Plasma flux expansion control for heat flux regulation on the DIII-D tokamak

Future reactors, including ITER, will have to withstand severe steady state high heat flux loads on many plasma-facing components (PFCs). Thus, robust, reliable and simplified physics-based RT models for monitoring and effective feedback control strategies for PFC heat load control are mandatory on future fusion reactors. Flux expansion control in the scrape-off layer of a reactor offers the possibility of controlling heat flux and divertor detachment closer to the timescale of the ion equilibration time (\textasciitilde few to 10's of ms) in comparison to the slow response (\textasciitilde 1 s) anticipated for reactor gas valves. A new controller has been developed to make use of the flexible divertor poloidal field coil set of the DIII-D tokamak, and enable precise control of the flux expansion. The design ensures flexibility through a complementary set of orthogonal actuator directions to guarantee minimum effect on existing controlled variables, e.g. radial and vertical position of the X-point. A non-linear free-boundary simulation code (GSevolve) is used to study the closed loop response and to verify the implementation of the algorithm on the DIII-D plasma control system. First results of the experimental commissioning of the new controller during the 2020 DIII-D campaign are presented