What is the 'ignition cliff' and what causes it in laser-fusion experiments?

[Melbourne Guy]

This recent article on NIF's failure to replicate their breakthrough results of last year has a number of interesting points for me...and one confusing term: ‘ignition cliff’.

It is described by Riccardo Betti, who heads the laser-fusion centre at the University of Rochester in New York, in this way.

“If you are on one side of the cliff, you can get a lot of fusion output, and if you are on the other side of the cliff, you get very little.”

Is there a layman's description for what causes such dramatic uncertainty?

Also, based on this article, Omar Hurricane, chief scientist for Livermore’s inertial-confinement fusion programme, seems a pretty laid back type of guy.

https://www.nature.com/articles/d41586-022-02022-1

 

[hutchphd]

I believe the term describes what happens when one approaches break-even and the increase in produced energy aids in further fusion, providing positive feedback

https://www.lle.rochester.edu/media...ions/documents/APS17/Christopherson_APS17.pdf

 

[Nik_2213]

IMHO, they've run into the same sort of 'Unknown Unknown' that confounded poly-ethylene plastic and 'Fogbank' aerogel production.

IIRC, the former was made by accident, but reproducibility was impossible until the need for catalytic trace of oxygen was established. The latter, the 'packing' in 'classic' thermo-nukes, could not be made a-fresh using original recipe. Although details are [REDACTED], it seems modern 'Analar' reagents are just too pure, took a lot of expensive, urgent research to solve...

 

[PeterDonis]

Is there a layman's description for what causes such dramatic uncertainty?

It's not uncertainty, it's a large change in output power from a small change in initial conditions. The basic idea is the same as what happens if you gradually raise the temperature of a combustible fuel until you reach its ignition point. Just before the ignition point, nothing is happening; just after the ignition point, you have a large fire. In other words, a large change in output power from a small change in initial conditions. (The fact that the word "ignition" is used in both contexts is not a coincidence.)

 

[artis]

It's not uncertainty, it's a large change in output power from a small change in initial conditions. The basic idea is the same as what happens if you gradually raise the temperature of a combustible fuel until you reach its ignition point. Just before the ignition point, nothing is happening; just after the ignition point, you have a large fire. In other words, a large change in output power from a small change in initial conditions. (The fact that the word "ignition" is used in both contexts is not a coincidence.)

@Melbourne Guy
I would only add that there are vast differences in terms of how precise those "initial conditions" need to be in order to achieve ignition. For ordinary fossil fuels in order to ignite the chemical reaction one can have a wide set of conditions under which it will still work, better or worse, but work.
Meanwhile for experiments like NIF with a tiny fuel pellet you need extremely precise focusing of the lasers and very tight initial conditions and only those will result in an even heating and implosion which will result in high enough T in short enough time to produce significant fusion yield which will then produce alpha particles that will further aid in heating the fuel etc.

So although the terms are similar it's a lot different than throwing a lit match into a puddle of gasoline

 

[Melbourne Guy]

So although the terms are similar it's a lot different than throwing a lit match into a puddle of gasoline

Thanks, @artis, fusion is clearly hard science...and even harder engineering, from what I've read! @hutchphd's link was helpful for me, and I took @PeterDonis' words as a useful analogy to clarify what I took as uncertainty but isn't

 

[artis]

Thanks, @artis, fusion is clearly hard science...and even harder engineering, from what I've read! @hutchphd's link was helpful for me, and I took @PeterDonis' words as a useful analogy to clarify what I took as uncertainty but isn't

By the way , NIF from what I recall can do both "direct drive" as well as "indirect drive" shots, they differ in that in direct they shine the laser beams directly onto the surface of the pellet giving the pellet surface material so much heat in such rapid time that it quickly forms a plasma and material is ejected from the surface causing what is know as the "rocket effect" driving an implosion inwards.

These experiments that you talk about are I think all indirect drive where they shine the lasers onto the inner surfaces of a hollow high Z (gold) material where at the center is the fuel pellet. The laser heats up the material inner surface so that it starts to emit X rays and those X rays then cause the fuel pellet surface heating and ejection of material and compression.

From what i understand with this method it is easier to achieve a homogeneous irradiation of the pellet surface and therefore a better implosion, but i could be wrong on this.

This second process by X ray surface irradiation and rocket effect implosion also happens to be the same exact one happening within a thermonuke.
Also like in a nuke they seem to use a tamper within the outer layer of the fuel pellet. I once thought the pellet is all the same but it turns out it's not, the DT fuel is in the middle and they use various materials for the outer coating/layer. This outer layer is the one that ablates away and drives the rest of the sphere inwards.

Here are some diagrams you might find interesting
https://www.researchgate.net/figure/The-hohlraum-target-used-for-indirect-drive-CHSI-fusion-at-NIF_fig4_235664650