Divertor Heat Flux Control Design for High Heat Flux Tokamaks

The principal goal of the project, 'Divertor Heat Flux Control Design for High Heat Flux Tokamaks,' is to develop an integrated toolset for time-dependent divertor heat flux simulation and controller design. This toolset is intended to provide divertor heat flux control algorithms suitable for application to tokamaks with high unmitigated divertor heat fluxes, a problem that is common to compact spherical tokamaks (STs) and high-field standard aspect ratio tokamaks. The focal device for initial implementation of this toolset will be the SPARC tokamak. The proposal is a collaborative effort between Oak Ridge National Laboratory (ORNL) and General Atomics (GA), leveraging the expertise in boundary simulation and controller design and implementation from each institution. For a series of critical operational scenarios for SPARC identified by Commonweath Fusion Systems (CFS), the team members at ORNL will perform high-fidelity timedependent simulations of the tokamak boundary plasma using the SOLPS-ITER transport code. The team members at GA will develop and implement synthetic diagnostics as scripts or plugins for the SOLPS framework based on the proposed diagnostics measurement locations specified by the SPARC team. GA and ORNL will use the SOLPS simulation results, including the synthetic diagnostic data for developing reduced-order linear and non-linear models for the SPARC scrape-off layer plasma with real-time control capability. Different modelling methodologies will be applied (dynamic mode decomposition, sparse identification of nonlinear dynamics, occupation kernel, and first order plus dead time) to describe the evolution of key operational parameters, such as the divertor heat flux, upstream density, and volumetric quantities in response to external actuation, such as fuel ion and impurity gas puff magnitude, injected power, and strike point position. Simple (proportional-integral-derivative [PID] controller) and complex (model predictive controller [MPC]) control algorithms will be developed and their closed-loop performance will be evaluated based on the lightweight reduced order model. In addition, a subset of control variables depending on the type of the detachment regime will be identified and characterized based on the assumptions for associated noise and delay for diagnostics provided by the SPARC team. In preparation for SPARC experimental data, the reduced models, along with the developed PID and MPC systems will be implemented and tested in closed-loop, with the help of the full high-fidelity timedependent SOLPS model. The developed closed loop simulation environment will also be coupled with the Integrated Plasma Simulator framework for understanding the effect on the core-edge integration with the proposed divertor detachment controllers. Given the extreme challenge of detachment control in SPARC, which is predicted to have extremely narrow heat flux widths and high divertor fluxes, the proposed tool development of SPARC will prove applicable and valuable to devices with common physics challenges, specifically compact designs such as the ST.