The ITER divertor Thomson scattering system: engineering and advanced hardware solutions

A divertor Thomson scattering(TS) system being developed for ITER has incorporated proven solutions from currently available TS systems. On the other hand any ITER diagnostic has to operate in a hostile environment and very restricted access geometry. Therefore the operation in an environment of intensive stray light, plasma background radiation, the necessity meet the requirement using only a 20 mm gap between divertorcassettes for plasma diagnosis as well as to measure plasma temperatures as low as 1 eV severely constrain the divertorTS diagnostic design. The challenging solutions of this novel diagnostic system which has to ensure its steady performance and also the operability and maintenance are the focus of this report. One of the most demanding parts of the in-vessel diagnostic equipment development is the design assessment using different engineeringanalyses. The task definition and first results of thermal, e/m and seismic analyses are provided. The process of further improving of the design involves identification of susceptible areas and multiple iterations of the design, as needed. One of the key points for all Thomson scatteringdiagnostics are the laser capabilities. A high-performance and high-power laser system using a steady-state and high-repetitive mode Nd:YAG laser (2J, 50–100Hz, 3ns) has been developed. The reduced laser pulse duration matched with high-speed low-noise APD detector can be very important under high background light level. For diagnostics such as Thomson scatteringand Raman spectroscopy, a high-degree of discrimination against stray lightat the laser wavelength is required for successful detection of wavelength-shifted light from the laser-plasma interaction region. For this case of high stray lightlevel, a triple grating polychromatorcharacterized by high rejection and high transmission has been designed and developed. The novel polychromatordesign minimizes stray lightwhile still maintaining a relatively high transmission.