Time-domain response of displaced linear branch in long Josephson junction

At low fluxon density, the flux-flow state in a long Josephson tunnel junction is manifested by the so called displaced linear slope (DLS) appearing as a smooth resonance-free branch in current-voltage characteristics. For the DLS branch, we measured the Josephson radiation linewidth by factor of 104 larger than that of a equivalent lumped Josephson oscillator. With increasing the fluxon density, a sharp crossover is observed to a much smaller linewidth approaching the lumped oscillator limit. Numerical simulations using perturbed sine-Gordon model indicate that the observed behavior is related to the internal degrees of freedom in the weakly interacting fluxon chain which dynamics becomes intrinsically chaotic.