title: Direct observation of a "devil's staircase" in wave-particle interaction.
abstract: We report the experimental observation of a "devil's staircase" in a time-dependent system considered as a paradigm for the transition to large-scale chaos in the universality class of Hamiltonian systems. A test electron beam is used to observe its non-self-consistent interaction with externally excited wave(s) in a traveling wave tube (TWT). A trochoidal energy analyzer records the beam energy distribution at the output of the interaction line. An arbitrary waveform generator is used to launch a prescribed spectrum of waves along the slow wave structure (a 4 m long helix) of the TWT. The resonant velocity domain associated to a single wave is observed, as well as the transition to large-scale chaos when the resonant domains of two waves and their secondary resonances overlap. This transition exhibits a "devil's staircase" behavior for increasing excitation amplitude, due to the nonlinear forcing by the second wave on the pendulum-like motion of a charged particle in one electrostatic wave.
authors.affiliation: Physique des interactions ioniques et moléculaires, Unité 6633 CNRS-Université de Provence, Equipe turbulence plasma, case 321, Centre de Saint-Jérôme, F-13397 Marseille cedex 20. doveil@up.univ-mrs.fr
authors.forename: Fabrice
authors.initials: F
authors.lastname: Doveil
authors.forename: Alessandro
authors.initials: A
authors.lastname: Macor
authors.forename: Yves
authors.initials: Y
authors.lastname: Elskens