Development and demonstration of an active optical axis alignment method for divertor infrared thermography system in ITER

An active alignment method to compensate for optical axis misalignment between the in-vessel and ex-vessel optics caused by thermal expansion of the vacuum vessel during plasma operation was developed in this study. By observing the return light from an alignment pattern at the vacuum window and a calibration light source positioned at the first pupil plane inside the vacuum vessel, subsequent optical axis alignment during plasma operation can be conducted using only the return light from the alignment pattern. Testing with a prototype demonstrated that the developed alignment method can align the optical axis remotely and actively with accuracies of less than 0.03° for angular decentration, less than 1 mm for parallel decentration, and less than 5 mm for optical path length displacement.