Detonators

Detonators are used to initiate explosives where it is necessary to create a shockwave.
Creating a symmetric implosion shockwave for a nuclear weapon needs close
synchronization when the detonators are fired. The tolerance for timing differences is very
low: of the order of 100 nanoseconds is necessary.

With a conventional detonator, a wire is electrically heated. This causes a small quantity of a
sensitive primary explosive to detonate (for example lead azide, mercury fulminate etc). The
primary explosive then initiates a secondary explosive (for example PETN or tetryl), which
then fires the main high explosive charge.

This process of electrically heating the wire, and then heat conduction to the primary
explosive until it reaches detonation temperature, needs several milliseconds - and there
may be large timing errors. This means that conventional detonators do not have the
necessary timing exactness to fire an implosion system in a nuclear weapon.

Exploding Bridgewire Detonators (EBW)

Nuclear weapon implosion systems need nanosecond accuracy for firing the detonators and
exploding bridgewire (EBW) detonators are often used. One method to reduce the duration
of action of the detonator is to send a sudden powerful surge of electric current through a
very thin wire of gold or platinum in a specially designed detonator. The current heats the
wire until the point of vaporization. This specially designed EBW technique was invented by
Luis Alvarez at Los Alamos during the Manhattan Project.

Current surge rise times of a fraction of a microsecond are possible, and a spread in
detonation times can be several nanoseconds. This is sufficiently exact for very low
tolerance applications such as firing explosive lenses in nuclear weapons.

EBW detonator

It is possible to employ an exploding bridgewire detonator to initiate a primary explosive
which is usually lead azide exactly like in a conventional detonator. However, if the surge of
electrical current is sufficiently powerful, the exploding bridgewire can directly initiate a less
sensitive booster explosive like PETN. One advantage of this method is that there is much
less risk of accidental activation by heat, stray currents, or static electricity than for a
conventional detonator. Very fast and powerful surges of electrical current are necessary to
fire the detonators and initiate a less sensitive explosive though. This type of exploding
bridgewire detonator is one of the safest types of detonators known to scientists.

The disadvantage is that a typical EBW needs very fast and powerful surges of 5,000 volts,
with a peak current which consists of at least 500-1000 amps. Several kiloamps is more
typical for most EBW detonators, however a multi-EBW system would probably try to
minimize the current needed. It is possible to get timing accuracies which are better than 10
nanoseconds with sufficient attention in detonator design and construction. EBWs were
used in the American Trinity "Gadget" device. The EBW and the slapper detonator are the
safest known types of detonator.

Slapper Detonators

There are a number of more recent detonator designs which are based on exploding foils
which have been developed. Exploding foil detonators use a different design concept from
EBWs: this is called a slapper detonator. This is also sometimes called an exploding foil
initiator (EFI).

This idea was developed in America and elsewhere in the world. It uses the expanding foil
plasma to drive another thin foil or plastic film "flyer" to high velocity across a gap, which
initiates the explosive by means of impacting the surface at high velocity.

Slapper detonators are quite efficient for the conversion of electrical energy into kinetic
energy in the flyer. It can be possible to achieve 25-30% energy transfer. This delivers the
energy needed to initiate a detonation of the explosive.

A typical slapper detonator consists of an explosive pellet which is pressed to a high density
in order to gain maximum strength. It is also possible to use polymer bonded explosives for
this. There is an insulation disk which has a hole in the center next to the explosive pellet
and the hole is set against the explosive pellet. An insulating flyer film (for example Kapton
or Mylar) with a metal foil etched to one side is placed against the disk. A narrowed section
of the etched foil serves as the bridgewire.

The large current firing pulse causes the narrowed section of the foil to become vaporized.
Then this breaks the insulated flyer which accelerates down the barrel of the disk and
impacts the explosive pellet. As a result this impact transmits a shock wave into the
explosive which causes it to detonate.

A slapper detonator has the advantage of the ability to initiate an area of the explosive
surface rather than a singular point. It is possible that this will make the design of compact
implosion systems easier.

This system has several advantages in comparison with the EBW detonator:

  • The metal bridge becomes completely separated from the explosive by means of an
    insulating film and an air gap (the bridgewire of an EBW detonator is in direct contact
    with the explosive)
  • It is possible to pack the explosive to a very high density
  • The amount of energy which is needed to fire the detonator is less
  • Very insensitive explosives can be detonated - this is extremely difficult with regard
    to an EBW detonator

Typically the detonator bridgewire which is used in EBWs consists of highly pure gold or
platinum, is 20-50 microns wide and about 1 mm long. PETN is normally used as the
explosive, possibly with a tetryl booster charge. Slapper detonators use metal foils (usually
aluminum, but gold foil would work well also) which are put on a thin plastic film (this is
usually Kapton). A wider variety of primary explosives can be used. PETN or HMX may have
been used in slappers in earlier weapon systems, but weapons using insensitive high
explosives probably use the explosive HNS which is very stable to heat.

Spark-Gap Detonators

The spark-gap detonator is another type of very fast detonator. It employs a high voltage
(approximately 5,000 V) spark which is created across a narrow gap to initiate a primary
explosive. Using a very sensitive primary explosive (like lead azide or lead styphnate) the
amount of current necessary is relatively small; a small-sized capacitor can supply enough
power.

The main disadvantage for this type of detonator design is that it is quite unsafe. Static
charges or other induced currents can very easily fire a spark gap detonator by accident. For
this reason it is unlikely that they have been used in deployed nuclear weapons.

Spark-gap detonators use primary explosives like a normal (non-fast-acting) detonator. A
normal detonator uses a metal filament in contact with primary explosive. An electric
current heats the metal filament until it gets extremely hot (red) and heat transfers to the
near primary explosive by conduction. The explosive then detonates.

However, a spark-gap detonator has two electrodes with a gap between them, which is filled
with primary explosive. An electric spark is passed across this gap and this spark detonates
the explosive. It is extremely fast-acting and gives high timing exactness with a multi-
detonator system because it is not necessary to rely on thermal conduction of heat to
initiate the explosive.

Because primary explosives are used, it is not necessary for the spark to be particularly high
energy. Primary explosives can be detonated easily from a low energy input.

These types of detonators are used in the Russian RPG-7 HEAT ammunition. The
ammunition has a nose mounted piezoelectric part. When it strikes something hard it gives a
pulse of electricity. They are ideal for this type of application, because the ammunition is
needed to function immediately when it strikes the target.

Scientists examined the use of spark-gap detonators in the Manhattan Project and the Indian
Nuclear program, but they were replaced by EBWs, as a result of safety questions.

Power Sources

For these types of detonators special power sources are necessary to produce the large
current surges which are needed.

For a nuclear weapon, a quite compact and light high-speed pulse power supply is
necessary for the detonation system. A power source which is very powerful and able to discharge
extremely quickly is necessary to achieve accurate timing and a fast response. Fast, accurate
and reliable switching components are also needed, and it is also necessary to manage the
inductance of the whole system.

The usual method for providing the power for an EBW multi-detonator system is through the
discharge of a high capacitance, high voltage, low inductance capacitor through a suitable
switch and into the bridgewire. The voltage range which is necessary is on the order of
several kilovolts (5 KV is normal). Silicone oil filled capacitors with the use of Kraft paper,
polypropylene, or Mylar dielectrics are suitable, ceramic-type capacitors are also. Compact
power supplies for charging capacitors are easy to acquire.

The capacitor also needs a switch which can handle high voltages and currents. The switch
must also be able to transition from a safe, non-conducting state to a fully conducting state
very rapidly, and without adding unnecessary inductance to the circuit.

A variety of switch technologies is available: triggered spark gaps, krytrons, sprytrons,
thyratrons
and explosive switches are some which can be used.

KN2 Kryton switch tube

Triggered spark gaps are sealed devices which are filled with high pressure air, argon, or SF6.
A non-conducting gap between electrodes becomes closed by means of applying a triggering
potential to a wire or grid in the gap. Typically, compact versions of these devices rate at 20-
100 KV, and 50-150 kiloamps, and the triggering potential is one-half to one-third the
maximum voltage. Switch current rise times last 10-100 nanoseconds.

Krytrons are a type of cold-cathode trigger discharge tubes which are filled with gas. The gas
is normally hydrogen, although it is possible to use krypton. Some of these contain a small
amount of Ni-63, which is a weak beta emitter which keeps the gas in a slightly ionized state.
Applying a trigger voltage causes an ionization cascade to close the switch. They are used as
an extremely high speed switch. These devices have maximum voltage ratings from 3 to 10
KV, and peak current rating of 300-3000 amps. This makes them unsuitable for directly firing
multiple EBW detonators. Krytrons are employed commercially in powerful xenon flash lamp
systems, and have other uses. They have faster response times than other types of trigger
discharge tubes.

The vacuum variant is called a vacuum krytron or sprytron. It is very similar and has a very
high radiation resistance. Sprytrons have faster switching times than krytrons. It is probably
the sprytron that is used in nuclear weapons.

Detonators in nuclear weapons are normally wired in parallel for reliability and to reduce
inductance. In the American "Fat Man" atomic bomb scientists wired the detonators in
parallel in triggered spark-gap circuits. There were four detonating circuits. Any two of these
circuits provided enough power for all 32 detonators. Each detonator was wired to two
different circuits in order that a failure of any one detonator circuit would not affect the
implosion. The whole system was fired by a spark-gap cascade. The triggered spark-gap
supplied a current surge to fire the four main circuits at the same time.