A fusion reactor is like an orchestra, where the plasma is the melody and the magnetic coils are the musicians. If one instrument plays out of tune, the whole symphony falls apart. In the under-construction SPARC tokamak (a compact device with a record-breakingly strong magnetic field), even a tiny imperfection in the coil shape can trigger dangerous plasma oscillations that could disrupt the reaction.
Engineers at Commonwealth Fusion Systems have figured out how to keep the melody in tune. They designed additional coils that compensate for the asymmetry of the main magnetic field, like a conductor who gently corrects an out-of-tune violin with a subtle gesture. The key achievement is the calculation of the optimal geometry for these coils: they precisely "hit the note" of plasma oscillations and damp them with minimal energy cost.
Special attention was paid to the most troublesome type of perturbation — the so-called "locked modes," which can completely stop the plasma. Simulations showed that the new coils handle them even under the worst assembly tolerances. This gives confidence that SPARC can reach new operating regimes without the risk of sudden disruptions.
Now that the orchestra is tuned, we can start rehearsing the main concert — a sustained fusion reaction that could one day provide humanity with clean energy.
SPARC's magnetic coils are made from a high-temperature superconductor — a material that conducts electricity without loss at relatively "warm" temperatures around -200°C, rather than near absolute zero like classical superconductors.
The idea of taming plasma with magnetic fields evokes the plot of the movie "Spider-Man 2," where the hero tries to contain an artificial sun in his laboratory — though there, it doesn't end happily.