Mutual-friction induced instability of normal-fluid vortex tubes in superfluid helium-4

Highlights: • In superfluid helium-4, the effect of ground-state vortices on normal-fluid vortices is to make the latter unstable. • It is shown that this key result offers a structural understanding of fully developed superfluid turbulence phenomenology. • A new statistical closure of mutual-friction forces in homogeneous, isotropic superfluid turbulence is proposed. - Abstract: It is shown that, as a result of its interactions with superfluid vorticity, a normal-fluid vortex tube in helium-4 becomes unstable and disintegrates. The superfluid vorticity acquires only a small (few percents of normal-fluid tube strength) polarization, whilst expanding in a front-like manner in the intervortex space of the normal-fluid, forming a dense, unstructured tangle in the process. The accompanied energy spectra scalings offer a structural explanation of analogous scalings in fully developed finite-temperature superfluid turbulence. A macroscopic mutual-friction model incorporating these findings is proposed.