Because to achieve 1MW of energy from fusion, you put 200MW of power into the lasers. And, you have to fire these lasers with unfathomable precision at a tiny piece of gold, in order to heat up an even tinier pellet of hydrogen, which then heats up enormously for a few milliseconds before fizzing out.
ICF is a good way of studying what happens inside a hydrogen bomb, but it is in no way imaginable how you could use it as a weapon in itself. At this point, you'd be much, much better off just firing the lasers at your target (though even that wouldn't achieve much, unless you target is kind enough to step in front of a highly sensitive, gigantic laser).
Edit: corrected a typo graciously pointed out by GP.
What stops you from using the "even tinier piece pellet of hydrogen"[sic] from initiating fusion in a slightly less tiny pellet of hydrogen that it's sitting on top of, which initiates fusion in a slightly less tiny pellet of hydrogen, and so on? Aside from concern for your own survival, of course.
Maybe if you can't fathom the precision required to irradiate the NIF hohlraum sufficiently isotropically to achieve ignition in the first place, you shouldn't be trying to answer this question.
Nuclear fusion requires both compression and heating of the fuel. In nuclear weapons, this is accomplished with a combination of radiation pressure and a fissile sparkplug, respectively. In inertial confinement fusion, there are two distinct laser pulses with different characteristics. A fusion pellet detonating would release radiation that could compress another pellet, but there would be no method of heating that pellet at the appropriate moment.
There may be an engineering method to overcome this, but it would be way beyond the difficulty of getting that first pellet to ignite, which already is a bleeding edge technological development.
> What stops you from using the "even tinier piece pellet of hydrogen"[sic] from initiating fusion in a slightly less tiny pellet of hydrogen that it's sitting on top of, which initiates fusion in a slightly less tiny pellet of hydrogen, and so on? Aside from concern for your own survival, of course.
The same thing that stops you from igniting the initial pellet with the hohlraum - you don't have anything creating the kind of confinement necessary to keep the plasma together.
The only thing allowing the plasma to get hot enough for fusion is the initial velocity of the inward-spreading shockwave from the initial explosion of the outer shell of the pellet. As the velocity of this shockwave inevitably decreases, confinement is inevitably lost and the plasma dissipates and cools down.
Probably in principle you could use the energy of the first pellet's plasma to cause similar shockwaves in a second, larger pellet and so on, but that requires an entirely different geometry, its not just a matter of putting the second pellet close to the first one.
Between the primary fission bomb and the secondary fusion bomb there is a huge shield, so the shockwave of the first one hit's the second one at the same time everywhere, instead of hitting the top.
My guess is that to put a ternary fusion bomb you will need another even bigger shield, but IANANBS.
No, the ignition here is small - on the order of a stick of dynamite. There's no reasonable way to scale it up to the size of a hydrogen bomb. Even if the lasers could just be scaled up larger & still work, which they can't b/c the lasers would make too dense of plasma & block their own beams, the number you'd need & the geometry of trying to use it on a full scale bomb would be totally impractical even for a single test. Also, a city sized laser ignition source w/ a bomb at the middle wouldn't be a useful weapon even if it were possible
Man-made nuclear fusion is not self-sustaining, requires massive infrastructure to ignite very little of material.
Nuclear fission of unstable isotopes is self-sustaining chain reaction that converts a lot of matter into energy without much of hardware - just put some sub-critical mass of Plutonium into a sphere lined with conventional explosives.
Well, of course that's always been true in the past, but isn't "ignition" precisely the point at which it becomes self-sustaining? Isn't that the distinction between "ignition" and not "ignition"? I mean, you're not the right person to ask (you apparently think plutonium is a brand name), but maybe somebody reading this understands the issues.
You don't even need explosives to get a self-sustaining nuclear fission chain reaction if you don't want a bomb; Harry Daghlian did it accidentally, Fermi did it underneath Stagg Field in Chicago in 01942, we do it routinely to generate electricity, and 16 fossil natural nuclear fission reactors have been discovered in Oklo. The explosives are only there to keep a rapid chain reaction from driving the pieces apart before you get enough yield for a weapon.
It's true that the NIF would not make a very useful bomb, being difficult to deliver to enemy territory even by ship, and probably inflicting more damage on the funding agency than the destroyed enemy city. But a significant part of that is non-recurring engineering costs, and it's probably possible to miniaturize it to a significant degree.
I read Freeman Dyson's autobiography recently, and he claims (contrary to the report of continuing DOE research in the Wikipedia article) one of the things they stopped working on in the 01960s due to the arms treaties was specifically hydrogen bombs that didn't require fission igniters.
Ignition in the case of ICF means that, for the brief time while the shockwave from the initial laser burst is still keeping the plasma together, you get to fuse all of the D+T in your pellet. Once the initial velocity is lost, the high-temperature He dissipates away.
Not ahcieving ignition means that the plasma cools too rapidly and the fusion reaction stops even before the brief microseconds of inertial confinement are lost.
Perhaps if you could deliver enough energy to a large enough pellet, you could use this to build a bomb, but today it is far too small for that, and the reaction wouldn't work with a larger fuel pellet (the geometry that allows the extreme pressures needed for fusion would not be easily achieved with a larger pellet, since even the wave-length of the laser is relevant at this level).
If not, why not?