Using APCS for Plasma Vertical Control at TCV

Early tokamaks with circular cross-section plasmas were vertically stable in nature and no problems were reported on the control of the vertical position. However, the concept of vertically elongated plasma cross-section, with benefits to the energy confinement time, led to vertical instabilities. To overcome this problem, complex closed feedback loop control systems with a vertical position measurement, signal processing, control algorithm, power supplies and active actuating coils are used. The Tokamak a Configuration Variable (TCV) permits the stabilization of highly elongated plasmas, with a powerful vertical position control system and fast power supplies capable of response times under 0.1 ms. The introduction of the new Advanced Plasma Control System (APCS) in TCV has required the development of a new vertical position observer built to optimally exploit the capabilities of the new system and correct for the changes that were detected by the introduction of signal digitizing in the feedback loop. This paper discusses the need for a new measurement of the plasma vertical position, the method to build the observer and the validation of the measured position by comparing with previous methods and observers. A summary of the ongoing work is provided. 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.