Measurement of the Current Profile in Fusion Tokamaks using the Motional Stark Effect Diagnostic

Abstract

Determining the current distribution in fusion plasmas is of paramount importance for avoiding performance limiting instabilities, and for developing plasma scenarios for future fusion power plants. Measurements of the current profile in the core plasma are obtained using the motional Stark effect (MSE) diagnostic. This diagnostic measures polarised light emission from injected fast neutrals, to obtain the pitch angle of the magnetic field. These measurements are used as a constraint in plasma equilibrium solvers, from which the toroidal current density distribution is inferred.

This thesis looks to assess the capabilities of two MSE diagnostic techniques to recover the current profile of tokamak plasmas. We first evaluate the performance of the conventional MSE diagnostic on MAST at measuring the edge current density evolution during edge localised mode cycles. Using the plasma equilibrium reconstruction code EFIT++, we infer the toroidal current density. We find that this current density depends strongly on the equilibrium settings and on the quality of the data in the pedestal region.

We describe a forward model, which considers spectral line broadening effects, to produce synthetic measurements from an imaging MSE (IMSE) diagnostic. We present in detail the design of an IMSE system for MAST Upgrade, as informed by this model. The predicted performance of the modelled diagnostic is assessed for a variety of MAST-U plasma scenarios.

We benchmark an IMSE diagnostic against a conventional MSE diagnostic on the high field side of the DIII-D tokamak. Observations in various plasma scenarios are presented, where IMSE measurements in forward field plasmas were consistent with the conventional MSE system. In reverse field plasmas, inconsistencies between the measured and EFIT predicted polarisation angles arise. This is attributed to a source of partially polarised emission at the plasma edge. We conclude by presenting potential developments of the calibration and design of future IMSE diagnostics.