Pseudospark switch
Encyclopedia
The pseudospark switch, also known as a cold-cathode
thyratron
due to the similarities with regular thyratrons, is a gas-filled tube
capable of high speed switch
ing. Advantages of pseudospark switches include the ability to carry reverse currents (up to 100%), low pulse, high lifetime, and a high current rise of about 1012 A
/sec. In addition, since the cathode is not heated prior to switching, the standby power
is approximately one order of magnitude
lower than in thyratrons. However, pseudospark switches have undesired plasma phenomena at low peak currents. Issues such as current quenching, chopping, and impedance
fluctuations occur at currents less than 2-3 kA while at very high peak currents (20-30 kA) a transition to a metal vapor arc occurs which leads to erosion
of the electrode
s. Pseudospark switches are functionally similar to triggered spark gaps.
) is contained between the electrodes.
While a pseudospark switch is generally fairly simple in construction, engineering a switch for higher lifetimes is more difficult. One method of extending the lifetime is to create a multichannel pseudospark switch to distribute the current and as a result, decrease the erosion. Another method is to simply use cathode materials more resistant to erosion.
Typical electrode materials include copper
, nickel
, tungsten
/rhenium
, molybdenum
, tantalum
, and ceramic
materials. Tantalum, however, cannot be used with hydrogen due to chemical erosion affecting the lifetime adversely. Of the metals, tungsten and molybdenum are commonly used, though molybdenum electrodes show issues with reignition behavior. Several papers which compare electrode materials claim tungsten is the most suitable of the metal electrodes tested. Some ceramic materials such as silicon carbide
and boron carbide
have proven to be excellent electrode materials as well, with lower erosion rates than tungsten in certain cases.
In a pseudospark discharge a breakdown is first triggered between the electrodes by applying a voltage. The gas then breaks down as a function of the pressure, distance, and voltage. An "ionization avalanche
" then occurs producing a homogeneous discharge plasma
confined to the central regions of the electrodes.
In the figure above, the various stages of the pseudospark discharge can be seen. Stage (I) is the triggering or low current phase. The discharges in both stage (II), the hollow cathode phase, and stage (III), the borehole phase, are capable of carrying currents of several hundred amps
. The transition from the borehole phase to the high current phase (IV) is very fast, characterized by a sudden jump in switch impedance. The last phase (V) only occurs for currents of several 10 kA and is unwelcome as it results in high erosion rates.
Cold cathode
A cold cathode is a cathode used within nixie tubes, gas discharge lamps, discharge tubes, and some types of vacuum tube which is not electrically heated by the circuit to which it is connected...
thyratron
Thyratron
A 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...
due to the similarities with regular thyratrons, is a gas-filled tube
Gas-filled tube
A gas-filled tube, also known as a discharge tube, is an arrangement of electrodes in a gas within an insulating, temperature-resistant envelope. Although the envelope is typically glass, power tubes often use ceramics, and military tubes often use glass-lined metal...
capable of high speed switch
Switch
In electronics, a switch is an electrical component that can break an electrical circuit, interrupting the current or diverting it from one conductor to another....
ing. Advantages of pseudospark switches include the ability to carry reverse currents (up to 100%), low pulse, high lifetime, and a high current rise of about 1012 A
Ampere
The ampere , often shortened to amp, is the SI unit of electric current and is one of the seven SI base units. It is named after André-Marie Ampère , French mathematician and physicist, considered the father of electrodynamics...
/sec. In addition, since the cathode is not heated prior to switching, the standby power
Standby power
Standby power, also called vampire power, vampire draw, phantom load, or leaking electricity , refers to the electric power consumed by electronic and electrical appliances while they are switched off Standby power, also called vampire power, vampire draw, phantom load, or leaking electricity...
is approximately one order of magnitude
Order of magnitude
An order of magnitude is the class of scale or magnitude of any amount, where each class contains values of a fixed ratio to the class preceding it. In its most common usage, the amount being scaled is 10 and the scale is the exponent being applied to this amount...
lower than in thyratrons. However, pseudospark switches have undesired plasma phenomena at low peak currents. Issues such as current quenching, chopping, and impedance
Electrical impedance
Electrical impedance, or simply impedance, is the measure of the opposition that an electrical circuit presents to the passage of a current when a voltage is applied. In quantitative terms, it is the complex ratio of the voltage to the current in an alternating current circuit...
fluctuations occur at currents less than 2-3 kA while at very high peak currents (20-30 kA) a transition to a metal vapor arc occurs which leads to erosion
Erosion
Erosion is when materials are removed from the surface and changed into something else. It only works by hydraulic actions and transport of solids in the natural environment, and leads to the deposition of these materials elsewhere...
of the electrode
Electrode
An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit...
s. Pseudospark switches are functionally similar to triggered spark gaps.
Construction
A pseudospark switch's electrodes (cathode and anode) have central holes approximately 3 to 5 mm in diameter. Behind the cathode and anode lie a hollow cathode and hollow anode, respectively. The electrodes are separated by an insulator. A low pressure (less than 50 Pa) "working gas" (typically hydrogenHydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
) is contained between the electrodes.
While a pseudospark switch is generally fairly simple in construction, engineering a switch for higher lifetimes is more difficult. One method of extending the lifetime is to create a multichannel pseudospark switch to distribute the current and as a result, decrease the erosion. Another method is to simply use cathode materials more resistant to erosion.
Typical electrode materials include copper
Copper
Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...
, nickel
Nickel
Nickel is a chemical element with the chemical symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel belongs to the transition metals and is hard and ductile...
, tungsten
Tungsten
Tungsten , also known as wolfram , is a chemical element with the chemical symbol W and atomic number 74.A hard, rare metal under standard conditions when uncombined, tungsten is found naturally on Earth only in chemical compounds. It was identified as a new element in 1781, and first isolated as...
/rhenium
Rhenium
Rhenium is a chemical element with the symbol Re and atomic number 75. It is a silvery-white, heavy, third-row transition metal in group 7 of the periodic table. With an average concentration of 1 part per billion , rhenium is one of the rarest elements in the Earth's crust. The free element has...
, molybdenum
Molybdenum
Molybdenum , is a Group 6 chemical element with the symbol Mo and atomic number 42. The name is from Neo-Latin Molybdaenum, from Ancient Greek , meaning lead, itself proposed as a loanword from Anatolian Luvian and Lydian languages, since its ores were confused with lead ores...
, tantalum
Tantalum
Tantalum is a chemical element with the symbol Ta and atomic number 73. Previously known as tantalium, the name comes from Tantalus, a character in Greek mythology. Tantalum is a rare, hard, blue-gray, lustrous transition metal that is highly corrosion resistant. It is part of the refractory...
, and ceramic
Ceramic
A ceramic is an inorganic, nonmetallic solid prepared by the action of heat and subsequent cooling. Ceramic materials may have a crystalline or partly crystalline structure, or may be amorphous...
materials. Tantalum, however, cannot be used with hydrogen due to chemical erosion affecting the lifetime adversely. Of the metals, tungsten and molybdenum are commonly used, though molybdenum electrodes show issues with reignition behavior. Several papers which compare electrode materials claim tungsten is the most suitable of the metal electrodes tested. Some ceramic materials such as silicon carbide
Silicon carbide
Silicon carbide , also known as carborundum, is a compound of silicon and carbon with chemical formula SiC. It occurs in nature as the extremely rare mineral moissanite. Silicon carbide powder has been mass-produced since 1893 for use as an abrasive...
and boron carbide
Boron carbide
Boron carbide is an extremely hard boron–carbon ceramic material used in tank armor, bulletproof vests, and numerous industrial applications...
have proven to be excellent electrode materials as well, with lower erosion rates than tungsten in certain cases.
The Pseudospark Discharge
Electron avalanche
An electron avalanche is a process in which a number of free electrons in a medium are subjected to strong acceleration by an electric field, ionizing the medium's atoms by collision , thereby forming "new" electrons to undergo the same process in successive cycles...
" then occurs producing a homogeneous discharge 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...
confined to the central regions of the electrodes.
In the figure above, the various stages of the pseudospark discharge can be seen. Stage (I) is the triggering or low current phase. The discharges in both stage (II), the hollow cathode phase, and stage (III), the borehole phase, are capable of carrying currents of several hundred amps
Ampere
The ampere , often shortened to amp, is the SI unit of electric current and is one of the seven SI base units. It is named after André-Marie Ampère , French mathematician and physicist, considered the father of electrodynamics...
. The transition from the borehole phase to the high current phase (IV) is very fast, characterized by a sudden jump in switch impedance. The last phase (V) only occurs for currents of several 10 kA and is unwelcome as it results in high erosion rates.