I mean, youâ(TM)re right that itâ(TM)s not here, but this is probably the safest fusion bet there is. We know really quite well how well Tokamaks scale. We have loads of examples of them, and we can predict really quite accurately how theyâ(TM)ll perform at this point. We know that there power density can be described as
(Beta_N ^ 2 R B^4) / q_* ^ 2
Beta_N and q_* are design features of the geometry etc of the tokamak, we know whatâ(TM)s achievable, and have plenty of examples already. That leaves you with two levers to pull - R (the major radius), and B (the magnetic field strength). When we first started building tokamaks we used copper for the magnets, we moved on to super conductors and increased the magnetic field strength, and then hit a wall. So that left us with only one lever to pull - major radius. Thatâ(TM)s why when you look at ITER, you see this *giant* device that costs billions. We know exactly what we need to do to break even - we need to increase the radius, and the power density will go up linearly, and weâ(TM)ll get there.
The thing is, in relatively recent years weâ(TM)ve got good at producing a bunch of new superconductors. We can now make REBCO tapes of significant length. The nice thing about REBCO, aside from not needing to be cooled as much as traditional superconductors, they can stand much higher magnetic field strength before they give up on superconducting. CFSâ(TM)s approach here is simple - build a device the size of existing tokamaks that weâ(TM)ve built at universities and research centres, but build it with REBCO superconducting magnets. That gets you double the field strength, which gives you 16 times the power density. JET already got to a Q factor of 0.72. And that was with copper magnets. Itâ(TM)s really really clear that SPARC *will* get to a burning plasma, because it can be more than 16 times as efficient as JET.
Thereâ(TM)s basically one question for whether ARC will work or not. When (not if) SPARC achieves a burning plasma, will all our techniques and algorithms for controlling the plasma still work, or will the new regime mean we have to completely re-learn how to do all of that stuff.
Long story short, can it fail? Yes - itâ(TM)s not done it yet, of course it can. Is it likely to? Not very. The physics is pretty well nailed down for most of this stuff.