First simultaneous observation of co- and counter-current fast-ion losses in the ASDEX Upgrade tokamak

In ITER and future fusion power plants, fusion-born alpha particles will have an isotropic pitch distribution, thus having a co- and counter-current populations. Understanding the mechanisms responsible for the fast-ion losses is critical for future magnetically confined fusion power plants. To further study the interplay of fast ions with plasma instabilities and improve their confinement, a double pinhole collimator has been developed for a Fast-Ion Loss Detector (FILD) in the ASDEX Upgrade tokamak. This new FILD opens the operational window to simultaneous measurements of the co- and counter-current ion velocity-space. The commissioning of this new probe is carried out in H-mode plasmas with an on-axis magnetic field B_0 = -2.5 T, and a plasma current I_p = 0.7 MA. Simultaneous co- and counter-current fast-ion losses have been measured. Both have shown a similar dependence on ICRH power, where the main difference is the intensity of the losses, with the co-losses being an order of magnitude larger. TAE-coherent ICRH-only losses have been identified for the co-current ions. Additionally, the presence of ELMs during the discharge were shown to increment NBI prompt losses, while partially mitigating ICRH-driven losses on both co- and counter- sides of the velocity-space. Finally, a very trapped and high gyroradius losses, with an unclear origin, have been measured in the co- and counter-current velocity-space. The computed ion trajectories show that these ions remain permanently near the vessel wall, suggesting that they are accelerated within the scrape-off layer.