A Scrape-off Layer Electron Temperature and Density Profile Diagnostic at the Helicon Antenna at DIII-D*

Electron density in the scrape-off layer plays an important role in coupling power from a helicon wave antenna into tokamak plasma. At DIII-D, the scrape-off layer (SOL) in front of the helicon antenna sits behind multiple limiting structures, and is likely to possess more than a single decay length. Direct measurements of profiles of electron temperature and density in this region using inverted helium atomic line emission from helium puffs will be obtained with a new helium thermal beam diagnostic installed at DIII-D. Measurements on field lines running in front of the antenna from locations above and below the antenna will enable measurements in reverse and forward toroidal field directions. Local gas feeds will enable fast duty cycling of the helium (~<10ms) and correspondingly fast measurements. Modeling of the expected signals using magnetic reconstructions of nominal target discharges, including helicon fiducial discharges, shows measurements up to the separatrix should be possible for a range of conditions. This new capability will enable research into the dynamics of the SOL conditions during helicon operations and improved knowledge of the physics of coupling of helicon power into the plasma.