Magnetic Diagnostics for Equilibrium Reconstruction
Flux loops, magnetic probes, and Rogowski coils supply the measurements that equilibrium reconstruction codes fit, and their modeling is part of the code chain.
The sensor set
Equilibrium reconstruction is only as good as the magnetic measurements it fits. The core sensor set includes flux loops that measure poloidal flux at fixed points, magnetic probes (small pickup coils) that measure the local field direction and magnitude, Rogowski coils that integrate to give enclosed current, and diamagnetic loops that measure the change in toroidal flux due to plasma pressure.
From signals to constraints
Each sensor is represented in the reconstruction code by a response function: given a trial equilibrium and coil currents, the code predicts what that sensor should read. The reconstruction adjusts the free functions until predicted and measured signals agree within their uncertainties. Accurate sensor geometry and calibration are therefore embedded in the code's model.
The role of conducting structures
Currents induced in the vacuum vessel and other passive conductors add to the measured field. Reconstruction codes carry a model of these structures, either as lumped current filaments or a finite-element mesh, so that induced currents are accounted for rather than mistaken for plasma current.
Integration drift and calibration
- Magnetic signals are often integrated from voltages, so slow drift must be corrected
- Cross-calibration against known vacuum shots isolates sensor errors
- Redundant sensors let the code detect and down-weight faulty channels
Why it belongs in the code discussion
The diagnostic model is not separate from the reconstruction code; it is part of it. Adding a new diagnostic, such as an internal pitch-angle measurement, means adding a new response term and re-tuning the fit. The quality of every downstream analysis traces back to how faithfully these responses are modeled.
In design studies, synthetic versions of these diagnostics are built to test whether a planned sensor set will constrain the equilibrium well enough for control and analysis.