Torsional Oscillator Experiments under DC Rotation with Reduced Vibration

Abstract

We describe a highly stable, rotating cryostat designed for torsional oscil- lator experiments under DC rotation, where vortex lines penetration has been studied for 3D superfluids made of monolayer He films as well as for the supersolid state in hcp solid 4 He. Especially, torsional oscillator experiments on hcp solid 4 He are known to be quite sensitive to small vibrations or linear velocities on the order of 10 µm/s or less. Thus, vibrations of the apparatus may destroy the measurements if they are not smaller than or equal to those of the building or the ground itself. The torsional oscillator performance described here often gives better data under steady rotation at moderate speeds than under stationary conditions. The article describes briefly a design idea shared by the two rotating cryostats at ISSP, the University of Tokyo, and discusses the torsional oscillator (TO) experiments under DC rotation. This is truly a high speed rotating cryostat with maximum rotational speed of at least 6 revolutions per second for TO experiments. It gives also much higher stability at reasonably low rotational speed because of the well-planned structure of the dou- ble frame construction with a lot of mass for the upper drive frame for rotation and the almost mechanically isolated, except for the drive mode motion, rotating cryo- stat with much higher stability of the inner frame for the cryostat mount. Phenomena of quantized vortex lines penetration through a macroscopic superfluid give unique information about the superfluidity itself. A method for detection of vortex lines pen- etration events using TO technology is also briefly reviewed.