Cecil,
You might have mentioned that A-bombs are necessarily used to trigger the fusion in H-bombs, since they’re the only means by which the required temperatures and pressures can be attained. If I’m not mistaken, plutonium is used in the former to produce fission while tritium gas is used the latter to produce fusion.
TC
LINK TO COLUMN: What’s the difference between a hydrogen bomb and an atom bomb? - The Straight Dope
Titrium gas is used in boosted fission bombs.In H-Bombs the neutronss released from the initial fission bombard the lithium-6 deuteride of the secondary producing titrium and detrium who fuse due to the heat of the fission.
As (or more) important: pressure from soft x-ray flux and mechanical compression. The tamper is a layer of inert (depleted) uranium that absorbs the very hard radiation from the initial fission reaction and becomes a dense plasma radiating at x-ray frequencies. The tamper is accelerated inwards by the sudden plasma ablation of its outer surface, mechanically compressing the fusion core. The accelerated mass of the tamper, plasma and the x-rays provide the pressure and heat to force and sustain the fusion reaction of the tritium (created by neutron bombardment of the lithium in the Li/De fuel mixture) and deuterium.
The fusion reaction won’t start without a lot of pressure, and can’t be sustained without lots of continued pressure; otherwise, the heat and energy generated by the beginning of the reaction would disperse the fuel so quickly it would be over before it got very far. In general, the great advances in nuclear weapons have been advances in containment and compression.
I am not a nuclear scientist or engineer. I have learned a few things reading. As far as I can tell, this wikipedia page is accurate and fairly readable. Or at least was when I looked at it.
I think we’re supposed to avoid gratuitous comments about breasts on the SDMB these days.
Another point that I think he missed is that for many fusion (hydrogen) bombs, the majority of the energy released is from fission, not from fusion.
Cite? The contribution of the fission core isn’t negligible but my understanding is that it’s no larger than it need be to sufficiently trigger the D/T fuel.
I believe the practical limit for a fission weapon is 80-90 kilotons; that means it could be no more than 45% of a 2 megaton detonation, and less than 10% of something like Castle Bravo. I don’t have figures right at hand but I doubt CB’s fission core was anywhere near that large, and I’d bet the current generation of strategic hydrogen weapons are similarly proportioned (10-20% of total yield).
Isn’t it nice that this is all a theoretical discussion? 
The fusion stage makes the fission stages more efficient.
More to the point, the neutron flux from the fusion stage can induce fission of isotopes that are essentially inert if you tried to build a single-stage fission device from these.
They won’t chain-react on their own but if you blast them with neutrons you get fission reactions that release a lot of energy, but not enough neutrons to sustain the reactions.
This boosting was neglected in the Castle Bravo design phase, which is why the yield was so much higher than anticipated. Isotopes that were thought/intended to be inert ended up fissioning.
Once when I was young and stupid I read a lot at nuclear weapons archive. The largest pure fission device tested was 500kt. The largest ever deployed was a French 120kt missile warhead. Nuclear weapons archive stated that a fission device of Plutonium could practically be on the order of high hundreds of KT and a HEU based device of several megatons.
A boosted fission device can also approach H Bombs in yield.
Well, see here. Specifically, it’s not the “core” (pit, in the parlance) but the outer shell or “tamper” that provides extra yield:
[QUOTE=Wikipedia]
Summary
A simplified summary of the above explanation would be:
An implosion assembly type of fission bomb is exploded. This is the primary stage. If a small amount of deuterium/tritium gas is placed inside the primary’s core, it will be compressed during the explosion and a nuclear fusion reaction will occur; the released neutrons from this fusion reaction will induce further fission in the plutonium-239 or uranium-235 used in the primary stage. The use of fusion fuel to enhance the efficiency of a fission reaction is called boosting. Without boosting, a large portion of the fissile material will remain unreacted; the Little Boy and Fat Man bombs had an efficiency of only 1.4% and 17%, respectively, because they were unboosted.
Energy released in the primary stage is transferred to the secondary (or fusion) stage. The exact mechanism whereby this happens is unknown. This energy compresses the fusion fuel and sparkplug; the compressed sparkplug becomes critical and undergoes a fission chain reaction, further heating the compressed fusion fuel to a high enough temperature to induce fusion, and also supplying neutrons that react with lithium to create tritium for fusion.
The fusion fuel of the secondary stage may be surrounded by depleted uranium or natural uranium, whose U-238 is not fissile and cannot sustain a chain reaction, but which is fissionable when bombarded by the high-energy neutrons released by fusion in the secondary stage. This process provides considerable energy yield (as much as half of the total yield in large devices), but is not considered a tertiary “stage”. Tertiary stages are further fusion stages (see below), which have been only rarely used, and then only in the most powerful bombs ever made.
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Bolding mine.
Ivy Mike, the first staged fusion weapon, got the majority of its yield from fissioning of the U-238 tamper:
[QUOTE=Wikipedia]
The test was carried out at 07:15 A.M local time on November 1, 1952 (19:15 October 31 GMT). It produced a yield estimated in the range of 10.4–12 megatons of TNT. However, 77% of the final yield came from fast fission of the uranium tamper, which meant that the device produced large amounts of fallout.
[/QUOTE]
Again, bolding mine.
For more on this concept, I highly recommend Richard Rhodes’ book, Dark Sun.
Okay, thanks. I guess I need to reread Making and Dark Sun… all of the info you posted seemed familiar but I’d clearly forgotten the gist of it.