The document discusses streamer theory of gas breakdown, which addresses some limitations of Townsend's theory. It explains that streamer theory involves additional mechanisms like photoionization and space charge effects. The total time lag of breakdown has two components - statistical time lag and formative time lag. An avalanche develops across the gap due to ionization, leaving a positive space charge. Secondary avalanches form near the anode due to field enhancement. As the streamer crosses the gap, a conducting channel is formed. Streamer theory predicts faster breakdown times and dependence on pressure/geometry compared to Townsend's theory.

1. CONDUCTION AND
BREAKDOWN IN GASES –
STREAMER THEORY
K. Karthikeyan,
Associate Professor/EEE,
Ramco Institute of Technology

2. TIME LAG FOR BREAKDOWN
 Actually, there is a time difference between the
application of a voltage sufficient to cause breakdown
and the occurrence of breakdown itself. This time
difference is called the time lag.
 The time t which lapses between the application of the
voltage sufficient to cause breakdown and the
appearance of the initiating electron is called a
statistical time lag (ts) of the gap.
 The appearance of electrons is usually statistically
distributed. After the appearance of the electron, a time
tt is required for the ionization processes to develop
fully to cause the breakdown of the gap, and this time
is called the formative time lag (tt).
 The total time ts + tt = t is called the total time lag.
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3. BREAKDOWN IN GASES – STREAMER
THEORY
 Townsend mechanism when applied to breakdown
at atmospheric pressure was found to have certain
drawbacks.
 Current growth occurs as a result of ionization processes
only. But in practice, breakdown voltages were found to
depend on the gas pressure and the geometry of the gap.
 The mechanism predicts time lags of the order of 10-5s,
while in actual practice break-down was observed to
occur at very short times of the order of 10-8s.
 Townsend mechanism predicts a very diffused form of
discharge, in actual practice, discharges were found to
be filamentary and irregular.
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4. BREAKDOWN IN GASES – STREAMER
THEORY
 Streamer theory postulated by Reather and Meak
removes the limitations and drawbacks of Townsend’s
theory.
 According to the Townsend’s theory, electric spark
discharge is due to the ionization of gas molecule by
the collision of electron and release of electron from
cathode due to positive ion bombardment.
 But according to the streamer’s theory, it is not only
ionization process responsible for breakdown of gas
but such as formation of photon and space charge into
the avalanche also are root cause of breakdown.
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5. BREAKDOWN IN GASES – STREAMER
THEORY
 Reather explains the formation of spark due to
streamer mechanism in which the secondary
mechanism of photo ionization of gas molecule is
involved.
 This theory predict the development of a spark
discharge directly from a single avalanche in which the
space charge developed by the avalanche itself is said
to transform the avalanche into a plasma streamer.
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6. BREAKDOWN IN GASES – STREAMER
THEORY
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o A single electron starting at
the cathode by ionization
builds up an avalanche that
crosses the gap.
o The electrons in the
avalanche move very fast
compared with the positive
ions.
o By the time the electrons
reach the anode the positive
ions are in their original
positions and form a positive
space charge at the anode.

7. BREAKDOWN IN GASES – STREAMER
THEORY
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8. BREAKDOWN IN GASES – STREAMER
THEORY
 This enhances the field, and the secondary
avalanches are formed from the few electrons
produced due to photo-ionization in the space
charge region.
 This occurs first near the anode where the space
charge is maximum and a further increase in the
space charge.
 This process is very fast and the positive space
charge extends to the cathode very rapidly resulting in
the formation of a streamer.
 Comparatively narrow luminous tracks occurring at
breakdown at pressures are called streamers. 8

9. BREAKDOWN IN GASES – STREAMER
THEORY
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Instantaneous recombination between positive ions and
electrons releases photons which in turn generate
secondary electrons by the photoionization process.

10. BREAKDOWN IN GASES – STREAMER
THEORY
 When the avalanche in the gap reaches a certain
critical size the combined space charge field and
externally applied field lead to intense ionization and
excitation of the gas particles in front of the avalanche
head.
 These electrons under the influence of the electric field
in the gap develop into secondary avalanches as
shown in Fig.
 Since photons travel with the velocity of light, the
process leads to a rapid development of conduction
channel across the gap.
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11. STREAMER THEORY
• As soon as the streamer tip approaches the cathode,
a cathode spot is formed and a stream of electrons
rush from the cathode to neutralize the positive space
charge in the streamer; the result is a spark, and the
spark breakdown has occurred.
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12. The three successive stages in
the development of the
streamer are shown
diagrammatically in Fig. in
which
(a) shows the stage when
avalanche has crossed the gap,
(b) shows that the streamer has
crossed half the gap length, and
(c) shows that the gap has been
bridged by a conducting
channel.
STREAMER THEORY
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13. TOWNSEND’S VS STREAMER
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Sl.
No.
Townsend’s Theory Streamer’s Theory
1.
Theory states, current growth occurs
as a result of ionization processes
only.
Theory states, breakdown voltages
were bound to depend on gas pressure
and geometry of the gap.
2.
This mechanism predicts a very
diffused form of discharge.
This mechanism predicts discharges
were found to be filamentary irregular.
3.
Theory gives equation to prove
Townsend’s criterion
γ [exp (αd) – 1] = 1
Theory gives equation
αd + ln (α/p) = 14.5 + ln (E/p) + 1/2
ln(d/p)
this tells streamer’s criterion.
4.
Primary and secondary ionization
causes avalanche breakdown.
The space charges fields play an
important role in growth of avalanche in
corona a spark discharges in non-
uniform field gaps.
5.
The mechanism predicts time lags of
order of 10-5 sec.
Breakdown occurs at very short times
of the order of 10-8 sec.
6.
Townsend’s mechanism operates for
pd values below 1000 torr-cm and
gas pressures varying from 0.01 to
350 torr.
Streamer’s mechanism operates for
higher values of pressure and higher
value of pd.

14. PASCHEN'S LAW
 It has been shown earlier that the breakdown criterion
in gases is given as
This equation shows a relationship between V and pd,
and implies that the breakdown voltage varies as the
product pd varies.
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15. PASCHEN'S LAW
• This equation is known
as Paschen's law.
• It is seen that the
relationship between
V and pd is not linear
and has a minimum
value for any gas.
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16. BREAKDOWN IN NON-UNIFORM FIELDS AND
CORONA DISCHARGES
 If the electric field is uniform, a gradual increase in
voltage across a gap produces a breakdown of the gap
in the form of a spark without any preliminary
discharges.
 On the other hand, if the field is non-uniform, an
increase in voltage will first cause a discharge in the gas
to appear at points with highest electric field intensity,
namely at sharp points or where the electrodes are
curved or on transmission lines.
 This form of discharge is called a corona discharge and
can be observed as a bluish luminescence.
 This phenomenon is always accompanied by a hissing
noise, and the air surrounding the corona region
becomes converted into ozone.
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17. THANK YOU
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