Design for Thomson Scattering measurements in the DIII-D SAS Divertor using in-vessel optics*

The Small Angle Slot (SAS) divertor configuration in the DIII-D tokamak presents a challenging geometry for obtaining Thomson scattering measurements of electron temperature and density in this region. Divertor carbon tiles were previously modified to allow the Thomson-scattered light to reach the ex-vessel collection lens. These modifications still restrict the measurement points to no closer than 65mm from the base of the divertor slot. Measurement positions closer to the base are accessed using in-vessel optical components directing Thomson-scattered light from near the slot base to the collection lens. High stray laser light levels, which affect the calibration of the electron density, are mitigated using a Raman-scattering calibration method. This technique collects absolute system response data using a non-monoatomic gas to scatter light to wavelengths further away from the overwhelming intensity at the laser wavelength.