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

Abstract

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.

Experimental observation of reverse-sheared Alfvén eigenmodes (RSAEs) in ELMy H-mode plasma on the EAST tokamak

“…Based on the theory, the α particle driven modes observed in weak magnetic shear plasmas in TFTR D-T experiment, which were originally explained as toroidal Alfvén eigenmodes (TAEs), have been reinterpreted as RSAEs [12]. From then on, these modes have been observed in other devices, including DIII-D [13], NSTX [14], MAST [15], AUG [16] and LHD [17]. The RSAEs were also observed in sawtooth plasmas with the minimum value of q profile (q min )1 [18][19][20].…”

Destabilization of reversed shear Alfvén eigenmodes driven by energetic ions during NBI in HL-2A plasmas withqmin∼ 1

“…There has been a lot of study on conventional RSAEs, both in experiments and theory. The RSAEs have been detected in many experiments such as JT-60U [9], TFTR [10], DIII-D [11], C-MOD [12], JET [3], ASDEX-U [13], NSTX [14], MAST [15] and LHD [16], etc. The significant characteristic of conventional RSAEs is that the mode frequency usually sweeps upwards from low frequency to TAE frequency ranges when the minimum of q-profile (q min ) drops in time.…”

Determination ofqduring sawtooth from inverse evolution of BAEs in Tore Supra

“…This deviation may be interrelated with the role of fast ions after the crash, since equation ( 5) is a first order approximation that does not take into account higher order corrections [32,40]. These results were also observed in [41], which discuss in detail the effects of fast ions on the BAE frequency, showing results from LIGKA software simulations, that exhibits a decrease in BAE frequency in time.…”