Thyratron

A thyratron is a type of gas-filled tube used as a high-power electrical switch and controlled rectifier. Thyratrons can handle much greater currents than similar hard-vacuum tubes. Electron multiplication occurs when the gas becomes ionized, producing a phenomenon known as Townsend discharge. Gases used include mercury vapor, xenon, neon, and (in special high-voltage applications or applications requiring very short switching times) hydrogen. Unlike a vacuum tube (valve), a thyratron cannot be used to amplify signals linearly.

In the 1920s, thyratrons were derived from early vacuum tubes such as the UV-200, which contained a small amount of argon gas to increase its sensitivity as a radio signal detector; and the German LRS Relay tube, which also contained argon gas. Gas rectifiers, which predated vacuum tubes, such as the argon-filled General Electric "Tungar bulb" and the Cooper-Hewitt mercury-pool rectifier, also provided an influence. Irving Langmuir and G. S. Meikle of GE are usually cited as the first investigators to study controlled rectification in gas tubes, about 1914. The first commercial thyratrons didn't appear until around 1928.

A solid-state device with similar operating characteristics is the thyristor, also known as the silicon controlled rectifier (SCR). The term "thyristor" was derived from a combination of "thyratron" and "transistor". Since the 1960s thyristors have replaced thyratrons in most low- and medium-power applications.

A typical hot-cathode thyratron uses a heated filament cathode, completely contained within a shield assembly with a control grid on one open side, which faces the plate-shaped anode. In the off situation the voltage on the control grid is negative with respect to the cathode. When positive voltage is applied to the anode, no current flows. When the control electrode is made less negative, electrons from the cathode can travel to the anode because the positive attraction from the anode prevails over the negative repulsion caused by the slightly negative voltage on the control grid. The electrons will ionize the gas by collisions with the gas in the tube, and an avalanche effect results, causing an arc discharge between cathode and anode. The shield prevents ionized current paths that might form within other parts of the tube. The gas in a thyratron is typically at a fraction of the pressure of air at sea level; 15 to 30 millibars (1.5 to 3 kPa) is typical. For a cold-cathode thyratron the trigger voltage on the control grid will typically be positive, and a flash-over from control grid to cathode will initiate the arc discharge in the tube.

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