Multiple discrete TI states of superfluid turbulence

We report a most unusual new observation in thermal counterflow of liquid helium II in a weakly nonuniform circular channel: the appearance of multiple discrete states of superfluid turbulence. These states are in the TI regime of heat currents but exhibit a lower level of dissipation than the ordinary TI state present in both uniform and nonuniform channels. Interestingly, these new states are only observed for diverging heat flow and are completely suppressed in converging flow. In contrast to the ordinary stable TI state, these new states are metastable. Transitions between the metastable states and to the TI state can be induced by weak mechanical perturbation. We have observed as many as five discrete metastable states at 1.75 K, but the number of states decreases markedly with temperature. Confirmatory data were obtained with a second channel of different size and shape. Although current theories of superfluid turbulence do not provide any obvious explanations for these new states, we offer some speculations based upon competing normal fluid flow profiles and a quantized vortex mill.