Electric discharge in gases
Encyclopedia
Electric discharge in gases occurs when electric current
flows through a gas
eous medium. Depending on several factors, the discharge may radiate visible light.
s:
Glow discharge is facilitated by electrons striking the gas atoms and ionizing them. For formation of glow discharge, the mean free path
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 krytron
s) or as addition to the alloy of the electrodes (e.g. thorium
), 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
, can also be used. Ignition electrodes and keepalive discharge electrodes can also be employed.
The
ratio between 
the electric field
and
the concentration of neutral particles is often used, because the mean energy of electrons (and therefore many other properties of discharge) is a function of 
. 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
, but the Townsend unit (Td) is frequently used.
Electric current
Electric current is a flow of electric charge through a medium.This charge is typically carried by moving electrons in a conductor such as wire...
flows through 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...
eous medium. Depending on several factors, the discharge may radiate visible light.
Discharge types
In cold cathode tubes, the electric discharge in gas has three regions, with distinct current-voltage characteristicCurrent-voltage characteristic
A current–voltage characteristic is a relationship, typically represented as a chart or graph, between an electric current and a corresponding voltage, or potential difference.-In electronics:...
s:
- I: Townsend dischargeTownsend dischargeThe 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...
, below the breakdown voltageBreakdown voltageThe breakdown voltage of an insulator is the minimum voltage that causes a portion of an insulator to become electrically conductive.The breakdown voltage of a diode is the minimum reverse voltage to make the diode conduct in reverse...
. 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 avalancheAvalanche 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...
. In this regime, the current increases from femtoamperes to microamperes 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, 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 stabilizers. 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 tubeNixie tubeA nixie tube is an electronic device for displaying numerals or other information. The glass tube contains a wire-mesh anode and multiple cathodes. In most tubes, the cathodes are shaped like numerals. Applying power to one cathode surrounds it with an orange glow discharge...
s, decatrons, neon lampNeon lampA neon lamp is a miniature gas discharge lamp that typically contains neon gas at a low pressure in a glass capsule. Only a thin region adjacent to the electrodes glows in these lamps, which distinguishes them from the much longer and brighter neon tubes used for signage...
s) 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, ignitronIgnitronAn ignitron is a type of controlled rectifier dating from the 1930s. Invented by Joseph Slepian while employed by Westinghouse, Westinghouse was the original manufacturer and owned trademark rights to the name "Ignitron"....
, thyratronThyratronA thyratron is a type of gas filled tube used as a high energy electrical switch and controlled rectifier. Triode, tetrode and pentode variations of the thyratron have been manufactured in the past, though most are of the triode design...
and krytronKrytronThe krytron is a cold-cathode gas filled tube intended for use as a very high-speed switch, and was one of the earliest developments of the EG&G Corporation. It is somewhat similar to thyratron...
(and its vacuum tubeVacuum tubeIn electronics, a vacuum tube, electron tube , or thermionic valve , reduced to simply "tube" or "valve" in everyday parlance, is a device that relies on the flow of electric current through a vacuum...
derivate, sprytron, using vacuum arcVacuum arcA vacuum arc can arise when the surfaces of metal electrodes in contact with a good vacuum begin to emit electrons either through heating or via an electric field that is sufficient to cause field electron emission...
), high-power mercury-arc valves and high-power light sources, e.g. mercury-vapor lampMercury-vapor lampA mercury-vapor lamp is a gas discharge lamp that uses an electric arc through vaporized mercury to produce light. The arc discharge is generally confined to a small fused quartz arc tube mounted within a larger borosilicate glass bulb...
s and metal halide lampMetal halide lampMetal-halide lamps, a member of the high-intensity discharge family of lamps, produce high light output for their size, making them a compact, powerful, and efficient light source. By adding rare earth metal salts to the mercury vapor lamp, improved luminous efficacy and light color is obtained...
s, 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
Mean free path
In physics, the mean free path is the average distance covered by a moving particle between successive impacts which modify its direction or energy or other particle properties.-Derivation:...
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
Electrical breakdown
The term electrical breakdown or electric breakdown has several similar but distinctly different meanings. For example, the term can apply to the failure of an electric circuit....
voltage for the glow discharge depends nonlinearly on the product of gas pressure and electrode distance according to Paschen's law
Paschen's law
Paschen's Law, named after Friedrich Paschen, was first stated in 1889. He studied the breakdown voltage of gas between parallel plates as a function of pressure and gap distance. The voltage necessary to arc across the gap decreased up to a point as the pressure was reduced. It then increased,...
. 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 krytron
Krytron
The krytron is a cold-cathode gas filled tube intended for use as a very high-speed switch, and was one of the earliest developments of the EG&G Corporation. It is somewhat similar to thyratron...
s) or as addition to the alloy of the electrodes (e.g. thorium
Thorium
Thorium is a natural radioactive chemical element with the symbol Th and atomic number 90. It was discovered in 1828 and named after Thor, the Norse god of thunder....
), 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
Krypton 85 is a radioisotope of krypton.It decays into rubidium-85, with a half-life of 10.756 years and a maximum decay energy of 0.687 MeV.Its most common decay is by beta particle emission with maximum energy of 687...
, can also be used. Ignition electrodes and keepalive discharge electrodes can also be employed.
The
ratio between the electric fieldElectric 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...
and
the concentration of neutral particles is often used, because the mean energy of electrons (and therefore many other properties of discharge) is a function of . 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
, but the Townsend unit (Td) is frequently used.