Ion Heating by Fast-Particle-Induced Alfvén Turbulence

A novel mechanism that directly transfers energy from super-Alfvénic energetic ions to thermal ions in high- $\beta$ plasmas is described. The mechanism involves the excitation of compressional Alfvén eigenmodes in the frequency range with $\omega \lesssim {\omega }_{\mathrm{ci}}$. The broadband turbulence resulting from the large number of excited modes causes stochastic diffusion in velocity space, which transfers wave energy to thermal ions. This effect may be important on National Spherical Torus Experiment [M. Ono et al., Nucl. Fusion 40, 557 (2000)] and may scale up to reactor scenarios. This has important implications for low-aspect-ratio reactor concepts, since it potentially allows for increased fusion reactivity in such a device.