# Vlasov-Poisson-Fokker-Planck system

The linear Fokker–Planck operator is a model for a certain type of collision in a gas or plasma of particles, which are assumed to be described by a kinetic distribution function . This means that the number of particles having their positions and velocities at time is given by . The linear Fokker–Planck collision process has been introduced by S. Chandrasekhar [a6]. The main assumption is that the collisional effects take the form of a stochastic perturbation in Newton's laws of classical mechanics, which are written as

Here, denotes the standard Wiener process, is a diffusion coefficient, is a friction parameter and is an external force (per mass unit). This random perturbation can be interpreted as the result of interactions with a thermal bath at temperature , with the mass of particles and the Boltzmann constant. Writing the local conservation of the number of particles , one obtains the Vlasov–Fokker–Planck equation

The Fokker–Planck term

can also be viewed as a simplified version of the Fokker–Planck–Landau collision operator, which is quadratic as the Boltzmann collision operator, see [a9] or [a7].

There are two situations where the Vlasov–Fokker–Planck equation is relevant: for charged particles and for gravitational systems. When dealing with a self-consistent force field , one obtains in both cases the Poisson equation

The constant is positive in the Coulombic case and negative in the Newtonian case. See [a8], [a10] for discussions on this model in the latter gravitational case.

Concerning the mathematical study of the Vlasov–Poisson–Fokker–Planck system, the situation is comparable with the Vlasov–Poisson system, which is obtained when . The main differences are that there are no characteristics, and that the Laplacian term gives rise to smoothing effects. Actually, the Vlasov–Fokker–Planck operator is hypo-elliptic as soon as ; when , the Green function can be computed explicitly.

Weak solutions in the whole space , can be obtained exactly as for the Vlasov–Poisson case, using an estimate of the energy

Existence of strong solutions (with a bounded force ) is obtained in [a2], and smoothing effects are provided in [a3]. For a bounded domain , with boundary conditions, existence of weak solutions is obtained in [a5]. Concerning the asymptotics, it is proved in [a4] that tends to a stationary solution when (at least in the Coulombic case), by using the decrease of the free energy

which satisfies

The same analysis has been generalized to the case of a bounded domain in [a1].

#### References

[a1] | L.L. Bonilla, J.A. Carrillo, J. Soler, "Asymptotic behavior of an initial-boundary value problem for the Vlasov–Poisson–Fokker–Planck system" SIAM J. Appl. Math. , to appear (1997) |

[a2] | F. Bouchut, "Existence and uniqueness of a global smooth solution for the Vlasov–Poisson–Fokker–Planck system in three dimensions" J. Funct. Anal. , 111 (1993) pp. 239–258 |

[a3] | F. Bouchut, "Smoothing effect for the non-linear Vlasov–Poisson–Fokker–Planck system" J. Diff. Eq. , 122 (1995) pp. 225–238 |

[a4] | F. Bouchut, J. Dolbeault, "On long time asymptotics of the Vlasov–Fokker–Planck equation and of the Vlasov–Poisson–Fokker–Planck system with coulombic and newtonian potentials" Diff. Int. Eq. , 8 (1995) pp. 487–514 |

[a5] | J.A. Carrillo, "Global weak solutions of the absorption and reflection-type initial-boundary value problems for the Vlasov–Poisson–Fokker–Planck system" submitted (1996) |

[a6] | S. Chandrasekhar, "Stochastic problems in physics and astronomy" Rev. Mod. Phys. , 15 (1943) pp. 1–89 |

[a7] | S. Chapman, T.G. Cowling, "The mathematical theory of non-uniform gases" , Cambridge Univ. Press (1939) |

[a8] | M.K.H. Kiesling, "On the equilibrium statistical mechanics of isothermal classical gravitating matter" J. Stat. Phys. , 55 (1989) pp. 203–257 |

[a9] | E.M. Lifshitz, L.P. Pitaevskii, "Physical kinetics" , Pergamon (1981) |

[a10] | T. Padmanabhan, "Statistical mechanics of gravitating systems" Phys. Rep. , 188 (1990) pp. 285–362 |

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Vlasov-Poisson-Fokker-Planck system.

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