  We report an experimental investigation on the influence of an external
magnetic field on forced 3D turbulence of liquid gallium in a closed vessel. We
observe an exponential damping of the turbulent velocity fluctuations as a
function of the interaction parameter N (ratio of Lorentz force over inertial
terms of the Navier-Stokes equation). The flow structures develop some
anisotropy but do not become bidimensional. From a dynamical viewpoint, the
damping first occurs homogeneously over the whole spectrum of frequencies. For
larger values of N, a very strong additional damping occurs at the highest
frequencies. However, the injected mechanical power remains independent of the
applied magnetic field. The simultaneous measurement of induced magnetic field
and electrical potential differences shows a very weak correlation between
magnetic field and velocity fluctuations. The observed reduction of the
fluctuations is in agreement with a previously proposed mechanism for the
saturation of turbulent dynamos and with the order of magnitude of the Von
Karman Sodium dynamo magnetic field.
