TURBULENT AND BALLISTIC DRAG OF SUPERFLUID-HELIUM AT LOW-TEMPERATURESON AN OSCILLATING MICROSPHERE

Transverse oscillations of a microsphere in liquid helium below 0.3 K are investigated. For small amplitudes we observe ballistic drag by ph onons and He-3 impurities. At a threshold velocity amplitude the drag force increases discontinuously to a value determined by turbulence in the superfluid and varies with the square of the velocity amplitude. The turbulent drag coefficient is determined by the density of the liq uid and the radius of the sphere, similar to the case of uniform motio n in a classical liquid. The threshold velocity for the onset of turbu lent friction is found to be independent of the temperature.