Divertor infrared thermography on COMPASS
Introduction
The COMPASS tokamak [1] is equipped with an open lower graphite divertor. Direct observation of approx. 190 mm × 152 mm of the divertor region from the top of the machine is secured by the divertor infra-red (IR) system which was commissioned in 2018. This system provides measurements of the full radial profile of the divertor surface temperature with the pixel size ∼0.6–1.1 mm/px on the target surface, ∼0.04–0.12 mm/px mapped to the outer midplane (OMP), frame rate up to 57 kHz (320 px × 4 px) and up to 90 kHz with a limited radial divertor coverage (64 px × 4 px). See Figure 8 in [2] for an overview of the field of view (FoV) of the system.
Section snippets
IR system overview
The IR system consists of Telops FAST-IR 2K IR camera sensitive to wavelength range 3–5.5 μm, ∼1 m long IR endoscope, sapphire vacuum window and a special graphite divertor tile optimized for IR thermography. Furthermore the IR camera is placed inside a magnetic shielding box made of 8 mm thick soft iron with embedded water cooling. The IR camera is mounted to the main tokamak support structure via a massive holder enabling precise positioning of the camera (radial and toroidal movement and
Optical performance
The optical performance of the IR endoscope was tested in laboratory during the commissioning of the system. The global transmission of the endoscope combined with the sapphire vacuum window was measured using a calibration black body source in the range 100–600 °C. The average transmission τ ≈ 40–50% was found.
The spatial resolution of the system was tested using the slit experiment as described in [3]. A 1 mm wide rectangular slit placed 720 mm from the sapphire window was viewed by the IR
L-mode
A set of ohmic L-mode plasma discharges was diagnosed using the new IR system. IR signal was acquired for 320 px × 20 px at 8 kHz framerate and the divertor tile was preheated to 100 °C. An example of the radial temperature profile across the divertor surface is shown for discharge #17697 in Fig. 7 (Bt = 1.38 T; Ip = 300 kA, ne = 2.5e19 m−3).
Time evolution of the perpendicular (surface) heat flux profile was calculated using the THEODOR code [4]. Surface effects (deposited layers, surface micro
Conclusion
A new divertor IR system provides routine fast measurement of time evolution of the divertor temperature and heat load distribution for both the inner and the outer divertor target on COMPASS. Global transmission of optical part of the system τ ≈ 40–50% was found during its commissioning. Re-manufacturing of at least one of the lenses with lower surface error will be needed in order to improve the spatial resolution.
First experimental divertor heat flux measurements using the new system were
Acknowledgments
This work was co-funded by MEYS projects number 8D15001 and LM2015045, by Czech Science Foundation project GA16-14228S and by projet No. CZ.02.1.01/0.0/0.0/16_019/0000768. This work has been carried out within the framework of the EUROfusion Consortium and has received funding from the Euratom research and training programme 2014-2018 under grant agreement No 633053. The views and opinions expressed herein do not necessarily reflect those of the European Commission.
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