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Axisymmetric modes (toroidal mode n=0) are considered to play an influential role in the stability of future tokamak fusion plasmas where the large presence of fusion alpha particles can potentially drive these modes [1]. Among them, two possible ones are the well-known GAE (Global Alfven Eigenmode), and the newly emerged VDOM (Vertical Displacement Oscillatory Modes) [2]. Being global in nature these modes may couple the plasma core, the probable region of confinement of alphas, with the plasma edge, impacting the stability of ELMs and subsequently the plasma dynamics in the divertor region through the formation of current sheets in the vicinity of magnetic X-points. During the final DT campaign on JET, dedicated experiments were conducted to observe and diagnose the n=0 modes driven by the fusion born alpha particles, and indeed a bunch of n=0 oscillations was identified in the spectrogram of the experimental magnetic data [3]. To characterize the observed n=0 modes on JET, we have carried out linear simulations with the NIMROD extended MHD code [4] based on the reconstructed experimental profiles from one of the relevant discharges, the shot # 104408 at 7.35s. The primary mode comes out to be VDOM in our simulation with its frequency to decrease with steepening of the density pedestal, as found in the experiment [3]. Besides, another mode of GAE type adjacent to the VDOM peak in the fast Fourier spectra is noticed, which has more prominent structure at the edge pedestal region. The detailed characteristics of these two modes found in our simulation study will be presented and their relationship with the modes observed in the JET D-T experiment will be discussed.
[1] Kiptily, V.G., et al., Evidence for Alfvén eigenmodes driven by alpha particles in D-3He fusion experiments on JET, Nuclear Fusion, 2021. 61(11).
[2] Barberis, T., Yolbarsop, A. and Porcelli, F., Vertical displacement oscillatory modes in tokamak plasmas. Journal of Plasma Physics, 2022. 88(5).
[3] Oliver, H.J.C. et al, Axisymmetric eigenmodes excited by alpha particle energy gradients in JET D-T plasmas, submitted to Phys. Rev. Lett., July 2025.
[4] Sovinec, C.R., et al., Nonlinear magnetohydrodynamics simulation using high-order finite elements. Journal of Computational Physics, 2004. 195(1): p. 355-386.
