# Real-Time Detection of Overloads on the Plasma-Facing Components of Wendelstein 7-X
## 5 Towards Feedback Control of Thermal Loads  
With increasing plasma performance in subsequent operational phases, W7-X will require a feedback control system able to prevent plasma interruptions due to overloading to achieve high-performance long-plasma operation. The feedback control will have to trigger countermeasures when the risk of overheating reaches a threshold limit [17]. It has to be informed of the current development of the thermal events, their type, cause, and risk. This requires a high-level scene understanding which can only be achieved through advanced computer vision and machine learning techniques [18]. The Thermal Overload Detection algorithm can protect the machine through the interlock, but it cannot provide enough information on the thermal events for effective feedback control. We foresee a dual detection scheme with two algorithms running in parallel (see Figure 13). The Thermal Overload Detection algorithm guarantees that the PFCs operational limits are not exceeded and the Thermal Event Detection algorithm (still under research), detects promptly the thermal events, tracks them over time, classifies them and evaluates their risk. It informs the feedback control system of the ongoing events through a real-time network. The advantage of this dual detection scheme is that the Thermal Event Detection algorithm does not need to comply with the strict real-time and high-reliability requirements of a safety system and it can use more advanced state-of-the-art machine learning techniques. Figure 12: Divertor overload example. (**a**) Maximum of the temperature over time. (**b**) Maximum of the estimated risk over time. (**c**) Maximum temperature for each PFCs over time; the low-iota divertor target reaches 1200 \({}^{\circ}\)C at 0.4 s. (**d**) Time evolution of the maximum temperature normalized by the temperature limit over the image, time evolution of the maximum averaged temperature normalized by the temperature limit over the image, and Fast Interlock Signal (0.5 corresponding to a warning and 1.0 to an alarm); We can observe that an alarm is triggered 0.12 s before the maximum temperature reaches the limit (when the divertor temperature reaches 1200 \({}^{\circ}\)C) just in time to compensate the 0.11 s of the system delay. The Thermal Event Detection algorithm can evaluate the estimated risk \(\hat{r}_{i}\) of each event \(i\) using Equation (8). \[\hat{r}_{i}=\max_{(x,y)\in A_{i}}\hat{r}_{xy} \tag{8}\]  
where \(\hat{r}_{xy}\) is the estimated risk computed with Equation (7) for each pixel within the event segmented region \((x,y)\in A_{i}\). The temperature threshold \(T_{xy}^{th}\) in Equation (3) is computed with a larger reaction time to allow the feedback control to take action (\(t_{r}=0.5\) s). When the estimated risk \(\hat{r}_{i}\) reaches 1 for an event, the feedback control has to take the proper action according to the type of event to avoid the PFCs overloading and the interruption of the plasma by the Thermal Overload Detection algorithm. In a first development version of the feedback control, the system will act on the heating systems, ramping down the ECRH (Electron Cyclotron Resonance Heating) power until a safe operation is reached or stopping NBI (Neutral Beam Injection) operation if fast particle losses are detected. In later stages, we will introduce more advanced feedback control of thermal loads by plasma detachment or by controlling the strike-line shape and positioning employing the control coils below each divertor unit. The strike-line control is of special importance at W7-X. It is well known that the developing bootstrap current will move the strike-line towards the pumping gap in long discharges [19]. Also, beta effects can put some components at risk and this will require advanced feedback control. We can control the strike-line shape and position by employing the control coils located underneath the divertor targets of W7-X.