Giant Radiation Linewidth of Multifluxon States in Long Josephson Junctions

Millimeter-wave band radiation measurements of flux-flow states in long Josephson junctions are reported. At low fluxon density, the radiation linewidth is found by factor of ${10}^{4}$ larger than that of a 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 show that the observed behavior is related to internal degrees of freedom in the moving fluxon chain.