Townsend discharge
From Wikipedia, the free encyclopedia
Avalanche effect between two electrodes
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: when the number of free charges
drops or the electric field weakens, the phenomena ceases. It is a process characterized by
very low current densities: in common gas filled tubes, typical magnitude of currents
flowing during this process range from about 10
18
A to about 10
5
A, while applied voltages
are almost constant. Subsequent transition to ionisation processes of dark discharge, glow
discharge, and finally to arc discharge are driven by increasing current densities: in all these
discharge regimes, the basic mechanism of conduction is avalanche breakdown. Townsend
discharge is named after John Sealy Townsend, and is also commonly known by
practitioners as a "Townsend avalanche".
Contents
[hide]
1 Quantitative description of the phenomenon
o 1.1 Gas ionisation caused by motion of positive ions
o 1.2 Cathode emission caused by impact of ions
2 Avalanche
3 Conditions
4 Applications
5 See also
6 References
[edit] Quantitative description of the phenomenon
The basic setup of the experiments investigating ionization discharges in gases consist of a
planar parallel plate capacitor filled with a gas and a continuous current high voltage source
connected between its terminals: the terminal at the lower voltage potential is named
cathode while the other is named anode. Forcing the cathode to emit electrons (eg. by
irradiating it with a X-ray source), Townsend found that the current I flowing into the
capacitor depends on the electric field between the plates in such a way that gas ions seems
to multiply as they moved between them. He observed currents varying over ten or more
orders of magnitude while the applied voltage was virtually constant. The experimental
data obtained from his experiments are described by the following formula
where
I is the current flowing in the device,
I
0
is the photoelectric current generated at the cathode surface,
e is the Euler number
n
is the first Townsend ionisation coefficient, expressing the number of ion pairs
generated per unit length (e.g. meter) by a negative ion (anion) moving from
cathode to anode,
d is the distance between the plates of the device.
The almost constant voltage between the plates is equal to the breakdown voltage needed to
create a self-sustaining avalanche: it decreases when the current reaches the glow discharge
regime. Subsequent experiments revealed that the current I rises faster than predicted by the
above formula as the distance d increases: two different effects were considered in order to
explain the physics of the phenomenon and to be able to do a precise quantitative
calculation.
[edit] Gas ionisation caused by motion of positive ions
Townsend put forward the hypothesis that positive ions also produce ion pairs, introducing
a coefficient
p
expressing the number of ion pairs generated per unit length by a positive
ion (cation) moving from anode to cathode. The following formula was found
since
p
< <
n
, in very good agreement with experiments.
The first Townsend coefficient ( ), 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.
[edit] Cathode emission caused by impact of ions
Townsend, Holst and Oosterhuis also put forward an alternative hypothesis, considering
augmented emission of electrons by cathode caused by positive ions impact, introducing
Townsends second ionization coefficient , the average number of electrons released from
a surface by an incident positive ion, and working out the following formula:
These two formulas may be thought as describing limiting cases of the effective behavior of
the process: note that they can be used to well describe the same experimental results. Other
formulas describing, various intermediate behaviors, are found in the literature, particularly
in reference 1 and citations therein.
[edit] 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 2
n
free
electrons.
[edit] 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.
[edit] 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.
Neon lamp/cold-cathode gas diode relaxation oscillator
The presence of Townsend discharge and glow discharge breakdown voltages
shapes the V
A
I
A
characteristic of any gas diode or neon lamp in a way such that it
has a negative differential resistance region of the S-type. This occurrence is
typically used to generate electrical oscillations and waveforms, as in the relaxation
oscillator whose schematic is shown in the picture on the right. The sawtooth
shaped oscillation generated has frequency
where
V
GLOW
is the glow discharge breakdown voltage,
V
TWN
is the Townsend discharge breakdown voltage,
C
1
, R
1
and V
1
are respectively the capacitance, the resistance and the supply
voltage of the circuit.
Since temperature and time stability of the characteristics of gas diodes and neon
lamps is low, and also the statistical dispersion of breakdown voltages is high, the
above formula can only give a qualitative indication of what the real frequency of
oscillation is.
Townsend avalanche discharges are exploited in devices such as Geiger counters
and Proportional counters to detect and measure the energy of an ionizing radiation.
Incoming radiation ionizes one of the atoms or molecules in the medium. When the
electrons reach the anode, a current is induced, which is amplified further with
electronics. This detects and measures some of the characteristics of the incident
ionizing radiation.
