TRANSITION TO NORMAL FLUID TURBULENCE IN HELIUM-II

Turbulence in the heat flow of helium II manifests itself as a tangle of quantized vortex lines. It has been known for some time that there are two different forms of turbulence, called the T-l and the T-2 turb ulent states. Experiments show that when a critical velocity is exceed ed there is a large increase of the superfluid vortex line density and a transition occurs from T-l to T-2. Until now, the nature of the two turbulent states and of the critical velocity has been a mystery. We present a model which solves this problem by addressing for the first time the issue of the stability of the normal fluid. The computed tran sition is found to be in good agreement with the observations.