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.

• Received 5 January 1996

DOI:https://doi.org/10.1103/PhysRevLett.77.3617