Negative Triangularity

Negative triangularity (δ = −0.30) shapes the plasma cross-section to suppress edge instabilities — the design invariant at the core of the Kronos MetroVolt architecture.

Symbol

δ (triangularity)

MetroVolt value

−0.30 (held by X-point radial control)

Why it matters

ELM-free edge protects direct-energy-conversion grids

Gate

G1 confinement testbed

Triangularity (δ) describes how the D-shaped cross-section of a tokamak plasma is tilted. Conventional tokamaks use positive triangularity, which points the plasma's corners toward the central column. Negative triangularity reverses that shape. Kronos MetroVolt holds a strongly negative value, δ = −0.30, as a conserved design invariant.

Three properties make negative triangularity load-bearing for the Kronos design:

• An ELM-free edge. The negative-triangularity edge suppresses Edge-Localized Modes by demonstrated regime physics. A single reactor-scale Type-I ELM would deliver tens of megajoules in sub-millisecond bursts — lethal to electrostatic direct-energy-conversion collector grids. Negative triangularity and direct energy conversion are therefore co-dependent.
• Core confinement with an L-mode edge. The 2023 DIII-D campaign demonstrated H-mode-grade confinement (H_98,y2 ≥ 1, β_N > 2.5) with an L-mode-like edge intact. Kronos treats negative triangularity as recovering confinement an L-mode edge would otherwise forfeit — not as a bonus multiplier.
• Density headroom. Sustained non-disruptive operation up to 1.8× the Greenwald density anchors the plant's operating choice.