Abstract

A demonstration of real-time feedback control for autonomous tracking and stabilization of m/n = 2/1 tearing modes in a tokamak using Electron Cyclotron Resonance Heating and Current Drive (ECRH/ECCD) is reported. The prototype system on TEXTOR combines in the same sight-line an Electron Cyclotron Emission (ECE) diagnostic for tearing mode sensing and a steer-able ECRH/ECCD antenna. The mode location is retrieved from the ECE measurements and serves as input for a control loop, which aligns the ECRH/ECCD deposition with the tearing modes by steering of a launcher mirror. The alignment is achieved by matching the mode location in the sensor spectrum with the fixed ECRH/ECCD actuator frequency. The control response is dominated by the response of the mechanical launcher. Analysis of the launcher dynamics receives special emphasis in the control design. In addition, the ECRH/ECCD power is modulated in phase with the rotation frequency of the O-point of the tearing modes using a feedback loop, which extracts the modes frequency and phase from the ECE data. The 'line-of-sight' concept has advantages over conventional tearing mode suppression schemes as it provides information of the modes location and phase in the direct vicinity of the EC-deposition. It avoids the use of ray-tracing in the control loop and guarantees accurate alignment in the presence of disturbances. The experimental results demonstrate the capabilities of the control system to track and suppress tearing modes in real-time. A relatively simple control design suffices to meet the performance requirements demanded for effective tearing mode suppression. In addition, a control-oriented model for the tearing mode control problem is being developed. This model includes tearing mode and plasma dynamics, models for the actuators, diagnostics, data-processing and control algorithms. Simulation results for typical TEXTOR conditions will be discussed and validated with experimental results.

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