The Townsend discharge or Townsend avalanche is a gas ionisation process where free electrons are accelerated by an electric field, collide with gas molecules, and consequently free additional electrons. Those electrons are in turn accelerated and free additional electrons. The result is an avalanche multiplication that permits electrical conduction through the gas. The discharge requires a source of free electrons and a significant electric field; without both, the phenomenon does not occur.
The Townsend discharge is named after John Sealy Townsend, who discovered the fundamental ionisation mechanism by his work circa 1897 at the Cavendish Laboratory, Cambridge.
https://en.wikipedia.org/wiki/Townsend_discharge
The multipactor effect is a phenomenon in radio-frequency (RF) amplifier vacuum tubes and waveguides, where, under certain conditions, secondary electron emission in resonance with an alternating electric field leads to exponential electron multiplication, possibly damaging and even destroying the RF device.
Description[edit]
The multipactor effect occurs when electrons accelerated by radio-frequency (RF) fields are self-sustained in a vacuum (or near vacuum) via an electron avalanche caused by secondary electron emission. The impact of an electron to a surface can, depending on its energy and angle, release one or more secondary electrons into the vacuum. These electrons can then be accelerated by the RF fields and impact with the same or another surface. Should the impact energies, number of electrons released, and timing of the impacts be such that a sustained multiplication of the number of electrons occurs, the phenomenon can grow exponentially and may lead to operational problems of the RF system such as damage of RF components or loss or distortion of the RF signal.
Mechanism[edit]
The mechanism of multipactor depends on the orientation of an RF electric field with respect to the surface. There are two types of multipactor: two-surface multipactor on metals and single-surface multipactor on dielectrics.
Two-surface multipactor on metals[edit]
This is a multipactor effect that occurs in the gap between metallic electrodes. Often, an RF electric field is normal to the surface. A resonance between electron flight time and RF field cycle is a mechanism for multipactor development.
The existence of multipactor is dependent on the following three conditions being met: The average number of electrons released is greater than or equal to one per incident electron (this is dependent on the secondary electron yield of the surface) and the time taken by the electron to travel from the surface from which it was released to the surface it impacts with is an integer multiple of one half of the RF period and the average secondary electron yield is greater than or equal to one.
Single-surface multipactor on dielectrics[edit]
There is a multipactor effect that occurs on a dielectric surface. Often, an RF electric field is parallel to the surface. The positive charge accumulated on the dielectric surface attracts electrons back to the surface. A single-surface multipactor event is also possible on a metallic surface in the presence of a crossed static magnetic field.
Frequency-gap product in two-surface multipactor[edit]
The conditions under which multipactor will occur in two surface multipactor can be described by a quantity called the frequency-gap product. Consider a two surface setup with the following definitions:
- , distance or gap between the surfaces
- , angular frequency of the RF field
- , peak plate-to-plate RF voltage
- , peak electric field between the surfaces, equal to /.
The RF voltage varies sinusoidally. Consider the time at which the voltage at electrode A passes through 0 and starts to become negative. Assuming that there is at least 1 free electron near A, that electron will begin to accelerate to the right toward electrode B. It will continue to accelerate and reach a maximum velocity ½ of a cycle later just as the voltage at electrode B begins to become negative. If the electron(s) from electrode A strike electrode B at this time and produce additional free electrons, these new free electrons will begin to accelerate toward electrode A. The process may then repeat causing multipactor. We now find the relationship between the plate spacing, RF frequency, and RF voltage that causes the strongest multipactor resonance.
Consider a point in time at which electrons have just collided with electrode A at position -d/2. The electric field is at zero and is beginning to point to the left so that the newly freed electrons are accelerated toward the right. Newton's equation of motion of the free electrons is
The solution to this differential equation is
where we’ve assumed that when the electrons initially leave the electrode they have zero velocity. We know that resonance happens if the electrons arrive at the rightmost electrode after one half of the period of the RF field, . Plugging this into our solution for we get
Rearranging and using the frequency instead of the angular frequency gives
- .
The product is called the frequency-gap product. Keep in mind that this equation is a criterion for greatest amount of resonance, but multipactor can still occur when this equation is not satisfied.
History[edit]
This phenomenon was first observed by the French physicist Camille Gutton, in 1924, at Nancy.
Multipactor was identified and studied in 1934 by Philo T. Farnsworth, the inventor of electronic television, who attempted to take advantage of it as an amplifier. More commonly nowadays, it has become an obstacle to be avoided for normal operation of particle accelerators, vacuum electronics, radars, satellite communication devices, and so forth. A novel form of multipactor has been proposed (Kishek, 1998), and subsequently experimentally observed, in which charging of a dielectric surface considerably changes the dynamics of the multipactor discharge.
References[edit]
- ^ Romanov, Gennady (2011). "Update on Multipactor in Coaxial Waveguides Using CST Particle Studio" (PDF). Proceedings of 2011 Particle Accelerator Conference: 2.
Simulations of electron multipactor discharge in the coaxial waveguide have been performed using CST Particle Studio, with a primary goal to verify the effect of multi-particle approach combined with advanced probabilistic emission model on the discharge thresholds. Most simulations agree with analytical results and the results from more simplified numerical codes
https://en.wikipedia.org/wiki/Multipactor_effect
A spark chamber is a particle detector: a device used in particle physics for detecting electrically charged particles. They were most widely used as research tools from the 1930s to the 1960s and have since been superseded by other technologies such as drift chambers and silicon detectors. Today, working spark chambers are mostly found in science museums and educational organisations, where they are used to demonstrate aspects of particle physics and astrophysics.
https://en.wikipedia.org/wiki/Spark_chamber
A corona discharge is an electrical discharge caused by the ionization of a fluid such as air surrounding a conductor carrying a high voltage. It represents a local region where the air (or other fluid) has undergone electrical breakdown and become conductive, allowing charge to continuously leak off the conductor into the air. A corona occurs at locations where the strength of the electric field (potential gradient) around a conductor exceeds the dielectric strength of the air. It is often seen as a bluish glow in the air adjacent to pointed metal conductors carrying high voltages, and emits light by the same mechanism as a gas discharge lamp.
In many high voltage applications, corona is an unwanted side effect. Corona discharge from high voltage electric power transmission lines constitutes an economically significant waste of energy for utilities. In high voltage equipment like cathode ray tube televisions, radio transmitters, X-ray machines, and particle accelerators, the current leakage caused by coronas can constitute an unwanted load on the circuit. In the air, coronas generate gases such as ozone (O3) and nitric oxide (NO), and in turn, nitrogen dioxide (NO2), and thus nitric acid (HNO3) if water vapor is present. These gases are corrosive and can degrade and embrittle nearby materials, and are also toxic to humans and the environment.
Corona discharges can often be suppressed by improved insulation, corona rings, and making high voltage electrodes in smooth rounded shapes. However, controlled corona discharges are used in a variety of processes such as air filtration, photocopiers, and ozone generators.
https://en.wikipedia.org/wiki/Corona_discharge
Runaway breakdown is a theory of lightning initiation proposed by Alex Gurevich in 1992.[1][2]
Electrons in air have a mean free path of ~1 cm. Fast electrons which move at a large fraction of the speed of light have a mean free path up to 100 times longer. Given the long free paths, an electric field can accelerate these electrons to energies far higher than that of initially static electrons. If they strike air molecules, more relativistic electrons will be released, creating an avalanche multiplication of "runaway" electrons. This process, relativistic runaway electron avalanche, has been hypothesized to lead to electrical breakdown in thunderstorms, but only when a source of high-energy electrons from a cosmic ray is present to start the "runaway" process.
The resulting conductive plasma trail, many tens of meters long, is suggested to supply the "seed" which triggers a lightning flash.
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