Diffusion of electrons by coherent wavepackets

Abstract

The momentum transfer and velocity diffusion of electrons periodically interacting with a coherent longitudinal wavepacket is considered. Applying the resonance overlap criterion, we -establish the threshold for intrinsic stochasticity and the width of the stochastic region AVstoch in velocity space.Direct numerical integration of the singleparticle dynamics and an approximate discrete mapping are used to corroborate the resonance overlap results and to find the short-and long-term momentum transfer and diffusion in the field. After the onset of stochasticity, we find a net induced current j ~Av stoch and in the weakfield regime (autocorrelation time< i bounce time) an initial rate of change of the variance <6 v>/2t equal to the quasilinear-theory diffusion coefficient.In the strong-field regime momentum transfer and stochasticity persist owing to non-adiabatic transitions between trapped and untrapped states as the electron traverses the wavepacket. The diffusion coefficient substantially deviates from the quasilinear (~LE ) as well as from resonance broadening (~E3 /2) scaling, while the scattering in velolocity space tends to lose the local, diffusive, nature characteristic of weak fields. 1.2.

Interaction of charged particles with localized electrostatic waves in a magnetized plasma

“…The motion of particles in the presence of adiabatically varying waves has been studied in [10]. The interaction of particles, moving along the magnetic field, with periodic, spatially localized, static, coherent, electrostatic wave packets has been studied in [11] while the single-pass interaction was discussed in [12,13]. The energy transfer between particles and wave packets has been analytically formulated, in the Born approximation, in [14] as well as in [15].…”

Generation of ultrafast electrons by simultaneous stimulated Raman backward and forward scattering

“…A simple model [4] [6,7]. The behavior of the electrons can also be bistable: they either get a large velocity increment &om the wave or no velocity increment at all.…”

Magnetic Field Line Diffusion at the Onset of Stochasticity

“…But fo r weak fields the difference is negligible, as can be seen form (6) The last factor gives zero unless m = p, and from (14) we conclude then that in the latter case also n = q holds. …”

“…Then one w ould expect chaotic particle orbits in the interval < £\m I otherw ise (14) and no " h eatin g " outside this interval. O f course, the reasoning with overlap o f resonances is only qualitative -we used only /"in stea d o f a definite num ber -, and the heating process m ay be dif ferent from ordinary quasilinear diffusion.…”