Our journal article entitled “Shear Alfvén wave continuum spectrum with bifurcated helical core equilibria” has been published in the peer-reviewed Nuclear Fusion journal. Jointly published by IAEA and IOP Publishing, Nuclear Fusion is one of the renowned journals specializing in fusion plasmas.
The paper reports on the first modelling results aimed at understanding low-frequency Alfvén eigenmodes called beta induced Alfvén eigenmodes (BAEs) observed during monster sawtooth crashes in ASDEX Upgrade tokamak plasmas and their comparison with the experimental observations.
In tokamak plasmas when the magnetic winding index, i.e. the so-called safety factor q, is in the vicinity of unity, the self-consistent excursion of the magnetic axis due to saturated internal kink-modes distorts axial symmetry and bifurcates MHD equilibria into 3D helical core and axisymmetric mantle. In the equilibrium calculations we find that the helically distorted MHD equilibria may exist in the axisymmetric devices if q = 1 rational surfaces are present.
In this work, the radial structure of the continuum spectrum of shear Alfvén 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. The continuum calculations with the bifurcated equilibria lead to the frequency splitting via coupling to 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.
The weblink of the paper is here.