Tokamak plasma confinement degrades with collisionality. As plasma becomes denser or cooler, particles collide more frequently, scattering across magnetic field lines and leaking energy outward. This is one of the foundational scaling laws of fusion research: higher collisionality means worse confinement. The entire field optimizes for low-collisionality regimes to maximize energy confinement time.
Negative triangularity plasmas — where the cross section of the plasma is shaped with an inward-pointing D instead of the conventional outward-pointing D — break this scaling.
The paper (arXiv:2603.12494, March 2026) measures normalized energy confinement in negative triangularity discharges and finds that confinement weakly improves with increasing collisionality. The relationship inverts. The trend that damages confinement in conventional tokamaks helps in negative triangularity geometry.
The mechanism involves edge turbulence suppression. In conventional tokamaks, the edge region drives turbulent transport that worsens with collisionality. In negative triangularity, the reversed shaping changes the magnetic field curvature at the edge, suppressing the turbulent modes that normally drive transport. As collisionality increases, it enhances the collisional stabilization of these edge modes — the same process that hurts confinement in the core helps at the edge when the geometry is reversed.
The structural lesson: a scaling law is a relationship between variables within a specific geometry. Change the geometry, and the relationship can reverse. “Higher collisionality worsens confinement” is not a law of plasma physics — it is a law of conventional tokamak geometry. In a different geometry, the same physics produces the opposite outcome. The scaling law encoded the shape of the container, not just the physics of the contents.