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Description
Axisymmetric modes (toroidal mode n=0) have been observed in recent experiments on TCV with both co-current and counter-current directions injection of neutral beam [1]. These energetic particle driven global modes, prominently the GAE (Global Alfven Eigenmode), and the VDOM (Vertical Displacement Oscillatory Modes) [2], could potentially be driven by fusion alpha particles in future fusion tokamaks and may play a pivotal role in the stability of those plasmas. Indeed, a recent D-T experiment on JET D-T has demonstrated the excitation of these modes by alpha particles [3]. We have carried out linear simulations with the initial value extended MHD code, NIMROD [4], based on reconstructed experimental profiles from a recent TCV discharge, the shot # 72101. In the first phase of the study, we compared the frequency and the structure of the n=0 modes simulated with NIMROD with those obtained in another study done with the eigen value code, MISHKA, on the same TCV discharge [1]. A satisfactory agreement of the results is found between the two codes for the case of a conducting wall placed at the plasma boundary (ideal case). In the second phase, the advanced feature of NIMROD in simulating the open field line region is utilized to study the case of the actual shape and position of the TCV wall that is separated from the plasma boundary by a low density and temperature halo plasma (real case). The detailed characteristics of the principal modes found in our simulation study will be presented and their relationship with the modes observed in the experiment will be discussed.
[1] Dreval M. et al, Axisymmetric global Alfvén eigenmodes in the TCV tokamak, Nuclear Fusion, 66, 086025 (2026).
[2] Barberis, T., Yolbarsop, A. and Porcelli, F., Vertical displacement oscillatory modes in tokamak plasmas. Journal of Plasma Physics, 88 (5) (2022).
[3] Oliver, H.J.C. et al, Axisymmetric eigenmodes excited by alpha particle energy gradients in JET D-T plasmas, Physical Review Letters, 136, 055101 (2026).
[4] Sovinec, C.R., et al., Nonlinear magnetohydrodynamics simulation using high-order finite elements. Journal of Computational Physics, 195(1): p. 355-386 (2004).