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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[DDopson]

Can any of the plasma experts explain with more precision what it means for a fusion reaction to be "self-sustaining"?

I've long been under the impression that these fusion reactions happen at such incredibly high temperatures that they radiate heat outwards (as various forms of light) faster than the reaction can replenish that heat. For example, if you detonate a nuclear weapon in the middle of an infinitely large tank of D+T gas, maybe you get a little bit of fusion, but you don't get a self-propagating wave of fusion that matches the intuitions people have for self-sustaining chemical combustion waves. Not unless the D+T gas has been implosively compressed to extreme densities that can only be sustained transiently during a bomb or an inertial confinement test.

There's also the minor inconvenience that 82% of the energy released by D+T fusion goes into the 14 MeV neutron, which is unlikely to interact with the plasma, at least in a tokamak. Between this and the radiative heat loss, my (possibly naive?) intuition was that even if a tokamak is producing several Q of net heat, it still needs continuous electrical input to keep the plasma from cooling off.

Self-sustaining fusion seems to require extreme densities. Perhaps this is possible in the brief transient conditions of an inertial confinement? But then presumably, this can't be scaled because a larger fuel pellet would have more inertia and thus wouldn't achieve the same degree of compression. Am I thinking about this correctly?

Like, is there some critical density where the fusion reaction becomes a positive feedback cycle where it creates heat faster than heat is lost, resulting in a runaway reaction rate that consumes a substantial fraction of the fuel pellet at a temperature much higher than can be explained by the input power?

Also, do we have a sense of how much Q and / or other aspects it would take for a tokemak to operate continuously without electrical heating input to the plasma? And is that considered important?

 

[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).