# Real-Time Detection of Overloads on the Plasma-Facing Components of Wendelstein 7-X
## 1 Introduction  
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]. With a heating power of 10 MW, protecting the plasma-facing components (PFCs) is of vital importance in preventing damage to the device in steady-state. W7-X experimental campaigns define a step-by-step road map with increasing performance to reach steady-state operation. The operation phase named OP1.2 [2] was designed to test the island divertor concept in which the target plates intersect the helical field lines inside the magnetic islands at the edge for power and particle exhaust [3]. W7-X has a three-dimensional helical shape with a five-fold modular symmetry (see Figure 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. 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 steady-state operation phase (OP2) is planned to start in 2022. By then, all the PFCs will be water-cooled and the new high-heat flux water-cooled divertor will sustain heat fluxes of up to 10 MW/m\({}^{2}\). The divertor tiles are made of a Carbon Fibre Composite (CFC) layer joined to a CuCrZr cooling structure. The divertor's maximum operational temperature is limited by a Cu interlayer, which should not exceed a sustained temperature of 475 \({}^{\circ}\)C. This temperature is reached at 10 MW/m\({}^{2}\) when the surface temperature is 1200 \({}^{\circ}\)C [4; 5]. The other water-cooled PFCs are the baffles, the heat shields, the wall and pumping gap panels, and the poloidal closures. The baffles and the heat shields are made  
Figure 1: W7-X five-fold modules with its 10 divertor units, 5 upper and 5 lower divertor units. The divertor targets intersect the magnetic islands at the edge for power and particle exhaust. Figure 2: The divertor units are 5 m long and 1 m wide with several target modules: vertical targets (TM1V-TM3V), low-iota targets (TM1H-TM4H), low-load target modules (TM5H-TM6H) and high-iota targets (TM7H-TM9H). The divertor water-cooled tiles are made of CFC (Carbon Fibre Composite) except the two low-load central target modules, which are made of fine-grain graphite. of fine-grain graphite tiles, which are limited to 400 \({}^{\circ}\)C because of the brazed joint between the cooling pipe and the heat sink. The wall and pumping gap panels and the poloidal closure are made of water-cooled stainless steel, which cannot exceed 200 \({}^{\circ}\)C. See Figure 3 for an overview of the PFCs and their maximum operational temperatures. 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. The interlock is part of the safety system of the machine and stops the plasma heating systems when it is triggered. 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. In Section 2 the system to protect the PFCs of W7-X in OP2 is described. The thermography diagnostic, used for the detection of thermal events, is introduced together with the architecture of the real-time image analysis system. Section 3 goes into detail on the image analysis algorithms, the scene models concept (Section 3.1), the processing pipeline (Section 3.2), and the overload detection algorithm (Section 3.3). Section 4 illustrates the results obtained with the analysis of the data from the OP1.2 campaign.