Thomson Scattering Measurements Using In-vessel Optics in the DIII-D Small Angle Slot Divertor

Divertor Thomson scattering using in-situ optics has been extended to the DIII-D Small Angle Slot (SAS) divertor to characterize plasma dissipation phenomena, measure cold dissipative conditions with T_e  <1 eV near the target surface, and provide critical data for validation of boundary modeling. The SAS divertor geometry is particularly challenging for obtaining Thomson scattering measurements. The tight enclosure, situated behind a protrusion in the wall boundary, precludes typically used ex-vessel collection optics from accessing the magnetic field strike point region, a vital measurement position for evaluating the performance of this divertor configuration. In-vessel lenses and optical fiber bundles have been installed to collect Thomson-scattered light to a vacuum feedthrough, where additional ex-vessel fiber bundles transport the light to remotely located polychromators for spectral analysis. The lenses and fiber bundles are mounted beneath modified wall tiles and are designed to withstand the 375°C vessel baking process. Stray laser light is mitigated through the viewing angles of the lens-fiber system. First results from SAS divertor plasma experiments, including measurement of sub-eV electron temperatures and consistency between neutral pressures, Langmuir probes, and volumetric spectroscopy, are presented.