Ignitrons are high-current switches that can open and close very quickly using vaporized-metal plasma arcs to complete a circuit. Mercury is typically used because it does not tend to plate out the internal surfaces of the ignitron structure and cause a short-circuit pathway, permanently closing the switch. Mercury is toxic, making the ignitrons difficult to manufacture, dispose of, and service.
Gallium and its alloys have been considered as alternatives to mercury. Not only are they liquid at typical use temperatures like mercury but they exhibit a lower vapor pressure to withstand even higher voltages. The key obstacle is that using gallium or its alloys in typical ignitron designs results in rapid plating of internal surfaces and premature failure of the switch. NASA Marshall uses a novel internal structure of the ignitron that prevents plating of the liquid metal on inner surfaces.
Ignitrons are capable of conducting high currents and withstanding high voltages, thus providing high, instantaneous power over a very short time. Such pulsed-power applications as pulsed lasers, pulsed fusion, and power rectification are important to NASA.
Ignitron electrical switches have traditionally been used in a number of industrial applications in which the high-speed switching of electrical current under high voltage is needed. Today, many of these applications are served by solid-state, semiconductor-type switches, in part due to the toxicity of the mercury and the lack of suitable alternative designs. On the whole, however, ignitrons can offer much greater durability and reliability over solid-state switch designs and can handle even higher speeds and higher voltages.