Plasma models for real-time control of advanced tokamak scenarios

Abstract. An integrated plasma profile control strategy, ARTAEMIS, is being developed for extrapolating present-day advanced tokamak (AT) scenarios to steady state operation. The approach is based on semiempirical (grey-box) modeling. It was initially explored on JET, for current profile control only (D. Moreau, et al., Nucl. Fus. 48 (2008) 106001). The present paper deals with the generalization of this strategy to simultaneous magnetic and kinetic control. The determination of the device-specific, control-oriented models that are needed to compute optimal controller matrices for a given operation scenario is discussed. The methodology is generic and can be applied to any device, with different sets of heating and current drive actuators, controlled variables and profiles. The system identification algorithms take advantage of the large ratio between the magnetic and thermal diffusion time scales and have been recently applied, in their full version, to both JT-60U and DIII-D data. On JT-60U, an existing series of high-bootstrap-current (∼70%), 0.9 MA noninductive AT discharges was used. The actuators consisted of four groups of neutral beam injectors aimed at perpendicular injection (on-axis and off-axis), and co-current tangential injection (also on-axis and off-axis). On DIII-D, dedicated open-loop modulation experiments were carried out. The reference plasma state was that of a 0.9 MA AT scenario which had been optimized to combine non-inductive current fractions near unity with 3.5 < βN < 3.9, bootstrap current fractions larger than 65%, and H98(y,2)=1.5. DIII-D was operated in the loop voltage (Vext) control mode (as opposed to current control) to avoid feedback in the response data from the primary circuit. Actuators other than Vext were co-current, counter-current and balanced neutral beam injection, and electron cyclotron current drive. Power and loop voltage modulations resulted in dynamic variations of the plasma current between 0.7 and 1.2 MA. It is concluded that the response of several essential plasma parameter profiles to the specific actuators of a given device can be satisfactorily identified from a small set of experiments. This provides, for control purposes, a readily available alternative to first-principle plasma modeling.