Utilization of Libeam Polarimetry for Edge Current Determination On DIII-D

Local details of the edge current profile can have profound effects on the stability and ultimate performance of many advanced tokamak (AT) modes1-3. This is true for both bootstrap and externally driven currents that may be used to tailor the edge shear. Absent a direct local measurement of j(r), the best alternative is a determination of the poloidal field. On the DIII-D tokamak4 we are using precision polarimetry of an injected lithium beam (LIBEAM) to make measurements of the necessary precision in the region of interest (ρ = 0.7–1.1)5. Combined polarimetry/spectroscopy of the various Zeeman- split 2S-2P lithium resonance line components gives direct information on the local magnetic field components. Because of the negligible Stark mixing of the relevant atomic levels, this method of determining j(r) is insensitive to the large local electric fields typically found in enhanced confinement (H-mode) edges, and thus avoids an ambiguity present in motional Stark effect (MSE) measurements of B6,7. Beam intensities and energy are suitable for measurements in the edge region of even high density discharges. Key issues for utilizing this technique include good beam quality, an optimum viewing geometry, and a suitable optical pre-filter to isolate the polarized emission line. Details of the existing DIII-D diagnostic are presented. We also discuss details of the analysis necessary to determine the field component from these measurements, and our plans for future improvements to the system.