A positive ion approaching a metallic cathode can cause emission of electrons from the cathode by giving up its kinetic energy on impaCT's If the total energy of the positive ion, namely, the sum of its kinetic energy and the ionization energy, is greater than twice the work function of the metal, then one electron will be ejected and a second electron will neutralize the ion. The probability of this process is measured as , which is called the Townsend's secondary ionization coefficient due to positive ions and is defined as the net yield of electrons per incident positive ion. , increases with ion velocity and depends on the kind of gas and electrode material used.
(ii) Electron Emission due to Photons
To cause an electron to escape from a metal, it should be given enough energy to overcome the surface potential barrier. The energy can also be supplied in the form of a photon of ultraviolet light of suitable frequency. Electron emission from a metal surface occurs at the critical condition
where cp is the work function of the metallic electrode. The frequency (v) is given by the relationship
Is known as the threshold frequency For a clean nickel surface
With φ = 4.5 eV, the threshold frequency will be that corresponding to a wavelength λ = 2755 Aº. If the incident radiation has a greater frequency than the threshold frequency ', then the excess energy goes partly as the kinetic energy of the emitted electron and partly to heat the surface of the electrode. Since φ is typically a few electrons volts, the threshold frequency lies in the far ultraviolet region of the electromagnetic radiation spectrum.
(iii) Electron Emission due to Metastable and Neutral Atoms
A metastable atom or molecule is an excited particle whose lifetime is very large (10-3 s) compared to the lifetime of an ordinary particle (10-8 s). Electrons can be ejected from the metal surface by the impact of excited (metastable) atoms, provided that their total energy is sufficient to overcome the work function. This process is most easily observed with metastable atoms, because the lifetime of other excited states is too short for them to reach the cathode and cause electron emission, unless they originate very near to the cathode surface. Therefore, the yields can also be large nearly 100%, for the interactions of excited He atom with a clean surface of molybdenum, nickel or magnesium. Neutral atoms in the ground state also give rise to secondary electron emission if their kinetic energy is high
(= 1000 eV). At low energies the yield is considerably less.
· Electron Attachment Process
The types of collisions in which electrons may become attached to atoms or molecules to form negative ions are called attachment collision. Electron attachment process depends on the energy of the electron arid-the nature of the gas and is a very important process from the engineering point of view. All electrically insulating gases, such as O2, CO2, C12, F2, .C2F6, C3F8, C4F10, CC12F2, and SF6 exhibit this property. An electron attachment process can be represented as:
Atom + e- → negative atomic ion + (Ea + K) 11
The energy liberated as a result of this process
is the kinetic energy K plus the electron affinity Ea. In the
attaching or insulating gases, the atoms or molecules have vacancies in their
outermost shells and, therefore, have an affinity for electrons. The attachment
process plays a very important role in the removal of free electrons from an
ionized gas when arc interruption occurs in gas-insulated Switchgear.
· TOWNSEND'S CURRENT GROWTH EQUATION
Referring to Fig. 1 let us assume that no electrons are emitted from the cathode. When one electron collides with a neutral particle, a positive ion and an electron are formed. This is called an ionizing collision. Let α be the average number of ionizing collisions made by an electron per centimeter travel in the direction of the field (α depends on gas pressure p and E/p, and is called the Townsend's first ionization coefficient). At any distance x from the cathode, let the number of electrons be nx. When these nx electrons travel a further distance of dx they give
d nx/dx = α nx ; nx=n0 e αx
Then, The number of electrons reaching the anode (x=d)
The number of new electrons created, on the average, by each electron is
e(α.d) -1 = (nd - no) / no 12
Therefore, the average current in the gap, which
is equal to the number
I =Io. e(α.d) 13
where 10 is the initial current at the cathode.