Divertor infrared thermography on COMPASS

Abstract

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\u202fmm/px on the target surface and temporal resolution better than 20\u202fμs. The system consists of fast IR camera TELOPS Fast-IR 2K placed in a magnetic shielding box, a positionable holder, a 1\u202fm 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\u202f°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\u202f≈\u202f40–50%. 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 λ q omp = 2.1 − 3.3 \u202fmm and average H-mode heat flux profiles in an inter-ELM period and during an ELM heat flux maximum with λ q omp ≈ 0.6 \u202fmm and λ q omp ≈ 6.7 \u202fmm, respectively.