From Helium Beams to Alpha Particles: Unlocking ICE Diagnostics for Burning Plasmas

Helium neutral beam injection (He NBI) in the Large Helical Device (LHD) provides a unique platform for simulating the behavior of fusion-born alpha particles in burning plasmas. Due to their similar charge-to-mass ratio and fast-ion dynamics, He ions can emulate the slowing-down phase of alpha particles before thermalization. In this study, we observe sub-Alfvénic harmonic Ion Cyclotron Emissions (ICEs) excited by He NBI, accompanied by ~20 kHz modulations in core electron cyclotron emission (ECE) and global density fluctuations measured by phase contrast imaging (PCI). These observations indicate strong coupling between fast ions, ICE, and bulk plasma instabilities. The experiment demonstrates that He NBI–driven ICEs can serve as a proxy for understanding alpha particle behavior in the early slowing-down phase. This result highlights the potential of using RF emissions to diagnose fast-ion dynamics in burning plasma scenarios such as ITER and DEMO. Owing to their passive, non-invasive, and radiation-tolerant nature, ICE diagnostics are promising tools for monitoring energetic alpha particles in future fusion reactors.

This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (No. RS-2023-00281272).