Transformation of electric energy into sound energy in Josephson junctions

Charge fluctuations of superconducting and normal carriers exist in Josephson junction, connected with the thermal activation processes. These fluctuations excite damping acoustic fluctuations in the electrodes across the junction. Application of the current causes the decrease of damping while at the current of definite value damping disappears completely. After further increase of current the acoustic fluctuations become unsteady their amplitudes start to increase and if at this regime the sound wave passes through the junction it is amplified. This paper deals with the physical basis of the sound wave amplification by Josephson junction with introduction of relaxation time of the coherent phase (CP) as the output of a self-consistent solution of the Fokker-Planck equation. The problem deals with a wide bound of frequencies and other parameters. The question of experimental measurement of the lifetime of Josephson junction in metastable state is also discussed.