A. Puig Sitjes

Wendelstein 7-X near real-time image diagnostic system for plasma facing components protection

Abstract The 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 min, which means that the thermal control of the plasma-facing components (PFCs) is of vital importance to prevent damage to the device. In this paper an overview of the design of the Near Real-Time Image Diagnostic System (hereinafter called “the 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 to 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 graphics processing unit–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 online 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 three-dimensional 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 regions of interest–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 the System to be tested without risk of damaging the divertors in preparation for when water-cooled high-heat-flux divertors will be used.

A divertor scraper observation system for the Wendelstein 7-X stellarator

Two graphite divertor elements called scrapers have been installed on the Wendelstein 7-X stellarator in the throat of the magnetic island divertor. To diagnose one, we have designed, built, calibrated, and installed a new infrared/visible imaging endoscope system to enable detailed observations of the plasma interactions and heat loads at one of the scrapers and the nearby divertor surfaces. The new system uses a shuttered pinhole-protected pair of 90° off-axis 228 mm focal length aluminum parabolic mirrors, and two flat turning metal mirrors, to send light to a sapphire vacuum window 1.6 meters away, beyond which we have co-located telephoto lens-based infrared and visible cameras. The back-to-back off-axis parabolas serve to cancel out most aberrations, enabling the use of off-the-shelf commercial optics outside of the vessel. For the infrared, we use a 3-5 μm 1-megapixel FLIR SC8303HD camera and for the visible, a 5-megapixel CMOS PCO 5.5 edge camera. A short 1-m quartz pickoff fiber is used to send 200-1100 nm light to a compact spectrometer, also located in the same iron shield box as the cameras. The camera field of view covers the 700 mm length of the scraper, and includes locations monitored by thermocouples and Langmuir probes embedded in some of the scraper tiles. Predicted and actual optical test performances of the overall system are compared.