Using APCS for plasma vertical control at TCV

Early Tokamaks with circular cross-section plasmas were intrinsically vertically stable and no problems were reported on the control of the vertical position. However, the new concept of vertically elongated plasma cross-section with benefits to the energy confinement time led to vertical instabilities. Such instabilities can only be overcome by means of a complex closed feedback loop control system with a vertical position measurement, signal processing, control algorithm, power supplies and active actuating coils. The Tokamak Configuration Variable (TCV), with a powerful vertical position control system, with fast power supplies capable of response times under 0.1 ms, permits the stabilization of highly elongated plasmas. With the introduction of the new Advanced Plasma Control System (APCS) in the TCV a new vertical position observer was built to further explore the capabilities of the new system and solve the limitations that were detected by the introduction in the feedback loop of signal digitalization. The need for a new measure of the plasma vertical position is discussed as well as the method for building the observer and the validation of the measured position which is compared with the previous methods and measures. A summary of the ongoing work is made. We also discuss its aim and the goals that are expected to be achieved by exploring the new digital non-linear control algorithm capabilities of APCS.