Krytrons... again! :0)

[Majorana]

Hello, everybody! Just finished watching (for perhaps the 90th time...) "Frantic", it's broadcast at least twice a year, every year... Of course, the scene where Harrison Ford almost falls from the roof and then retrieves the krytron (INTACT!! ) from the broken statue is my favourite. And every time, the usual question comes back to my mind: why is a krytron so special? At the end, it's just another triggered spark gap! There are triggered spark gaps that can handle much higher currents than a krytron, that are not subjected to any special export regulations, ITAR etc... used every day to trigger lamps, spark igniters and all sort of things. As far as I can understand, they can do the same things than krytrons, but without a ton of stamped paper in trail... You zap a krytron, it fires, you zap a triggered spark gap, it fires too... and the latter can be even more sturdy. So why all this fuss around krytrons?? (...apart from being capable of remaining intact after falling on a concrete floor from a roof in Paris... )

 

[Borek]

My understanding was that their specialty is a very precise timing (they are very fast), allowing well synchronized ignition of charges. But I can be completely wrong.

 

[Baluncore]

Krytrons are exceptionally fast switches. They were originally used as the TR switch in radar systems, where one antenna is used for both transmit and receive. The high power transmit pulse causes the krytron to conduct, which "shorts out" the input, protecting the receiver, while the outgoing pulse is being transmitted. That radar application operated with quarter wavelength lines, so the krytron became an RF reflector after it fired, and did not absorb further transmit RF.

The TR switch is a performance critical part of a pulse radar set, hence the need to protect it.

 

[DaveE]

Very fast rise time switches with (sort of) high power. Most have a β emitter to "pre-ionize" the gas. It's a spark gap, but not a "normal" one, they are biased right up to the edge of conduction. We used them in a laser Q-Switch many years ago for a DoD contractor. Now days that's not a great application, although they worked fine for us. They have a short lifetime, are a PITA to drive compared to semiconductors, and are expensive and hard to buy. You won't find them in any lasers now, most favor AO Q-switches not EO.

KN-22 is the most common one (in my experience anyway):
https://lampes-et-tubes.info/rs/EG&G_KRYTRONS.pdf