Abstract
Wendelstein 7-X (W7-X) is the leading experiment on the path of demonstrating that stellarators are a feasible concept for a future power plant. One of its major goals is to prove quasi-steady-state operation in a reactor-relevant parameter regime. The surveillance and protection of the water-cooled plasma-facing components (PFCs) against overheating is fundamental to guarantee a safe steady-state high-heat-flux operation. The system has to detect thermal events in real-time and timely interrupt operation if it detects a critical event. The fast reaction times required to prevent damage to the device make it imperative to automate fully the image analysis algorithms. During the past operational phases, W7-X was equipped with inertially cooled test divertor units and the system still required manual supervision. With the experience gained, we have designed a new real-time PFC protection system based on image processing techniques. It uses a precise registration of the entire field of view against the CAD model to determine the temperature limits and thermal properties of the different PFCs. Instead of reacting when the temperature limits are breached in certain regions of interest, the system predicts when an overload will occur based on a heat flux estimation, triggering the interlock system in advance to compensate for the system delay. To conclude, we present our research roadmap towards a feedback control system of thermal loads to prevent unnecessary plasma interruptions in long high-performance plasmas.
Highlights
Wendelstein 7-X (W7-X) is the largest drift optimized stellarator
Note that the estimated risk image is normalized for the different temperature limits and heat fluxes on the different plasma-facing components (PFCs), allowing to detect overloaded regions in the entire field of view naturally, avoiding the use of regions of interest
The protection of W7-X during steady-state plasmas requires checking in real-time the temperature of the PFCs against their limits, and triggering an alarm to the interlock and interrupt operation when these limits are compromised
Summary
Wendelstein 7-X (W7-X) is the largest drift optimized stellarator. It aims at demonstrating reactor-relevant plasma performance of the stellarator line with nearly steady-state plasmas of up to 30 min [1]. The divertor is composed of 10 units with several target modules (see Figure 2) During this experimental phase, we used an inertially cooled test divertor made of fine-grain graphite tiles. We used an inertially cooled test divertor made of fine-grain graphite tiles This allowed us to test the image analysis system designed to protect the PFCs in steady-state without danger of damaging the high-heat flux divertor. The imaging system will have to guarantee a safe steady-state operation of the device by monitoring the PFCs in real-time and prevent overheating by triggering the interlock system if their temperature limits are compromised. Several similar safety systems have been developed for other fusion devices with the same goal [6,7,8,9] These systems typically rely on pre-defined regions of interest and it triggers the interlock when the temperature limits of these regions are breached.
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