Electric discharge in gases occurs when
electric current
An electric current is a stream of charged particles, such as electrons or ions, moving through an electrical conductor or space. It is measured as the net rate of flow of electric charge through a surface or into a control volume. The movi ...
flows through a
gaseous medium due to
ionization
Ionization, or Ionisation is the process by which an atom or a molecule acquires a negative or positive charge by gaining or losing electrons, often in conjunction with other chemical changes. The resulting electrically charged atom or molecul ...
of the gas. Depending on several factors, the discharge may radiate visible light. The properties of electric discharges in gases are studied in connection with design of lighting sources and in the design of high voltage electrical equipment.
Discharge types
In
cold cathode tubes, the electric discharge in gas has three regions, with distinct
current–voltage characteristic
A current–voltage characteristic or I–V curve (current–voltage curve) is a relationship, typically represented as a chart or graph, between the electric current through a circuit, device, or material, and the corresponding voltage, or ...
s:
[Reference Data for Engineers: Radio, Electronics, Computers and Communications](_blank)
By Wendy Middleton, Mac E. Van Valkenburg, p. 16-42, Newnes, 2002
*I:
Townsend discharge
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 an ...
, below the
breakdown voltage. At low voltages, the only current is that due to the generation of charge carriers in the gas by cosmic rays or other sources of ionizing radiation. As the applied voltage is increased, the free electrons carrying the current gain enough energy to cause further ionization, causing an
electron avalanche
An electron avalanche is a process in which a number of free electrons in a transmission medium are subjected to strong acceleration by an electric field and subsequently collide with other atoms of the medium, thereby ionizing them (impact ioniza ...
. In this regime, the current increases from femtoamperes to microamperes, i.e. by nine orders of magnitude, for very little further increase in voltage. The voltage-current characteristics begins tapering off near the breakdown voltage and the glow becomes visible.
*II:
glow discharge
A glow discharge is a plasma formed by the passage of electric current through a gas. It is often created by applying a voltage between two electrodes in a glass tube containing a low-pressure gas. When the voltage exceeds a value called the st ...
, which occurs once the breakdown voltage is reached. The voltage across the electrodes suddenly drops and the current increases to milliampere range. At lower currents, the voltage across the tube is almost current-independent; this is used in glow discharge
voltage-regulator tubes. At lower currents, the area of the electrodes covered by the glow discharge is proportional to the current. At higher currents the normal glow turns into
abnormal glow, the voltage across the tube gradually increases, and the glow discharge covers more and more of the surface of the electrodes. Low-power switching (glow-discharge thyratrons), voltage stabilization, and lighting applications (e.g.
Nixie tubes,
decatrons,
neon lamps) operate in this region.
*III:
arc discharge, which occurs in the ampere range of the current; the voltage across the tube drops with increasing current. High-current switching tubes, e.g.
triggered spark gap,
ignitron,
thyratron and
krytron (and its
vacuum tube
A vacuum tube, electron tube, valve (British usage), or tube (North America), is a device that controls electric current flow in a high vacuum between electrodes to which an electric potential difference has been applied.
The type known as ...
derivate,
sprytron, using
vacuum arc), high-power
mercury-arc valves and high-power light sources, e.g.
mercury-vapor lamps and
metal halide lamps, operate in this range.
Glow discharge is facilitated by electrons striking the gas atoms and ionizing them. For formation of glow discharge, the
mean free path
In physics, mean free path is the average distance over which a moving particle (such as an atom, a molecule, or a photon) travels before substantially changing its direction or energy (or, in a specific context, other properties), typically as ...
of the electrons has to be reasonably long but shorter than the distance between the electrodes; glow discharges therefore do not readily occur at both too low and too high gas pressures.
The
breakdown voltage for the glow discharge depends nonlinearly on the product of gas pressure and electrode distance according to
Paschen's law. For a certain pressure × distance value, there is a lowest breakdown voltage. The increase of strike voltage for shorter electrode distances is related to too long mean free path of the electrons in comparison with the electrode distance.
A small amount of a radioactive element may be added into the tube, either as a separate piece of material (e.g.
nickel-63 in
krytrons) or as addition to the alloy of the electrodes (e.g.
thorium
Thorium is a weakly radioactive metallic chemical element with the symbol Th and atomic number 90. Thorium is silvery and tarnishes black when it is exposed to air, forming thorium dioxide; it is moderately soft and malleable and has a high ...
), to preionize the gas and increase the reliability of electrical breakdown and glow or arc discharge ignition. A gaseous radioactive isotope, e.g.
krypton-85
Krypton-85 (85Kr) is a radioisotope of krypton.
Krypton-85 has a half-life of 10.756 years and a maximum decay energy of 687 keV. It decays into stable rubidium-85. Its most common decay (99.57%) is by beta particle emission with maximum energy ...
, can also be used. Ignition electrodes and keepalive discharge electrodes can also be employed.
[Handbook of optoelectronics, Volume 1](_blank)
by John Dakin, Robert G. W. Brown, p. 52, CRC Press, 2006
The E/N ratio between the
electric field
An electric field (sometimes E-field) is the physical field that surrounds electrically charged particles and exerts force on all other charged particles in the field, either attracting or repelling them. It also refers to the physical field ...
E and the concentration of neutral particles N is often used, because the mean energy of electrons (and therefore many other properties of discharge) is a function of E/N. Increasing the electric intensity E by some factor q has the same consequences as lowering gas density N by factor q.
Its
SI unit is V·cm
2, but the
Townsend unit (Td) is frequently used.
Application in analog computation
The use of a glow discharge for solution of certain mapping problems was described in 2002.
According to a Nature news article describing the work,
[
] researchers at Imperial College London demonstrated how they built a mini-map that gives tourists luminous route indicators. To make the one-inch London chip, the team etched a plan of the city centre on a glass slide. Fitting a flat lid over the top turned the streets into hollow, connected tubes. They filled these with helium gas, and inserted electrodes at key tourist hubs. When a voltage is applied between two points, electricity naturally runs through the streets along the shortest route from A to B – and the gas glows like a tiny glowing strip light. The approach itself provides a novel visible
analog computing
''ANALOG Computing'' (an acronym for Atari Newsletter And Lots Of Games) was an American computer magazine devoted to the Atari 8-bit family of home computers. It was published from 1981 until 1989. In addition to reviews and tutorials, ''ANAL ...
approach for solving a wide class of maze searching problems based on the properties of lighting up of a glow discharge in a microfluidic chip.
References
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