Speaker
Description
Instabilities such as Edge Localised Modes (ELMs) can degrade confinement in tokamak plasmas [1, 2]. Externally applied Magnetic Perturbations (MPs) are used to suppress these instabilities [3]. This technique is still under investigation in many tokamaks, and its understanding is crucial for future fusion devices, as applying these perturbations can also have a detrimental effect on fast-ion confinement [4]. The MAST-U spherical tokamak is equipped with an upper and lower sets of MP coils, and fast-ion losses are directly measured and analysed in this device using a scintillator-based Fast-Ion Loss Detector (FILD) [5]. FILD measurements reveal a clear modulation of the total losses with the applied configuration of the MPs (perturbation amplitude, poloidal spectrum, and phase) and with the velocity-space of the fast-ion losses [6].
However, FILD measurements only reflect 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 [8, 9]. To this end, a temporal modulation of the NBI source that provides the fast-ion distribution has been employed. The IR measurements enable the qualitative identification of the regions where the fast-ion losses are deposited. The application of this method indicates that the local FILD measurements should not be treated as indicative of the MP configuration that optimises fast-ion confinement: The IR data shows that, under several configurations, the applied perturbation leads to lost fast-ions impinging on different regions of the wall, and not necessarily to a global improvement or deterioration of the confinement. The experimental results are compared with calculations performed using MARS-F to model the plasma response to the 3D perturbation and ASCOT to reproduce the changes in the global deposition patterns of the fast-ion losses.
[1] M. Garcia-Muñoz et al 2013 PPCF 55 124014
[2] J. Coenen et al 2015 JNM 463 7884
[3] T.E. Evans et al 2004 PRL 92(23) 235003
[4] L. Sanchis et al 2021 NF 61 046006
[5] J. F. Rivero-Rodríguez et al 2018 RSI 89 10I112
[6] L. Velarde et al 2025 NF 65 112003
[7] L. Velarde et al 2025 FED 220 11
[8] C.J. Lasnier et al 2014 RSI 85 11D855