Shear Alfvén wave continuum spectrum with bifurcated helical core equilibria

Allah Rakha, Ph Lauber, M. J. Mantsinen, D. A. Spong

Research output: Contribution to journalArticlepeer-review

Abstract

The radial structure of the continuum spectrum of shear Alfvén and Alfvén-acoustic waves in the beta-induced Alfvén eigenmode (BAE) frequency range is modeled for tokamak plasmas in the presence of 3D effects obtained from the bifurcated MHD equilibrium reconstruction. Plasma compressibility and geodesic curvature effects responsible for the low-frequency continuum spectrum calculations are invoked. In the equilibrium calculations we find that the helically distorted MHD equilibria may exist even for the axisymmetric devices if q = 1 rational surfaces are present. The continuum calculations with the bifurcated equilibria lead to a frequency splitting between the highest frequency branch and the lowest frequency branch continua at the frequency accumulation point. Radially localised shifting of modes happens via coupling of the adjacent n - 1 continuum around an accumulation point. Our modelling (including 3D effects) correctly reproduces the phenomenon of continuum frequency splitting and provides a possible solution for the differences of few kHz in frequency splitting, which remained unexplained with the 2D kinetic calculations (Curran et al 2012 Plasma Phys. Control. Fusion 54 055001). The pressure scaling confirms the increase of helical excursion of the magnetic axis in equilibrium reconstruction and hence the range of continuum frequency splitting. In our calculations, the existence of low-frequency continua is in agreement with the experimentally observed low-frequency modes.

Original languageEnglish
Article number106042
JournalNuclear Fusion
Volume59
Issue number10
DOIs
StatePublished - Sep 9 2019

Funding

FundersFunder number
Horizon 2020 Framework Programme633053

    Keywords

    • 3D effects
    • Alfvén continuum
    • frequency splitting
    • helical core
    • tokamak

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