One of the challenges of the snowflake divertor (SFD) configuration is finding a reliable means of reconstructing the magnetic field geometry in the divertor. Since the SFD (and other advanced divertors) have multiple field nulls, there is a large region with shallow flux gradients that is difficult to resolve accurately using external diagnostics. In this work we present a technique that uses heat flux measured by the infrared television (IRTV) camera to improve SFD reconstruction. This is relevant for purposes of control, since the SFD is topologically unstable and requires active feedback on the shape [E. Kolemen, et. al., Nucl. Fusion, 58, 6 (2018)], and analysis, since reconstructions provided by other algorithms such as EFIT [L. Lao, et. al., Nucl. Fusion, 25, 11 (1985)] can mis-characterize the shape and even the snowflake type (plus or minus). The technique identifies the spatial position of the two x-points located in the SFD based on characteristics of the heat flux such as the strike point location and power distribution. The inferred x-point positions are then used as a constraint in fitting new equilibria using the TokSys suite of software. \red{This procedure is applied to $\sim$800 DIII-D SFD timeslices and reduces the summed strike point errors from an average 9.4cm to 0.9cm. The newly-created x-point constrained equilibria are compared to kinetic reconstructions and an average 16\% reduction in the edge current is observed.} This is correlated via a simple linear relationship to the shape constraints. Other changes in the pedestal structure are observed, but more work must be done to incorporate the IRTV constraint directly into kinetic solvers to obtain integrated solutions. 

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 xpoint 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.