The motional Stark effect (MSE) diagnostic has been successfully applied to many tokamak plasmas to yield internal magnetic field measurements which contribute significant information to equilibrium reconstruction. As commonly implemented, the diagnostic measures the line polarization of a single component of the Stark spectrum and returns a spatially resolved profile of the magnetic field pitch angle. The line shifts in the Stark spectrum contain information about the magnetic field magnitude which is not typically exploited, and has not previously been used in equilibrium reconstruction. In this paper, we examine the utility of the MSE line shift (LS) measurement as compared with the MSE line polarization (LP) by analysis with the equilibrium and stability code (ESC)–equilibrium reconstruction variance (ERV) code system, a program which employs the theory of variances to evaluate the relationship between specified diagnostics and equilibrium reconstruction. Comparisons of the code results are made for an analytic solution of a toroidal plasma with circular cross-section and high aspect ratio, and for previous experimental results of q-profile reconstruction using VMEC on TFTR. ESC–ERV is then used to evaluate MSE-LS and MSE-LP in conditions like those expected in ITER, and for a configuration similar to the National Spherical Torus experiment. The analysis suggests that both the MSE-LP and the MSE-LS approaches, given sufficient measurement accuracy, can be used for reconstruction of q-profiles and pressure profiles.
Sensitivity of equilibrium profile reconstruction to motional Stark effect measurements