Hoop Stress and Radial Support
Field pushes each turn outward; hoop tension and centering loads must be carried by structure, not by the brittle REBCO layer.
The hoop and the centering force
A current loop in its own field experiences an outward hoop force that puts the winding in tension, plus a net centering force that pushes the toroidal-field coils toward the machine axis. In a compact spherical tokamak both are large because the field is high and the center stack is small. The structure exists to intercept these loads before they reach the conductor.
Load path
Hoop tension is reacted along the length of the conductor and its co-wound structure, where REBCO is strongest. Centering force is reacted by a wedged or bucked center stack that transfers load into a central column and the vessel structure. The design goal is to keep the REBCO layer in a controlled, mostly-tensile strain state and avoid transverse or compressive loading that the ceramic tolerates poorly.
- Hoop stress: outward tension, carried along the tape's strong axis.
- Centering force: inboard push, carried by the bucked center column.
- Overturning moment: from interaction with the poloidal field, carried by the case.
- Local peaking: highest at the inboard midplane where field is greatest.
Why the center stack is the crux
The inboard leg of the toroidal-field system sits where field and force are both highest, in the least space. It shares that volume with shielding and the center-post. Every millimeter is contested, which is why center-post lifetime and magnet stress are treated as coupled problems rather than separate ones.
This page documents a design and simulation study, not a built machine. Construction begins Q2 2027; first-of-a-kind first tritium is targeted near 2030. Figures are computed, reproducible targets, not measurements.