This note is currently being revised in the light of new information
\nsupplied by Lindl’s ICF paper. 24\/11\/1995<\/p>\n
TELLER-ULAM CONSTRUCTION<\/p>\n
“… it is my judgement in these things that when you see something that
\n is technically sweet you go ahead and do it and you argue about what to
\n do about it only after you have had your technical success. That is the
\n way it was with the atomic bomb. I do not think anyone opposed making it;
\n there were some debates about what to do with it after it was made.”<\/p>\n
\t\t\t\t\tRobert J. Oppenheimer
\n\t\t\t\t\t on the H-bomb<\/p>\n
“Don’t bother me with your conscientious scruples. After all, the thing’s
\n \t\t\tsuperb physics.”<\/p>\n
\t\t\t\t\tEnrico Fermi on the H-bomb<\/p>\n
The basic problem of the H-bomb is to use the energy and particles
\nreleased in a fission device to firstly compress and secondly heat
\na mass of fusion fuel. Fusion can only occur under temperatures,
\npressures, and densities at, or exceeding, those found at the centre
\nof the sun. The latter is the case for a H-bomb since the reactions
\nin the bomb occur on a much shorter scale than those in the sun.<\/p>\n
You have to have extremely fast moving nuclei to overcome
\nelectrostatic repulsion of the positive proton charges. You need
\nabout 1 trillion atmospheres (8,000,000,000 tonnes\/square inch) or
\nabout 1 million megabars. This leads to extremely densely packed
\natoms and molecules, which increases the likelihood and frequency
\n(rate) of collisions. High compactification of fissile material
\nalso reduces the mean free path of fast neutrons. To achieve these
\ngoals, you have to configure the secondary just right. The Teller-
\nUlam multistage configuration does precisely this. It is thought
\nthat three main concepts are involved in this design. <\/p>\n
You should think of a H-bomb as a multistage engine, with 3 explosive
\nstages. Since the explosions occur so quickly, it seems like only
\none flash occurs, whereas 3 actually do. These correspond to the
\ninitial fission of the primary, the fusion of the secondary, and the
\nfission of the casing or fusion tamper. In the case of a neutron
\nbomb, the casing may be made out of a non-fissionable material like
\nlead, so you would only get two explosions.<\/p>\n
Separation of Stages<\/p>\n
Much detail as to what goes in inside a H-bomb was gained in 1954
\nduring the Ivy Mike fallout. By a careful analysis of the fallout
\nproducts, you could work out roughly where the energy came from.
\nIn particular, you looked at the ratio of higher Z radioisotopes in
\nthe fallout. You tried to find evidence as to whether these products
\nhad been exposed to unusually high neutron fluxes. Compression of
\nthe U-235 sparkplug in the secondary would increase the probability of
\nmultiple neutron exposure. Hence the formation of elements like
\ntransuranic Einsteinium and Fermium, which were first detected in the
\nIvy Mike fallout. See the references for evidence of massive Li6D compression
\nand multiple neutron exposure.<\/p>\n
The British designed their first H-bomb after examining American supplied
\nRussian fallout from the Joe-4 test.<\/p>\n
Around 50% of the H-bomb energy comes from fusion. The other 50% is
\nfrom fission of the U-238 fusion capsule tamper or weapons casing.
\nThe fusion-boosted implosion core just serves as a trigger, and gives
\nat most a few hundred kT of energy. Ted Taylor has done calculations
\nshowing it is possible to get into the megaton range for extremely
\nefficient fusion-boosted imploders. Tritium gas is injected into the
\ncore during implosion to achieve boosting.<\/p>\n
For a given volume of Pu or U, you would find an equivalent volume of
\nLi6D to be 25 times less massive, due to differing densities. If you
\nfused this amount of Li6D, you would get 3 times as much energy as
\nyou would fissioning the equivalent amount of Pu or U, taking into
\naccount the energy released per reaction. Note that although a single
\nfission releases more energy than a single fusion event, the fission
\nreleases the binding energy of 235 nucleons, whereas the fusion does
\nthe same for five or six nucleons. If you had 235\/6 = 40 fusions, you
\nwould release more energy overall than fission of 235 nucleons. In a H-bomb
\nit follows you need about 10x the volume of Li6D than Pu or U, to
\nachieve a 50% energy release ratio. In other words, H-bombs have
\na small mass of U or Pu, and a much larger mass of Li6D. In a reaction,
\n100% of the material never fuses. With experience, 10% is an outstanding
\nresult. For a beginner, 1% is a good start.<\/p>\n
The Failed Classical Super Design<\/p>\n
Historically, the first theoretical designs for a H-bomb began with the
\nclassical Super. This was a boosted trigger surrounded by a mass of fusion
\nfuel. When the trigger went off, the heat and shockwave were supposed to
\nset off an outwardly propagating thermonuclear reaction in the fusion
\nmaterial. This didn’t work. Calculations by Ulam and von Neumann showed
\nthat temperatures and pressures weren’t high enough to sustain such a
\nreaction. It would ‘fizzle’. The design was based on what happens in a
\nsupernova. Here, when material collapses into a neutron star, there is
\nan amount of ‘bouncing’ off the core. When the material is reflected, a
\nchain thermonuclear fusion reaction is set off, releasing a good percentage
\nof that ever fused by the star over its lifetime.<\/p>\n
A new idea was called for. This is where Teller, Ulam, and de Hoffmann came
\nin. Rough calculations showed that sustained fusion could occur if the Li6D
\nmass was separated from the trigger, possibly in the form of a concentric
\ncylinder, surrounding a U-235 sparkplug, and surrounded itself by a U-238
\npusher. An ablation layer made up of a low-Z hydride surrounds this pusher.
\nIt is possible that primary and secondary are at two foci of an ellipsoid.<\/p>\n
The main unknowns to the public are currently the design of the casing,
\nand the shape and size of the secondary, relative to the primary.<\/p>\n
Compression<\/p>\n
The problem then is to transfer the energy from the implosion to
\nthis Li6D cylinder, firstly compressing it, and then heating it.
\nCompression must precede heating since hot materials tend to expand
\nmore than cold ones. This energy transfer is the crucial idea in
\na H-bomb. You must compress the Li6D in under a shake, or else the
\nexpanding bomb debris will take everything apart before fusion has
\nsubstantially gone underway.<\/p>\n
The Greenhouse George test showed that a small quantity of D-T could
\nbe ignited by a fission device.<\/p>\n
Radiation Coupled Implosion<\/p>\n
Ed Teller has stated that the transfer of energy from the primary to
\nthe secondary is primarily via radiation in the form of soft X-rays,
\nwhich travel at light speed. X-rays released by the trigger travel across
\nthe air gap separating the casing from the trigger, and strike the
\nheavy (high-Z) bomb casing. Radiation pressure generated by the X-rays
\nis decoupled from the fluid pressure of the fission fragments, which travel
\nmuch more slowly.<\/p>\n
We can learn a lot from Teller’s statement. Mechanical (fluid) pressure isn’t
\nthe transfer mechanism. Nor are hard (MeV) X-rays straight from nuclear
\nreactions. Indeed, soft X-rays come from the ionization of a reasonably high-Z
\nmaterial. The only place this high-Z material could be is the bomb casing,
\nwhich is responsible for most of the bomb’s weight.<\/p>\n
It is possible that a blackbody radiation mechanism is responsible
\nfor the tamper implosion.<\/p>\n
For a few millionths of a second, the insides of the bomb become like
\na blackbody. Since the casing is so massive compared to the rest of
\nthe components (including the secondary), it expands relatively
\nslowly. During the time the vaporised casing expands, a phenomenon known as
\nX-ray fluorescence causes the casing ions to generates secondary X-rays.
\nSince the casing atoms have been ionised, when the sea of electrons fall back
\ninto their shells, a uniform emission of secondary soft X-rays is released.
\nIf the casing is machined just right, it is possible to direct these
\nonto the secondary fuel mass from all directions, leading to a very even
\ncompression. The X-rays act as a photon gas, which equilibriates at light
\nspeed, much more quickly than a material gas made up of fission particles
\nwould (this would equilibriates at the speed of sound). The problem of
\nthe H-bomb is the calculation of the hydrodynamics, not the nuclear physics.<\/p>\n
It doesn’t have to be soft X-rays which cause the fluorescence. Anything with
\nenough kinetic energy will do the job – fission fragments or neutrons can do
\nit. All that needs to be done is to ionise the casing atoms.<\/p>\n
What happens is that the secondary X-rays deposit their energy onto the
\nablation layer almost instantaneously and uniformly from all sides. The
\nresult is instantaneous heating. The surface layer of the fusion target
\nis vaporised, forming a surrounding plasma envelope. The layer undergoes
\na blowoff with great force. This causes the inner part of the wrapper
\nto compress (Newton’s 3rd law) due to rocket recoil. This tamper pushes against
\nthe secondary Li6D fuel mass, and the mass is compressed to a fraction
\nof its original width. If there is an air gap (levitation) between tamper
\nand fuel, the tamper can develop more momentum to do the job. This is what
\nhappens in the levitated cores of fission triggers.<\/p>\n
Since the ablator is composed of low-Z, light material, the blowoff will
\nput a lot of energy into the expanding plasma. This prevents preheating of
\nthe Li6D fusion fuel before adequate compression is achieved, while still
\nallowing for inward momentum coupling. In other words, the impulse is high.<\/p>\n
By this time, the neutrons from the fission will have reached the sparkplug.
\nThe fissioning sparkplug ignites the Li6D annular cylinder from the inside,
\nwhile compression occurs on the outside. Burning starts from the inner
\nedge of the Li6D and, in under 1 ns, a large fraction of the Li6D is ignited.
\nThe core reaches 1000-10,000x the original density, igniting at 100 million
\ndegrees C. <\/p>\n
The high energy neutrons (> 1 MeV) released by fusion radiate out and
\nstrike the U-238 atoms of the pusher and expanding casing, causing more
\nfission.<\/p>\n
The casing acts as a heavy gas, whose inertia slows the expansion of the
\nexplosion. However, it plays no part in confinement of the fusion fuel. The
\ncompression caused by the imploding tamper does that job. The interatomic
\nforces between the casing atoms are negligible.<\/p>\n
The bomb tamper is crucial in confining the reactions until they develop
\nappreciably.<\/p>\n
To direct energy onto the secondary, you need firstly to
\ninteract with the casing. All this happens in under 10 shakes.<\/p>\n
In ICF, a typical fusion sphere consists of layers of: (1) Be or LiH ablator,
\n(2) a high Z polymer shield, (3) the main Li6D fuel, (4) the U-238 pusher,
\n(5) a void, and (6) a Li6D ignitor.<\/p>\n
Note that it’s not the fission trigger X-rays which cause the blowoff,
\nbut the secondary X-rays due to the X-ray fluorescence of the high-Z
\nheavy bomb casing. The casing acts like a hohlraum target. Nothing is
\nreflected as such. Unlike visible light, which is coupled to optical
\nbandstates on the surface of metals, X-rays are absorbed due to their
\nmuch higher energy. <\/p>\n
The X-rays come mainly from the L->K and M->K shell transitions as the
\nelectrons drop down into the K shell vacancy, and hence lose energy.<\/p>\n
Another possibility for an X-ray source is bremmstrahlung from deccelerating
\nelectrons in the ionised plasma.<\/p>\n
Eventually, the X-rays manage to diffuse through the expanding bomb casing,
\nand are released in a huge flux. This causes the initial light burst of a
\nnuclear explosion, and is responsible for immediate deaths. Considering this
\nlight is 1000x brighter than the sun, this is no surprise! The temperature
\nsoars to over 1000 deg C in microseconds.<\/p>\n
The mechanism of a H-bomb bears an uncanny relation to indirect drive
\nICF. Implosions driven by this method are relatively insensitive to the
\nnature of the primary beams (they could be lasers or ions just as well).
\nThey are also hydrodynamically more stable. This is important, since the
\nfusion fuel mass must be compressed symmetrically and evenly. <\/p>\n
X-ray – Plasma Interactions<\/p>\n
This method tends to produce a large volume of target plasma through which
\nthe X-rays must propagate, however. Although it would be more efficient if
\nthe plasma were transparent to this radiation, it is not absolutely
\nnecessary. A diffuse photon gas due to absorption, scattering, and re-
\nemission by the target plasma will do.<\/p>\n
A number of physical effects must be considered. These include:<\/p>\n
\tAbsorption:<\/p>\n
\t– X-ray absorption by target
\n\t\t– inverse bremsstrahlung (generates collisional low temp
\n\t\t\telectrons)
\n\t\t– parametric instabilities (bremsstrahlung induced
\n\t\t\tcollisionless hot electrons)
\n\t\t– resonance absorption (collisionless hot electrons)<\/p>\n
Hot electrons lead to target expansion, which is not good for compression,
\nfor it takes more energy to compress a hot gas than a cold one.<\/p>\n
Other undesirable effects include:<\/p>\n
\t– stimulated Brillouin scattering
\n\t– stimulated Raman scattering<\/p>\n
These also generate preheat and hot electrons in the target.<\/p>\n
We also need to look at:<\/p>\n
– thermal conduction (energy absorbed in a critical layer can be
\n\t\tinihibited from flowing into the ablation region)<\/p>\n
Conversion Efficiences<\/p>\n
For planar hohlraums, about 70-80% of the incident energy can be
\nconverted into X-rays. You get better target coupling at short wavelengths.<\/p>\n
Other Forms of Compression<\/p>\n
Instead of radiation, could it be a material shockwave which does
\nthe compression? Or a combination of both? It is known that at the
\ncentre of the earth, iron is compressed to 30% its volume, subject to
\nabout 5 Mbars. So we are way beyond the non-compressible regime, into
\nnonlinear effects. In fact, Ulam proposed using shock waves, but this
\nwould have resulted in less even compression. Compression of the fusion
\nfuel can get as high as 1000x solid density, at 100 million degrees C.<\/p>\n
Ulam is said to have come up with the solution to the energy transfer
\nproblem when he was looking at ways to improve the efficiency of the
\ntrigger. The joint Teller-Ulam paper talked about “hydrodynamic lenses
\nand radiation mirrors”. Could there be some sort of lensing or baffle
\nsystem inside the hohlraum, which focusses radiation onto the Li6D via the
\ncasing? I find this highly unlikely. Note that the shorter the wavelength,
\nthe less refracted light gets. It is very hard to bend X-rays, let alone
\ngamma rays. Also, wouldn’t the lens system vaporise before enough radiation
\nwas focussed? “Hydrodynamic lenses” is reminiscent of the shaped charges
\nused in achieving a spherical shockwave in the trigger implosion.<\/p>\n
Possible focussing systems include hohlraums shaped like ellipsoids, or
\nparabaloids with the primary at the focus. It is very difficult to shape
\nthe secondary like a cylinder, and get a compression wave travelling just
\nbefore fast neutrons from the sparkplug cause fission – although not
\nimpossible. Another problem with the cylindrical shape is that compressing
\nfrom the sides is like squeezing a tube of toothpaste. If the compression
\nis not fast enough, the material will squirt out the ends.<\/p>\n
Laser fusion using X-rays to compress pellets of D-T fuel is used in
\nLivermore’s NOVA. Ten pulsed lasers give a temperature of about 10^8 K, and
\nincrease particle density by a factor of 10^3. Each pellet is smaller than
\na grain of sand, and absorbs about 200kJ of energy in < 1 ns. Delivered
\npower is about 2 x 10^14 W, about 100 times the entire world's electric
\npower generating capacity. This is a peaceful example of inertial confinement
\nfusion.<\/p>\n
Neutrons Causing Compression?<\/p>\n
Neutrons expand out at a slightly greater rate as the fission fragments.
\nCan they compress the Li6D in time, before the fragments tear everything
\napart? A shockwave is just a longitudinal compression of the propagation
\nmedium. Energy is transferred in collisions between the atoms or molecules.<\/p>\n
If this worked (a classical super design), then the most efficient
\nway to capture these fission neutrons would be to surround a fission
\nbomb with fusion fuel, and hope to cause an outward propagating shock wave.
\nIf you didn't surround it, then you'd be wasting lots of neutrons.
\nThe fact that H-bombs don't look like this (big, fat, and round) is evidence
\nagainst he idea.<\/p>\n
Other Theories<\/p>\n
From: merlin <\/p>\n
The basic idea is the primary is detonated — neutrons escape in all
\ndirections — the secondary could be a hollowed out sphere of U-238
\nwith a Li6D core — though usually the secondary is elongated to hold
\nmore Li6D. The neutrons convert Li6D to TD. They also cause fast
\nfissions in the U-238 wrapper around the Li6D — these fast fissions
\nrelease an enormous amount of energy — the energy causes the U-238
\nto expand (about 2\/3 of energy causes expansion outward from center
\nof the sphere — but about 1\/3 of energy goes into inward compression
\n— thereby compressing the TD core) — the shock compression and
\nheating of the TD core reaches thermonuclear temperature and pressure
\n— then a recursive reaction begins — fast neutrons from the TD core
\ncause fast fissions in the U-238 wrapper — fast fissions in the U-238
\nwrapper cause additional shock compression and heating of the core —
\nif optimum fusion temperature or pressure are exceeded the fusion
\nreaction slows down, fewer neutrons are produced, fewer fast fissions
\noccur, the U-238 wrapper releases some pressure — until optimum
\nfusion temp and pressure is reached again and the recursive reaction
\nstabilizes (at least until you run out of TD to burn). This is why
\nin the traditional hydrogen bomb about half of the yield is fusion
\nand half of the yield is fission — the energy has to be balanced in
\norder to hold the device together long enough to burn as much of the
\nTD fuel as possible. In the neutron bomb you get more waste tritium
\nbecause most of the U-238 mantle has been stripped away — and the
\ndevice disassembles faster — with much lower explosive yield.<\/p>\n
The following diagram is adapted from Matt Kennel’s :<\/p>\n
——————————————————-
\n \/ | \t\t\t |
\n \/ oooooo |===========fusion fuel========================
\n| oa-bombo –fission spark plug—————————
\n oooooo |==============================================
\n | |
\n ——————————————————-<\/p>\n
implosion\trepetition of fusion cells clad in U-238 tampers
\n primary<\/p>\n
1994<\/p>\n
