Motional Stark effect upgrades on DIII‐D

The measurement and control of the plasma current density profile (or q profile) is critical to the advanced tokamak program on DIII‐D. A complete understanding of the stability and transport properties of advanced operating regimes requires detail poloidal field measurements over the entire plasma radius from the core to the edge. In support of this effort, we have recently completed an upgrade of the existing motional Stark effect (MSE) diagnostic, increasing the number of channels from 8 to 16. A new viewing geometry has been added to the outer edge of the plasma which improves the radial resolution in this region from 13 to ∼2 cm. This view requires the use of a reflector that has been designed to minimize polarization effects. Vacuum‐compatible polarizers have also been added to the instrument for in situ calibration. Future use of the MSE diagnostic for feedback control of the q profile will also be discussed.