project title: New Experimental Techniques for the Investigation of Superfluid Turbulence at Very Low Temperatures
project leader: Prof. W. Schoepe
access given (in days): 92
access used (in days): 76
local host: Prof. Matti Krusius
home institution: Physics Department, University of Regensburg
country of institution: Germany
starting date (yyyy-mm-dd):
Prof. Wilfried Schoepe has performed extensive research on turbulence in superfluid 4He in the University of Regensburg. His technique to generate turbulence is to oscillate a small magnetic sphere of 100 micrometers in diameter within the superfluid bath between two superconducting plates. If the sphere is charged, it can be driven electrostatically with an oscillating electric field. This approach has turned out successful, but a number of unsolved problems remain. Furthermore the same measurements should be extended to 3He superfluids, but so far the practical problems with his experimental approaches have been too demanding for the complicated measurements at sub-millikelvin temperatures. For this reason prof. Schoepe is interested in exploring simpler alternatives, to replace his oscillating microsphere with some other more manageable type of mechanical oscillator. If the superfluid sample is in addition in rotation, the turbulence created around the oscillator could be studied as a function of the vortex polarization produced by the rotation.
Prof. Schoepe has explored different new alternatives for studying the dynamics of quantized vortex lines and for generating superfluid turbulence at low temperatures in the regime of ballistic quasiparticle flight. For this the rotating cryostat provides more possibilities than a quiescent helium bath. One of his proposals was to study the spectrum of helical Kelvin wave excitations which expand on a vortex, when it is attached to the crystal by one of its two ends. The detection would be by means of NMR techniques, making use of certain coherently precessing order parameter modes. So far such measurements have not been attempted in practice. Prof. Schoepe has also participated in the study of the quartz tuning fork oscillator as a sensor of vortices in uniform rotation. A further project has been to explain the critical velocity for starting turbulence with a vibrating oscillator. Experimentally the velocity has been found to be proportional to the square root of the oscillation frequency. A simple scaling explanation has been found which confirms the experimental conclusion. Reports: Blazkova, M., Clovecko, M., Eltsov, V.B., Gazo, E., de Graaf, R., Hosio, J.J., Krusius, M., Schmoranzer, D., Schoepe, W., Skrbek, L., Skyba, P., Solntsev, R.E., and Vinen, W.F., Vibrating quartz fork - a tool for cryogenic helium, Journal of Low Temperature Physics, 150, p. 525-535 (2008). Hanninen, R. and Schoepe, W., Phys. Rev. Lett., submitted.