IFS Seminar

Abstract: Instabilities of Alfvén eigenmodes (AEs) are of significant

concern because they can enhance the cross-field transport of alpha

particles beyond the neoclassical level in fusion plasmas. The thresh-

old value of alpha-particle pressure for exciting AEs depends critically

on the damping rate of AEs. The damping mechanisms include ki-

netic damping due to interactions with thermal particles, continuum

damping due to Alfvén  continuum crossing, and radiative damping

due to emitting kinetic Alfvén  waves (KAWs). The radiative damping

is substantial and can even prevail in high-temperature burning plas-

mas. We revisit the radiative damping theory for TAEs in low-shear

plasmas, considering TAE with an eigenfrequency near the bottom of

the TAE gap. In contrast to earlier papers, we provide the damping

calculations in real space rather than Fourier space. In the real-space

formalism, the Alfvén  eigenmode represents a smooth radially localized

source that emits a short-wavelength KAW. Unlike the source itself, the

radiated wave is tractable via WKB approximation. The separation of

spatial scales between the AE and KAW makes the radiative losses depend on this separation exponentially. This approach is straightforward technically and more enlightening from a physics standpoint for

benchmarking numerical calculations of radiative damping.