Measuring the inboard side scrape-off layer of DIII-D plasmas using Swing-Probes

The scrape-off layer (SOL) plasma of a tokamak often has a complicated spatial dependence. The temperatures, densities and flow speeds can vary significantly on the same magnetic flux tube at different poloidal locations. To fully understand the plasma variation, we must make active measurements along the full length of the flux tubes by expanding our diagnostic capability to include the critically under-diagnosed inboard side. To accomplish this, a new pair of in-situ reciprocating Mach probes called Swing-Probes have been developed and deployed on the DIII D centerpost. This design is unique in that the probe swings vertically through the SOL plasma, taking measurements along a 180° arc with a 20 cm radius. Two electrodes maintain a Mach-pair orientation throughout the swing and provide measurements of saturation current, electron temperature, and parallel flow speeds. The probes can handle very high heat fluxes and have taken measurements up to the Last Closed Flux Surface (LCFS) in high-powered H-Mode at 10 MW. The Swing-Probe temperature and density measurements have been verified against the floor Langmuir probes, the core Thomson scattering and the divertor Thomson scattering systems in DIII-D for conditions where poloidal variation are expected to be small. Measurements have been taken across a wide range of plasma conditions and provide informative relationships between the plasma parameters at the entrance of the inner divertor and the crown of the plasma.The low-turbulence plasmas on the inboard scrape-off layer make it possible to clearly quantify the sheath-expansion around Langmuir probes. An I-V fitter has been developed which can account for sheath-expansion in a theoretically consistent way, improving the reliability of Langmuir probe data analysis. In an inner-wall limited experiment in DIII-D requested by ITER, the Swing-Probes made the first Langmuir probe measurements of an enhanced heat flux feature just outside the LCFS. These measurements match those made using infrared camera images of the centerpost in DIII-D and in other tokamaks and confirm that this feature should be accounted for in the designs of plasma facing surfaces in ITER.