G2: The Magnet Gate

The second gate proves the high-field magnet system reaches and holds 16.84 T peak field with 8 T on axis under operational load.

What G2 proves

G2 is the magnet gate. It demonstrates that the superconducting magnet system reaches the design fields, 16.84 T peak at the conductor with 8 T on axis, and holds them stably under the mechanical and thermal loads of operation. Field is what confines the plasma; without it there is no gain.

The engineering challenge

High field in a compact spherical tokamak concentrates enormous stress in the centerpost and toroidal-field structure. The magnet must carry current density high enough to produce 16.84 T while withstanding Lorentz forces and staying superconducting under quench-protected conditions. This is the hardest engineering test in the machine.

• Measured quantity: achieved peak field and on-axis field under load
• Pass criterion: 16.84 T peak / 8 T on axis, held stably through an operational cycle
• Registered prediction: field, conductor temperature margin, and quench behavior

Sequence

G2 is demonstrated during commissioning, before full plasma operation, because the field must be proven before the plasma that depends on it. Magnet energization, temperature-margin verification, and controlled quench testing are part of the gate. Only after the magnet holds design field is the machine cleared to pursue G1 confinement at the full operating point.

Status

The magnet is a design informed by REBCO high-temperature superconductor practice, but the specific coil set is not yet built. G2 is where conductor supply, winding quality, and structural design meet reality. The 16.84 T figure is a design target until the gate is passed.