Summary:
The mechanism of the electric spark
by: Leonard B Loeb, Professor of Physics ,University of
California at Berkeley; and John M Meek, Research Engineer;
Publisher: Stanford University, Calif., Stanford University Press, 1941.
"This was the first real monograph you could get hold of on
sparks." (IEEE)
Preface Comment
Sent: Friday, June 04, 2004 6:30 PM
Subject: Mechanism of the Electric Spark, Pupoff and
Mark McKay
Dear Sterling,
Attached is the summary of
" The Mechanism of The Electric Spark" by
Loeb and Meek.
The work of Loeb and Meek
basically states that in a spark gap
exposed to open air consisting of a simple cathode and
anode,
there is a huge current gain due to avalanche. The work of
Loeb and
Meek is significant for two reasons; 1)During the
ionization of
air molecules into positive ions and negative ions, (free
electrons)
a quanta of EMR is required to liberate each electron.
When the free electrons are absorbed by the high voltage
anode
the free electrons become bound again, and give up a
quanta
of electromagnetic radiation. Because of the huge increase
in
free electrons developed in avalanche the amount of
EMR given
off by the high voltage anode is in the range of
hundred to thousand
fold increase. Thus the huge "Radiant Event".
2) In both circuits the current from avalanche is recycled
back to the
primary battery and the secondary battery. Please look at
the circuit and
trace the path. Thus a never ending supply of current.
Loeb and Meek were
professors of physics at Berkeley and
their work is well established by many test.
When Mark [Mckay] and
I were first doing research on the Gray Circuit
we (especially he) kept blowing up components. It was a
year before
I read the rare book by Loeb and Meek that explained why
the
components were blowing up. There was a current gain of
500%
with the open air, high voltage spark gap.
The use of steel for
the high voltage anode is also critical to
a large 'Radiant Event'. I (believe) that the magnetic
properties of
steel play an important role in the release of this energy.
Keep up the good work!
Gary Magratten
'The Mechanism of The Electric Spark'
by Loeb and Meek
A Summary
The Generation of Electric Power by High Voltage Avalanche
Also available in PDF with
formatting from www.faraday.ru "Excessive Output by Means of Air
Ionization: The Mechanism of the Electric Spark"
Dedicated to:
Professor J. S. Townsend
Whose pioneer research and theory laid the whole foundation for the study of the
mechanism of the electrical spark discharge.
Preface
Although the electric spark has been known to
mankind in its various manifestations from time immemorial, its mechanism has to
date been little understood. The initial clarification of the mechanisms
involved is due to J.S. Townsend as a result of his brilliant researches in the
early nineteen hundreds. On the basis of his theory of ionization by
collision by electrons and positive ions, the fundamental mechanisms active and
especially the coefficients required in their application were made available.
In 1936 the present senior author was forced to
describe the mechanism of spark discharge in terms of a modified but distinctly
unsatisfactory Townsend theory. In 1935 the discovery of photo-ionization in air
by corona discharge indicated a solution was not far off. The turning point of a
more successful theory came in the discovery of streamers in positive point to
plane corona in 1936. The quantitative analysis of the self-propagating positive
streamer in all breakdown phenomena became clearly evident as a result of the
data concerning electron avalanches. As a result a qualitative mechanism of
sparking by streamer propagation from anode to cathode functioning by means of
photo-ionization in the gas was established.
The Townsend Sparking Criteria
It will not be necessary here to derive the famous
equation of Townsend for the current [i] in a gap between electrodes as a
function of the photoelectric current [io] from the cathode, the gap length [x]
and the coefficients [a] and [B]. For this the reader can go to any standard
text.
(a-B)x
i=io(a-B)e
----------------
(a-B)x
a-Be
In this equation the first Townsend coefficient [a]
represents the number of new electrons created in the gas by an intial electron
in its advance of 1 cm along the field axis from the cathode.
The second Townsend coefficient [B] in Townsend's
original theory was the number of new electrons created by a single positive ion
in its advance of 1 cm along the field from the anode.
The quantity [a] has been extensively studied in
various gases. It varies with the ratio of field strength to pressure, X/p,
where [X] is in volts per centimeter and [p] is in millimeters of Hg.
note: the reason we are going through this is to determine
the actual increase in current provided by the spark gap,
and thus be able to design the circuit to avoid
blowing out semiconductor components.
It also provides a sound and already proven scientific
theory to work from giving us a good foundation and the
confidence to proceed with technical design work.
The quantity [B] has been evaluated, albeit rather
inaccurately, from the
variations of [i] with [x] at various higher values of X/p, by many observers in
different gases. Inasmuch as it has now been shown that there are numerous
other mechanisms other than impact with positive ions which can liberate
the secondary electron needed in discharge.
There has been an inclination to give up the mechanism
of impact
ionizations by positive ions in gas. The discovery of measurable photoelectric
ionization in gas has now made it possible to explain such cases.
The exact way in which photo-ionization in the gas could operate to cause a
spark,
was not clear until the development of the present streamer theory.
Two of these equations are given below, together with
Townsend's original
equation for comparison.
1)
(a-B)x
i=io (a-B)e
__________
(a-B)x
a-Be
________________________________________________________________
2)
ax
i = io
e
_________________
ax
1-y(e -1)
____________________________________________________________
3)
ax
i= io
e
___________________
(a-u)x
a- nQg [
e -1]
Equation 2 is one for the liberation of secondary
electrons at the cathode by positive ion bombardment. I this equation [y] is the
chance that a positive -ion will liberate an electron from the cathode on
impact.
Equation 3 is the equation for liberation of electrons
by photoelectric action at the cathode.; [Q] is the number of photons created
per centimeter path of advance of an intial electron from the cathode. [g]
is a geometrical factor .5. which depends on the fraction of photons reaching
the cathode. [n] is the fraction of the photons reaching the cathode that
succeed in actually liberating electrons from the cathode so they do not diffuse
back. [u] is the absorption coefficient of the photons in the gas.
The Streamer Theory of Spark Discharge
Anode Space-Charge Field Due to an Avalanche
Assume a spark gap of 1 cm in length. Assume that in
air at atmospheric pressure the potential across the plates is 31,600 volts,
which is the conventionally observed sparking potential [Vs].
Let us then calculate what happens in the field to one
of those electrons. It starts across the gap, quickly acquiring an average
random energy of some E=1/2mC2= 3.6 electron volts and a drift velocity [v] in
the field direction of about 1.5 to 2 times 10(7) centimeters per second. As it
moves it creates new electrons at a rate of [a] per centimeters in the field
direction so that in a distance [x] it and its progeny amount to e(ax)
electrons, forming what is called an electron avalanche.
Therefore, e(ax) positive ions have been left behind by
the electron group, virtually where they were formed in the 10(-7) second of
advance for the electrons in the distance x=q across the plates. As the electron
avalanche advances, its tip is spreading laterally by the random diffusive
movement of the electrons.
From these data it is possible to compute the density
of positive-ion space charge left behind at any point [x]. The value of
[a] under these conditions is about 17, making e(aq)=e(17). The first ion pair
is created at .0407 cm from the cathode. At .5 cm from the cathode there are
4914 ions, at .75 cm there are 3.66 times 10(5) ions, and within .0407 cm from
the anode there are 1.2 times 10(7) ions. Most electrons will be drawn to the
anode except for some few that are bound by the positive ions, making a sort of
a conducting discharge plasma in the avalanche.
Such a distribution of ions does not make a conducting
filament of charges across the gap, and hence in itself an avalanche that has
crossed does not constitute a breakdown of the gap. Thus one must look
further for the mechanism of the spark.
If Loeb and Meek are correct then if we assume
a spark gap of 3 mm and a voltage of 5,000 volts
there are roughly 2,000 electrons created by avalanche
for every one electron leaving the cathode.
They state that most of these 'free electrons' are
absorbed by the anode.
[This would certainly explain why the semiconductor components
can not handle the current gain.]
NOTE: Loeb and Meek make little reference to initial amperage.
There are only two values they refer to 10(-5) ampere and 10(-12)
ampere.
In conclusion: Sparks and Arcs are two different beast. My initial research
into the amperage necessary to form an arc does not apply to spark and the
process of avalanche where this huge gain mechanism is possible.
The Generation of Electric Power by High Voltage
Avalanche pg.6
Photoelectric Ionization in Gas
as a Secondary Mechanism
Accompanying the cumulative ionization there is
produced by electrons from
four to ten times as many excited atoms and molecules. Some are excited to
an energy exceeding the ionizing potential of some of the atoms and molecules
present, either by excitation of an inner shell, by ionization and excitation,
or
in a mixed gas like air by the excitation of molecules of higher ionizating
potential, e.g., N2. These excited atoms or molecules emit radiations of
very short wave length in some 10(-8) second. This short ultraviolet radiation
is highly absorbed in the gas and leads to ionization of the gas. In fact,
the whole gas and the cathode as well are subjected to a shower of photons
of all energies traveling from the region of dense ionization with the velocity
of light. Thus nearly instantaneously in the whole gap and from the cathode
new photoelectrons are liberated which almost at once begin to ionize
cumulatively.
The Mechanism of
Positive Streamer Formation
The photoelectrons created at points in the gas and
at the cathode at any
great radial distance from the avalanche axis will merely create other
avalanches.
Those in the gas will be short and those coming from the cathode region will be
long and like that of the initial avalanche. Being smaller and, in any case,
later
in creation than the parent avalanche, such avalanches will be of no interest
in breakdown. However, those photoelectrons created near the spave-charge
channel of positive ions, and especially near the anode, will be in an enhanced
field which exerts a directive action drawing them into itself. If the
space-charge
field [X1] is in the order of magnitude of the imposed field [X], this action
will
be very effective. In addition the values of [a] will be much enhanced.
The electrons from the intense cumulative ionization of
such
photoelectron avalanches in the combined fields [X] and [X1] which
are drawn into the positive space charge feed into it, making it a
conducting PLASMA which starts at the anode. The added fields will
be most effective along [X] and so will the ionization. The positive ions
they leave behind will therefore extend the space charge towards the cathode.
These electrons also create photons which produce electrons to continue
this process. In this fashion the positive space charge develops
toward the cathode from the anode as a self-propagating positive
space-charge streamer.
The Generation of Electric Power by High Voltage
Avalanche pg.7
As the streamer advances towards the cathode it
produces a filamentary
region of intense space-charge distortion along a line parallel to the field.
The conducting streamer of a plasma consisting of electrons and ions
extending to the anode thus makes a very steep gradient at the cathode end
of the streamer tip. As this advances toward the cathode the photoelectron
avalanches produced by radiation at the cathode, especially at the intercept
of the extended streamer axis at the cathode, begins to produce an intense
ionization near the cathode. Hence the positive ions created there may increase
the secondary emission. Thus, as the space-charge streamer approaches the
cathode, a cathode spot is forming which may become a source of visible
light. When the streamer reaches the cathode there is a conducting filament
bridging the gap. As the streamer tip reaches the cathode the high field
produces a rush of electrons towards the end of the streamer. This if followed
by a current of electrons, gives a high-potential wave which passes up the
preionized conducting channel to the anode, multiplying the electrons
present by a large factor. The channel is thus rendered highly conducting.
If the metal can emit a copious supply of electrons because of the
formation of an efficient cathode spot, the current of electrons continues the
channel maintaining its high conductivity and ever increasing in it.
This current , unless limited by external resistance, will then develop into
an arc. It is, however, the intense increase in ionization by the potential
wave which gives the highly conducting channel characterizing the spark.
Page posted by SDA,
June 4, 2004
Last updated June 04, 2004
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