Pure fusion bombs

[Ma Deuce]

Quite simply, would there be any practical (as in practical to use in a warhead) way to initiate an explosive fusion reaction without a fission trigger as is required on modern thermonuclear warheads, and if so, how?

[Kuroneko]

No. If we could start fusion at a whim without the immense energies delivered by fission triggers, we wouldn't have nearly so much trouble with getting fusion power plants to work, but more importantly, the energetic conditions required to overcome electrostatic repulsion in fusion components are so immense making any compact (warhead-like) method of generating it would require something with usable energy densities comparable to fissionable materials. To put it in perspective, a 1keV characteristic energy, which is pretty much a lower limit for any interesting nuclear reactions, is already around ten million kelvin. (Practically, it should be at least an order of magnitude higher.)

[GrandMasterTerwynn]

Quite simply, would there be any practical (as in practical to use in a warhead) way to initiate an explosive fusion reaction without a fission trigger as is required on modern thermonuclear warheads, and if so, how?

You have to compress and heat the hell out of the fusion fuel. Conventional explosives aren't going to cut it. So you need some sort of immensely powerful cluster of lasers, or a huge, powerful electromagnetic field generator. Both of these tend to be the size of large buildings, making them rather impractical as bombs. Collapsing bubbles in a fluid can briefly generate the temperature and pressures high enough to cause fusion, but to get a fusion reaction big enough to be of any use as a bomb, you'll need a bubble the size of a large swimming pool . . . and enough fluid to fill a very large swimming pool. So, the best way to create a pure fusion bomb is to do the following.

A) Gather approximately six solar masses of hydrogen into one place. Mutual gravitational attraction will cause this immense quantity of hydrogen to collapse into a big, bright ball plasma which will undergo fusion at a furious rate.

B) Gradually move the six solar masses of hydrogen towards your target. There aren't many ways to do this very quickly, but that's alright. You have ten million years to do so.

C) At the end of ten million years, the fusion reactor will have a core of iron, which takes more energy to fuse than is outputted by the resulting fusion reaction. The whole mess will collapse and release more energy being produced by all the stars of the galaxy combined for a short period of time. Your target will be entirely incinerated by the ensuing blast. However, you should expect significant collateral damage, and occupying the territory afterwards will be entirely impossible, as it will no longer be there.

[Darth Wong]

We have plenty of non-fission ways of initiating fusion, but none that work for weapons purposes. But if we're talking sci-fi, it wouldn't be too hard to imagine absurdly energetic laser ICF initiators being used as a trigger for a pure-fusion bomb.

[Admiral Valdemar]

Aside from masers and lasers working to initiate the reaction like NOVA and NIF which were used to model such third generation nukes, a fourth generation pure fusion weapon is beyond us. Some have brought up wacky ideas, but so far you still need fissionable material to get a thermonuke.

[HyperionX]

You could potentionally use a small fusion bomb to initiate a larger fusion bomb. Yes, that sound absurd at first but it is concievable that a small nuclear explosion is more readily produced than a larger. Thus, something like using laser induced fusion or bubble fusion that's impractical for make a huge explosion may still work.

[Darth Wong]

In sci-fi, with artficial gravity being a commonplace technology, it should be possible to compress the living shit out of a ball of D-T in order to make a fusion bomb. Not that this would be applicable to real-life.

[Admiral Valdemar]

You could potentionally use a small fusion bomb to initiate a larger fusion bomb. Yes, that sound absurd at first but it is concievable that a small nuclear explosion is more readily produced than a larger. Thus, something like using laser induced fusion or bubble fusion that's impractical for make a huge explosion may still work.

Since the problem is initiating fusion in the first place, how do you propose you start even this small fusion bomb?

In sci-fi, with artficial gravity being a commonplace technology, it should be possible to compress the living shit out of a ball of D-T in order to make a fusion bomb. Not that this would be applicable to real-life.

Yes, that or having some very compact lasers or other inertial cinfinement process. You could use exotic matter of like A/M, but then you may as well have a pure A/M bomb then (and we all know how dangerous that can be).

The difficulty in getting H-bombs to start is a blessing and curse somewhat.

[Ma Deuce]

Alright, so the concensus it if you're trying to be realistic, stick to fission triggers for thermonuclear warheads. As for the fission trigger, would there be any potential (probably made) fissionable element that would be more efficient than the P239 currently used, provided it could be manufactured in sufficient quantities?

Now on another matter, what about directing (at least part) the energy of a nuclear explosion in a single direction (call it a "shaped nuclear charge"). Right now I'm leaning toward a nuclear-pumped laser, as this was a concept that was seriously considered for SDI, but I'm interested in learning if this concept was truly workable or if there would be other methods of "directing" the energy of a nuclear explosion...

[Patrick Degan]

Alright, so the concensus it if you're trying to be realistic, stick to fission triggers for thermonuclear warheads. As for the fission trigger, would there be any potential (probably made) fissionable element that would be more efficient than the P239 currently used, provided it could be manufactured in sufficient quantities?

Past that point in the periodic table, you're talking about transuranic elements which exist only in laboratory experiments and for a very few seconds at best.

[tharkûn]

Alright, so the concensus it if you're trying to be realistic, stick to fission triggers for thermonuclear warheads. As for the fission trigger, would there be any potential (probably made) fissionable element that would be more efficient than the P239 currently used, provided it could be manufactured in sufficient quantities?

A good fissile primary will be dense, have a a high neutron cross sectional area, an efficient chain reaction, give off lots of X-rays, be relatively stable, have few non-fission neutron absoprtion reactions and be relatively workable.

All of the elements up to Einstenium last long enough for theoretical weaponization. I think some of the Cm isotopes have good neutron/X-ray production, I'm unsure about their neutron absoprtion reactions under supercritical conditions. Cm does give off a good number of alphas, but if you dope it with LiD those give off lots of energy and more heat. I'm pretty sure Am has bad neutron configurations; it is ubiquitious in smoke detectors, but I've never even heard is chain reaction properties discussed. Cf and Bk getting down to a thousand year half-life which makes for short shelflives or variable implosion settings; given the price of Cf and Bk little research has been done and I don't think the nuclear chain reaction potentials are all that impressive, but I haven't studied this in a long time.

Frankly though long, long before you look for a better primary fissile material you will want to fine tune the impregnation and make a more reactive tamper.

Now on another matter, what about directing (at least part) the energy of a nuclear explosion in a single direction (call it a "shaped nuclear charge"). Right now I'm leaning toward a nuclear-pumped laser, as this was a concept that was seriously considered for SDI, but I'm interested in learning if this concept was truly workable or if there would be other methods of "directing" the energy of a nuclear explosion...

To make a thermonuclear explosion you direct the x-ray energy of a fissile explosion. You can direct the X-rays up until the shockwave destroys whatever you are using to direct them - that or the x-rays completely ablate the material. Almost certainly a directed weapon is going to waste massive amounts of energy off axis, but you could get a decent directed x-ray pulse. The mass requirements may be extremely prohibitive without some type of unobtanium.

[Xon]

B) Gradually move the six solar masses of hydrogen towards your target. There aren't many ways to do this very quickly, but that's alright. You have ten million years to do so.

You only need 1 solar mass and you get 10 billion years.