Hamiltonian and Brownian systems with long-range interactions: IV. General kinetic equations from the quasilinear theory

TL;DR

This paper develops a comprehensive kinetic theory for Hamiltonian systems with long-range interactions, deriving equations that account for inhomogeneity, memory effects, and relaxation processes, bridging microscopic dynamics and macroscopic states.

Contribution

It introduces a general kinetic equation from quasilinear theory applicable to inhomogeneous systems, extending previous models and linking phase mixing, violent relaxation, and statistical theories.

Findings

Derived a kinetic equation valid at order 1/N for inhomogeneous systems.

Connected the kinetic equation to the Vlasov equation in the large N limit.

Formulated a Fokker-Planck equation for test particle relaxation in a thermal bath.

Abstract

We develop the kinetic theory of Hamiltonian systems with weak long-range interactions. Starting from the Klimontovich equation and using a quasilinear theory, we obtain a general kinetic equation that can be applied to spatially inhomogeneous systems and that takes into account memory effects. This equation is valid at order 1/N in a proper thermodynamic limit and it coincides with the kinetic equation obtained from the BBGKY hierarchy. For N tending to infinity, it reduces to the Vlasov equation describing collisionless systems. We describe the process of phase mixing and violent relaxation leading to the formation of a quasi stationary state (QSS) on the coarse-grained scale. We interprete the physical nature of the QSS in relation to Lynden-Bell's statistical theory and discuss the problem of incomplete relaxation. In the second part of the paper, we consider the relaxation of a test particle in a thermal bath. We derive a Fokker-Planck equation by directly calculating the diffusion tensor and the friction force from the Klimontovich equation. We give general expressions of these quantities that are valid for possibly spatially inhomogeneous systems with long correlation time. We show that the diffusion and friction terms have a very similar structure given by a sort of generalized Kubo formula. We also obtain non-markovian kinetic equations that can be relevant when the auto-correlation function of the force decreases slowly with time. An interest of our approach is to develop a formalism that remains in physical space (instead of Fourier space) and that can deal with spatially inhomogeneous systems.