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This note is currently being revised in the light of new information
supplied by Lindl's ICF paper. 24/11/1995
TELLER-ULAM CONSTRUCTION
"... it is my judgement in these things that when you see something that
is technically sweet you go ahead and do it and you argue about what to
do about it only after you have had your technical success. That is the
way it was with the atomic bomb. I do not think anyone opposed making it;
there were some debates about what to do with it after it was made."
Robert J. Oppenheimer
on the H-bomb
"Don't bother me with your conscientious scruples. After all, the thing's
superb physics."
Enrico Fermi on the H-bomb
The basic problem of the H-bomb is to use the energy and particles
released in a fission device to firstly compress and secondly heat
a mass of fusion fuel. Fusion can only occur under temperatures,
pressures, and densities at, or exceeding, those found at the centre
of the sun. The latter is the case for a H-bomb since the reactions
in the bomb occur on a much shorter scale than those in the sun.
You have to have extremely fast moving nuclei to overcome
electrostatic repulsion of the positive proton charges. You need
about 1 trillion atmospheres (8,000,000,000 tonnes/square inch) or
about 1 million megabars. This leads to extremely densely packed
atoms and molecules, which increases the likelihood and frequency
(rate) of collisions. High compactification of fissile material
also reduces the mean free path of fast neutrons. To achieve these
goals, you have to configure the secondary just right. The Teller-
Ulam multistage configuration does precisely this. It is thought
that three main concepts are involved in this design.
You should think of a H-bomb as a multistage engine, with 3 explosive
stages. Since the explosions occur so quickly, it seems like only
one flash occurs, whereas 3 actually do. These correspond to the
initial fission of the primary, the fusion of the secondary, and the
fission of the casing or fusion tamper. In the case of a neutron
bomb, the casing may be made out of a non-fissionable material like
lead, so you would only get two explosions.
Separation of Stages
Much detail as to what goes in inside a H-bomb was gained in 1954
during the Ivy Mike fallout. By a careful analysis of the fallout
products, you could work out roughly where the energy came from.
In particular, you looked at the ratio of higher Z radioisotopes in
the fallout. You tried to find evidence as to whether these products
had been exposed to unusually high neutron fluxes. Compression of
the U-235 sparkplug in the secondary would increase the probability of
multiple neutron exposure. Hence the formation of elements like
transuranic Einsteinium and Fermium, which were first detected in the
Ivy Mike fallout. See the references for evidence of massive Li6D compression
and multiple neutron exposure.
The British designed their first H-bomb after examining American supplied
Russian fallout from the Joe-4 test.
Around 50% of the H-bomb energy comes from fusion. The other 50% is
from fission of the U-238 fusion capsule tamper or weapons casing.
The fusion-boosted implosion core just serves as a trigger, and gives
at most a few hundred kT of energy. Ted Taylor has done calculations
showing it is possible to get into the megaton range for extremely
efficient fusion-boosted imploders. Tritium gas is injected into the
core during implosion to achieve boosting.
For a given volume of Pu or U, you would find an equivalent volume of
Li6D to be 25 times less massive, due to differing densities. If you
fused this amount of Li6D, you would get 3 times as much energy as
you would fissioning the equivalent amount of Pu or U, taking into
account the energy released per reaction. Note that although a single
fission releases more energy than a single fusion event, the fission
releases the binding energy of 235 nucleons, whereas the fusion does
the same for five or six nucleons. If you had 235/6 = 40 fusions, you
would release more energy overall than fission of 235 nucleons. In a H-bomb
it follows you need about 10x the volume of Li6D than Pu or U, to
achieve a 50% energy release ratio. In other words, H-bombs have
a small mass of U or Pu, and a much larger mass of Li6D. In a reaction,
100% of the material never fuses. With experience, 10% is an outstanding
result. For a beginner, 1% is a good start.
The Failed Classical Super Design
Historically, the first theoretical designs for a H-bomb began with the
classical Super. This was a boosted trigger surrounded by a mass of fusion
fuel. When the trigger went off, the heat and shockwave were supposed to
set off an outwardly propagating thermonuclear reaction in the fusion
material. This didn't work. Calculations by Ulam and von Neumann showed
that temperatures and pressures weren't high enough to sustain such a
reaction. It would 'fizzle'. The design was based on what happens in a
supernova. Here, when material collapses into a neutron star, there is
an amount of 'bouncing' off the core. When the material is reflected, a
chain thermonuclear fusion reaction is set off, releasing a good percentage
of that ever fused by the star over its lifetime.
A new idea was called for. This is where Teller, Ulam, and de Hoffmann came
in. Rough calculations showed that sustained fusion could occur if the Li6D
mass was separated from the trigger, possibly in the form of a concentric
cylinder, surrounding a U-235 sparkplug, and surrounded itself by a U-238
pusher. An ablation layer made up of a low-Z hydride surrounds this pusher.
It is possible that primary and secondary are at two foci of an ellipsoid.
The main unknowns to the public are currently the design of the casing,
and the shape and size of the secondary, relative to the primary.
Compression
The problem then is to transfer the energy from the implosion to
this Li6D cylinder, firstly compressing it, and then heating it.
Compression must precede heating since hot materials tend to expand
more than cold ones. This energy transfer is the crucial idea in
a H-bomb. You must compress the Li6D in under a shake, or else the
expanding bomb debris will take everything apart before fusion has
substantially gone underway.
The Greenhouse George test showed that a small quantity of D-T could
be ignited by a fission device.
Radiation Coupled Implosion
Ed Teller has stated that the transfer of energy from the primary to
the secondary is primarily via radiation in the form of soft X-rays,
which travel at light speed. X-rays released by the trigger travel across
the air gap separating the casing from the trigger, and strike the
heavy (high-Z) bomb casing. Radiation pressure generated by the X-rays
is decoupled from the fluid pressure of the fission fragments, which travel
much more slowly.
We can learn a lot from Teller's statement. Mechanical (fluid) pressure isn't
the transfer mechanism. Nor are hard (MeV) X-rays straight from nuclear
reactions. Indeed, soft X-rays come from the ionization of a reasonably high-Z
material. The only place this high-Z material could be is the bomb casing,
which is responsible for most of the bomb's weight.
It is possible that a blackbody radiation mechanism is responsible
for the tamper implosion.
For a few millionths of a second, the insides of the bomb become like
a blackbody. Since the casing is so massive compared to the rest of
the components (including the secondary), it expands relatively
slowly. During the time the vaporised casing expands, a phenomenon known as
X-ray fluorescence causes the casing ions to generates secondary X-rays.
Since the casing atoms have been ionised, when the sea of electrons fall back
into their shells, a uniform emission of secondary soft X-rays is released.
If the casing is machined just right, it is possible to direct these
onto the secondary fuel mass from all directions, leading to a very even
compression. The X-rays act as a photon gas, which equilibriates at light
speed, much more quickly than a material gas made up of fission particles
would (this would equilibriates at the speed of sound). The problem of
the H-bomb is the calculation of the hydrodynamics, not the nuclear physics.
It doesn't have to be soft X-rays which cause the fluorescence. Anything with
enough kinetic energy will do the job - fission fragments or neutrons can do
it. All that needs to be done is to ionise the casing atoms.
What happens is that the secondary X-rays deposit their energy onto the
ablation layer almost instantaneously and uniformly from all sides. The
result is instantaneous heating. The surface layer of the fusion target
is vaporised, forming a surrounding plasma envelope. The layer undergoes
a blowoff with great force. This causes the inner part of the wrapper
to compress (Newton's 3rd law) due to rocket recoil. This tamper pushes against
the secondary Li6D fuel mass, and the mass is compressed to a fraction
of its original width. If there is an air gap (levitation) between tamper
and fuel, the tamper can develop more momentum to do the job. This is what
happens in the levitated cores of fission triggers.
Since the ablator is composed of low-Z, light material, the blowoff will
put a lot of energy into the expanding plasma. This prevents preheating of
the Li6D fusion fuel before adequate compression is achieved, while still
allowing for inward momentum coupling. In other words, the impulse is high.
By this time, the neutrons from the fission will have reached the sparkplug.
The fissioning sparkplug ignites the Li6D annular cylinder from the inside,
while compression occurs on the outside. Burning starts from the inner
edge of the Li6D and, in under 1 ns, a large fraction of the Li6D is ignited.
The core reaches 1000-10,000x the original density, igniting at 100 million
degrees C.
The high energy neutrons (> 1 MeV) released by fusion radiate out and
strike the U-238 atoms of the pusher and expanding casing, causing more
fission.
The casing acts as a heavy gas, whose inertia slows the expansion of the
explosion. However, it plays no part in confinement of the fusion fuel. The
compression caused by the imploding tamper does that job. The interatomic
forces between the casing atoms are negligible.
The bomb tamper is crucial in confining the reactions until they develop
appreciably.
To direct energy onto the secondary, you need firstly to
interact with the casing. All this happens in under 10 shakes.
In ICF, a typical fusion sphere consists of layers of: (1) Be or LiH ablator,
(2) a high Z polymer shield, (3) the main Li6D fuel, (4) the U-238 pusher,
(5) a void, and (6) a Li6D ignitor.
Note that it's not the fission trigger X-rays which cause the blowoff,
but the secondary X-rays due to the X-ray fluorescence of the high-Z
heavy bomb casing. The casing acts like a hohlraum target. Nothing is
reflected as such. Unlike visible light, which is coupled to optical
bandstates on the surface of metals, X-rays are absorbed due to their
much higher energy.
The X-rays come mainly from the L->K and M->K shell transitions as the
electrons drop down into the K shell vacancy, and hence lose energy.
Another possibility for an X-ray source is bremmstrahlung from deccelerating
electrons in the ionised plasma.
Eventually, the X-rays manage to diffuse through the expanding bomb casing,
and are released in a huge flux. This causes the initial light burst of a
nuclear explosion, and is responsible for immediate deaths. Considering this
light is 1000x brighter than the sun, this is no surprise! The temperature
soars to over 1000 deg C in microseconds.
The mechanism of a H-bomb bears an uncanny relation to indirect drive
ICF. Implosions driven by this method are relatively insensitive to the
nature of the primary beams (they could be lasers or ions just as well).
They are also hydrodynamically more stable. This is important, since the
fusion fuel mass must be compressed symmetrically and evenly.
X-ray - Plasma Interactions
This method tends to produce a large volume of target plasma through which
the X-rays must propagate, however. Although it would be more efficient if
the plasma were transparent to this radiation, it is not absolutely
necessary. A diffuse photon gas due to absorption, scattering, and re-
emission by the target plasma will do.
A number of physical effects must be considered. These include:
Absorption:
- X-ray absorption by target
- inverse bremsstrahlung (generates collisional low temp
electrons)
- parametric instabilities (bremsstrahlung induced
collisionless hot electrons)
- resonance absorption (collisionless hot electrons)
Hot electrons lead to target expansion, which is not good for compression,
for it takes more energy to compress a hot gas than a cold one.
Other undesirable effects include:
- stimulated Brillouin scattering
- stimulated Raman scattering
These also generate preheat and hot electrons in the target.
We also need to look at:
- thermal conduction (energy absorbed in a critical layer can be
inihibited from flowing into the ablation region)
Conversion Efficiences
For planar hohlraums, about 70-80% of the incident energy can be
converted into X-rays. You get better target coupling at short wavelengths.
Other Forms of Compression
Instead of radiation, could it be a material shockwave which does
the compression? Or a combination of both? It is known that at the
centre of the earth, iron is compressed to 30% its volume, subject to
about 5 Mbars. So we are way beyond the non-compressible regime, into
nonlinear effects. In fact, Ulam proposed using shock waves, but this
would have resulted in less even compression. Compression of the fusion
fuel can get as high as 1000x solid density, at 100 million degrees C.
Ulam is said to have come up with the solution to the energy transfer
problem when he was looking at ways to improve the efficiency of the
trigger. The joint Teller-Ulam paper talked about "hydrodynamic lenses
and radiation mirrors". Could there be some sort of lensing or baffle
system inside the hohlraum, which focusses radiation onto the Li6D via the
casing? I find this highly unlikely. Note that the shorter the wavelength,
the less refracted light gets. It is very hard to bend X-rays, let alone
gamma rays. Also, wouldn't the lens system vaporise before enough radiation
was focussed? "Hydrodynamic lenses" is reminiscent of the shaped charges
used in achieving a spherical shockwave in the trigger implosion.
Possible focussing systems include hohlraums shaped like ellipsoids, or
parabaloids with the primary at the focus. It is very difficult to shape
the secondary like a cylinder, and get a compression wave travelling just
before fast neutrons from the sparkplug cause fission - although not
impossible. Another problem with the cylindrical shape is that compressing
from the sides is like squeezing a tube of toothpaste. If the compression
is not fast enough, the material will squirt out the ends.
Laser fusion using X-rays to compress pellets of D-T fuel is used in
Livermore's NOVA. Ten pulsed lasers give a temperature of about 10^8 K, and
increase particle density by a factor of 10^3. Each pellet is smaller than
a grain of sand, and absorbs about 200kJ of energy in < 1 ns. Delivered
power is about 2 x 10^14 W, about 100 times the entire world's electric
power generating capacity. This is a peaceful example of inertial confinement
fusion.
Neutrons Causing Compression?
Neutrons expand out at a slightly greater rate as the fission fragments.
Can they compress the Li6D in time, before the fragments tear everything
apart? A shockwave is just a longitudinal compression of the propagation
medium. Energy is transferred in collisions between the atoms or molecules.
If this worked (a classical super design), then the most efficient
way to capture these fission neutrons would be to surround a fission
bomb with fusion fuel, and hope to cause an outward propagating shock wave.
If you didn't surround it, then you'd be wasting lots of neutrons.
The fact that H-bombs don't look like this (big, fat, and round) is evidence
against he idea.
Other Theories
From: merlin <merlin@neuro.usc.edu>
The basic idea is the primary is detonated -- neutrons escape in all
directions -- the secondary could be a hollowed out sphere of U-238
with a Li6D core -- though usually the secondary is elongated to hold
more Li6D. The neutrons convert Li6D to TD. They also cause fast
fissions in the U-238 wrapper around the Li6D -- these fast fissions
release an enormous amount of energy -- the energy causes the U-238
to expand (about 2/3 of energy causes expansion outward from center
of the sphere -- but about 1/3 of energy goes into inward compression
-- thereby compressing the TD core) -- the shock compression and
heating of the TD core reaches thermonuclear temperature and pressure
-- then a recursive reaction begins -- fast neutrons from the TD core
cause fast fissions in the U-238 wrapper -- fast fissions in the U-238
wrapper cause additional shock compression and heating of the core --
if optimum fusion temperature or pressure are exceeded the fusion
reaction slows down, fewer neutrons are produced, fewer fast fissions
occur, the U-238 wrapper releases some pressure -- until optimum
fusion temp and pressure is reached again and the recursive reaction
stabilizes (at least until you run out of TD to burn). This is why
in the traditional hydrogen bomb about half of the yield is fusion
and half of the yield is fission -- the energy has to be balanced in
order to hold the device together long enough to burn as much of the
TD fuel as possible. In the neutron bomb you get more waste tritium
because most of the U-238 mantle has been stripped away -- and the
device disassembles faster -- with much lower explosive yield.
The following diagram is adapted from Matt Kennel's <mbk@lyapunov.UCSD.EDU>:
-------------------------------------------------------
/ | |
/ oooooo |===========fusion fuel========================
| oa-bombo --fission spark plug---------------------------
\ oooooo |==============================================
\ | |
-------------------------------------------------------
<----------><---------------------------...>
implosion repetition of fusion cells clad in U-238 tampers
primary
1994