Two regimes of self-consistent heating of charged particles.

An experimental illustration of the transition between two basic transport regimes for wave-particle interaction is reported. A striking feature is that chaos, although present in both regimes, does not need to be invoked to explain the observed behavior. This experimental realization for a theoretical paradigm opens the possibility to check the validity of basic models, as is normally required in physics.

Direct observation of a "devil's staircase" in wave-particle interaction.

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.

Electron climbing a "devil's staircase" in wave-particle interaction.

Numerous nonlinear driven systems display spectacular responses to forcing, including chaos and complex phase-locking plateaus characterized by "devil's staircase," Arnold tongues, and Farey trees. In the universality class of Hamiltonian systems, a paradigm is the motion of a charged particle in two waves, which inspired a renormalization group method for its description. Here we report the observation of the underlying "devil's staircase" by recording the beam velocity distribution function at the outlet of a traveling wave tube versus the amplitude of two externally induced waves.