Abstract
Injecting frozen deuterium pellets into an ELMy H-mode plasma is a well established scheme for triggering edge localized modes (ELMs) before they naturally occur. This paper presents non-linear simulations of spontaneous type-I ELMs and pellet-triggered ELMs in ASDEX Upgrade performed with the extended MHD code JOREK. A thorough comparison of the non-linear dynamics of these events is provided. In particular, pellet-triggered ELMs are simulated by injecting deuterium pellets into different time points during the pedestal build-up described in A Cathey et al (2020 Nuclear Fusion 60 124007). Realistic ExB and diamagnetic background plasma flows as well as the time dependent bootstrap current evolution are included during the build-up to accurately capture the balance between stabilising and destabilising terms for the edge instabilities. Dependencies on the pellet size and injection times are studied. The spatio-temporal structures of the modes and the resulting divertor heat fluxes are compared in detail between spontaneous and triggered ELMs. We observe that the premature excitation of ELMs by means of pellet injection is caused by a helical perturbation described by a toroidal mode number of n = 1. In accordance with experimental observations, the pellet-triggered ELMs show reduced thermal energy losses and a narrower divertor wetted area with respect to spontaneous ELMs. The peak divertor energy fluence is seen to decrease when ELMs are triggered by pellets injected earlier during the pedestal build-up.
Highlights
Type-I edge localized modes (ELMs) are expected to induce large losses and excessive transient divertor heat loads in large machines like ITER, which can affect the lifetime of the components [1]
This paper presents non-linear simulations of spontaneous type-I ELMs and pellet-triggered ELMs in ASDEX Upgrade performed with the extended MHD code JOREK
For pellet ELM triggering to be successful in mitigating the impact of ELMs on the divertor lifetime, the properties of spontaneous and pellet-triggered ELMs need to be investigated in direct comparison because experimental observations suggest that a reduced extent of the wetted area can cancel out the beneficial effects of the decreased energy expelled by pellet-triggered ELMs [7]
Summary
Type-I edge localized modes (ELMs) are expected to induce large losses and excessive transient divertor heat loads in large machines like ITER, which can affect the lifetime of the components [1]. Low-frequency large type-I ELMs must be avoided in ITER either by small-ELM scenarios [2], ELM-free scenarios like QH-mode [3], active control via external resonant magnetic perturbations (RMPs) [4] or pellet ELM triggering [5, 6]. It has been shown experimentally that pellets allow to increase the ELM frequency and reduce the thermal energy losses associated to an individual ELM crash [5]. For pellet ELM triggering to be successful in mitigating the impact of ELMs on the divertor lifetime, the properties of spontaneous and pellet-triggered ELMs need to be investigated in direct comparison because experimental observations suggest that a reduced extent of the wetted area can cancel out the beneficial effects of the decreased energy expelled by pellet-triggered ELMs [7]
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