Halo Current and Force Limits

A disruption drives halo and eddy currents through the vessel; the safety envelope caps the forces these can exert on the structure.

Currents that push on the machine

When the breeder plasma moves or terminates rapidly, it drives halo currents through the plasma edge into the vessel and induces eddy currents in conducting structures. These currents, crossing the strong magnetic field, produce large mechanical forces. The safety envelope treats the peak force and its asymmetry as protected limits, because exceeding them is a structural hazard rather than merely a lost pulse.

def structural_ok(halo_fraction, toroidal_peaking, field_T, limits):
    # force scales with halo current * field; asymmetry concentrates it
    effective_load = halo_fraction * toroidal_peaking * field_T
    return effective_load <= limits['max_load']

# both magnitude and asymmetry (peaking) matter for the local force
assert structural_ok(0.3, 1.5, 8.0, {'max_load': 4.5})

Why mitigation shape matters

A symmetric, well-mitigated shutdown spreads the halo current and its force around the vessel; an asymmetric one concentrates it. This is the structural reason injection timing and uniformity are safety-critical, not just performance-critical. Toroidally distributed injectors reduce the peaking factor that drives local forces.

• Protected quantity: peak halo current fraction and its toroidal peaking factor.
• Guarded by: mitigation that terminates the plasma before halo currents grow large.
• Monitored by: distributed structural and current diagnostics feeding the reflex tier.

The forces are also orientation-dependent, so the diagnostics feeding the reflex tier resolve not just the magnitude of the halo current but its toroidal distribution, since a symmetric current of a given size is far more benign than the same current concentrated on one sector. Keeping the peaking factor low is therefore a live objective of the mitigation, and the structural envelope is expressed in terms of both magnitude and asymmetry rather than a single scalar limit.

Halo-force limits are part of the breeder fail-safe state definition: a mitigated shutdown is only 'safe' if it stays inside the force envelope during ramp-down. These loads are being characterized in simulation as part of the pre-FOAK safety case, and validated against the structural model during commissioning.