Editing Thyratron (section)

==Description==
[[File:Thyratron Symbols.svg|thumb|alt=Thyratron Symbols|Most commonly used symbols in the US and Europe of a thyratron (variations are usually related to the representation of the filament and the cathode)]]

Thyratrons resemble [[vacuum tube|vacuum tubes]] both in appearance and construction but differ in behavior and operating principle. In a vacuum tube, conduction is dominated by [[Electron|free electrons]] because the distance between [[anode]] and [[cathode]] is small compared to the [[mean free path]] of electrons. A thyratron, on the other hand, is intentionally filled with gas so that the distance between anode and cathode is comparable with the mean free path of electrons. This causes conduction in a thyratron to be dominated by [[Plasma (physics)|plasma]] conductivity. Due to the high conductivity of plasma, a thyratron is capable of switching higher currents than vacuum tubes which are limited by [[space charge]]. A vacuum tube has the advantage that conductivity may be modulated at any time whereas a thyratron becomes filled with plasma and continues to conduct as long as a [[voltage]] exists between the anode and cathode. A [[pseudospark switch]] operates in a similar regime of the [[Paschen's law|Paschen curve]] as a thyratron and is sometimes called a [[cold cathode]] thyratron.

A thyratron consists of a [[hot cathode]], an anode, and one or more [[control grid|control grids]] between the anode and cathode in an airtight glass or ceramic envelope that is filled with gas. The gas is typically [[hydrogen]] or [[deuterium]] at a pressure of 300 to 500&nbsp;m[[Torr]] (40 to 70&nbsp;[[Pascal (unit)|Pa]]). Commercial thyratrons also contain a [[titanium hydride]] reservoir and a reservoir heater that together maintain gas pressure over long periods regardless of gas loss.

Conductivity of a thyratron remains low as long as the control grid is negative relative to the cathode because the grid repels electrons emitted by the cathode. Space charge limited electron current flows from the cathode through the control grid toward the anode if the grid is made positive relative to the cathode. Sufficiently high space charge limited current initiates [[Townsend discharge]] between anode and cathode. The resulting plasma provides high conductivity between anode and cathode and is not limited by space charge. Conductivity remains high until the current between anode and cathode drops to a small value for a sufficiently long time that the gas ceases to be [[ionization|ionized]]. This recovery process takes 25 to 75&nbsp;μ[[second|s]] and limits thyratron repetition rates to a few&nbsp;k[[Hertz|Hz]].
<ref>{{cite book
|title=Gas Discharge Closing Switches
|publisher=Springer Science+Business Media, LLC
|isbn=978-1-4899-2132-1
|date=1990}}</ref>