On Third Sound Amplification by Stimulated Condensation

Abstract

We describe the process by which condensing ⁴He vapor atoms are expected to contribute coherently to the macroscopic superfluid motion of a third sound resonance. This phenomenon is similar to the experimentally observed thickening of a film in the fixed velocity state of a persistent current, though with an interesting and subtle difference. As before, the condensing atoms contribute to the kinetic energy of the macroscopic state by the coherent normal-to-superfluid conversion. In the present case, however, the macroscopic state is an oscillatory third sound mode. This allows the energy to remain in the third sound mode even if superfluid is removed by film flow in order to keep the film thickness constant. Thus, a continuous process of third sound amplification by stimulated condensation should accompany a condensing vapor flux under the right conditions.