SACHIN BHATKARIYA

B.Tech 3rd year 6th Sem.

09mubee070, B

Content

Paschen’s law

Streamer Mechanism

Comparison b/w streamer and townsend

Paschen’s Law

Sustained Townsend discharge leads to spark then arc (flashover). Formulated mathematically by Paschen, (see p 52)

The flashover voltage is a function of the product of the gas pressure and the gap length for an uniform field

Implications in practice:

- Altitude effect

- Compressed gases

- Vacuum contactors

Approximation for curve:

0.1

1

10

100

1000

10000

0.001 0.01 0.1 1 10 100

pressure x gap length (cm bar) B

reakd

ow

n V

olt

ag

e (

kV

)

Empirical Formula (Eq. 3.11)

Paschen Equation ( Eq. 3.10)

pdpdVc

36.2472.6

31.1 25.5

Townsend Streamer

pdpdUd 36.2472.6

a) Low pressure(few collisions:low ionization)

b) High pressure(low kinetic energy:low ionization)

c) Medium pressure( optimal: highionization)

+ - + - + -

Paschen’s Law

Low gas density - more kinetic energy gained but less collisions High gas density – more collisions but less energy gained

Streamer discharges

A self-sustaining discharge can develop from a single avalanche

Space charge (ions) distort and enhance field

Photons cause further avalanches in high field regions

Streamer discharges occur if n 5 .108

Occurs for non-uniform long gaps and at high pressures

GASES – FLASHOVER - STREAMER MECHANISM –

MEDIUM GAPS (> 5 Bar.mm)

Cath

ode (

-)

Ano

de (

+)

E - Field

Photons

Avalanche with x = 20

Asymmetrical, non uniform gaps:

The polarity Effect

_ +

_+

Region of high field strength near the sharp point, in both cases

Avalanches are formed in these regions, leaving a positive space charge in this region.

In the case of the positive tip the space charge has the same polarity as the electrode and assists in increaing the field.

In the case of the negative tip the space charge opposes the polarity of the tip.

A lower flashover voltage is thus obtained for the positive tip, compared to the negative one.

Comparison of Townsend vs. Streamer Mechanism