Shielded Superconducting Linear Motor for Towed-Grid Studies of Quantum Turbulence

摘要

The purpose of ongoing low-temperature quantum turbulence research is to produce turbulence similar to that studied in classical fluids to compare the experimental data and theories. Specifically, in the absence of viscosity, through what path does the turbulence decay? Homogeneous isotropic turbulence (HIT) is the best characterized classical situation. To produce HIT in a quantum fluid, we must tow a grid through a channel of superfluid helium at 20 mK. A grid motion of 1 cm at a nearly constant speed up to 1 μS is required. To avoid Joule and eddy current heating of the liquid helium, a magnetically shielded superconducting linear motor has been built. The grid is attached to the end of a 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 that accelerates the rod (and grid) in 1 mm, moves the rod and grid at constant speed for 10 mm, and then decelerates it in 1 mm. The motor was built guided by simulations that demonstrated the design and current pulses required are quite reasonable. The simulation and control program is written in LabView with an embedded C compiler. Using the simulator, various designs of solenoid (with and without shielding) and armature were investigated. We compare the simulation and the experimental results. By writing a pulse-generating program in LabView, we can apply virtually any pulse shape required to produce the desired motion. This is necessary because of the almost purely inductive (zero resistance) load of the motor circuit. The simulations, design process, and the experimental data demonstrating the functioning motor will be presented.