Decay of Turbulence Generated by Spin-Down to Rest in Superfluid 4He

Abstract

We report on the extension of the experiments (P. M. Walmsley et al., Phys. Rev. Lett. 99, 265302 (2007)) on the decay of quasiclassical turbulence generated by an impulsive spin-down from angular velocity Ω to rest of superfluid 4 He in a cubic container at temperatures 0.15 K -1.6 K. The density of quantized vortex lines L is measured by scattering negative ions. Following the spin-down, the maximal density of vortices is observed after time t ∼ 10Ω −1 . By observing the propagation of ions along the axis of the initial rotation, the transient dynamics of the turbulence spreading from the perimeter of the container into its central region is investigated. Nearly homogeneous turbulence develops after time t ∼ 100Ω −1 and decays as L ∝ t −3/2 . The effective kinematic viscosity in T = 0 limit is ν = 0.003κ, where κ = 10 −3 cm 2 s −1 is the circulation quantum.

A compact rotating dilution refrigerator

“…We describe the design and performance of a new rotating dilution refrigerator that will primarily be used for investigating the dynamics of quantized vortices in superfluid 4 He. All equipment required to operate the refrigerator and perform experimental measurements is mounted on two synchronously driven, but mechanically decoupled, rotating carousels.…”

“…The behavior of a high-Q mechanical resonator during rapid changes in rotation has also been investigated. Rotation has proven to be a vital tool for investigating the properties and dynamics of quantized vortices in superfluid 4 He 1 , the superfluid phases of 3 He 2 and Bose-Einstein condensates 3 . The equilibrium state of a bulk superfluid rotating with angular velocity, Ω, is an array of rectilinear vortices with areal density 2Ω/κ, where κ is the quantum of circulation (κ = h/m = 1.0 × 10 −3 cm 2 s −1 for superfluid 4 He).…”

“…The ability to rotate cryogenic apparatus can also be useful for some branches of astrophysics and particle physics 8 . Studies of superfluid 4 He under rotation now span over 60 years [9][10][11][12][13] . There have been two approaches used that permit rotation and these are dependent on the temperatures required.…”

“…Any nuisance rotational or vibrational noise will produce unwanted excitation of vortex waves (such as Kelvin waves) due to the very low levels of dissipation at low temperatures which, at sufficient amplitudes, would result in reconnections between neighboring vortices and the destruction of the vortex array. Packard and co-workers 15 failed to observe a vortex array in pure 4 He at low temperatures, presumably due to excessive vibrational or rotational noise. Instead, they had to add a small amount of 3 He to damp nuisance vortex motion so that images of vortex arrays could be obtained.…”

“…Instead, they had to add a small amount of 3 He to damp nuisance vortex motion so that images of vortex arrays could be obtained. This resulted in a widespread misconception that vortex arrays in pure 4 He could not be obtained in the zero temperature limit due to the extremely small damping. More recently, Walmsley and Golov 6 were able to create vortex arrays in a rotating container of isotopically pure 4 He at 0.2 K. They investi-gated the stability of the vortex array by adding a small oscillatory AC component of rotation to the steady value.…”