This is a posting I made on a mailing list at work, that many people told me they enjoyed reading. It is reproduced verbatim here.
Alasdair Mackintosh wrote:
> [power valves]
Please don’t ask why I’m interested in these. (It has to do with time machines.)
The reason all these things work is that mercury gives up electrons easily when heated. Carbon doesn’t, because it’s short on electrons, so even though it’s really hot, it doesn’t emit a space charge when heated. This means that the electrons find it easier to go from the pool of mercury to the carbon electrodes than vice versa.
Photographs don’t really capture the weirdness of a real mercury arc rectifier. The light is flickery and weird; the plasma bends and warps as it tries to wrap itself around the magnetic field produced by the huge current flowing through it. And yes, it’s bluish-purple, depending on what buffer gas is in the envelope.
Very high current density mercury rectifiers sometimes use tungsten or molybdenum electrodes, but the rectifying performance isn’t as good as with carbon.
> Special types include “the thyratron, the Ignitron and the Excitron.”
These are all what I’d call “triggered rectifiers”, rather than straight rectifiers.
The Eight Metre Theta Pinch Experiment- an early controlled fusion experiment (at Harwell? or Culham?)- used a very large ignitron which was housed in a large metal cabinet, proudly decorated with a large sign that said “THETA PINCH CLAMPING IGNITRON”, which photograph in a school science textbook fired my childhood imagination for many years.
Ignitrons are unpopular these days because their primary failure mode is to explode with great violence in a cloud of conducting, toxic, very hot liquid and gas. They’re mostly used in scientific applications, though they’re sometimes still used in manufacturing applications that require HUGE pulses of current (like implosion welding of large metal assemblies).
Thyratrons have largely been replaced by thyristors and other forms of silicon controlled rectifiers. SCRs are used in almost all large power-conversion systems, because they are easy to make large- they do not suffer from the very high current densities of transistors. You can also make them such that they are directly optically triggered, which is useful when you’re talking about switching 475 kV with them.
Excitrons use an excitation anode to keep the arc permanently struck, whereas in a conventional mercury arc rectifier each arc spends at least 50% of its time ‘out’. This gives a higher frequency capability (mercury arc rectifier losses rapidly increase when you exceed about 100 Hz, and the devices become essentially unusable above about 120 Hz, which would be a problem in aviation applications which traditionally use 440 Hz power) and a lower voltage drop (at 50 Hz, the drop of an excitron is about 20-25% less than an equivalent mercury arc rectifier; this is a considerable saving in I2R losses.) Excitrons also reach lower peak envelope gas pressures and can therefore tolerate higher current densities. The current density of the “active pool” of an excitron can exceed 50 000 amps per square centimetre.
All of these devices are pretty much obsolete with the development of large thyristors, semiconductor diodes, etc., etc. They tended to be unreliable, sometimes exploded and were generally very bad to be around. They also needed to be kept hot or they’d stop working, and the mercury they contained sometimes ended up in bad places- I remember when our school was closed for refurbishment, they found traces of mercury from where an arc rectifier had been dropped thirty years previously. It had been cleaned up and decontaminated several times, but mercury was still soaked into the concrete floors…
Thyratrons and krytrons are interesting because they can be made to trigger very quickly and repeatably, which makes them useful for initiating the implosion of a nuclear bomb’s “pit”. This means that krytrons are on the controlled list of the IAEA.
However, krytrons are also used in some photocopiers for exposure control…