High frequency Alfvén eigenmodes detected with ion-cyclotron-emission diagnostics during NBI and ICRF heated plasmas on the ASDEX Upgrade tokamak: Paper

R. Ochoukov*, R. Bilato, V. Bobkov, S.C. Chapman, R. Dendy, M. Dreval, H. Faugel, A. Kappatou, Ye.O. Kazakov, M. Mantsinen, K.G. McClements, D. Moseev, S.K. Nielsen, J.M. Noterdaeme, M. Salewski, P. Schneider, M. Weiland

*Corresponding author for this work

Research output: Contribution to journalJournal articleResearchpeer-review

Abstract

The paper presents the first reported observation of high frequency Alfvén eigenmode excitation on the ASDEX Upgrade tokamak. The mode is driven in a novel way using radio frequency (RF) wave acceleration of either beam-injected deuterium ions or thermal He-3 minority ions in a three-ion heating scenario. In the case of beam ion acceleration, the instability only appears during deuteron acceleration at the third beam ion cyclotron harmonic (wave frequency ω = 3ΩD where ΩD is the deuterium cyclotron frequency), as the mode is not detected during the more commonly used second harmonic/minority heating scenario or in the absence of beam-injected ions. The mode frequency is around 0.6–0.7ΩD, where ΩD is evaluated in the low-field side plasma edge, and tracks the magnetic field B and the edge plasma electron density ne via the Alfvénic relation ω ∼ B ne−1/2. The mode does not appear as a single frequency wave but as a bundle of closely spaced (in frequency) sub-modes. When the parallel beam ion velocity component is increased, the sub-mode frequency spacing is observed to decrease, possibly due to a change in the eigenmode structure. Under certain conditions, typically in discharges with a relatively low plasma current, IP <0.7 MA, the mode appears to be driven directly by sub-Alfvénic deuterium beam ions. Absolute measurements of the mode amplitude show that at least 1% of the beam-injected power is transferred non-collisionally to the instability. While this is too low for practical alpha-channeling applications, discharges are planned with the aim of increasing the level of power transferred non-collisionally between fast ions and the instability.
Original languageEnglish
Article number126043
JournalNuclear Fusion
Volume60
Issue number12
Number of pages10
ISSN0029-5515
DOIs
Publication statusPublished - 2020

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