Abstract
Edge localized modes (ELMs) are magnetohydrodynamic (MHD) instabilities that cause fast periodic relaxations of the strong edge pressure gradient in tokamak fusion plasmas. Magnetic pick-up coils allow the extraction of toroidal mode numbers n during the ELM cycle including the nonlinear crash on ASDEX Upgrade, providing a good comparability to nonlinear 3D MHD codes. This paper investigates how the mode numbers before and during the ELM crash change with a variation of plasma parameters. It is found that the toroidal structure size during the crash is similar to the one existing slightly before and always has a low n = 1–7 range. Furthermore, in the nonlinear phase of the ELM n does not show a clear trend with most peeling-ballooning relevant parameters such as normalized pressure gradient, bootstrap current density or triangularity, whereas a strong decrease of n with edge safety factor q95 is observed in agreement with nonlinear modeling in the here investigated high collisionality region. A simple geometric model is presented, which is capable of explaining the q scaling by existence of ballooned structures that minimize n.
Highlights
Edge localized modes (ELMs) are periodically occurring instabilities that cause fast relaxations of the strong edge pressure gradient in the high-confinement regime (H-mode) of tokamak fusion plasmas [1,2,3]
Magnetic pick-up coils allow the extraction of toroidal mode numbers n during the ELM cycle including the nonlinear crash on ASDEX Upgrade, providing a good comparability to nonlinear 3D MHD codes
This paper investigates how the mode numbers before and during the ELM crash change with a variation of plasma parameters
Summary
Edge localized modes (ELMs) are periodically occurring instabilities that cause fast relaxations of the strong edge pressure gradient in the high-confinement regime (H-mode) of tokamak fusion plasmas [1,2,3]. These crashes induce significant heat and particle losses and thereby create intense heat fluxes towards the divertor tiles. Fusion 60 (2018) 125011 not resolvable by the magnetic diagnostic It couples into low n modes forming the nonlinear crash phase [13]
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