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BalderSion t1_j0twoza wrote

Reply to comment by Graekaris in How do X-rays “compress” a nuclear fusion pellet? by i_owe_them13

Ablation pressure is basically the rocket equation. Radiation boils off the outermost layer, pushing that layer away from the pellet as a gas with some thermal energy. Equal and opposite reaction pushes the pellet in the opposite direction. Now make this evenly around the pellet and all the pellet can do is compress into a higher density.

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JanB1 t1_j0u1c35 wrote

So it's not just force exerted by the photons, but instead a different mechanic is going on?

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BalderSion t1_j0u6rwl wrote

Right. It's true that photons have momentum, but not much as these things go. It's rather more efficient that the photons boil the outer layer, and the reaction force from the gases boiling off push the pellet inward radially.

This sets up a situation where a light fluid is pushing against a heavy fluid (not unlike putting vinegar on top of oil in a salad dressing) so a slight nonuniformity amplifies because of the Rayleigh–Taylor instability, so some of the fuel squirts out, rather than compress uniformly and your target won't fuse. This is why the targets have to be so smooth and the radiation needs to be uniform.

There are some so called fast ignition schemes that aim to relax these requirements, but they haven't been demonstrated yet. We're on the path.

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JanB1 t1_j0u9egt wrote

Nice, thank you very much for this explanation! You seem to be knowing quite a lot about this.

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nicuramar t1_j0u7zv2 wrote

It’s the reaction force (Newton’s) resulting from material flying away at extremely high acceleration and thus force.

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