Wendelstein 7-X Near Real-Time Image Diagnostic System for Plasma-Facing Components Protection

Wendelstein 7-X (W7-X) fusion experiment is aimed at proving that the stellarator concept is suitable for a future fusion reactor. Therefore, it is designed for steady-state plasmas of up to 30 minutes, which means that the thermal control of the Plasma Facing Components (PFCs) is of vital importance to prevent damages to the device. In this paper an overview of the design of the Near Real-time Image Diagnostic System for PFCs protection in W7-X is presented. The goal of the System is to monitor the PFCs with high risk of permanent damage due to local overheating during plasma operations and send alarms to the Interlock System. The monitoring of the PFCs is based on thermographic and video cameras and their video streams are analyzed by means of GPU-based computer vision techniques to detect the strike-line, hot spots and other thermal events. The video streams and the detected thermal events are displayed on-line in the control room in the form of a thermal map and permanently stored in the database. In order to determine the emissivity and maximum temperature allowed, a pixel-based correspondence between the image and the observed device part is required. The 3D geometry of W7-X makes the System particularly sensitive to the spatial calibration of the cameras since hot spots can be expected anywhere and a full segmentation of the field of view is necessary, in contrast to other ROI-based systems. A precise registration of the field of view and a correction of the strong lens distortion caused by the wide-angle optical system are then required. During the next operation phase the uncooled graphite divertor units will allow to test the System without risk of damaging the divertors in preparation when watercooled high-heat flux divertors will be used.