In order to answer key physics questions arising from the Spherical Tokamak concept, and to eventually provide a compact design path for future fusion power reactors, the National Spherical Torus Experiment Upgrade (NSTX-U) program at the Princeton Plasma Physics Laboratory (PPPL) is interested in developing high-confinement, low-disruptivity, advanced scenarios characterized by large bootstrap fractions and high βN values leading to steady-state, high-performance operation with enhanced magnetohydrodynamic stability. The realization and sustainment of this type of advanced plasma scenarios in NSTX-U will demand robust integrated control of a large number of plasma properties, which include the shape of the plasma spatial profiles. In particular, active control of the current profile, together with appropriate rotation and pressure profile control, has been demonstrated to be a key condition for the realization and sustainment of high- confinement, MHD-stable, steady-state scenarios. The control of the plasma spatial profiles arises as one of the most challenging control problems in tokamaks due to the high dimensionality and nonlinearity exhibited by high-performance plasmas. A model-based approach emerges as the only possible way of tackling this challenging, nonlinear, high-dimensional, control-design problem, while enabling model-based scenario planning, real-time estimation, control integration, actuator sharing, and real-time optimization. The Lehigh University (LU) Plasma Control Group (PCG) has pioneered many control-oriented modeling and control-design techniques for advanced scenario control, strongly impacting not only the domestic programs but also major experiments around the world.
The long-term research goal of the LU-PCG is to contribute from academia to the development and sustainment of disruption-free, high-performance, steady-state, advanced plasma scenarios in present tokamaks as a major pre-requisite for both ITER and prospective tokamak-based fusion power reactors. This goal is fulfilled by (i) developing both physics-oriented and control-oriented predictive dynamic models, (ii) introducing advanced control techniques as tools to elucidate the physics of fusion plasmas and to enable fusion power generation, (iii) developing fusion-driven control theory, (iv) supporting the work in this area at the U.S. fusion experimental facilities, (v) providing the expertise developed here in the U.S. to international tokamaks through collaboration agreements, the International Tokamak Physics Activity, and the ITER Scientist Fellow Network. The overall objective of this project, which is well aligned with LU-PCG’s long-term goal, is to continue and extend LU-PCG’s work on integration of physics and operation in NSTX-U, with the ultimate goals of developing control solutions to enable the realization of advanced scenarios, increasing the physics productivity of experimental time by providing improved and expanded control capabilities, and contributing to the resolution of important physics issues related to advanced burning plasmas.
By building both on the long history of collaboration between LU-PCG and NSTX-U and on LU-PCG’s expertise arising from present and past work on model-based scenario optimization and control at different devices around the world (e.g., DIII-D, EAST, KSTAR, ITER), the work plan will focus on the following research priorities at NSTX-U: (1) Further development and validation of the Multi-Mode Model (MMM) anomalous-transport code; (2) Incorporation of enhanced version of MMM into PPPL’s TRANSP code to tackle several key physics research tasks; (3) Refinement and extension of control-oriented plasma-response models to enhance the prediction capabilities of LU’s Control-Oriented Transport SIMulator (COTSIM); (4) Model-based curation of diagnostic data for effective real-time reconstruction of those plasma properties needed for control and forecasting; (5) Development of integrated profile/scalar feedforward+feedback control schemes via off-line and on-line model-based optimization; (6) Coupling of COTSIM with MHD equilibrium solver to enable studies on integrated profile/scalar and shape/position control; (7) Coupling of COTSIM with reduced model of the scrape-off-layer and divertor regions to enable design of active control schemes for simultaneously attaining high-performance core-plasma conditions and safe divertor operation.
