Electron avalanche
Encyclopedia
An electron avalanche is a process in which a number of free electrons in a medium
(usually a gas
) are subjected to strong acceleration by an electric field
, ion
izing the medium's atoms by collision (called impact ionization
), thereby forming "new" electrons to undergo the same process in successive cycles. This uncontrolled positive feedback loop leads to a type of chain reaction
.
Electron avalanches are essential to the dielectric breakdown process within gases. The process can culminate in corona discharge
s, streamer
s, leader
s, or in a spark
or continuous arc
that completely bridges the gap. The process extends to huge sparks — streamers in lightning
discharges propagate by formation of electron avalanches created in the high potential gradient
ahead of the streamers' advancing tips. Once begun, avalanches are often intensified by the creation of photoelectrons as a result of ultraviolet
radiation emitted by the excited medium's atoms in the aft-tip region.
The process can also be used to detect ionizing radiation that initiates the process, in as much as the passage of a single particles can amplified to large discharges. This is the mechanism of a Geiger counter
and also the visualization possible with a spark chamber and other wire chamber
s.
. If this event occurs within an area that has a high potential gradient
, the positively charged
ion
will be strongly attracted toward, or repelled away from, an electrode
depending on its polarity, whereas the electron will be accelerated
in the opposite direction. Because of the huge mass difference, electrons are accelerated to a much higher velocity
than ions.
High-velocity electrons often collide with neutral atoms inelastically, sometimes ionizing them. In a chain-reaction
— or an 'electron avalanche' — additional electrons recently separated from their positive ions by the strong potential gradient, cause a large cloud of electrons and positive ions to be momentarily generated by just a single initial electron. However, free electrons are easily captured by neutral oxygen or water vapor molecules (so-called electronegative
gases), forming negative ions. In air at STP
, free electrons exist for only about 11 nanosecond
s before being captured. Captured electrons are effectively removed from play — they can no longer contribute to the avalanche process. If electrons are being created at a rate greater than they are being lost to capture, their number rapidly multiplies, a process characterized by exponential growth
. The degree of multiplication that this process can provide is huge, up to several million-fold depending on the situation. The multiplication factor M is given by
Where X1 and X2 are the positions that the multiplication is being measured between, and α is the ionization constant. In other words, one free electron at position X1 will result in M free electrons at position X2. Substituting the voltage gradients into this equation results in
Where V is the applied voltage, VBR is the breakdown voltage and n is an empirically derived value between 2 and 6. As you can see from this formula, the multiplication factor is very highly dependent on the applied voltage, and as the voltage nears the breakdown voltage of the material, the multiplication factor approaches infinity and the limiting factor becomes the availability of charge carriers.
Avalanche sustenance requires a reservoir of charge to sustain the applied voltage, as well as a continual source of triggering events. A number of mechanisms can sustain this process, creating avalanche after avalanche, to create a corona
current. A secondary source of plasma
electrons is required as the electrons are always accelerated by the field in one direction, meaning that avalanches always proceed linearly toward or away from an electrode
. The dominant mechanism for the creation of secondary electrons
depends on the polarity of a plasma. In each case, the energy
emitted as photon
s by the initial avalanche is used to ionise a nearby gas molecule creating another accelerable electron. What differs is the source of this electron. When one or more electron avalanches occur between two electrodes of sufficient size, complete avalanche breakdown
can occur, culminating in an electrical spark
that bridges the gap.
Transmission medium
A transmission medium is a material substance that can propagate energy waves...
(usually a gas
Gas
Gas is one of the three classical states of matter . Near absolute zero, a substance exists as a solid. As heat is added to this substance it melts into a liquid at its melting point , boils into a gas at its boiling point, and if heated high enough would enter a plasma state in which the electrons...
) are subjected to strong acceleration by an electric field
Electric field
In physics, an electric field surrounds electrically charged particles and time-varying magnetic fields. The electric field depicts the force exerted on other electrically charged objects by the electrically charged particle the field is surrounding...
, ion
Ion
An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...
izing the medium's atoms by collision (called impact ionization
Impact ionization
Impact ionization is the process in a material by which one energetic charge carrier can lose energy by the creation of other charge carriers...
), thereby forming "new" electrons to undergo the same process in successive cycles. This uncontrolled positive feedback loop leads to a type of chain reaction
Chain reaction
A chain reaction is a sequence of reactions where a reactive product or by-product causes additional reactions to take place. In a chain reaction, positive feedback leads to a self-amplifying chain of events....
.
Electron avalanches are essential to the dielectric breakdown process within gases. The process can culminate in corona discharge
Corona discharge
In electricity, a corona discharge is an electrical discharge brought on by the ionization of a fluid surrounding a conductor that is electrically energized...
s, streamer
Streamer
Streamer or streamers may refer to:* Pennon, a small pointed flag* Campaign streamer, flag used by military units* Paper streamer* Positive streamer, lightning bolt* Streamer moth, the geometer moth Anticlea derivata...
s, leader
Leader (spark)
A leader is a hot, highly conductive channel of plasma that plays a critical part during dielectric breakdown within in a long electric spark.-Mechanism:...
s, or in a spark
Electric spark
An electric spark is a type of electrostatic discharge that occurs when an electric field creates an ionized electrically conductive channel in air producing a brief emission of light and sound. A spark is formed when the electric field strength exceeds the dielectric field strength of air...
or continuous arc
Electric arc
An electric arc is an electrical breakdown of a gas which produces an ongoing plasma discharge, resulting from a current flowing through normally nonconductive media such as air. A synonym is arc discharge. An arc discharge is characterized by a lower voltage than a glow discharge, and relies on...
that completely bridges the gap. The process extends to huge sparks — streamers in lightning
Lightning
Lightning is an atmospheric electrostatic discharge accompanied by thunder, which typically occurs during thunderstorms, and sometimes during volcanic eruptions or dust storms...
discharges propagate by formation of electron avalanches created in the high potential gradient
Potential gradient
A potential gradient is the local space rate of change of the potential with respect to displacement.In electrostatics then, it is the local space rate of change of the electric potential:Units are volts per meter...
ahead of the streamers' advancing tips. Once begun, avalanches are often intensified by the creation of photoelectrons as a result of ultraviolet
Ultraviolet
Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...
radiation emitted by the excited medium's atoms in the aft-tip region.
The process can also be used to detect ionizing radiation that initiates the process, in as much as the passage of a single particles can amplified to large discharges. This is the mechanism of a Geiger counter
Geiger counter
A Geiger counter, also called a Geiger–Müller counter, is a type of particle detector that measures ionizing radiation. They detect the emission of nuclear radiation: alpha particles, beta particles or gamma rays. A Geiger counter detects radiation by ionization produced in a low-pressure gas in a...
and also the visualization possible with a spark chamber and other wire chamber
Wire chamber
A multi-wire chamber is a detector for particles of ionizing radiation which is an advancement of the concept of the Geiger counter and the proportional counter....
s.
Analysis
A plasma begins with a rare natural 'background' ionization event of a neutral air molecule, perhaps as the result of photoexcitation or background radiationBackground radiation
Background radiation is the ionizing radiation constantly present in the natural environment of the Earth, which is emitted by natural and artificial sources.-Overview:Both Natural and human-made background radiation varies by location....
. If this event occurs within an area that has a high potential gradient
Potential gradient
A potential gradient is the local space rate of change of the potential with respect to displacement.In electrostatics then, it is the local space rate of change of the electric potential:Units are volts per meter...
, the positively charged
Electric charge
Electric charge is a physical property of matter that causes it to experience a force when near other electrically charged matter. Electric charge comes in two types, called positive and negative. Two positively charged substances, or objects, experience a mutual repulsive force, as do two...
ion
Ion
An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...
will be strongly attracted toward, or repelled away from, an electrode
Electrode
An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit...
depending on its polarity, whereas the electron will be accelerated
Acceleration
In physics, acceleration is the rate of change of velocity with time. In one dimension, acceleration is the rate at which something speeds up or slows down. However, since velocity is a vector, acceleration describes the rate of change of both the magnitude and the direction of velocity. ...
in the opposite direction. Because of the huge mass difference, electrons are accelerated to a much higher velocity
Velocity
In physics, velocity is speed in a given direction. Speed describes only how fast an object is moving, whereas velocity gives both the speed and direction of the object's motion. To have a constant velocity, an object must have a constant speed and motion in a constant direction. Constant ...
than ions.
High-velocity electrons often collide with neutral atoms inelastically, sometimes ionizing them. In a chain-reaction
Chain reaction
A chain reaction is a sequence of reactions where a reactive product or by-product causes additional reactions to take place. In a chain reaction, positive feedback leads to a self-amplifying chain of events....
— or an 'electron avalanche' — additional electrons recently separated from their positive ions by the strong potential gradient, cause a large cloud of electrons and positive ions to be momentarily generated by just a single initial electron. However, free electrons are easily captured by neutral oxygen or water vapor molecules (so-called electronegative
Electronegativity
Electronegativity, symbol χ , is a chemical property that describes the tendency of an atom or a functional group to attract electrons towards itself. An atom's electronegativity is affected by both its atomic number and the distance that its valence electrons reside from the charged nucleus...
gases), forming negative ions. In air at STP
Standard conditions for temperature and pressure
Standard condition for temperature and pressure are standard sets of conditions for experimental measurements established to allow comparisons to be made between different sets of data...
, free electrons exist for only about 11 nanosecond
Nanosecond
A nanosecond is one billionth of a second . One nanosecond is to one second as one second is to 31.7 years.The word nanosecond is formed by the prefix nano and the unit second. Its symbol is ns....
s before being captured. Captured electrons are effectively removed from play — they can no longer contribute to the avalanche process. If electrons are being created at a rate greater than they are being lost to capture, their number rapidly multiplies, a process characterized by exponential growth
Exponential growth
Exponential growth occurs when the growth rate of a mathematical function is proportional to the function's current value...
. The degree of multiplication that this process can provide is huge, up to several million-fold depending on the situation. The multiplication factor M is given by
Where X1 and X2 are the positions that the multiplication is being measured between, and α is the ionization constant. In other words, one free electron at position X1 will result in M free electrons at position X2. Substituting the voltage gradients into this equation results in
Where V is the applied voltage, VBR is the breakdown voltage and n is an empirically derived value between 2 and 6. As you can see from this formula, the multiplication factor is very highly dependent on the applied voltage, and as the voltage nears the breakdown voltage of the material, the multiplication factor approaches infinity and the limiting factor becomes the availability of charge carriers.
Avalanche sustenance requires a reservoir of charge to sustain the applied voltage, as well as a continual source of triggering events. A number of mechanisms can sustain this process, creating avalanche after avalanche, to create a corona
Corona discharge
In electricity, a corona discharge is an electrical discharge brought on by the ionization of a fluid surrounding a conductor that is electrically energized...
current. A secondary source of plasma
Plasma (physics)
In physics and chemistry, plasma is a state of matter similar to gas in which a certain portion of the particles are ionized. Heating a gas may ionize its molecules or atoms , thus turning it into a plasma, which contains charged particles: positive ions and negative electrons or ions...
electrons is required as the electrons are always accelerated by the field in one direction, meaning that avalanches always proceed linearly toward or away from an electrode
Electrode
An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit...
. The dominant mechanism for the creation of secondary electrons
Secondary electrons
Secondary electrons are electrons generated as ionization products. They are called 'secondary' because they are generated by other radiation . This radiation can be in the form of ions, electrons, or photons with sufficiently high energy, i.e. exceeding the ionization potential...
depends on the polarity of a plasma. In each case, the energy
Energy
In physics, energy is an indirectly observed quantity. It is often understood as the ability a physical system has to do work on other physical systems...
emitted as photon
Photon
In physics, a photon is an elementary particle, the quantum of the electromagnetic interaction and the basic unit of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force...
s by the initial avalanche is used to ionise a nearby gas molecule creating another accelerable electron. What differs is the source of this electron. When one or more electron avalanches occur between two electrodes of sufficient size, complete avalanche breakdown
Avalanche breakdown
Avalanche breakdown is a phenomenon that can occur in both insulating and semiconducting materials. It is a form of electric current multiplication that can allow very large currents within materials which are otherwise good insulators. It is a type of electron avalanche...
can occur, culminating in an electrical spark
Electrostatic discharge
Electrostatic discharge is a serious issue in solid state electronics, such as integrated circuits. Integrated circuits are made from semiconductor materials such as silicon and insulating materials such as silicon dioxide...
that bridges the gap.
See also
- Avalanche breakdownAvalanche breakdownAvalanche breakdown is a phenomenon that can occur in both insulating and semiconducting materials. It is a form of electric current multiplication that can allow very large currents within materials which are otherwise good insulators. It is a type of electron avalanche...
- Avalanche diodeAvalanche diodeIn electronics, an avalanche diode is a diode that is designed to go through avalanche breakdown at a specified reverse bias voltage. The junction of an avalanche diode is designed to prevent current concentration at hot spots, so that the diode is undamaged by the breakdown...
- Corona dischargeCorona dischargeIn electricity, a corona discharge is an electrical discharge brought on by the ionization of a fluid surrounding a conductor that is electrically energized...
- Multipactor
- Geiger counterGeiger counterA Geiger counter, also called a Geiger–Müller counter, is a type of particle detector that measures ionizing radiation. They detect the emission of nuclear radiation: alpha particles, beta particles or gamma rays. A Geiger counter detects radiation by ionization produced in a low-pressure gas in a...
- spark chamber
- wire chamberWire chamberA multi-wire chamber is a detector for particles of ionizing radiation which is an advancement of the concept of the Geiger counter and the proportional counter....