Krytron

The keep-alive has a low positive voltage applied, which causes a small area of gas to ionize near the cathode.

[4] In place of or in addition to the keep-alive electrode some krytrons may contain a tiny amount of radioactive material (usually less than 5 microcuries (180 kBq) of nickel-63), which emits beta particles (high-speed electrons) to make ionization easier.

The gas filling provides ions for neutralizing the space charge and allowing high currents at lower voltage.

Once the tube is triggered, it conducts until the arc is interrupted by the current falling too low for too long (under 10 milliamperes for more than 100 microseconds for the KN22 krytrons).

It is designed for use in environments with high levels of ionizing radiation, which might trigger a gas-filled krytron spuriously.

The trigger pulse has to be much more intense, as there is no preionized gas path for the electric current, and a vacuum arc must form between the cathode and anode.

[9] This design, dating from the late 1940s, is still capable of pulse-power performance that even the most advanced semiconductors (even IGBTs) cannot match easily.

A given krytron tube will give very consistent performance to identical trigger pulses (low jitter).

However, the formation of the keep-alive glow discharge is more sensitive, which necessitates the use of a radioactive source to aid its ignition.

They are best known for their use in igniting exploding-bridgewire and slapper detonators in nuclear weapons, their original application, either directly (sprytrons are usually used for this) or by triggering higher-power spark gap switches.

They are also used to trigger thyratrons, large flashlamps in photocopiers, lasers and scientific apparatus, and for firing ignitors for industrial explosives.

One prominent case was that of Richard Kelly Smyth, who allegedly helped Arnon Milchan smuggle 15 orders of 810 krytrons total to Israel in the early 1980s.

[10] Krytrons and sprytrons handling voltages of 2,500 V and above, currents of 100 A and above, and switching delays of under 10 microseconds are typically suitable for nuclear weapon triggers.

The device in the film was actually a Krytron-Pac, which consisted of a Krytron tube along with a trigger transformer encased in black epoxy.

[12] The krytron, incorrectly called a "kryton", also appeared in the Tom Clancy nuclear terrorism novel The Sum of All Fears.

Optically triggered solid-state switches based on diamond are a potential candidate for krytron replacement.