Description
Instabilities such as Edge Localised Modes (ELMs) can degrade confinement in tokamak plasmas, potentially leading to damaging heat fluxes on the first wall [1, 2]. Externally applied Magnetic Perturbations (MPs) are used to mitigate, and even suppress, these instabilities [3]. This suppression technique is still under investigation in many tokamaks, and its understanding is crucial for future fusion devices like ITER, as applying these perturbations can also have an effect on fast-ion confinement [4]. MAST-U is equipped with an upper and a lower set of MPs. Moreover, fast-ion losses are directly measured and analysed in the MAST-U spherical tokamak, using a scintillator-based Fast-Ion Loss Detector (FILD) [5]. FILD measurements reveal a clear modulation of the losses with the applied configuration of the MPs (perturbation amplitude, spectrum, and phase) and with the velocity-space of the fast-ion losses [6].
However, FILD measurements only reflect a local behaviour of the losses. Infrared (IR) thermography can be used to complement FILD local measurements [7], as it monitors a wider region of the tokamak wall, allowing a more global understanding of the behaviour and deposition pattern of the fast-ion losses. To this end, a temporal modulation of the NBI, that provides the fast-ion distribution, has been employed, so that Fourier analysis of the data recorded by the IR cameras can be applied [8]. This enables the qualitative identification of the regions where the fast-ion losses are deposited. The application of this method indicates that the local FILD measurements, in terms of the configuration of the perturbation that optimises energetic-particle confinement, should not be treated as representative of their global behaviour: The IR data shows that, under several configurations, the applied perturbation leads to a redistribution of the fast-ion losses on the wall, and not necessarily to a global improvement or deterioration of the confinement.
[1]M.Garcia-Muñoz et al.,PPCF (2013) 55 124014
[2]J.Coenen et al.,JNM (2015) 463 7884
[3]T.E.Evans et al.,PRL (2004) 92(23) 235003
[4]L.Sanchis et al.,NF (2021) 61 046006
[5]J.F.Rivero-Rodríguez et al.,RSI (2018) 89 10I112
[6]L.Velarde et al.,NF (2025) 65 112003
[7]L.Velarde et al.,FED (2025) 220 11
[8]C.J.Lasnier et al.,RSI (2014) 85 11D855