Thyratron

A thyratron is a type of gas-filled tube used as a high-power electronic switch. It is a hot-cathode gas-discharge tube similar to a vacuum tube but containing a low-pressure gas, typically mercury vapor, hydrogen, or a noble gas like xenon. Thyratrons are capable of switching much larger currents and voltages than vacuum tubes, making them suitable for applications such as radar transmitters, high-voltage power supplies, and pulsed power systems.

The key feature of a thyratron is its ability to act as a controlled rectifier. It remains non-conductive until a sufficient voltage is applied to the control grid. Once the grid voltage reaches a critical value (the "grid firing voltage"), the gas ionizes, creating a plasma that allows a large current to flow between the anode and cathode. Unlike a vacuum tube, once the thyratron is triggered, the grid loses control, and the current can only be stopped by reducing the anode voltage below the extinguishing potential, essentially turning the device off by interrupting the anode current. This "latching" behavior is a fundamental characteristic of thyratron operation.

While similar in appearance to ignitrons, thyratrons utilize grid control whereas ignitrons rely on an igniter electrode to initiate conduction. Thyratrons also differ from krytrons, which are also gas-filled switches but operate with much lower currents and faster switching speeds.

Modern solid-state devices, such as silicon-controlled rectifiers (SCRs) and insulated-gate bipolar transistors (IGBTs), have largely replaced thyratrons in many applications due to their higher efficiency, longer lifespan, smaller size, and more precise control capabilities. However, thyratrons remain in use in some specialized high-power, high-voltage applications where their robustness and ability to handle large surge currents are advantageous.