Mise en page 1 - Laboratoire National des Champs Magnétiques ...

2009 MAGNETO-SCIENCELarge Alfvén Waves in liquid sodiumAlfvén waves are hydromagnetic waves involving fluid velocityand magnetic field. They are a key phenomenonin many geophysical and astrophysical area, such as theEarth’s liquid core, the magnetosphere, solar wind and interstellarplasma dynamics. They are difficult to observe inexperiments but it would be useful to reach a state of interactingAlfvén waves as a key example of weak turbulence.Here, we use liquid sodium (figure 154). Compared togalinstan (gallium/indium/tin liquid metal alloy) it is a betterelectrical conductor (ohmic dissipation of Alfvén wavesis reduced) and its density is much less (hence Alfvénwaves are faster). The geometry of the liquid sodium cavityis a cylinder of diameter 10 cm and of length 20 cm.This was inserted in an available diameter of 16 cm witha maximum value of 16 T. 7 coils are placed around thiscylinder at regular intervals along the length. In addition, aso-called ‘emission’ coil is placed at one end of the cylinderto provide a magnetic excitation of Alfvén waves. This is ashort electrical pulse current producing a poloïdal magneticfield. The Alfvén wave can then be observed on the signalof the 7 measuring coils. The characteristic dimensionlessLundquist number (propagation time divided by dissipationtime) changed from a maximum of 60 with galinstanto around 500 with liquid sodium. Hence we expect to observeAlfvén waves very clearly. In figure 155, the arrivalof an Alfvén wave is recorded at the end of the cylindricalcavity, while it was produced at the other end by a pulsein a coil. On the left-hand side, one can see the electricalpulse (black curve) followed by the arrival of a main oscillationwith a shorter delay when the applied B is stronger,according to the Alfvén propagation time. On the righthandside, time is made dimensionless using the theoreticaltime of propagation of an Alfvén wave from one end of thecylinder to the other: all arrivals collapse around a dimensionlesstime of unity. In figure 156, the 7 measured signalsare shown. It is possible to follow the signal from the endwhere it was created to the other end. There is however anadditional component to the signal which is the signatureof structural vibrations. The linearity of the response signal(figure 157) shows however that the excitation is not yetlarge enough to reach a turbulent state.Figure 155: Propagation of an Alfvén wave recorded on the coilfarthest from the ‘emission’ coil for different intensities of magneticfield. Same plot using the dimensionless time scale based ontheoretical Alfvén speed.Figure 156:a pulse.The 7 signals of the 7 coils are plotted together, afterFigure 154: These photographs show the process of filling thecontainer with sodium, the bare setup and finally the setup withinits thermal insulator equipped with pressure and temperature sensors.Figure 157: Linear response of electromotive force with respectto the intensity of the excitation pulse. Energy losses during reflections.F. DebrayTh. Alboussiere, P. Cardin, P. La Rizza, J.P. Masson, H.C. Nataf, F. Plunian, N. Schaeffer, D. Schmitt(LGIT/CNRS/OSUG/UJF, Grenoble)109