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Question
Why does Kronos use negative triangularity?
Three load-bearing reasons: an ELM-free edge that protects the converter, H-mode-grade core confinement with an L-mode edge, and density headroom.
Negative triangularity is not a tuning knob in the Kronos design — it is a conserved invariant, and three arguments make it load-bearing:
- Grid survival. The negative-triangularity edge is ELM-free. A single reactor-scale Type-I ELM would dump tens of megajoules in under a millisecond — lethal to the electrostatic direct-energy-conversion collector grids. The shape and the converter are co-dependent.
- Core quality with an L-mode edge. The 2023 DIII-D campaign demonstrated H-mode-grade confinement (H98,y2 ≥ 1, βN > 2.5) with the L-mode-like edge intact.
- Density headroom. Sustained non-disruptive operation to 1.8× the Greenwald density anchors the plant's operating window.
Questions & answers
Why negative triangularity instead of the usual shape?
Three reasons carry the choice. (1) Grid survival: the negative-triangularity edge is ELM-free, and a single large ELM would destroy the electrostatic direct-energy-conversion grids. (2) Core quality: DIII-D showed H-mode-grade confinement with an L-mode edge. (3) Density headroom: negative-triangularity plasmas run non-disruptively to 1.8× the Greenwald density.
What happens if the confinement bet fails?
A negative result at the G1 confinement testbed re-sizes the machine along a stated fallback rather than killing the concept — but it would materially affect the economics, and Kronos says so openly.
Is the shape negotiable?
No. δ = −0.30 is treated as a conserved design invariant, because negative triangularity and direct energy conversion are co-dependent.
Honest gapThe first programmatic gate is a sub-scale NT-ST confinement experiment. A negative result re-sizes the machine along the stated fallback rather than killing the concept — but it would materially damage the economics, and the design series says so.