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Instability in the Frequency Range of Alfvén Eigenmodes Driven by Negative-Ion-Based Neutral Beams in JT-60U

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  • Instability in the Frequency Range of Alfven Eigenmodes Driven by Negative-Ion-Based Neutral Beams in JT-60U

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Abstract

Alfvén eignemode experiments have been carried out by using the negative-ion-based neutral beam in JT-60U. For bursting instabilities in the frequency range of Alfvn Eigenmodes, two types of mode activity have been observed. In one type called Abrupt Large-amplitude Event (ALE), enhanced transport of the energetic ions was detected. The enhanced transport showed the clear energy dependence based on measurements performed using a natural diamond detector. In another type of bursting event called the fast Frequency Sweeping (fast FS) mode, frequency up-down chirping is observed on the time-scale of a few ms, much faster than equilibrium changes in the background plasma. This frequency chirping was well reproduced by the non-linear simulation code, MEGA, for magnetohydrodynamics (MHD) with energetic particles. The slow up-frequency sweeping of the Reversed-Shear-induced Alfvén Eigenmode (RSAEs), due to the slow evolution of qmin was also observed. These experiments were analyzed by using the NOVA-K code. It is shown that the most unstable modes are localized around the minimum in the magnetic safety factor (qmin) in the analysis.

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