Krytron

Key takeaways

Krytron belongs to physics; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Krytron to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Krytron from memory before moving on to harder problems.

Reference excerpt

The krytron is a cold-cathode gas-filled tube intended for use as a very high-speed switch, somewhat similar to the thyratron. It consists of a sealed glass tube with four electrodes. A small triggering pulse on the grid electrode switches the tube on, allowing a large current to flow between the cathode and anode electrodes. The vacuum version is called a vacuum krytron, or sprytron. The krytron was one of the earliest developments of the EG&G Corporation.

Description Unlike most other gas switching tubes, the krytron conducts by means of an arc discharge, to handle very high voltages and currents (reaching several kilovolts and several kiloamperes), rather than the low-current glow discharge used in other thyratrons. The krytron is a development of the triggered spark gaps and thyratrons originally developed for radar transmitters during World War II. The gas used in krytrons is hydrogen; noble gases (usually krypton), or a Penning mixture can also be used.

Operation

A krytron has four electrodes. Two are a conventional anode and cathode. One is a keep-alive electrode, placed near the cathode. The keep-alive has a low positive voltage applied, which causes a small area of gas to ionize near the cathode. High voltage is applied to the anode, but primary conduction does not occur until a positive pulse is applied to the trigger electrode ("Grid" in the image above). Once started, arc conduction carries a considerable current. The fourth is a control grid, usually wrapped around the anode, except for a small opening on its top. 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 radiation source serves to increase the reliability of ignition and formation of the keep-alive electrode discharge. The gas filling provides ions for neutralizing the space charge and allowing high currents at lower voltage. The keep-alive discharge populates the gas with ions, forming a preionized plasma. This can shorten the arc formation time by 3–4 orders of magnitude in comparison with non-preionized tubes, as time does not have to be spent on ionizing the medium during formation of the arc path. The electric arc is self-sustaining. 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). Krytrons and sprytrons are triggered by a high voltage from a capacitor discharge via a trigger transformer, in a similar way flashtubes for e.g. photoflash applications are triggered. Devices integrating a krytron with a trigger transformer are available.

Sprytron A sprytron, also known as vacuum krytron or triggered vacuum switch (TVS), is a vacuum, rather than a gas-filled, version. It is designed for use in environments with high levels of ionizing radiation, which might trigger a gas-filled krytron spuriously. It is also more immune to electromagnetic interference than gas-filled tubes. Sprytrons lack the keep alive electrode and the preionization radioactive source. The trigger pulse must be stronger than for a krytron. Sprytrons are able to handle higher currents. Krytrons tend to be used for triggering a secondary switch, e.g., a triggered spark gap, while sprytrons are usually connected directly to the load. 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. An arc first forms between the cathode and the grid, then a breakdown occurs between the cathode–grid conductive region and the anode. Sprytrons are evacuated to hard vacuum, typically 0.001 Pa. As kovar and other metals are somewhat permeable to hydrogen, especially during the 600 °C bake-out before evacuation and sealing, all external metal surfaces must be plated with a thick (25 microns or more) layer of soft gold. The same metallization is used for other switch tubes as well. Sprytrons are often designed similar to trigatrons, with the trigger electrode coaxial to the cathode. In one design the trigger electrode is formed as metallization on the inner surface of an alumina tube. The trigger pulse causes surface flashover, which liberates electrons and vaporized surface discharge material into the inter-electrode gap, which facilitates formation of a vacuum arc, closing the switch. The short switching time suggests electrons from the trigger discharge and the corresponding secondary electrons knocked from the anode as the initiation of the switching operation; the vaporized material travels too slowly through the gap to play significant role. The repeatability of the triggering can be improved by special coating of the surface between the trigger electrode and the cathode, and the jitter can be improved by doping the trigger substrate and modifying the trigger probe structures. Sprytrons can degrade in storage, by outgassing from their components, diffusion of gases (especially hydrogen) through the metal components, and gas leaks through the hermetic seals. An example tube manufactured with internal pressure of 0.001 Pa will exhibit spontaneous gap breakdowns when the pressure inside rises to 1 Pa. Accelerated testing of storage life can be done by storing in increased ambient pressure, optionally with added helium for leak testing, and increased temperature storage (150 °C) for outgassing testing. Sprytrons can be made miniaturized and rugged. Sprytrons can be also triggered by a laser pulse. In 1999 the laser pulse energy needed to trigger a sprytron was reduced to 10 microjoules. Sprytrons are usually manufactured as rugged metal/ceramic parts. They typically have low inductance (10 nanohenries) and low electrical resistance when switched on (10–30 milliohms). After triggering, just before the sprytron switches fully on in avalanche mode, it briefly becomes slightly conductive (carrying 100–200 amperes); high-power MOSFET transistors operating in avalanche mode show similar behavior. SPICE models for sprytrons are available.

… excerpt ends here. Continue reading the full article.

Illustrations

Worked examples

Example 1 — a first encounter with Krytron

Start with the simplest possible case. Write down what Krytron claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In physics, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Krytron before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Krytron ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Krytron

In research: Krytron appears in physics research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Krytron in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Krytron is common in secondary-school and first-year university syllabi. It links to neighbouring topics Gas-filled tubes, Nuclear weapons, Pulsed power, so understanding it makes those chapters shorter.
In everyday life: Look for Krytron outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

How to study Krytron in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Krytron means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Krytron out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Krytron in simple terms?

The krytron is a cold-cathode gas-filled tube intended for use as a very high-speed switch, somewhat similar to the thyratron. It consists of a sealed glass tube with four electrodes.

Why does Krytron matter?

Because it connects several physics ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Krytron?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Krytron.