Vertical control modeling and simulation for the STEP prototype reactor

Vertical stability control systems that are robust in radiative environments are a critical need for the design of the Spherical Tokamak for Energy Production (STEP) prototype reactor. Diagnostic models and control schemes are developed and simulated using the TokSys[1] suite of tools in order to evaluate feasibility and controllability of the proposed diagnostic and actuator combination. Modeling shows that having dz/dt calculated from the induced coil current in passive stabilizing hoops, combined with z position estimation using double-null diverter heat flux imbalance measurements, can achieve adequate vertical control of STEP’s flat-top scenario, as long as the heat flux-based z estimation can achieve an effective response time shorter than 0.1s. Simulations are used to find the optimal controller gain settings for a range of effective time constants, with and without simulated diagnostic noise. TokSys simulations are further used to scan elongation to evaluate the controllability of alternative plasma shapes and establish a range of plausible flattop plasma shapes. The maximum controllable displacement control metric is evaluated for the range of parameters simulated.

1. Humphreys, D.A., et al., Nuclear Fusion, 47(8), (2007), 943–951.