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Comment by ajnin | Hacker News Reader

That's basically what's going on with ITER, it needs to be so massive because the technology available at the time of conception was not capable of generating more intense magnetic field. There is a very strong relationship between the strength of the field and the required size of the tokamak (hence time and cost of construction). By the time you're done better technology will be available, but that's how progress goes.

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Unfortunately, it's not so simple. SPARC is just the latest attempt to benefit from stronger magnet technology and thus smaller designs, but another big problem remains unsolved: turbulence. Since smaller designs and stronger magnets directly imply higher field strength gradients, the plasma becomes harder to control for the necessary burn time to achieve Q>1. Lockheed also thought they could simply use stronger magnets to build smaller reactors faster, but in the end they discovered that smaller is not automatically better. I wouldn't be surprised if we also see the eventual SPARC redesign that's 100 times more massive. Unless we get a major theoretical breakthrough in the near future, ITER will probably remain the most likely design candidate for near term net energy production.

Can you provide a source for that? Because here's the head of MIT's fusion program, claiming the exact opposite: https://www.youtube.com/watch?v=KkpqA8yG9T4&t=2221s

See here for example: https://www-pub.iaea.org/mtcd/publications/pdf/csp_019c/pdf/... - but note that this stuff is not trivial. The intricacies of turbulence in MHD are incredibly complex and hard to study (near-impossible analytically and very hard numerically). Nevertheless, these scaling behaviours (see the figures in section 2) have been known for more than 20 years by now.

Also, beware that the head of MIT's fusion program (your source) is the head of SPARC. Since he siphoned a huge amount of money from gullible venture capitalists outside his field by now, you definitely shouldn't rely on him for an obective analysis of SPARC's fundamental design.

Well that paper is above my pay grade so I'll have to trust you on it.

I will push back on your second paragraph though. The presentation I linked was from two years before the founding of CFS. It seems likely to me that Whyte founded CFS because he believed in the physics he presented, rather than the reverse.

I see this sort of causation reversal and associated accusations all the time in internet discussions. For example, yesterday someone told me that Musk only likes electric cars because he owns lithium mining rights. Tesla bought those rights in 2020, because they produce lithium batteries.

SPARC is in fact a heavily marketed continuation of ARC, which has been around since 2014 (https://arxiv.org/abs/1409.3540 - note the same authors). So the commercialization ideas would have started way earlier. But a project lead believing in the project shouldn't matter in any objective analysis anyways. The guys over at Lockheed also certainly believed in their compact fusion reactor design when they started working on it (which was, incidentally, one year before ARC), but they too had to learn by now that belief alone is not enough when it comes down to the laws of physics.

Don't get me wrong, I'd love for SPARC to be a success. I just haven't seen any objective, external review that would confirm (or at least try to show) how they solved the problem of controlling turbulent plasma in a high gradient field. It's not even fully clear that ITER will be able to do that, and they'll have a much easier time due to the larger reactor radius.

The guys over at Lockheed weren't in charge of one of the leading academic fusion programs in the US. ARC in 2014 was just a design created by students in one of Whyte's classes.

At this point, as a woefully ignorant layman, I'm forced to weigh two contrary claims. One is by the head of MIT's fusion program. The other is by a commenter on HN, helpfully linking an impressive-looking paper I don't understand, which does not appear to say with layman-friendly clarity that higher magnetic fields make fusion plasmas less stable.

Hey, I'm not trying to convince you of anything. In fact I couldn't care less what people think of this specific project. It's not my money that these VC funds are spraying. I'm just trying to convey a modicum of common sense when it comes to evaluating sources for particularly extraordinary claims. I see Elon Musk get torn to shreds regulalry around here whenever he says something slightly optimistic about electric cars or rocket science, so I frankly find it astounding how easily people eat stuff that comes from guys behind private fusion companies. I guess plasma physics and continuum mechaniccs are less impugnable than rockets and cars for most people.

Well I was hoping you were trying to convince me, and would have an easier-to-digest source handy, or a simple explanation of the relevant part of the paper. You certainly don't owe me that, and I don't mean to doubt you, and for all I know you're a professional fusion researcher. Plenty of people like that are on HN. But I don't know, so I'm just muddling through as best I can, trying to get things right.

Personally, the claim that stronger fields make plasma more stable doesn't seem all that extraordinary to me, and certainly not one that's contrary to simple common sense. And when it comes to evaluating sources, Dennis Whyte seems like a fairly reputable one.

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