Plasma shape and position control in highly elongated tokamaks

Plasma shape and position control in elongated tokamaks is analysed, using the TSC code. The paper presents a new algorithm which allows the shape evolution of a discharge to be entirely preprogrammed. The algorithm computes poloidal field coil voltages as functions of time, using magnetic measurements made close to the vessel wall. No preprogrammed coil current wave forms are required. By simulating the start-up phase of typical TCV tokamak discharges, it is shown that the actual shape evolution follows closely the preprogrammed one. X-points can be specified at arbitrary positions. Active stabilization of the dominant vertical mode is achieved by a conventional PD feedback loop. The optimization of feedback coefficients, the trade-off between shape accuracy and power dissipation in the poloidal field coils, as well as the computing power requirements for implementing the algorithm in a real experiment are discussed.