NSTX-U employs a coordinated set of diagnostics to constrain magnetohydrodynamic force balance and reconstruct the plasma magnetic equilibrium, including the plasma boundary, flux surfaces, and derived quantities such as q-profiles and shaping parameters. A key strength of this diagnostic set is its constraint complementarity: magnetic sensors provide edge field/flux constraints and accurate plasma current (I_p) measurements, while Motional Stark Effect (MSE-CIF) adds internal magnetic pitch-angle constraints that reduce ambiguity in current-profile inference. In parallel, real-time implementations such as rt-MSE support low-latency inputs to the Plasma Control System (PCS) for scenario development and shape control.

Together, these diagnostics enable equilibrium reconstruction for offline analysis (e.g., EFIT-style workflows) and real-time equilibrium estimation used for boundary and position control. Their combined use improves reconstruction fidelity during rapid evolution (ramp-up, shaping changes, transients), strengthens constraints on internal profiles when available, and supports consistent comparisons between control-time and post-shot equilibria.

Magnetic diagnostics provide the backbone constraints for equilibrium reconstruction by measuring poloidal field components and poloidal flux at the boundary, along with high-fidelity measurements of plasma current (I_p) via Rogowski coils. Typical sensor sets include magnetic pickup coils (B-dot / Mirnov-style arrays for equilibrium and stability), flux loops, and current sensors. These measurements constrain the plasma boundary shape, position, and global parameters (e.g., stored energy and inductive quantities when combined with additional signals/models), and they serve as primary inputs to both offline equilibrium reconstruction and real-time control-oriented estimators.

On NSTX-U, the magnetics system is engineered to support real-time availability of a large subset of equilibrium-relevant channels. This reduces latency for the PCS and improves robustness of shape and position control, especially in regimes where rapid boundary evolution and conducting-structure effects require careful modeling and sensor coverage.

The MSE-CIF diagnostic measures the internal magnetic field pitch angle by observing polarized Dα emission from injected neutral beam atoms. In the beam-atom frame, the Lorentz electric field E = v × B produces Stark splitting and polarization of the emitted light; the polarization orientation encodes the local magnetic field direction. By collecting this emission across multiple sightlines in the midplane, MSE-CIF provides a pitch-angle profile that strongly constrains equilibrium reconstruction—especially the inferred internal current distribution and derived safety factor profile.

MSE constraints are particularly valuable in NSTX-U because low toroidal field and strong shaping can increase sensitivity of internal-profile inference to modeling assumptions. Adding MSE pitch-angle measurements reduces reconstruction degeneracy and improves consistency between equilibria used for stability, transport, and control-oriented analyses.

rt-MSE provides low-latency processing of MSE polarization signals and delivers control-relevant pitch-angle information to the Plasma Control System (PCS). In real-time workflows, internal magnetic constraints can be incorporated (directly or indirectly) into control-oriented equilibrium estimation, enabling improved boundary control and scenario reproducibility when the diagnostic and computational pipeline support the required timing.

On NSTX-U, real-time equilibrium reconstruction and shape control rely primarily on magnetics, with ongoing development and upgrades aimed at expanding the set of diagnostics available in real time. Real-time MSE processing aligns with this direction by enabling internal constraints to be used with reduced latency, complementing the magnetics-only real-time solution and the higher-fidelity post-shot equilibrium reconstructions.