Triggered spark gaps are very popular as fast high current switches. With proper design, a triggered spark gap can switch megawatts of power in a few microseconds, with jitters of less than a nanosecond. These devices make use of the very low impedance of an arc once the arc is established. Two electrodes are separated by sufficient distance that the gap doesn't spontaneously break down. The breakdown is initiated by a variety of means: UV irradiation from another spark or a laser, an overvoltage pulse, or reducing the gas pressure in the gap.
There is a lot of similarity to a xenon flash tube here. In a triggered spark gap, the idea is to switch a lot of current at high voltage, so the arc characteristics are optimized for that. In a flash tube, the idea is to generate light, so the design is optimized for that (i.e. use xenon (high lumens/watt), a long path (limit watts/volume), etc).
Some triggered gaps operate in a vacuum, for instance the GPV series sold by EG&G (now part of Perkin-Elmer). Others use a gas filling which breaks down, as in those sold by Maxwell Labs (and others sold by EG&G, and others). Both sealed and unsealed gaps are made.
Triggered spark gaps should be distinguished from other arc conduction devices in that they are bidirectional. Other devices like thyratrons and ignitrons have a preferred direction of current flow.
Field distortion - The voltage on an auxiliary electrode (typically between the main gap electrodes) causes a local increase in the E field strength above the breakdown field. This technique is often combined with the swinging cascade design.
Swinging Cascade -
Irradiation - A spark across an auxiliary spark gap near (or within) the main gap produces UV light which ionizes the air in the gap or causes the emission of photoelectrons from the main gap electrodes, either of which reduces the breakdown voltage below the impressed voltage. Laser triggered spark gaps use a pulsed laser instead of the auxiliary spark gap. The "Trigatron" developed by Meek, et. al., is an implementation of this technique.
Overvoltage - The voltage across the gap is momentarily raised above the breakdown voltage. This method is used in triggering high power xenon flash lamps by the "series" technique. A not so obvious approach to overvoltaging a gap is by reducing the pressure of the gas in the gap.
In a typical triggered gap, there are three terminals. Two are the main current carrying terminals and the third is the trigger. There are 4 basic combinations of the polarities of the voltage being switched and the trigger pulse, and they all result in somewhat different performance of the gap. If the gap is symmetrical (like a 3 sphere gap), some of the combinations may be equivalent.
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Früngel describes a triggered version of a quenched gap. Typically these are made with a number (i.e. 20) of disks, often copper, stacked with intervening insulating spacers (often mica). The gaps are very small, and there is a large thermal mass to conduct the heat away. Hydrogen is usually the working gas.
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revised 11 Jan 2000, 17 March 2001