Time Lags for Breakdown of Electronegative Gases

Time Lags for Breakdown of Electronegative Gases

• There is a time lag between the applications of a voltage sufficient to cause breakdown and the occurrence of breakdown itself. This laps time is known as statistical time lag (t_S). The appearance of electrons is usually statistically distributed. After breakdown i.e. after the appearance of the electron, a time (t_t) is required for the ionization processes to develop fully to cause the breakdown of the gap. This time is known as formative time lag (t_f).

The total time t_S + t_f = t  is called the total time lag as shown in Figure A below :

Figure A

Breakdown with a step function voltage pulse,

t_S = statistical time,

 t_f   = formative time,

t = total time.

For the breakdown to occur , the applied voltage V should be greater than the static breakdown voltage V_b as shown in Figure A. The difference in voltage △V = V – V_b is called the over voltage and the ratio V / V_S is called the impulse ratio. The variation of  t_f with over voltage (△V) is shown in Figure B.

Figure B

Point of distinction between Photon and Positive lon :

Photon : A quantum of electromagnetic radiations usually considered as elementary particle that is its own antiparticle and that has zero rest mass and charge and a spin of one. Symbol : γ

Positive ion : (Cation) –is created by electron loss and is attracted to the cathode in electrolysis.

Factor Affecting on Statistical Time (t_S).

1. Time required for an initiating electron to arrive in the gap.
2. Time required to excite an initiating electron due to irradiation, cosmic rays or sun rays.
3. Time required for the dust particle to force in the gap and get charged.

Factors Affecting on Time of Formation ( t_f )

1. Time required for the collision of an initiating electron with gas molecule.
2. ‘α’ which is Townsend’s primary ionization coefficient.
3. ‘γ‘ which is Townsend’s secondary ionization coefficient.

Break Down in Electronegative Gases 

1. Gases which are lacking in one or two electrons in their outer shell are known as electronegative gases. Typical examples are SF₆^, Freon, CO₂, halogens (CI, Br, 1).
2. These can accept free electron and become stable negative ion.
3. This process is known as direct attachment. This process is reversible i.e. the captured electron can be released by absorption of photon.
4. Direct attachment process can be described by following equation AB + e → AB^– + hv
5. In other case, the gas molecules are splitted into parts and electronegative atoms are formed which in deed forms negative ion. This process is known as dissociative attachment.
6. Dissociative attachment can be described by following equation. AB + e → A⁺ B^– + e
7. In above equations, A is carbon or sulphur atom and B is halogen or oxygen atom. The change in energy is demoted by hv.
8. The Townsend current growth equation for describing these processes is as below.