RF modified fast-ion distribution function in the full-orbit description

Fast ions generated by neutral beam injection (NBI) or radiofrequency (RF) heating can resonantly interact with background plasma and excite Alfvénic eigenmodes (AEs), in turn leading to enhanced transport and potential damage to reactor components. The combined use of RF and NBI heating exhibits mixed effects on AE activity—mitigating or enhancing mode excitation in NSTX(-U), and reducing AE-induced fast-ion losses in ASDEX Upgrade. Understanding the RF interplay with fast ions, e.g. from NBI or alpha particles in burning plasmas, is essential for the operation of future burning plasma devices.

In low-B field spherical tokamaks, and particularly for high-energy RF-accelerated ions, the guiding-center (GC) approximation breaks down, necessitating a full-orbit (FO) treatment. We present benchmarks of a full-orbit RF implementation in the ASCOT5 code against the GC-based ORBIT-RF mode, and validate the results against experimental data from ASDEX Upgrade using diagnostics such as FIDA and INPA.

This tool has been employed to analyze NSTX(-U) cases where the RF influences the AE stability. Significant differences between FO and GC approaches are highlighted, revealing critical impacts on the fast-ion distribution and AE stability, especially for highly energetic ions poorly confined in low B-field scenarios. To investigate the effects of the RF on the AE saturation regime, a perturbative method for AE mode amplitude and phase evolutions has been implemented in ASCOT5. We plan on using this tool to understand and characterize the AE properties, focusing on the chirping properties, and to develop strategies for employing RF waves as an AE control mechanism.