Nonquasilinear evolution of particle velocity in incoherent waves with random amplitudes

The one-dimensional motion of $N$ particles in the field of many incoherent waves is revisited numerically. When the wave complex amplitudes are independent, with a gaussian distribution, the quasilinear approximation is found to always overestimate transport and to become accurate in the limit of infinite resonance overlap.Comment: 8 pages Elsevier style. Communications in Nonlinear Science and Numerical Simulation accepted (2008) in pres

Diffusion limit for many particles in a periodic stochastic acceleration field

The one-dimensional motion of any number \cN of particles in the field of many independent waves (with strong spatial correlation) is formulated as a second-order system of stochastic differential equations, driven by two Wiener processes. In the limit of vanishing particle mass ${\mathfrak{m}} \to 0$, or equivalently of large noise intensity, we show that the momenta of all $N$ particles converge weakly to $N$ independent Brownian motions, and this convergence holds even if the noise is periodic. This justifies the usual application of the diffusion equation to a family of particles in a unique stochastic force field. The proof rests on the ergodic properties of the relative velocity of two particles in the scaling limit.Comment: 20 page

Kinetic limit of N-body description of wave-particle self- consistent interaction

A system of N particles eN=(x1,v1,...,xN,vN) interacting self-consistently with M waves Zn=An*exp(iTn) is considered. Hamiltonian dynamics transports initial data (eN(0),Zn(0)) to (eN(t),Zn(t)). In the limit of an infinite number of particles, a Vlasov-like kinetic equation is generated for the distribution function f(x,v,t), coupled to envelope equations for the M waves. Any initial data (f(0),Z(0)) with finite energy is transported to a unique (f(t),Z(t)). Moreover, for any time T>0, given a sequence of initial data with N particles distributed so that the particle distribution fN(0)-->f(O) weakly and with Zn(0)-->Z(O) as N tends to infinity, the states generated by the Hamiltonian dynamics at all time 0<t<T are such that (eN(t),Zn(t)) converges weakly to (f(t),Z(t)). Comments: Kinetic theory, Plasma physics.Comment: 18 pages, LaTe

Ornstein-Uhlenbeck limit for the velocity process of an NN-particle system interacting stochastically

An $N$-particle system with stochastic interactions is considered. Interactions are driven by a Brownian noise term and total energy conservation is imposed. The evolution of the system, in velocity space, is a diffusion on a $(3N-1)$-dimensional sphere with radius fixed by the total energy. In the $N\rightarrow\infty$ limit, a finite number of velocity components are shown to evolve independently and according to an Ornstein-Uhlenbeck process.Comment: 19 pages ; streamlined notations ; new section on many particles with momentum conservation ; new appendix on Kac syste

A symplectic, symmetric algorithm for spatial evolution of particles in a time-dependent field

A symplectic, symmetric, second-order scheme is constructed for particle evolution in a time-dependent field with a fixed spatial step. The scheme is implemented in one space dimension and tested, showing excellent adequacy to experiment analysis.Comment: version 2; 16 p