Imaging MSE measurements of the current density in the pedestal on the high field side of the DIII-D tokamak*

The imaging motional Stark effect (MSE) diagnostic measures the polarization angle of Stark-split beam emission to create a 2D image of the magnetic field pitch angle. The imaging MSE system presented here is designed to measure beam emission on the high-field side (HFS) of the DIII-D tokamak, including radial resolution of a few cm and sub-cm channel spacing in the pedestal region on the HFS. These are the first high-resolution measurements of the magnetic field pitch angle in the HFS pedestal region on the DIII-D tokamak. HFS pitch angle measurements are used to constrain the EFIT 2D equilibrium solver. As the local pitch angle is not a flux surface quantity, these new HFS measurements add valuable new constraints to preexisting measurements on the low-field side (LFS). Including HFS imaging MSE constraints produces EFITs more similar to kinetic EFITs than EFITs constrained only by the LFS MSE measurements. For example, including the HFS imaging MSE measurements produces EFITs with positive current density across the HFS of the plasma, unlike EFITs constrained with only the LFS MSE measurements.