GTP Workshop on Modeling MHD Turbulence; Applications to Planetary and Stellar dynamos at NCAR, 27-30 June, 2006, Boulder, CO, USA Induction measurements in the VKS2 experiment Romain Volk, Philippe Odier, Jean-Fran¸ cois Pinton (1) Michael Berhanu, Stephan Fauve, Nicolas Mordant, Fran¸ cois P´ etr´ elis (2) Florent Ravelet, Romain Monchaud, Arnaud Chiffaudel, Fran¸ cois Daviaud (3) (1) Laboratoire de Physique de l’ ´ Ecole Normale Sup´ erieure de Lyon, France (2) Laboratoire de Physique Statistique de l’ ´ Ecole Normale Sup´ erieure de Paris, France (3) Service de Physique de l’Etat Condens´ e, Direction des Sciences de la Mati` ere, CEA-Saclay, France Abstract We report recent results from the VKS2 experiment: response to an externally imposed homoge- neous magnetic field, and transport of a localized applied field. The VKS2 experiment The VKS project in Cadarache [1] is one of several experiments dedicated to the study of the dynamo effect in an unconstrained homogeneous flows of liquid metals [2]. The acronym “VKS” stands for “von K´ arm´ an sodium” and refers to the flow generated between two counterrotating impellers in a finite cylinder. The phenomenology of the time-averaged flow is as follows. Each impeller acts as a centrifugal pump: the fluid rotates with the impeller and is expelled radially. To ensure mass conservation the fluid is pumped in the center of the impeller and recirculates near the cylinder wall. In the exact counter-rotating regime, the mean flow is divided into two toric cells separated by an azimuthal shear layer. The kinetic Reynolds number is about 10 7 and the shear layer instability is a strong source of turbulence. The VKS2 evolution result from flow optimization and numerical inspection of its dynamo behavior [3]. With respect to the first version (VKS1[1]), the motor power has been increased to 300kW and the volume of the conducting domain is twice greater. A temperature regulation allows long measurements in stationary regime. Magnetic Reynolds number between 12 and 50 are reached. Figure 1: VKS2 flow vessel and driving impellers Figure 2: Mean flow geometry Response to a uniform applied field [4] We apply a large scale field with a pair of coils (B 0y =2.7G, too weak to modify the flow) in a direction transverse to the axis of rotation of the driving impellers – the direction expected for the dynamo neutral mode in the kinematic dynamo simulations [3]. Fig. 3 shows the evolution of the mean of the induced field b y in the direction of the applied field. Once R m > 20, 〈b y 〉 exceeds B 0y . In addition, the fluctuations of the induced component b y are non-Gaussian, – Fig. 4 – at all R m values. These features are in contrast with VKS1 measurements, where the induced field b y saturated at 0.4B 0y , and its fluctuations were Gaussian. However, no self-sustained dynamo regime has been reached, and at the largest R m values we have measured a linear growth of the mean and rms values of the induced field. Note, in Fig. 3, that the measured mean values of induction deviate significantly from the ones predicted by induction from the mean flow velocity.