Velocity diffusion and stochastic acceleration of charged particles in turbulent electric fields

Statistics of charged particles in turbulent electric fields has been studied numerically, and compared with quasilinear and resonance broadening diffusion processes. Particle trajectories are followed by solving the equation of motion for an ensemble of particles where the turbulent electrostatic fields having amplitude sigma are assumed to have gaussian spectrum k/sub j/ = (j-1)Δk, j = 1,2,3,..., with spectrum width deltak and random phases /theta//sub j/. We recover the results obtained by Doveil and Gresillon (1982), namely the quasilinear theory predicts the correct particle diffusion in velocity space in weak turbulent fields and both quasilinear and resonance broadening theory fail to predict the correct diffusion coefficients in strong turbulent electric fields. Our extended calculation reveals the following: The velocity variance <Δv2>, defined by <(v-v/sub o/)2> where v/sub o/ is the initial velocity, exhibits two different linear stages in time: the initial linear rise is followed by the second slow (but still linear) rise characterized by a longer duration and shorter correlation time. The diffusion coefficient estimated from the initial linear rise of <Δv2> substantially differs from those predicted by either quasilinear or resonance broadening theory even at modest field amplitudes. The observed friction coefficient is in good agreement with the prediction of quasilinear theory only for weak turbulence fields. For stronger turbulence fields, the acceleration of some particles to high energy is observed