Fusion energy breakthrough by US scientists boosts clean power hopes

First-ever net energy gain from fusion raises hopes for zero-carbon alternative.

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[cerberusTI]

That is not correct. Reducing field strength will cause the ignition temperature to skyrocket to impossible to reach temperatures. As soon as ignition temperature increases beyond the temperature of the plasma fusion will stop. We are talking from full power to zero fusion activity on a scale of microseconds.



We do have good control over it. It is called field strength. Control of the neutral beams would not be positive safety. As an example if the cooling pumps fail then even shutting the neutral beams off completely would not halt the rise of the temperature inside the reactor. Neither would stopping fuel injection (at least not in a timely manner). Temperature would rise to a point that you have catastrophic failure. Reducing the field strength though would skyrocket the ignition temperature and almost instantly (sub second scale) stop all fusion and heat generation.

Probably one of the only advantages to the fact that the conditions for controlled fusion at anything less than stellar mass scales are so incredibly tough means that stopping fusion is a simple already solved problem. It is so incredibly hard and requires continual active effort to maintain those conditions that not actively maintaining those conditions will halt the fusion.

Microseconds may seem like a short time, but in this case it really is not.

Magnetic confinement exerts pressure to balance the pressure from the plasma as it reaches higher temperature, but it is not the only way to exert pressure. The acceleration imparted to particles leaving the reaction has an equal but opposite reaction which also contains the reaction, keeping the remaining reactants in. As the number of fusion events you can expect increases with temperature, this can produce a runaway effect where the temperature increases fast enough to overcome expansion to a lower pressure.

You get increasingly more reactions as the temperature rises, and it is hot and dense enough to rise without input if you have infinite Q. I am not saying you could not control it or make it safe somehow, but that is the point at which the reaction wants to run away at the current temperature and pressure, with the reaction rate naturally increasing at an ever higher rate.

Maybe it would end up more like pulsing than exploding, but my guess is that this is a line we avoid crossing for practical reasons in a power reactor (I am sure they would love to have that problem right now).

 

[Chuckstar]

Not really, but I bet the person whining about downvotes for insisting we spent a "ton of money" on fusion does.

I don’t care. That’s why I said “who cares”. See how that works? You’re the one who brought up Musk, as though him spending $44 billion on Twitter must mean that’s not a ton of money.

And no one complained about downvotes… nice strawman.

 

[DDopson]

Self-sustaining means that the fusion itself heats the plasma enough so that no other heating is required Confinement is still necessary, though.
It's hard to define what "self-sustaining" means in case of inertial confinement with spherical symmetry, because the conditions are rather uniform and the duration of fusion conditions is extremely brief. It's about as self-sustaining as a nuclear detonation. In case of so-called "fast ignition", self-sustaining would indeed mean that the fusion that starts in a hot spot propagates as a kind of a wave through the fuel pellet.
In case of magnetic confinement, the (charged) fusion products heat the plasma enough to maintain the fusion-relevant temperature. This reaction is not an uncontrolled explosion because the temperature can get too high for given fuel type and the heat would be radiated away faster, cooling down the plasma. You still need the magnetic fields to confine the plasma and possibly some neutral beams or RF sources to support plasma currents (in a tokamak or FRC; a stellarator doesn't need a plasma current, so it can be even more "self-sustaining").

Ignition means infinite (scientific) Q, because you don't have to heat the plasma. The engineering Q will of course be finite, because you have to expend energy on cooling, etc and, in case of tokamak, on maintaining the toroidal plasma current I don't remember exactly how much that is, but some numbers I vaguely remember are about an order of magnitude smaller than the heating.

Thanks, that's a great answer. Yeah, I guess now that you've explained it, it's pretty obvious that ignition would imply Q=infinity. And the distinction of engineering Q versus scientific Q is helpful.

When I was asking "what Q would a tokamak need for ignition?", I'm now realizing that I really meant, "how much faster would the fusion heat production need to be to sustain ignition?", which then probably translates to better established questions like "how much better does the confinement need to be?", which the internet tells me is measured by the Tau parameter (although I would call this a "metric" as parameters mean inputs in my part of the engineering world).