Design of the divertor Thomson scattering system on DIII-D

Local measurements of ne and Te in the divertor region are necessary for a more complete understanding of divertor physics. We have designed an extension to the existing multipulse Thomson scattering system [Carlstrom et al., Rev. Sci. Instrum. 63, 4901 (1992)] to measure ne in the range 5×1018–5×1020 m−3 and Te in the range 5–500 eV with 1 cm resolution from 1 to 21 cm above the floor of the DIII-D vessel (eight spatial channels) in the region of the X point for lower single-null diverted plasmas. One of the existing, 20 Hz, Nd:YAG lasers will be redirected to a separate vertical port and viewed radially with a specially designed f/6.8 lens. Fiber optics carry the light to polychromators whose interference filters have been optimized for low Te measurements. Other aspects of the system, including the beam path to the vessel, polychromator design, real-time data acquisition, laser control, calibration facility, and DIII-D timing and data acquisition interface, will be shared with the existing multipulse Thomson system. An in situ laser alignment monitor will provide alignment information for each laser pulse.