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

  • Thread starter Thread starter Melbourne Guy
  • Start date Start date
Click For Summary

Discussion Overview

The discussion centers around the concept of the "ignition cliff" in laser-fusion experiments, particularly in the context of the National Ignition Facility (NIF) and its challenges in achieving reproducible fusion results. Participants explore the implications of this term, its causes, and related experimental techniques.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Some participants describe the ignition cliff as a threshold where a small change in initial conditions leads to a dramatic increase in fusion output, akin to reaching the ignition point of a combustible fuel.
  • Others suggest that the ignition cliff involves positive feedback mechanisms where increased energy production enhances further fusion reactions.
  • A participant draws parallels between the ignition cliff and historical challenges in material production, emphasizing the unpredictability and complexity involved in achieving fusion.
  • Some participants clarify that achieving ignition in fusion requires extremely precise initial conditions, contrasting it with the more forgiving conditions required for igniting fossil fuels.
  • Discussion includes details about different laser-fusion techniques, such as direct and indirect drive methods, and their implications for achieving homogeneous irradiation of fuel pellets.

Areas of Agreement / Disagreement

Participants express varying interpretations of the ignition cliff and its implications, with no consensus on a singular explanation or understanding of the term. The discussion remains unresolved regarding the precise nature and causes of the ignition cliff.

Contextual Notes

Participants note the complexity and precision required in laser-fusion experiments, highlighting the dependence on specific initial conditions and the challenges in achieving reproducibility in results.

Melbourne Guy
Messages
462
Reaction score
315
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
 
Engineering news on Phys.org
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...
 
Melbourne Guy said:
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.)
 
  • Like
  • Informative
Likes   Reactions: artis, Melbourne Guy and berkeman
PeterDonis said:
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
 
artis said:
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 👍
 
Melbourne Guy said:
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
 
  • Like
Likes   Reactions: Melbourne Guy