Speaker
Description
The standard magnetorotational instability (SMRI) has been regarded as the most promising instability responsible for the turbulence required to explain the fast accretion observed across the Universe. However, unlike other fundamental plasma processes such as Alfvén waves and magnetic reconnection, which have been subsequently detected and studied in space and the laboratory, SMRI remains unconfirmed even for its existence long after its proposal, despite its widespread applications in modeling, including recent black hole imaging. Its direct detection has been hindered in observations due to its microscopic nature at astronomical distances and in the laboratory due to stringent requirements and interferences from other processes. Here, we report the first direct evidence showing that SMRI exists in a novel laboratory setup where a uniform magnetic field is imposed along the axis of a differentially rotating flow of liquid metal confined radially between concentric cylinders and axially by copper end rings. Through in situ measurement of the radial magnetic field $B_r$ at the inner cylinder, the onset of the axisymmetric SMRI at magnetic Reynolds number Rm≥3 is identified from the nonlinear increase of $B_r$ beyond a critical magnetic Reynolds number. Experimental data also reveals that the axisymmetric SMRI is accompanied by a nonaxiymmetric m=1 mode, a linear instability with exponential growth at its onset. Further analysis excludes the possibility that the m=1 mode is the conventional Rayleigh instability or the Stewartson-Shercliff layer instability, implying that it could be a non-axisymmetric version of SMRI that breaks the rotational symmetry of the system. We will discuss a possible mechanism causing the SMRI to be excited at Rm lower than the theoretical prediction for an ideal Couette flow. The experimental results are reproduced by nonlinear three-dimensional numerical simulations, showing that SMRI causes the velocity and magnetic fields to contribute an outward flux of axial angular momentum in the bulk region, just as in accretion disks.
We are deeply grateful for the support that made this research possible. This work was supported by U.S. DoE (Contract No. DE-AC02-09CH11466), NASA (Grant No.NNH15AB25I), NSF (Grant No. AST-2108871) and the Max-Planck-Princeton Center for Plasma Physics (MPPC).
