Journal article

Optimization of the active MHD spectroscopy system on JET for the excitation of individual intermediate and high-n Alfven eigenmodes

The stability of Alfven eigenmodes (AEs) is studied experimentally in the JET tokamak by observing the plasma response to antenna-driven frequency-sweeping perturbations at the plasma edge. During the 2008/9 experimental campaigns, the complete set of the new antennas was operated and AEs with toroidal mode numbers (n) in the intermediate-n range were excited under various plasma conditions. In this paper, we describe the results of the work achieved on the technical aspects of the diagnostic. The antenna currents have been optimized to improve the antenna-plasma coupling. The mode-tracking system has been upgraded for real-time targeting of modes with specific n. As an example of the optimized performance of the diagnostic, the paper concludes with a report on the damping rates of n = 3-5 toroidal AEs that were measured dynamically while the background plasma parameters were evolving in time.

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