The Krytron
Krytrons are a highly specialized variety of cold cathode trigger tube. They
were one of the first products developed by the US based company EG&G. The
Krytron has 4 electrodes, and is filled with a gas at low pressure. A Krytron is
distinguished among cold cathode trigger tubes for a variety of reasons.
The Krytron is designed to switch moderately high impulse currents (up to
around 3kA) and voltages (Up to around 5kV) in an arc discharge mode, compare
this with the usual glow discharge of the standard trigger tube. Also, and
perhaps more importantly, the Krytron is able to turn on this arc discharge very
rapidly, the reason being that it relies on an already present plasma to support
the conduction, rather than waiting for the plasma to be formed as a result of
priming etc. This plasma is created and sustained by a keep-alive current
between the keep-alive electrode and the cathode of the device. When the trigger
is applied under the conditions of a high anode to cathode voltage, this plasma
forms an easy path for the main conduction between anode and cathode.
The fact that a conduction path is already established prior to triggering
makes a huge difference in the commutation time of these devices compared to
standard cold cathode trigger tubes. Commutation times below 1 nanosecond are
achievable with Krytrons and the time lag between application of trigger and the
commencement of switching may be less than 30ns with an optimized driver
circuit. (Note this delay is largely due to the fact that the ionized path will
need to spread from the keep alive terminal to the anode of the device) Compare
this delay time to that seen in the standard trigger tube which is dependent
upon many environmental factors and typically 3 or 4 orders of magnitude
greater. Note that the variation in time delay exhibited by the krytron is almost totally independent of environment, however the
time delay may be reduced up to a point with increasing trigger voltage.
Likewise the commutation time is generally decreased if the rise time of the
trigger pulse is also decreased. Given identical trigger pulses however a
krytron will have a very similar time delay from one shot to the next. This
variation is known as jitter and may be less than 5ns in optimal circumstances.
A Krytron contains a source of Beta radiation, Ni-63. The quantity in each
device is less than 5 microcuries and presents no significant hazard. Usually
the source is pulse welded to a piece of Nickel wire that is in turn welded to
one of the electrode supports. The purpose of this source is to increase the
reliability of the krytron by aiding the formation of the initial glow discharge
between the keep alive and the cathode. This initial keep alive current is very
much subject to environmental factors such as are seen in the formation of the
glow discharge in standard trigger tubes. It is for this reason that a
radioactive priming element is used, much as in the priming source employed in a
standard trigger tube (which is also occasionally a radioactive source).
Krytrons typically come in a small glass envelope somewhat similar to a neon
indicator bulb with more leads.
Krytrons require a high voltage pulse (500V to 2kV) to be applied to the
trigger electrode to fire successfully. This pulse is almost always generated by
a pulse transformer fired by a capacitor discharge in the primary (rather like a
simple strobe tube firing circuit).
The krytron often has only a short life expectancy if used regularly (often
as few as a couple of hundred shots) However when used within the appropriate
parameters and well within the expected life time they are extremely reliable,
requiring no warm up and being immune to many environmental factors to a large
extent (e.g. vibration, temperature, acceleration).
These properties, combined with the small size make the krytron ideal for
use in the detonating circuitry of certain types of missiles and smart bombs.
The krytron may be used directly to fire a high precision exploding wire, or
alternatively as part of the triggering circuitry for a triggered spark gap or
similar ultra high current triggering device as used in exploding foil slapper
type detonators and larger EBW circuits.
Krytrons are used in firing circuits for certain lasers and flash tubes and
also in some pulse welding applications, often as triggering devices for other
larger devices such as Thyratrons and spark gaps.