MHD simulations of small ELMs at low triangularity in ASDEX Upgrade. (1st May 2022)

• Record Type: Journal Article
• Authors: Cathey, A; Hoelzl, M; Harrer, G; Dunne, M G; Huijsmans, G T A; Lackner, K; Pamela, S J P; Wolfrum, E; Günter, S
• Is Part Of: Plasma physics and controlled fusion. Volume 64:Number 5(2022)
• Publication Date: 2022-05-01
• Subjects: small ELMs -- non-linear MHD -- macroscopic instabilities; Plasma (Ionized gases) -- Periodicals; Controlled fusion -- Periodicals; 530.44
• DOI: 10.1088/1361-6587/ac5b4b
• Languages: English
• ISSNs: 0741-3335

Abstract

Abstract: 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.