Saturation of the fan instability: Nonlinear merging of resonances

Abstract

A Hamiltonian self-consistent wave-particle model has been built in order to study the nonlinear interaction of a packet of waves with a nonequilibrium electron distribution in a magnetized background plasma. In particular, this model and the corresponding numerical code allow us to study in detail the excitation by the fan instability of lower hybrid waves interacting resonantly with a strongly anisotropic electron velocity distribution. This paper points out the essential role played by the process of “dynamical merging of resonances,” which results from an instability of the trapped particles’ motion, leading, in its explosive stage, to the amplification of the waves’ amplitudes. Moreover the relaxation phase of the fan instability is shown to lead to a universal distribution of the particles’ velocities, which does not depend on the number of waves and on their distribution in the k space.