Characterization of Alfvén eigenmodes using NBI during current ramp-up in the ASDEX Upgrade tokamak

Alfvén cascades (ACs) and beta-induced Alfvén eigenmodes (BAEs) have been studied in the ASDEX Upgrade tokamak during the current ramp-up phase of neutral beam heated (NBI) discharges using principally reflectometry, but also soft x-ray (SXR) and electron cyclotron emission imaging (ECEI). ACs have been observed on the tokamak high-field side and low-field side in reflectometer signals even in the absence of a cutoff. Under this condition it is shown that the response is not due to an interferometry effect but due to backscatter. The radial structure of BAEs and ACs has been obtained by cross-correlating the reflectometer with SXR, ECEI and magnetic signals. The reflectometer signals reveal a variety of Alfvén eigenmodes with different characteristics depending on the plasma heating scheme. Here, discharges with similar plasma parameters but varying NBI sources and/or additional electron cyclotron resonance heating were performed. It is shown that the bursting behaviour of ACs for q min < 2 depends on the NBI beam geometry. Also, a discrepancy in the n = 2 AC minimum frequency of a Grand Cascade is explained by the linear gyro-kinetic code LIGKA simulations which include energetic particle effects.

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