We've had thermonuclear weapons (h-bombs) for a long time already.

Harnessing the energy in a controlled and sustainable fashion is what's hard.

No, you can have a fusion booster, like in the "Sloika" [0] design, but for a Teller-Ulam design, that is a H-bomb, you use a nuclear primer to ignite a fusion reaction and by far the most energy comes from the fusion part. [1]

[0] https://www.atomicarchive.com/history/hydrogen-bomb/page-11....

[1] https://nuclearweaponarchive.org/Library/Teller.html

You are correct that in a Teller-Ulam design most of the energy can come from fusion, but as a note it's generally a fission-fusion-fission design in "modern" deployments with about 50% of the energy coming from fusion.

https://en.wikipedia.org/wiki/Thermonuclear_weapon

Only very large bombs, such as the Tsar Bomba (97% from fusion) ever where primarily fusion, and those are not in modern arsenals.

Half of the energy comes from the fissile "spark plug" that's inside the fusion material. Less than half of the energy comes from fusion.

It still very much is a fission weapon.

You have it backwards. The fission component is just the "trigger" for the fusion element, which produces the vast majority of the energy release.

Half of the energy comes from the fissile "spark plug" that's inside the fusion material. Less than half of the energy comes from fusion.

It still very much is a fission weapon.

Nope...

"thermonuclear bomb, also called hydrogen bomb, or H-bomb, weapon whose enormous explosive power results from an uncontrolled self-sustaining chain reaction in which isotopes of hydrogen combine under extremely high temperatures to form helium in a process known as nuclear fusion."

Yes, but if you get into the details... The second stage of a bomb is the fusion in which a cylinder of hydrogen and/or lithium is encased by U-238. The primary stage, a trinity-like a-bomb, forces the cylinder to compress by its x-rays. The fusion reaction creates a bunch of energy, but more importantly a huge amount of neutrons. These neutrons cause the u-238 to undergo fission, which is responsible for a majority of the energy and pretty much all of the fallout.

The fissile "spark plug" inside the fusion material is responsible for the majority of the energy...

The appropriate analogy for this technology would be that it may be possible to initiate a thermonuclear weapon without relying on fission at all. Currently we use a fission nuclear bomb just to generate the temperature and pressure needed to start the fusion reaction, same as the one on today's announcement.

So far it hasn't proven to be viable, but time will tell.

This is already used for nuclear weapons research, which is why it's under Dept. of Energy.

Not really. I mean, yes, nuclear weapons are a thing, but Dept. of Energy supports many many directions not related to nuclear weapons. Physics research is mostly funded by DOE Office of Science or the National Science Foundation.

It's an explicit goal of the NIF to better understand the physics of fusion for weapons research. Not the main one but it's pretty important!

My point was: NIF would be funded via DOE whether or not it's relevant for weapon research. Sorry for not have been clearer.

I'm no expert, but I think you have it backwards. My understanding is NIF raison d'etre is weapons research, with a power generation being a secondary concern. It got funded because of weapons research regardless of whether it was relevant to fusion power generation.

It may surprise people, but the DOE is the government body that is responsible for nuclear weapons research in the US.

If it was already decided to be funded, yes it would have been under DoE. Though I believe the weapons aspect had a very major contribution in deciding for it to be funded at all. It was proposed shortly after the nuclear testing ban and has been a big part in fulfilling that area.

I'm not trying to correct you, but adding context for the weapons aspect.

Yes, you are right, the funding decision was probably influenced by the weapons research.

That doesn’t follow. The DOE covers a bunch of energy research that has no relationship to nuclear weapons (e.g. solar).

Cheap and abundant energy has a multitude of military applications.

Peaceful ones too, probably.

Uhhh... it's definitely being used for "stockpile stewardship." The fusion crowd went splitsies with the weapons folks to get this funded in the first place.