Negative Triangularity

The breeder plasma is shaped with negative triangularity of delta -0.30 to manage heat exhaust while preserving core confinement.

Shaping the plasma

Triangularity describes the D-shape of the plasma cross-section. Negative triangularity turns the D outward, the reverse of the conventional inward shape. The breeder uses a negative triangularity of delta -0.30 as a canonical design choice.

Why negative

• Improved edge behavior, potentially avoiding certain edge instabilities
• Favorable heat-exhaust characteristics for the divertor
• Compatibility with the compact spherical geometry and its strong shaping capability

The trade

Negative triangularity is less conventional than positive, and its confinement and stability behavior at the design current and field is exactly what the confinement gate G1 must verify. The choice is made because its edge and exhaust benefits suit a compact high-field machine, but it places real demands on demonstrating core confinement at delta -0.30.

How it interacts with the machine

The shape does not stand alone. It is chosen together with the 9.66 MA plasma current, the 8 T on-axis field, and the compact spherical geometry so that core confinement and edge exhaust are balanced at the design gain point. The divertor design in particular is coupled to the triangularity, because the exhaust the divertor must handle depends on how the plasma edge is shaped.

Consistency in representation

The negative triangularity is canonical and must be represented consistently, the plasma D-shape opens outward. This matters in the machine's models and visualizations, where a triangularity of the wrong sign would misrepresent the design. The program tracks correct triangularity depiction as a documentation requirement, and its digital twin must show the outward-opening cross-section rather than omitting or reversing it.

The delta -0.30 shape is a design choice for a machine not yet built. Its confinement and stability performance is a prediction to be tested at the confinement gate G1, reported measured-versus-predicted.