First experimental insights into the transition from AMRI to HMRI


First experimental insights into the transition from AMRI to HMRI

Seilmayer, M.

In the last years the magnetorotational instability (MRI) was focused intensively by theory and experiments. It turns out that investigating the standard MRI (SMRI), with its magnetic field pointing perpendicular to the rotation direction, is very hard. It is only possible to get this type of instability in rather large (1 m-scale) or fast rotating (f > 10 Hz) cylinders. Beside that two other types of MRI were discovered[1, 2]. The helical and azimuthal MRI, which where successfully investigated in the laboratory with the PROMISE2 facility. This setup consists of two concentric cylinders, a current carrying rod on the axis producing Bϕ and a cylindrical coil providing Bz to the fluid. In the past, experiments proved the theory of each individual instability [3, 4].
With the improved magnetic field system the PROMISE3 setup gains advantage of a more homogeneous magnetic field. So it becomes possible now to observe the transition between these two instabilities. First we like to give a prove of function and show how the velocity distribution and other properties changed with the improved magnetic field. Next to that we like to present very first results on the mode transition form the AMRI unstable m = 1 regime to the HMRI unstable m = 0 case. Here we start in a AMRI unstable parameter set (Re = 1500, Ha = 100 and µ = Ωout /Ωin = 0.26). By increasing the axial magnetic field Bz the AMRI m = 1 mode is disturbed successively until it is completely damped. At a certain field strength the other m = 0 mode emerges which also disappears at higher fields.
Finally we can conclude that AMRI and HMRI still work in the modified experiment according to the theory. There is a possible transition region between these two instabilities. One open question is why the flow structure changes so significant for AMRI in comparison to the PROMISE2 campaign [3]. And would it be better to improve the field further?

References
[1] G. Rüdiger et al., AN, 328(10):1158-1161, 2007.
[2] G. Rüdiger et al., AN, 329(7):659-666, 2008.
[3] M. Seilmayer et al., PRL, 113(2):024505, 2014.
[4] F. Stefani et al., NJP, 9(8):295, 2007.

Keywords: AMRI; magnetic field; magnetohydrodynamic; experiment

  • Lecture (Conference)
    17th MHD Days, 30.11.-02.12.2016, Göttingen, Deutschland

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Publ.-Id: 24643