Control pathway for an advanced divertor on ITER

This paper presents the development of a coupled shape and divertor controller for ITER with capabilities to control the flux expansion and an advanced divertor configuration, the x-divertor (XD), in which a secondary x-point is placed in the downstream scrape-off layer. Due to the high-performance nature of ITER and its relatively few shaping coils, satisfying constraints on the coil currents, power supplies, and plasma shape is a challenge for this configuration. To meet these constraints the controller uses the constrained linear quadratic regulator (CLQR) framework [1], a variant of model predictive control (MPC). Previous work [2] has shown the existence of XD equilibria on ITER and in this study, we identify a control pathway for achieving the "pure" XD where the secondary x-point is placed at the outer strike point, representing a maximally flux-expanding scenario. Constraints are observed throughout the transition. One limitation is that at high flux expansion, the shallow angle of incidence of the magnetic field on the divertor results in a reduced plasma wetted area via tile shadowing. On the other hand, the high flux expansion and flaring characteristic of the XD facilitate enhanced detachment, which could negate the shadowing effect to some degree. In this case it would be desirable to operate as close to the flux expansion limit as possible. To this end we also demonstrate the controller's ability to track a trajectory for the flux expansion near the accepted angle of incidence limit.