Feasibility Study of Pedestal Plasma Current Measurements in DIII-D

At the DIII-D tokamak, a working group recently made a concerted effort to evaluate the feasibility of measuring the plasma current in and around the H-mode pedestal, and the findings of this working group are presented. The plasma current in this region is key to pedestal transport and stability, so measurements of this quantity are highly desirable as they would improve the physics understanding of issues that are key for tokamak performance. The working group first determed what physics questions stood to benefit from these measurements and then determined the associated measurement requirements (spatial and temporal resolution, accuracy, etc.). Diagnostic experts then submitted proposals for making the required measurements with multiple techniques (microwave polarimetry, spectroscopy, etc.). The content and evaluation of all these proposals are presented. No proposal was found to be superior overall, but lithium beam spectroscopy, deuterium beam spectroscopy (multiple techniques), and Doppler-free saturation spectroscopy are the most promising techniques.