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Particle interactions with spatially localized wavepackets

Wave-particle interaction is one of the most well studied subjects in plasma physics. Particle dynamics in the presence of electrostatic or electromagnetic waves has been one of the main paradigms on which the modern theory of nonlinear Hamiltonian dynamics and chaos has been applied [1]. However, almost all previous studies of wave-particle interactions from the point of view of Hamiltonian dynamics have been focused on waves having discrete spectra, namely, periodic waves. In a previous work [2] we have studied particle interactions with localized wavepackets propagating in the absence of a magnetic field or along a uniform magnetic field. In this work we study such interactions in the general case where the wavepackets propagate at an angle to a magnetic field. The wave fields range from ordinary wavepackets to ultra short few-cycle and subcycle transient pulses. For the latter, the assumption of adiabaticity for amplitude modulation, commonly adopted in previous works, does not hold. In the present work, the particle dynamics are analyzed on the basis of the canonical perturbation method [1] allowing for the construction of approximate invariants of motion for the nonintegrable Hamiltonian system describing particle motion. These invariants contain all the essential information for the strongly inhomogeneous phase space of the system. It is shown that the aperiodic character of the waves results in chaotic scattering and transient momentum variation. The latter depends strongly on the initial particle momentum, in terms of a resonant condition, involving all the parameters of the wavepacket (phase and group velocity, width and amplitude). Also there is a complex dependency on the initial particle position: spatially neighbouring particles having the same initial momentum end up with different momenta after their transition through the wavepacket. The collective particle behaviour as given by their distribution function is also described within our approach. Useful calculations on energy transfer through these interactions provide insight into heating, current drive, and transport in magnetized plasmas.