A Reference Governor for Vertical Control near Stability Limits

A reference governor has been designed to calculate target shapes that ensure safe tokamak operation at high vertical-instability growth rates. Due to their inherent vertical instability, the elongated plasmas envisioned for ITER and beyond are usually attained in present devices by means of feedback (FB). This FB control pursues elongated target shapes usually pre-defined before a discharge and rejects a certain range of disturbances in parameters such as I_p, l_i, or β_p. However, machine-condition changes and off-normal disturbances may make pre-defined shapes unattainable, sometimes resulting in disruptions. Advanced-control strategies that go beyond traditional FB loops can help minimize disruptions in ITER and fusion power plants. In this work, a reference-governor algorithm has been designed to update, in real time, the target shapes that keep the plasma below a safe growth-rate limit during the whole discharge. The algorithm adapts the target shapes in response to machine-condition changes as well as disturbance variations estimated from magnetic measurements. The algorithm uses gspert [1], which solves a perturbed Grad-Shafranov equation to model the plasma response. Initial DIII-D simulations show the advantages of this algorithm to ensure FB control near stability limits.

[1] Welander, A., et al, Fus. Eng. and Design 146 (2019) 2361