Outlook on a new combined MRI/TI experiment


Outlook on a new combined MRI/TI experiment

Seilmayer, M.; Stefani, F.; Gundrum, G.; Köppen, S.

In the project framework of DRESDYN a sodium based experiment investigating magneto rotational instability (MRI) an Tayler instability (TI) is going to be built. Since both instabilities are based on hydrodynamically stable but magnetized Taylor Couette flow, the magnetic field configuration distinguishes the different types of instabilities. The past experimental research successfully worked out helical MRI with a combination of axial and azimuthal magnetic field and azimuthal MRI with only a pure azimuthal field. In both cases the relevant field B[phi] ~ 1/r is
generated by a rather large insulated current running on the symmetry axis of the experiment. The most challenging type of instability still remains standard MRI (SMRI) which relays on pure axial magnetic field B[z]. Here the necessary high rotation rates and large dimensions do limit the experimental feasibility to trigger SMRI until now. Besides that, even a non-rotating fluid with zero velocity can be destabilized by Tayler instability. Here the current is driven through the liquid which gives a B[phi] ~ r dependence. Concluding, the aim of new experimental is to investigate the whole parameter space for the mentioned instabilities and corresponding transitions.

We like to present the latest stage of construction of a large scale liquid sodium Taylor-Couette experiment with a height of 2m and diameter of 0.8 m. In a rough approximation, the achievable boundary conditions will be up to 20 Hz rotation rate and up to 50kA of electrical current. In a more detail, there are some specific topics we like to discus.

First, there is the quasi coaxial system consisting of a central current carrying copper rod and five symmetric return paths which provides homogeneous magnetic field to the Taylor-Couette flow.
One challenging part is the design of the current distributor, which is supposed to divide the current into several equally weighted lines. Because of the individual characteristic resistance of all involved conductors an initial imbalance in the current distribution affecting the symmetry of the magnetic field is the result. So the adjustment of current distribution becomes mandatory to ensure maximum field homogeneity. An indirect access to set the current in all five return paths is to control the outflow temperature of the required water cooling. This is done by thermostatically operated valves in conjunction with the temperature dependent branch resistance. Finally, the calibrated system achieves less than 1% field in-homogeneity and works in a wide range of currents. Additional benefits of the presented approach will be the minimal stray field of the installation and cheap components.

Second, we designed a stacked magnetic field system to generate the axial component. Here 27 individual coils provide an almost homogeneous field with B[z] < 150 mT in the relevant volume. The most challenging aspect is the huge amount of electrical power (approximately 120 kW) which have to be cooled. The main advantage of the present geometry is the guaranteed access to all the sensors mounted on the outer cylinder surface. This is achieved by a special geometry to partially levitate the coil system.

[1]F. Stefani et al., PRE, 80(6), 2009.
[1]M. Seilmayer et al., PRL, 108(24), 244501, 2012.
[1]M. Seilmayer et al., IEEE Sensors Journal, DOI 10.1109/JSEN.2017.2765671, 2017.

Keywords: Experiment; Sodium; DRESDYN; Taylor-Couette

  • Poster
    GDRI DYNAMO MEETING, 26.-29.11.2017, Paris, Frankreich

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