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A. Cathey, M. Hoelzl, G. Harrer, M. Dunne, G. Huijsmans, K. Lackner, S. Pamela, E. Wolfrum, S. Günter, JOREK Team, . ASDEX Upgrade Team, EUROfusion MST1 Team:
"MHD simulations of small ELMs at low triangularity in ASDEX Upgrade";
Plasma Physics and Controlled Fusion, 64 (2022), 5; 05401101 - 05401115.

The development of small and no-ELM regimes for ITER is a high priority topic due to the risks
associated with type-I ELMs. By considering non-linear extended magnetohydrodynamic
(MHD) simulations of the ASDEX Upgrade tokamak with the JOREK code, we probe a regime
that avoids type-I ELMs completely, provided that the separatrix density is high enough. The
dynamics of the pedestal in this regime are observed to be qualitatively similar to the so-called
quasi-continuous exhaust regime in several ways. Repetitive type-I ELMs are substituted by
roughly constant levels of outward transport, caused by peeling-ballooning modes (with
dominant ballooning characteristics) which are localised in the last 5% of the confined region
(in normalised poloidal flux). The simulated low triangularity plasma transitions to a type-I
ELMy H-mode if the separatrix density is sufficiently reduced or if the input heating power is
sufficiently increased. The stabilising factors that play a role in the suppression of the small
ELMs are also investigated by analysing the simulations, and the importance of including
diamagnetic effects in the simulations is highlighted. By considering a scan in the pedestal
resistivity and by comparing the poloidal velocity of the modes to theoretical estimates for ideal
and resistive modes, we identify the underlying instabilities as resistive peeling-ballooning
modes. Decreasing the resistivity below experimentally-relevant conditions (i.e. going towards
ideal MHD), the peeling-ballooning modes that constrain the pedestal below the type-I ELM
stability boundary display sharply decreasing growth rates.

small ELMs, non-linear MHD, macroscopic instabilities

http://dx.doi.org/10.1088/1361-6587/ac5b4b

https://publik.tuwien.ac.at/files/publik_304143.pdf