Optimal tuning of tokamak plasma equilibrium controllers in the presence of time delays

Abstract

When designing the control loops for tokamaks, it is important to acknowledge the effects of time delays. An assumption sometimes made for tokamaks having superconducting coils is that these extra time delays will not have any undesirable effects on control. In fact, introducing extra delays into the axisymmetric control loops of certain supercon-ducting tokamaks can have significant detrimental consequences. Aside from qualitative observations, the detrimental effects of extra time delays in tokamak control loops are not always well understood outside the control community. This study exposes and quantifies the detrimental effects imposed by time delays in the control loop in superconducting tokamaks, by focusing on plasma current control and radial position control in a vertically stable circular plasma in the KSTAR tokamak. Delays in the power supplies, data acquisition, and vessel structure are taken into account. Extremum-seeking-based optimal tuning of PID controllers is proposed as a possible method for remediating the negative effects of time delays. The Nyquist dual locus technique is employed to assess stability of the optimally tuned closed-loop system in the presence of time delays.