Error Field Correction
Small unavoidable asymmetries in the field can lock plasma modes and trigger disruptions, so correction coils cancel these error fields.
What an error field is
An ideal tokamak field is perfectly axisymmetric, but real coils have small misalignments, joints, and manufacturing tolerances that add tiny non-axisymmetric field components. These error fields are a small fraction of the main field, yet they can resonate with the plasma and cause outsized harm, especially at low density.
Why it matters
An error field can penetrate the plasma and slow the rotation of a magnetic island until the mode locks to the wall. A locked mode grows and often leads to a disruption. Because the threshold error field for locking can be a very small fraction of the toroidal field, even minor asymmetries must be corrected, particularly during the vulnerable start-up phase.
Correction coils
Dedicated correction coils, arranged around the torus to produce controllable non-axisymmetric fields, cancel the intrinsic error field. Control sets their currents to minimize the plasma's response, using either a pre-characterized compensation from earlier operation or real-time feedback on the detected mode. The optimum correction can depend on plasma conditions, so it is adjusted through the pulse.
- Coil imperfections create small non-axisymmetric fields
- Error fields can lock modes and cause disruptions
- Locking threshold can be a tiny fraction of the field
- Correction coils cancel the intrinsic error field
Beyond correction
The same correction coils are often used deliberately to apply controlled non-axisymmetric fields for edge-instability control, so the hardware does double duty. Control must separate the intended applied field from the correction of intrinsic errors. In the Kronos breeder design study, field-symmetry control is part of a reliable start-up and steady-operation scenario for the modeled spherical tokamak; the machine is a design and simulation case.
Correcting a field a thousand times smaller than the main field is a good example of why fusion control must be precise.