Quantum turbulence: Decay of grid turbulence in a dissipationless fluid

We produced grid turbulence in liquid helium at 520 mK to compare with classical experiments and theories. Above T = 1 K, with viscosity present, it has been shown that grid turbulence is equivalent to homogeneous isotropic turbulence in a classical fluid. We seek to investigate the nature of grid turbulence when viscosity is zero. Specifically, in the absence of viscosity in a quantum fluid, through what path does the turbulence decay? To produce grid turbulence, an actuator was designed and built that can accelerate and decelerate the grid rapidly in a short distance (∼1 mm), and achieve glide speeds of up to 1 m/s. To avoid Joule and eddy current heating of the liquid helium, a magnetically shielded superconducting linear motor was built. The grid is attached to the end of a very light insulating armature rod which has two hollow cylindrical niobium cans fixed to it about 26 mm apart. This part of the rod is inside a superconducting solenoid which, when driven with the properly shaped current pulse, produces a magnetic field resulting in the required motion.;Detailed computer simulations guided the motor design. The simulation and motor control programs were written in LabView with an embedded C compiler. Using the simulator, various designs of solenoid (with and without shielding) and armature were investigated. We compared the simulation and the experimental results in which complex current pulse shapes were required to produce the desired motion.;The motor and grid (1 mm square hole array with 70% transparency) were mounted in a copper cell containing a pool of liquid helium cooled to 520 mK by dilution refrigeration. We measured the decay of the turbulence produced, after one 28 mm stroke of the grid, using calorimetry. Doped germanium thermometers less than 300 micrometer diameter immersed in the turbulent liquid helium allowed fast calorimetric measurements limited by the electronic time constant of 1 ms. The decay of turbulence was detected by the rate of temperature rise in the isolated cell after the grid was pulled. Recent theory suggests the decay occurs through a Kelvin-wave cascade on the vortex lines which couples the initially large turbulent eddies to the short wavelength phonon spectrum of the liquid, yielding a characteristic rate of temperature rise. Initial measurements support the Kelvin wave cascade theory.