Optimal trade-off between plasma shape control performance and current saturation problems in the next generation tokamaks

In this paper we present an approach for the design of a plasma current and shape feedback control system in a tokamak which takes explicitly into account the problem of current saturation in the coils designated for the control action. The proposed design technique is essentially aimed to enlarge the operating envelope in which the tokamak can operate without current saturation. The proposed controller structure consists of two feedback loops: one is a standard LTI controller designed to operate in the absence of current saturation; the other one gracefully degrades the system performance when the coil currents approach their saturation limits. A case study is presented.