Elsevier

Fusion Engineering and Design

Volume 146, Part A, September 2019, Pages 1003-1006
Fusion Engineering and Design

Divertor infrared thermography on COMPASS

https://doi.org/10.1016/j.fusengdes.2019.01.142Get rights and content

Highlights

  • A new fast divertor IR system was put into operation on the COMPASS tokamak. The system provides in-situ calibration possibility using a special heated divertor tile.

  • Radial profiles of the divertor heat flux are routinely measured for both the inner and the outer divertor target with spatial resolution ∼1 mm and frequency up to ∼60 kHz.

  • First experimental divertor heat flux measurements using the new system were successfully performed in both L-mode and H-mode.

Abstract

A new fast divertor infra-red thermography system was put into operation on COMPASS. It provides full radial coverage of the bottom open divertor with pixel resolution ∼0.6–1.1 mm/px on the target surface and temporal resolution better than 20 μs. The system consists of fast IR camera TELOPS Fast-IR 2K placed in a magnetic shielding box, a positionable holder, a 1 m long IR endoscope consisting of 14 Ge and Si lenses securing off-axis view from an upper inner vertical port and a special graphite divertor tile optimized for IR thermography. The tile is equipped with a heating system allowing tile preheating up to 250 °C. Embedded thermoresistors and a calibration target (a deep narrow hole acting as a black body radiator) allows in-situ calibration of the system including estimation of the target surface emissivity. Furthermore, a roof-top shaped structure on top of the tile increases magnetic field incidence angles above 3 degrees.

Laboratory tests of the system performed during its commissioning are presented. The global transmission of the optical system was found to be τ ≈ 40–50%. Poor spatial resolution compared to the design value was observed. Too large surface error of individual lenses was identified as the main cause and re-manufacturing of the most critical lens was suggested.

First experimental results obtained using the IR system are presented: divertor heat flux profiles in L-mode with the heat flux decay length λqomp=2.13.3 mm and average H-mode heat flux profiles in an inter-ELM period and during an ELM heat flux maximum with λqomp0.6 mm and λqomp6.7 mm, respectively.

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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