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Graekaris t1_j0tm696 wrote

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

Is ablation pressure just a special case of radiation pressure then? Utilising wavelengths with poor penetrative ability for higher efficiency in applying the pressure?

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Jon_Beveryman t1_j0tmfhb wrote

No. Radiation pressure is the pressure exerted by the radiation itself. The ablation pressure is a material response to the radiation heating.

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Graekaris t1_j0tpvtb wrote

I see. In this application, is the radiation pressure comparable in significance to the ablation pressure or is it negligible?

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Jon_Beveryman t1_j0u3bff wrote

In this application the radiation pressure is pretty minimal yeah. I haven't seen numbers for it myself, but in some other settings where you care about direct radiation pressure & ablation pressure, you usually discard the radiation pressure term entirely unless you are very close to the source or it's an incredibly potent source. For instance, in Teller-Ulam type thermonuclear bombs, the radiation pressure from the fission stage is assumed to provide virtually all of the implosion pressure for the fusion stage [going by unclassified sources only ofc, e.g Winterberg "The Physical Principles of Thermonuclear Explosive Devices"].

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nicuramar t1_j0u7rb3 wrote

The ablation pressure is much higher, making the radiation pressure mostly or entirely irrelevant.

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BalderSion t1_j0twoza wrote

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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