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of little consequence since the usual superfluid response, the formation of vortices, is absent. However, when flow is externally applied, for instance, by setting the superfluid sample in rotation, it turns out that various mechanisms exist by which vortices are formed at some critical rotation velocity. Thus the superfluid zero temperature limit has turned out not to be qualitatively very different from finite temperatures, although the zero-temperature dissipation mechanisms are still largely unknown.  

Our research group has provided much of the experimental proof which confirms the above scenario [1]. We use the isotropic B phase of superfluid 3He and a novel type of measurement for these studies. The new technique was developed for investigating vortex dynamics and turbulence in steady state conditions [2]. Here one or more vortex seed loops are injected by external means into a long cylin­der of 3He-B which rotates at constant conditions in the vortex-free state. The ensuing dynamics is observed non-invasively with external NMR pick-up coils as a function of time. At high temperatures and high vortex damping, the dynamics is laminar in the sense that the number of quantized vortices is conserved and the seeds ultimately evolve to recti­linear lines which are stable in uniform rotation. At low temperatures and low damp­ing, the seed loops become unstable, generate new vortices, which start interacting and give rise to a rapid turbulent burst. This process does not conserve the number of vortices, as shown by the evolution depicted in Fig. 1. The precursor to the turbulent burst is the single vortex instability in applied flow which was first identified in this context [3]. In the turbulent burst enough vortices are created so that the equilibrium vortex state can be reached. This is the final stable state where the dynamical evolution ends. It consists of a regular array of rectilinear vortex lines.

Fig. 1.  Vortex instability and turbulence in a rotating column of 3He-B in the turbulent tempera­ture regime. A seed vortex loop is injected in applied vortex-free flow and the subsequent evolu­tion is depicted. Different transient states are traversed, until the stable rotating equilibrium vor­tex state is reached.

After the turbulent burst the expansion of the vortices takes place in the form of a front followed by a helically twisted bundle of vortex lines [4]. A steady state with turbulent fluctuations can be monitored while the vortex front propagates along the rotating column at constant velocity. The twisted structure is formed by the spiral

Annual Report 2007

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