Speaker
Description
Alpha particles produced by D-T reactions play a crucial role in heating bulk plasma in fusion reactors, and their confinement is required to sustain fusion burning plasmas. However, fast ions such as alpha particles interact with Alfven Eigenmodes (AEs), leading to anomalous transport. Thus, the understanding of interactions between fast ions and AEs is essential for the realization of controlled nuclear fusion.
Phase-space dynamics of fast ions are important for resonant interactions between fast ions and AEs. The radial gradient of distribution function of fast ions destabilizes AEs. The resulting transport of fast ions is intensively investigated. In contrast, the gradient in energy can stabilize AEs through Landau damping, because the energy gradient is typically negative due to slowing-down process. The experimental investigation of the dynamics of fast ions in energy space has been limited.
Utilizing the Large Helical Device (LHD), we have investigated the interaction between AEs and fast ions produced by tangential neutral beam injection (NBI). Fast ions become energetic neutral particles by charge exchange, being detected by the Si-based Neutral Particle Analyzer (Si-NPA) with high time resolution (> 1 MHz). The line of sight of NPA was designated to observe the dynamics of fast ions in the core region, where AEs are commonly observed.
During the NBI injection, bursting activities of Toroidal Alfven Eigenmodes (TAE) were observed. TAE bursts were separated into growth and decay phase based on the temporal evolution of power spectrum. The fast-ion energy spectra were compared with those in the reference phase where TAE activities were negligible.
In both phases, the gradient in energy reduced at the energy where resonance condition of ($v_{\mathrm{fast}}\sim v_{\mathrm{A}}$) is satisfied, where $v_{\mathrm{fast}}$ is the velocity of fast ions and $v_{\mathrm{A}}$ is the phase velocity of TAE. This response suggests that Landau damping of TAE by fast ions has been directly observed.
Importantly, Landau damping was identified even during the growth phase of TAE. This result demonstrates that fast ions simultaneously contribute to TAE bursts in two competing ways: destabilization through the radial gradient, and stabilization through the energy gradient.