Conductor Critical-Current Margin

REBCO critical current falls with field, temperature, and field angle; the breeder must keep margin at its worst-case operating point.

Margin, not a single number

REBCO's critical current is not one value; it depends on operating temperature, local field magnitude, and the angle of the field to the tape face. The magnet must be designed with margin at its worst combination of these, which in the breeder is the high-field inboard leg. Running too close to the critical surface leaves no headroom for disturbances.

The angle trap

Because critical current is anisotropic, a coil can look fine on a magnitude-only check and still be marginal where the field runs across the tape face. The design orients the conductor so the worst angle does not fall on the peak-field region, and it verifies margin on the full field map rather than at nominal points.

Coupling to temperature

Higher operating temperature reduces critical current, so margin trades against cryogenic load. A colder magnet has more margin but a heavier refrigeration burden. The breeder's operating temperature is chosen to keep worst-case margin adequate while keeping the cryogenic plant tractable, a balance documented in the cryogenics pages.

Critical-current margin is inseparable from stress, quench, and cryogenics: they are four views of one magnet design.

Verification on the field map

Margin is not accepted at nominal points alone; it is verified across the full computed field map, including field angle, so a coil that passes a magnitude-only check but runs marginal at a bad angle is caught in analysis rather than in operation. This map-based verification is a prerequisite to trusting the magnet at the FOAK gate.

Construction begins Q2 2027; first-of-a-kind first tritium is targeted near 2030. Figures are computed, reproducible targets, not measurements.