Design of the soft x-ray spectrometer for observing high- Z elements at the full-metal COMPASS Upgrade tokamak

Abstract A compact and highly flexible soft x-ray (SXR) spectrometer for measuring electromagnetic radiation, primarily in the spectral range of about 2–8 keV, was designed with the aim of monitoring of highly ionized, mainly high- Z metallic impurities in the core of high temperature plasmas at the COMPASS Upgrade tokamak (currently under construction) with core plasma temperatures of ∼1–5 keV and core electron densities of ∼1–3 * 10 20 m −3 . These impurities will originate from plasma interaction with plasma-facing components made of tungsten and nickel–chromium-based alloys (Inconel), containing elements such as nickel, chromium, molybdenum and iron. The SXR spectrometer will enable the monitoring of each of these elements by resolving well their characteristic spectral lines in pre-selected spectral regions of interest. Additionally, information on the electron and ion temperature in the plasma core will be provided using artificially introduced impurities such as argon. The main technical parameters of the SXR spectrometer are as follows: Johann geometry using spherical Bragg crystals, back-illuminated charge-coupled device as a detector in a fixed position, pre-selected spectral regions of interest for metallic (W, Mo, Fe, Cr, Ni) and gaseous (Ar) impurities in the spectral region 2.4–8.1 keV (0.52–0.15 nm), spectral resolution about 2500, but better than 3000 for the Ar lines near 3.1 keV, time resolution about 40 ms, compact size (the core of the spectrometer of about 1 m by 1 m; 2.5 m long vacuum connection duct to the tokamak cryostat edge) and flexible configuration (remotely driven crystal exchanger, left–right spectrometer set-up flexibility, differential pumping). The contribution introduces the design of the spectrometer itself as well as future plans for its implementation in the tokamak hall in early operational phases with low hard radiation or in a shielded area outside the tokamak hall for high-performance plasmas at COMPASS Upgrade.