I'm not super knowledgeable, but my understanding is that ITER was designed around the best superconductors known at the time, which meant making the whole thing really big.

A benefit of stronger magnets is that you can make it a lot smaller, and presumably cheaper -- which is what MIT has been working on with SPARC.

I don't know if it's practical to upgrade the magnets on ITER, but I'd expect it to be really expensive -- especially if they've already manufactured/installed the old magnets.

It would be completely impractical, since ITER is not designed to withstand the much larger JxB forces (which scale as B^2).

In the high Tc high field designs, the mass of the metal supports for the magnets dominates the reactor mass.

Thank you, that's the kind of point I was looking for.

Why does JxB scale quadratically? Because a stronger field can contain (linearly) more current?

If you double the current J, you double the magnetic field B.

But the JxB force is now quadrupled, because you doubled both J and B.

It means nothing for ITER unfortunately. Changing the toroidal field strength implies a totally different plasma size, density, temperature, everything. ITER isn't designed for this strong of fields and the plasma they would confine.

Nope. And this is fine.

ITER is not a prototype of a commercial fusion reactor, it's a burning plasma laboratory. Its main goal is to study the plasma confinement technology.

ITER is also supposed to be the final step before a commercial reactor. If SPARC works well and they can keep the pace they're at to make ARC, then ITER might be a dead-end in that regard, no?

Iter's is supposed to have a successor, DEMO, which would be the final step before a commercial reactor. It is doubtful whether DEMO will ever be built.

We already have enough experience that makes ITER obsolete.

Newer designs will have a much thinner central column and stronger magnets, resulting in much better confinement. And high-temperature superconductors will an order of magnitude cheaper.

Right now, ITER is still needed to get information on plasma properties at the temperatures required for fusion.