Fast-ion dynamics at ITER-relevant densities in ASDEX Upgrade measured with collective Thomson scattering

Energy transfer from confined fast ions will play a key role in maintaining the core plasma at fusion-relevant conditions in future fusion devices such as ITER. The dynamics of such ions is experimentally well characterized in low-density plasmas, owing to good fast-ion coverage with diagnostics such as fast-ion D-α spectroscopy (FIDA), neutron/γ-ray spectrometry, and collective Thomson scattering. In contrast, this applies much less to highdensity discharges, in which the relevant diagnostic performance is more limited by the shorter slowing-down time of energetic particles, the reduced neutral beam (NBI) penetration, and the significantly increased Bremsstrahlung emission. Collective Thomson scattering (CTS) is less hampered by the latter of these limitations and will be the main diagnostic for measuring confined fast ions in ITER across their full energy range [1, 2, 3]. Although ITER will achieve a combination of high density n and low core collisionality ν ∝ nT ^−3/2 that cannot be accessed in present devices, it remains relevant to experimentally test predictions of energetic-ion dynamics at high density as a step in preparing for ITER operation. Here we present the first CTS measurements of fast-ion dynamics in ASDEX Upgrade (AUG) performed at a local density of n_e ≈ 9×10¹⁹ m⁻³ , i.e., similar to projections for the high-density ITER baseline scenario [4].