Heat Currents in Liquid Helium II

Abstract

Measurements on the deflection of a beam of negative ions due to a counterflow of superfluid and normal fluid in He II, show the existence of two thresholds for the velocity. The first one is interpreted as the critical velocity for creation of vorticity in the superfluid, while it is suggested that the second threshold corresponds to instability of the whole fluid with respect to turbulence. Measurements on the positive ions tell us the behavior of the normal-fluid velocity; when there is vorticity in the superfluid, these results indicate the existence of a normal-fluid velocity gradient in the flow direction. Parallel measurements on the temperature distribution show nonuniformity of the temperature gradient in the flow direction for large velocities. The assumptions on which the Gorter-Mellink expression for the mutual friction force is derived are not satisfied. The results are then interpreted as an indication that the higher order terms in the equations of motions may give instability at large velocities.