The Wave-Kinetic Landau Fluid

Efficient representation of kinetic effects such as Landau damping and particle trapping is crucial for the accuracy of reduced fluid models used to describe collisionless plasma turbulence. A new method for representing nonlinear resonance effects has been developed for Landau fluid [1] models. Wave-kinetic basis functions that focus velocity space resolution on wave-particle resonances naturally generate correct linear and nonlinear Landau damping amplitudes. Perhaps surprisingly, closely spaced resonances are accurately treated using ``inverse'' or ``pseudo'' moments [2] in velocity space. The closure for the fluid moment system is equivalent to the choice of a companion matrix that determines the linear response. This freedom can be used to generate multiple families of closures that generate the same Pad\'{e} approximation to the linear response [1], but have different nonlinear behavior. Results have been formally generalized to include trapped particle effects and collisions. \\[4pt] [1] G. W. Hammett and F. W. Perkins, Phys. Rev. Lett. \textbf{64}, 3019 (1990).\\[0pt] [2] P. Amendt, Phys. Plasmas \textbf{8}, 1437 (2001).