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. Because of the gas fill, thyratrons can handle much greater currents than similar hard vacuum valves/tubes since the positive ions carry considerable current. Gases used include mercury vapor, xenon, neon, and (in special high-voltage applications or applications requiring very short switching times) hydrogen.[1] Unlike a vacuum tube, a thyratron cannot be used to amplify signals linearly. Source Wiki.

The movie shows 3 of my thyratrons, while being energized. The 714 (Mercury Vapor and Gas), the 309CE/FG-17 and the 3C31/ELC1B. Here are the datasheets:
714_7021
309CE/FG17
3C31/ELC1B-A
The first two contain mercury. If stored or unused for a long period, the mercury can deposit on the internal walls and components. Extra care must be taken when warming such a tube, to avoid short circuits inside it. Also it is a must to first power the filament and then apply the switching voltage .

The thyratron is a gas-filled discharge chamber that contains a cathode filament, an anode plate, and one or more grids. An inert gas or metal vapour fills the discharge chamber. The grid controls only the starting of a current and thus provides a trigger effect. The normal grid potential is negative with respect to the cathode and prevents electrons from flowing to the plate and exciting a discharge. To cause a discharge, the grid potential is raised enough to start electrons flowing from the cathode. As free electrons stream toward the plate, they collide with gas molecules, freeing other electrons and ionizing the gas within the discharge chamber. When a sufficient number of ions and electrons are present, a “short” occurs, and a large current flows from the cathode to the plate, causing a discharge. The discharge can take place in a few hundred-millionths of a second. The discharge stops when the anode voltage has been sufficiently lowered. The grid can be used to initiate a discharge but can't be used to terminate one. After a discharge, a thyratron has a deionization time, needed for the ions to revert back to the neutral state. While ionized the Thyratron is conductive, so it will trigger even without a grid potential. For Mercury vapor thyratrons, this time is around 1mili-second, and for hydrogen thyratrons this gets a lot lower. Only by removing the plate potential or reducing it to the point where the electrons do not have enough energy to produce ionization will tube conduction and the production of positive ions stop. Only after the production of positive ions is stopped will the grid be able to regain control.

Thyratron tubes are used typically in radar pulse modulators, particle accelerators, lasers, and high-voltage medical equipment.

Here is a Radar Modulator schematic, using a Hydrogen Thyratron:

More details available here.

It is also possible to use a Thyratron as an oscillator in a Tesla Coil:

This is only a test Thyratron Tesla coil device and needs further improvements. I used the following:
- high voltage DC supply (using a 100W flyback transformer, and a Royer push and pull oscillator), providing 8KV 10mA.
- rewound mot supply , for tube filament
- a ferrite choke for charging the PFN
- a pulse forming network, as presented here.
- a hydrogen thyratron, TGI2 400/16
- a 200V pulse generator, more details here.
The oscillator itself is described here.

Here is another Tesla coil switched by a Thyratron, actually this is the first design I've ever seen on the Internet. Mine would be the second :)

Construction details (in german) here.

Some pictures showing a the 714 being energized:

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