Dynamics of the fast-ion acceleration in type-I ELMy H-mode scenarios on the TCV tokamak

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Abstract Fast ions have been observed to be accelerated in the presence of Edge Localised Modes (ELMs) and MHD activity on the Tokamak á Configuration Variable. The acceleration time, velocity-space and frequency dynamics have been resolved by analysing the fast-ion losses measured using a unique Fast Ion Loss Detector (FILD) that allows microsecond velocity-space mapping. The findings presented herein complement and extend previous studies done at the Asdex Upgrade Tokamak and show a decorrelation between the acceleration and the ELM crash. The experimental scenario is a high-confinement mode (H-mode) plasma, characterised by low density, high electron temperature and, hence, long slowing down times of the fast-ion population. Significant MHD activity has been observed in the inter-ELM-crash period with frequencies ranging from 50 to 250 kHz. These modes exhibit strong down-chirping and burst signatures. The empirical dependence of the modes’ frequency upon the plasma density identifies them as Alfvén Eigenmodes (AEs) and locates them in the outer region of the plasma, where the resonance conditions between the fast ions and modes are fulfilled. Their resonant interaction with the fast-ion, generated using a Neutral Beam Injector, changes during the ELM cycle according to the changes in the plasma parameters, such as density and temperature. The fast-ion losses are also identified using orbit following simulations, allowing us to distinguish the different contributions to the FILD signal. The pre-ELM AEs’ frequencies and the velocity-space of the MHD-induced fast-ion losses are preserved on the FILD signal during the first ∼800 microseconds of the ELM crash, suggesting a complex interaction between AEs, fast ions and ELMs.

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