Negative vs Positive Triangularity
Positive triangularity buys easy H-mode access with dangerous ELMs; negative triangularity trades that for an ELM-free edge. Why Kronos picked the reverse shape.
- Positive δ
- Easy H-mode access, but Type-I ELMs
- Negative δ (Kronos)
- ELM-free edge, L-mode-like edge, H-mode-grade core
- Kronos value
- δ = −0.30 (invariant)
- Demonstrated
- DIII-D 2023; density to 1.8× Greenwald
Triangularity is the tilt of the plasma's D-shaped cross-section. The two choices lead to very different machines.
Positive triangularity is the conventional choice. It gives easy access to H-mode — a high-confinement regime — but H-mode comes with Edge-Localized Modes (ELMs): periodic bursts that fling megajoules at the wall in under a millisecond. For most concepts ELMs are a wall-erosion nuisance; for a machine with electrostatic direct-energy-conversion grids they are fatal.
Negative triangularity gives that up on purpose. It runs an ELM-free, L-mode-like edge while recovering H-mode-grade core confinement — a combination DIII-D demonstrated in 2023 (H₉₈,y2 ≥ 1, βN > 2.5). It also runs to higher density (1.8× Greenwald) without disrupting. Kronos treats δ = −0.30 as a conserved invariant precisely because the ELM-free edge is what makes direct energy conversion survivable.