The D-shaped asymmetry of a tokamak plasma cross-section — with recent research showing that negative triangularity may offer superior confinement without the damaging edge instabilities of conventional H-mode.
Triangularity (δ) quantifies the D-shape of a tokamak’s plasma cross-section. Positive triangularity (δ > 0) means the plasma is wider on the outboard (low-field) side, creating the familiar D-shape pointing outward. Negative triangularity (δ < 0) reverses this, with the wider side pointing inward (toward the central column).[1]
Higher positive triangularity improves the Troyon beta limit and stabilises ballooning modes by reducing the pressure gradient on the bad-curvature (outboard) side. This has been the standard design choice since the 1980s and is used by ITER, JET, and most major tokamaks.[2]
Recent experiments at DIII-D and TCV have shown that negative triangularity plasmas can achieve H-mode-level confinement while remaining in L-mode, avoiding edge-localized modes (ELMs) entirely. Since ELMs are one of the most dangerous transient heat loads for plasma-facing components, this “ELM-free H-mode-quality confinement” is extremely attractive for reactor design.[3]
The negative triangularity approach is being actively investigated for next-generation reactor designs.