The Toroidal-Field Coil Set

The toroidal-field system sets the on-axis field and confines the plasma; its inboard leg is the most stressed structure in the breeder.

Function

The toroidal-field (TF) coil set produces the field that, with the plasma current of 9.66 MA, confines the plasma. It is designed for 8 T on axis, which requires 16.84 T on the inboard conductor because field rises toward the center stack. In a spherical tokamak the TF system is dominated by its inboard leg, packed into the narrow center column.

Inboard leg dominates

Unlike a conventional tokamak with room for a thick inboard structure, the compact spherical tokamak forces the TF inboard leg, shielding, and center-post into a small radius. This concentrates field, force, and neutron flux in the same place. The outboard return legs are comparatively benign; the design is set by the inboard region.

Design choices

• REBCO conductor to reach high field on a thin leg.
• Wedged or bucked center column to react centering force.
• Grading of conductor along the leg to match local field.
• Shared radial budget with shield and center-post cooling.

The TF set is the reason the machine can be compact and the reason the magnet gate exists. Its performance targets are frozen; its structural demonstration at full field is FOAK work.

Grading along the leg

Because field varies along the inboard leg, the conductor can be graded so its cross-section and current density match the local field rather than the worst-case peak everywhere. Grading saves precious center-stack space and improves overall current-carrying efficiency, but it complicates winding and joint placement, so it is a deliberate trade rather than a default.

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.