Given this knowledge, and given that ITER was designed with state-of-the-art magnets, you’d have to conclude SPARC had trouble doing this math. Except ITER was designed in the 1990s and 2000s. Since then, high-temperature superconductors that have much better performance have been discovered and brought into commercial production. Using these modern superconductors, the magnetic-field strength can be doubled, allowing the size of the tokamak to be reduced considerably.
The future’s so bright?
While both ITER and SPARC are entering unknown territory, there is quite a bit of difference between the two projects. ITER has been modeled and studied to the nth degree. Teams of scientists have worked on every aspect for decades to try to predict the performance of ITER. SPARC, as a smaller device, cannot simply transfer the numbers and scale everything down by a factor of two. The recent papers try to address the challenge of modeling the new design.
They show that, fundamentally, SPARC seems sound. The plasma should reach the right conditions. The plasma should be able to maintain itself—it can carry a current that generates a magnetic field that helps confine itself—for about the same time as similarly sized tokamaks. That seems OK.
On the other hand, instabilities are likely to be exacerbated because the plasma is denser. In particular, phenomena called edge-localized modes could develop faster and be harder to suppress (or reduce). These instabilities occur at the edge of the plasma and, at worst, lead to hot plasma exhausting itself on the vessel walls. Other instabilities are disruptive in different ways, leading to reduced confinement and lower temperatures, so these generally need to be controlled.
These instabilities, if not controlled, can result in massive currents flowing in the vacuum vessel with extensive damage. This is the sort of scenario that gives ITER engineers nightmares, and the situation is not much different for SPARC: large currents, the whole machine jumping off its foundations, and other enjoyably dynamic disasters are possible. However, SPARC also seems to behave similarly to existing tokamaks, meaning that the predicted instabilities should be controllable.
A diverting puddle of tungsten
Where things really seem marginal is in the diverter. In every tokamak, there is a null point in the magnetic field. Particles don’t just leak through the null—they spray like a firehose. The diverter is the chosen place where this spray of particles hit a surface.
Even in current-generation tokamaks, the diverter materials don’t survive very long. In ITER, the diverter is going to be subject to conditions that are even more extreme. SPARC may make ITER look like warm milk.
Under their most pessimistic scenario, tungsten bricks will cyclically melt and recrystallize. During this process, tungsten atoms will probably penetrate to the core plasma, cooling it, and may even quench the fusion reaction. Carbon, an alternative, is a sacrificial surface that doesn’t kill the plasma. So carbon may end up being used in SPARC so that they can demonstrate that fusion works.
But the end result of using carbon will be organic molecules with a high percentage of tritium—not something to be messed around with. And definitely not something that should be considered for a commercial reactor.