PPT OnTownsends Discharge in Gases

PRESENTED BY :-

RAJESH SAINI EE A2

The Townsend discharge is a gas ionization process where an initially very small amount of free electrons, accelerated by a sufficiently strong electric field, give rise to electrical conduction through a gas by avalanche multiplication.

DEFINITION : TOWNSEND’S DISCHARGE

Townsend discharge is named after John Sealy Townsend, and is also commonly known by practitioners as a "Townsend avalanche".

Avalanche effect between two electrodes

where

• is the current flowing in the device.

• is the photoelectric current generated at the cathode surface

• is the Euler number.

• is the first Townsend ionisation coefficient, expressing the

number of ion pairs generated per unit length by a negative ion

(anion) moving from cathode to anode.

• is the distance between the plates of the device.

Quantitative description of the phenomenon

Gas ionisation caused by motion of positive ions

The first Townsend coefficient ( α ), also known as first Townsend avalanche coefficient is a term used where secondary ionization occurs because the primary ionization electrons gain sufficient energy from the accelerating electric field, or from the original ionizing particle. The coefficient gives the number of secondary electrons produced by primary electron per unit path length.

Townsend, Holst and Oosterhuis also put forward an

alternative hypothesis, considering augmented emission

of electrons by cathode caused by positive ions impact,

introducing Townsend's second ionization coefficient ,

the average number of electrons released from a

surface by an incident positive ion, and working out the

following formula:

Cathode emission caused by impact of ions

Avalanche A Townsend avalanche is a cascade reaction involving electrons in a region with a sufficiently high electric field. This reaction must also occur in a medium that can be ionized, such as air. The positive ion drifts towards the cathode, while the free electron drifts towards the anode of the particular device. It accelerates in the electric field, gaining sufficient energy such that it frees another electron upon collision with another atom/molecule of the medium. The two free electrons then travel together some distance before another collision occurs. The number of electrons travelling towards the anode is multiplied by a factor of two for each collision, so that after n collisions, there are 2n free electrons.

Conditions A Townsend discharge can be sustained over a limited range of gas pressure and electric field intensity. At higher pressures, discharges occur more rapidly than the calculated time for ions to traverse the gap between electrodes, and the streamer theory of spark discharge is applicable. In highly non-uniform electric fields, the corona discharge process is applicable. Discharges in vacuum require vaporization and ionization of electrode atoms. An arc can be initiated without a preliminary Townsend discharge; for example when electrodes touch and are then separated

Applications Avalanche multiplication during Townsend discharge is naturally used in gas phototubes, to amplify the photoelectric charge generated by incident radiation (visible light or not) on the cathode: achievable current is typically 10~20 times greater respect to that generated by vacuum phototubes. The starting of Townsend discharge sets the upper limit to the blocking voltage a glow discharge gas filled tube can withstand : this limit is the Townsend discharge breakdown voltage also called ignition voltage of the tube.

RAJESH SAINI 3RD YEAR 6TH SEM. ROLL NO. – 09MUBEE064 BRANCH – ELECTRICAL ENGINEERING BATCH – A2 SUBJECT – HIGH VOLTAGE ENGINEERING TOPIC – TOWNSEND’S DISCHARGE

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