A. Ali

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.

Infrared imaging systems for wall protection in the W7-X stellarator (invited)

Wendelstein 7-X aims at quasi-steady state operation with up to 10 MW of heating power for 30 min. Power exhaust will be handled predominantly via 10 actively water cooled CFC (carbon-fiber-reinforced carbon) based divertor units designed to withstand power loads of 10 MW/m2 locally in steady state. If local loads exceed this value, a risk of local delamination of the CFC and failure of entire divertor modules arises. Infrared endoscopes to monitor all main plasma facing components are being prepared, and near real time software tools are under development to identify areas of excessive temperature rise, to distinguish them from non-critical events, and to trigger alarms. Tests with different cameras were made in the recent campaign. Long pulse operation enforces additional diagnostic design constraints: for example, the optics need to be thermally decoupled from the endoscope housing. In the upcoming experimental campaign, a graphite scraper element, in front of the island divertor throat, will be tested as a possible means to protect the divertor pumping gap edges during the transient discharge evolution.