High frequency Alfvén eigenmodes detected with ion-cyclotron-emission diagnostics during NBI and ICRF heated plasmas on the ASDEX Upgrade tokamak. (29th October 2020)

• Record Type: Journal Article
• Authors: Ochoukov, R.; Bilato, R.; Bobkov, V.; Chapman, S.C.; Dendy, R.; Dreval, M.; Faugel, H.; Kappatou, A.; Kazakov, Ye.O.; Mantsinen, M.; McClements, K.G.; Moseev, D.; Nielsen, S.K.; Noterdaeme, J.M.; Salewski, M.; Schneider, P.; Weiland, M.
• Is Part Of: Nuclear fusion. Volume 60:Number 12(2020)
• Publication Date: 2020-10-29
• Subjects: CAE -- ASDEX Upgrade -- ICRF -- NBI -- Alfven -- GAE; Nuclear fusion -- Periodicals; 621.48405
• DOI: 10.1088/1741-4326/abb79f
• Languages: English
• ISSNs: 0029-5515

Abstract

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.