MODIFICATION OF FLUID-FLOW AND HEAT-TRANSPORT IN VERTICAL BRIDGMAN CONFIGURATIONS BY ROTATING MAGNETIC-FIELDS

Applying a rotating magnetic field to an electrically conducting liqui d, a Lorentz force is induced which generates a melt rotation of a cer tain angular velocity. A cylindrical gallium melt (aspect ratio 2.5) h as been used as a model liquid. The melt has been heated from the bott om (Ra=10(6)) or from the top (Ra=-10(6)) and the resulting temperatur e fluctuations in the melt have been measured in dependence on the rot ating field strength (B-max=30 mT). In the case of the unstable gradie nt 0.8 mT are sufficient to dominate the buoyancy driven convection an d to reduce the amplitude of the buoyancy caused temperature oscillati ons for more than one order of magnitude. At the same time. the fluctu ation frequency increases with the field strength. In the case of the stabilizing temperature gradient: low amplitude/high frequency tempera ture fluctuations are generated by the rotating magnetic field, indica ting the transition to a time-dependent flow. In both cases we see an increase of the convective heat transport for magnetic inductions high er than approximately 5 mT. Applying the rotating magnetic field to th e Bridgman growth of gallium doped germanium, the same behavior can be seen: Growing with a top-seeded arrangement, the intensity of the dop ant striations is decreased and their frequency is increased. Growing with a bottom-seeded arrangement, the interface curvature changes from concave to convex and the flow becomes time-dependent.