Shifting and splitting of resonance lines due to dynamical friction in plasmas and gravitational systems

Wave-particle resonances underlie a variety of fundamental plasma physics processes, including the growth of instabilities. It has long been known that collisional scattering alters this interaction, leading to a broadened resonance. However, the effect of Fokker-Planck dynamical friction (e.g. drag) has not been previously determined. To this end, a quasilinear transport theory that incorporates convective drag and pitch angle scattering is self-consistently derived from first principles for a marginally-unstable mode resonating with a non-Maxwellian distribution [1]. It is found that drag fundamentally changes the structure of the resonance, breaking its symmetry and leading to the shifting and splitting of resonance lines. In contrast, scattering broadens the resonance in a symmetric fashion. Simulations show that the derived quasilinear system preserves the exact instability saturation amplitude and the corresponding particle redistribution of the fully nonlinear theory. Even in situations in which drag leads to a relatively small resonance shift, it still underpins major changes in the redistribution of resonant particles. This novel influence of drag is equally important in plasmas and gravitational systems. A deep connection exists between kinetic processes in plasmas and self-gravitating systems, as both are well described by mean field theories governed by long-range, inverse square laws. In plasmas, the effects of drag are especially pronounced in either low temperature laboratory experiments or fusion plasmas featuring relatively low turbulent scattering, as evidenced by past observations. The same theory directly maps to the resonant gravitational dynamics of the rotating galactic bar and orbiting bodies, providing new techniques for analyzing galactic dynamics.

[1] V.N. Duarte* and J.B. Lestz* et al, Phys. Rev. Lett. 130, 105101 (2023) *Equal contribution