Application of improved analysis of convective heat loads on plasma facing components to Wendelstein 7-X

Abstract

Within this research we present a method to speed up the simulation of convective heat loads onto the plasma-facing components (PFCs) of the Wendelstein 7-X (W7-X) stellarator by a factor of approximately 20 with the same statistical precision as compared to the previous standard. The geometric models developed for this are also designed to unravel the complex 3D PFCs onto a 2D picture-like input format which gives access to the full set of image analysis tools like for example wavelet analysis or the applicability of convolutional neural network (CNN) architectures. The significant speedup of heat load calculation allows to simulate a massive data set of heat loads for approximately 3 × 10 4 magnetic configurations in edge rotational transform-radial axis shift space. To first order, plasma dynamic effects like toroidal current development as well as beta effects mainly influence this space which motivates the simulation in this scope. A criterion to evaluate the safety of a magnetic configuration with respect to the convective heat load onto the plasma facing components has been developed taking statistical fluctuations of the simulation into account. This criterion, applied to the introduced data set, provides a map relevant for discharge planning and machine safety. The methods and concepts introduced herein could contribute to the safety evaluation of magnetic confinement devices in general and are not specific for W7-X.

Scenario with combined density and heating control to reduce the impact of the bootstrap current in Wendelstein 7-X

“…where P conv is the total available convective power [41] (e. g., for a worst-case estimate, the total heating power). Here, the segmentation size is chosen small enough to resolve the regions of the PFCs with different technical heat flux limits and the scale size of the heat load patterns.…”

“…The ratio q i /q d, i together with the number of hits N i out of N tot , on which q i is based, can be used for a statistical analysis of the probability of overloading a particular segment [41].…”

“…FIG.6: Mapping of a divertor unit to a 2D grid, as used for representing the power density in section IV. The colors indicate the maximum design heat load (Table for each surface element[41]. The magnified view from the inboard shows in particular the critical region adjacent to the pumping gap in a golden-orange color.…”

Assessment of potential heat flux overload of target and first wall components in Wendelstein 7-X finite-beta magnetic configurations and choice of locations for temperature monitoring

“…Since we only consider magnetic configurations and PFCs respecting the ten-fold symmetry described in section 1, we map all hit points into one half module of the device (and then need to reduce qi by the symmetry factor z= 10). We then proceed as described in [29]. As reported in that paper, ntot = 25000 ensures good statistics in the FLD calculations, allowing an acceptable computational time saving due to the choice of appropriate segment areas (see below).…”

“…Based on the results presented in section 3 and in the Appendix and on the analysis of further vacuum configurations [29], 25-29 locations in baffle heat sinks per divertor unit and 2-4 locations in the heat sinks of wall protection tiles per half module were chosen, as shown in Figure 7 and Figure 8. The thermocouples will be integrated before the next operational phase of W7-X into the heat sinks in the chosen locations (or, in few instances, in slightly shifted locations, due to space restrictions).…”