Central Solenoid and Poloidal-Field Coils

The solenoid drives plasma current and the poloidal coils shape the negative-triangularity plasma; both compete for scarce center-stack volume.

Two jobs, one tight space

Beyond the toroidal field, the breeder needs a central solenoid to induce and ramp the 9.66 MA plasma current, and a set of poloidal-field (PF) coils to position and shape the plasma. The chosen shape is a negative-triangularity D with triangularity -0.30, which the PF set must hold against the plasma's tendency toward other equilibria.

The spherical-tokamak squeeze

A compact center stack has little room for a large solenoid. This limits the available volt-seconds and pushes the design toward non-inductive current drive to sustain the plasma after ramp-up. The solenoid, TF inboard leg, shielding, and center-post all compete for the same central radius; the PF coils sit outboard where there is more room.

Shaping for negative triangularity

Negative triangularity is a deliberate choice that can suppress certain edge instabilities and ease the exhaust problem. Holding it requires the PF coils to maintain a shape that is less naturally stable than the conventional positive-triangularity D, so PF current and control margins are part of the design, not an afterthought.

• Central solenoid: current induction and ramp; volt-second limited on a compact stack.
• Poloidal-field coils: position, elongation, and the -0.30 triangularity.
• Non-inductive drive: sustains current beyond the solenoid's reach.
• Control margin: reserved to hold a negative-triangularity equilibrium.

Volt-second economy

A compact solenoid stores limited flux, so the breeder budgets its volt-seconds carefully: inductive ramp establishes the current, and non-inductive drive holds it. Running the solenoid harder to gain flux raises stress in an already-crowded stack, so the design leans on efficient current drive rather than a larger solenoid it does not have room for.

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.