Experts meet to improve infrared measurements for ITER

The temperature of the plasma inside the ITER vacuum vessel will reach more than 150 million °C during nuclear fusion. As a consequence, plasma-facing components such as the divertor and the first wall will be subjected to high heat loads—several times higher than those experienced by a spacecraft during reentry through Earth's atmosphere. These plasma heat loads can cause local "hot spots" on the surfaces of plasma-facing components, with temperatures of hundreds of degrees Celsius potentially resulting in fatigue and damage. In order to ensure the safe and reliable operation of the tokamak, it is necessary to constantly monitor the temperature of these plasma-facing components, and send early warning to operators as soon as the hot spots appear. For this purpose, ITER will have a set of infrared cameras that will monitor a large fraction of the surfaces in question.

ITER infrared diagnostics feature a labyrinth of optical components, which will transfer the infrared radiation emanating from the plasma-facing components to cameras located many metres away. Due to the involvement of so many optical components, the occurrence of errors in temperature measurements is inevitable. Further complexity comes from light reflections from tokamak metallic surfaces that can generate false hot spots on the camera images. In addition, the interpretation of the infrared data requires knowledge of the surface properties, which can evolve during plasma operation, causing measurement errors. In order to ensure accurate temperature measurements, the infrared diagnostics will be calibrated before their installation, but also periodically during ITER operation.

As the design of the ITER infrared diagnostics progresses, strategies for calibration need to be developed, tested, and incorporated in the diagnostics design and operation plans, taking into account specific ITER requirements and constraints. For this purpose, a workshop on calibration techniques for ITER infrared diagnostics was organized on 5 and 6 June at ITER Headquarters. Gathering more than 40 world experts in infrared diagnostics, the workshop started off with participants sharing experience with the calibration of infrared diagnostics installed in operational fusion experiments such as WEST, JET, Wendelstein 7-X and ASDEX Upgrade. The second part of the workshop focused on calibration strategies for the ITER infrared diagnostics developed by the European, United States and Japanese Domestic Agencies.

The workshop addressed pertinent topics such as cross-calibration techniques, calibration standardization, machine learning, the use of in-vessel calibration sources, and the impact of surface material properties on the measurements—invaluable input for the further development of ITER's infrared diagnostics and for increasing the fidelity of the measurements.