Proper orthogonal decomposition as an analytical approach to parametrisation of asymmetry and heat flux analysis in the Wendelstein 7-X stellarator

Abstract The complex dynamics of plasma and the heat flux through divertors in stellarator fusion devices, with a focus on the Wendelstein 7-X (W7-X), can be more easily understood by leveraging the proper orthogonal decomposition (POD) method. POD has been used to analyse discharges to identify divertor asymmetries indicative of underlying plasma transport phenomena. It has been demonstrated that the POD method, recognised for its dimension-reduction prowess, can effectively distill high-dimensional datasets into a manageable number of significant modes, facilitating the interpretation of intricate plasma behaviour. The findings reveal that the primary POD modes encapsulate the majority of the significance of the signal, underscoring the asymmetries caused by plasma drifts, and revealing the correlations of the POD modes linked to specific physical phenomena within the reactor vessel. Furthermore, an average symmetrisation parameter has been introduced as a diagnostic tool to quantify symmetry in heat flux deposition, revealing its potential to characterise and mitigate asymmetries.