Magnetic Diagnostics for Control
Flux loops, pickup coils, and saddle sensors - the fast, robust magnetic measurements that anchor real-time control.
Measuring the field
Magnetic diagnostics measure the field and flux around the plasma. Flux loops (toroidal loops of wire) measure poloidal flux via induced voltage; local pickup coils measure the local field component along their axis; saddle loops measure non-axisymmetric field for mode detection. All are simple, fast, and reliable - the foundation of magnetic control.
From voltage to flux
These sensors respond to the rate of change of flux, so their signals are integrated in real time to recover flux and field. Integration is exact in principle but accumulates drift from small offsets over long pulses, a practical limitation addressed by careful electronics and periodic re-baselining. For the millisecond-to-second loops of control, integrated magnetics are accurate enough.
What they enable
- Plasma current and boundary shape via equilibrium reconstruction
- Vertical and radial position for stabilization
- Flux at isoflux control points for shape control
- Locked- and rotating-mode detection via saddle loops
- Loop voltage for current and volt-second bookkeeping
Strengths and limits
Magnetics are fast, cheap, and every-shot reliable, which is why they carry the fast loops. But external magnetics constrain the plasma boundary and total current far better than the internal current profile - two internal profiles can look nearly identical from outside. Internal profile control therefore needs kinetic diagnostics to supplement the magnetics.
Placement and coverage
Sensor placement is a design choice: enough loops and coils, well distributed poloidally and toroidally, to reconstruct the boundary and detect the mode numbers that matter, with redundancy so that a failed sensor does not blind a loop. Coverage is set when the machine is designed, because these sensors live inside the vessel and cannot be added later.