Speaker
Description
Energetic-particle-driven instabilities can induce strong fast-ion redistribution and losses, thereby degrading plasma heating and confinement. This is a key concern for future burning plasmas such as ITER, where fusion-born alpha particles are expected to drive resonant wave-particle interactions. To limit these, it is important to understand the associated phase-space transport [1] and, more generally, the interplay between fast ions and MHD instabilities, to ultimately improve predictive capabilities [2] and developp robust control strategies for future reactors [3]. This work integrates multiple diagnostics to experimentally characterize fishbone-induced fast-ion transport in Neutral Beam Injection heated plasmas on the TCV tokamak across plasma shaping scans. Fishbone activity is identified from magnetic probes, electron cyclotron emission (ECE) and soft X-ray (SXR) diagnostics, providing the mode properties and timing of the instability cycle. To investigate the fast-ion response, event-synchronized conditional sampling and averaging over a large number of similar fishbone bursts is applied to Fast-Ion Deuterium-Alpha (FIDA) spectroscopy and Fast-Ion Loss Detector (FILD) measurements [4, 5] and combined with inversion techniques [6–8]. Confined and lost fast-ion velocity-space distributions are reconstructed and correlated throughout a typical fishbone cycle. Following the instability onset, reduced levels of FIDA emission are observed across the full wavelength range, suggesting a broad fast-ion redistribution over the measured energy domain. The depletion is stronger at higher energies and in the plasma core where the mode is localized. The dual-camera FILD system resolves the temporal velocity-space evolution of fast-ion losses. The signals exhibit coherent frequency signatures consistent with the activity observed in magnetic and ECE diagnostics. The velocity-space reconstructions show two distinct pitch-angle loss structures, with intense high-energy losses occurring near the mode onset, followed by a progressive shift toward lower energies and a decay of the loss amplitude. Together, these measurements reveal consistent signatures of core fast-ion redistribution and edge fast-ion losses, highlighting an energy-dependent transport process during fishbone cycles associated with resonant wave-particle interactions. This study establishes a general experimental framework for a systematic reconstruction of fast-ion redistribution and losses during MHD activity and supports the validation of predictive fast-ion transport models for future fusion devices, in different plasma configurations, including negative triangularity scenarios [9].
[1] Heidbrink W. W. Maxwell Prize lecture on measuring the phase-space transport of energetic particles in magnetic fusion experiments Physics of Plasmas 33 (2026) 030601
[2] Podestà M. et al A workflow for testing energetic particle transport models - from experiments to model validation (this conference)
[3] Jansen van Vuuren A. et al Experimental investigations of Alfvén eigenmode control in TCV fast-ion plasmas (this conference)
[4] Geiger B. et al Fast-ion transport in low density L-mode plasmas at TCV using FIDA spectroscopy and the TRANSP code Plasma Physics and Controlled Fusion 59 (2017) 115002
[5] Poley-Sanjuán J. et al First microsecond velocity-space resolved simultaneous measurements of co-andcounter-current fast-ion losses in forward and reverse magnetic field in a tokamak Nuclear Fusion 65 (2025) 076006
[6] Rud M. et al Bayesian velocity-space tomography with collision- and charge-exchange-physics prior from fast-ion D-alpha measurements at TCV with uncertainty quantification Plasma Physics and Controlled Fusion 68 (2026) 015035
[7] Rud M. et al Resonant mode-particle interaction priors for energetic particle phase-space tomography (this conferencee)
[8] Galdon-Quiroga J. et al Velocity-space sensitivity and tomography of scintillator-based fast-ion loss detectors Plasma Physics and Controlled Fusion 60 (2018) 105005
[9] Poley-Sanjuán J. et al Fast ion confinement in negative triangularity plasmas on the TCV tokamak Nuclear Fusion 66 (2026) 046012