Pulse Power Switching Devices most of the material here is by John Pasley
Ignitron Krytron Sprytron Thyratron Trigatron Febetron Triggered Spark Gaps
Klystrons ??? checking on this one
Hard-Tube Pulser: I'm still trying to identify this one, more info to follow.
Crossatron: (A version of a thyratron which can be triggered OFF)! I have got to find specs!
OII: (Orientation Independent Ignitron) Crossatron & OII developed by Hughes. The OII uses an amalgam of mercury and another material as its cathode and no mercury pool.
With one or two exceptions, switch tubes are devices that turn on when asked to but won't switch off until the current passing through them drops to essentially zero for long enough for them to de-ionize and "turn-off". Krytrons, Hydrogen Thyratons, Ignitrons and Triggered Spark Gaps can be triggered on but not off. You can think of them as "Closing Switches."
1.1 Switching basics and
terminology
The switch is possibly the most elementary device in the field of
electronics. A switch controls the flow of current in a circuit in a manner such
that either the current flows at a value determined by the other components in
series with it, or does not flow at all, as the case may be. However this ideal
behavior is actually never exactly what is seen in real life. A switch has it's
own parameters that determine how fast it can switch from open to closed, or how
rapidly it can interrupt the flow of current once it is has been opened. Also of
course there are more elementary considerations such as the current handling
capacity of the switch and the peak voltage it can cope with before damage or
other unwanted effects occur.
Mechanical switches such as are common in
the home are in actuality far from ideal in their behavior. The time taken to
switch from off to on ( the commutation time) is typically in the millisecond
range. Also spurious effects such as bouncing may occur as the switch fluctuates
rapidly from open to closed in the process of being physically manipulated by
the operator.
Electromagnetic relays and reed switches experience similar problems to those
seen in the humble light switch. Long commutation and switch bounce are standard
features of virtually all mechanical switching devices.
With the advent of transistors and similar devices such as thyristors one would
have thought that these slow switching problems would be things of the past.
This is in fact largely true. But semiconductors are limited in other ways, it
is very hard to find semiconductors capable of switching many kiloamperes
especially at potentials in the kilovolt region, and those devices that can
manage high currents such as the larger thyristors are troubled by overly high
commutation times. Whilst there are now semiconductors coming onto the market
capable of performing at these extremes of current and voltage there are some
requirements which put even these devices to shame. If you want to switch 50
kilo Amperes with a sub 20 nanosecond commutation time at 20kV you are going to
be in trouble if you are relying on semiconductor technology. However there is
an alternative class of devices that have been around long before the humble
transistor came on the scene. You might think that vacuum tubes and similar are
a thing of the past. But for problems of this magnitude they are the only things
on the market that will do the job.
1.2 Vacuum and Gas filled switching tubes, introduction and terminology
There are a great many different types of vacuum tube in existence, however it
is possible to group tubes according to some fairly basic criteria. There are
two primary distinguishing features, the source of free electrons within the
device and the gaseous filling (or lack of it) within the tube envelope. The
later of these two concepts we have already introduced by implication. A vacuum
tube is a device with a vacuum (very low pressure gas) filling. And a gas filled
device is, as the name would suggest, filled with gas that might be at a
pressure somewhat above or below atmospheric. The type of gas used is also an
important feature, particularly in switching tubes where a wide variety of
fillings are encountered.
The source of the free conduction electrons in the device may be either thermal
such as a heated filament physically associated with the cathode of the device -
a hot cathode, or alternatively a simple consequence of a high voltage gradient
across the device, resulting in auto-emission from the cathode. A device
employing this latter method is known as a cold cathode device. In high voltage
switching the presence of high voltages, and hence the possibility of large
voltage gradients within devices means that the cold cathode system, quite a
rarity in most other types of tubes, is the norm rather than the exception.
Other important terms encountered in gaseous state switching tubes:
Delay time - The delay time is the time taken between the application of a trigger pulse and
the commencement of conduction between the primary electrodes.
Jitter - Jitter is the variation of time delay from shot to shot given similar electrical
stimulus.
Commutation time - The commutation time is the time taken for the conduction to reach maximum once
it has commenced. (i.e. From the time from the end of the delay time to the time
at which the maximum level of conduction occurs.)
It should be pointed out that none of these switching tubes look very much like
the things in the back of an old radio set. Many are large, some exceptionally
so. Also, glass has largely given way to ceramic in the higher powered devices.
Before you go down your local electronics shop or radio shack, it should also be
pointed out that many of these devices besides costing $100's (often $1000's)
apiece, and are also largely unavailable to the general public due to their
application in advanced missile and nuclear weapon technologies. Of these
devices the most 'everyday' is the ignitron which finds much application in
industrial welding situations.
Another interesting vacuum tube is the TWT, (traveling wave tube), used for high power (~40 watts and up) microwave amplification. These are extensively used on commercial communications satellites, since they are lighter and more efficient than their solid state counterparts. Triton ETD ( formerly ITT electron Technology) makes these tubes and have some interesting technical info on their web.
EG&G Catalogues/ Material. (RE: Components)
Exploding Wires Vol. 4, Proc. of 4th Conf. on the Exploding Wire Phenomena. Ed. Chace and Moore -Plenum Press (RE: EBW's)
High Power Optically Activated Solid State Switches, ed. Rosen And Zutavern- Artech House (RE: Solid state devices)
High Speed Pulse Technology by Frank Frungel -Academic Press. (RE: EBW's, FCG's, components)
High Velocity Impact Phenomena by Ray Kinslow-Academic Press. (RE: Foil Slappers)
IEEE publications (please contact author for more details).
Maxwell Catalogues. (RE: spark gaps)
Mullard Valves and Tubes Book 2 Part 3 (RE: components)
Nuclear Weapons Frequently Asked Questions; Section 7 Nuclear Weapons Nations and Arsenals by Carey Sublette. (RE: smuggling info. relating to krytrons)
The Times (RE: smuggling info. relating to krytrons)