Laboratory Study of Magnetorotational Instability in a Swirling Partially Ionized Plasma*

The magnetorotational instability (MRI) has been proposed as a plasma instability to transport angular momentum to enable fast accretion in astrophysical disks, and its standard form (SMRI) has recently been detected in a laboratory setting [1]. However, for weakly-ionized protoplanetary disks, it remains unclear whether the combined non-ideal magnetohydrodynamic (MHD) effects of Ohmic resistivity, ambipolar diffusion, and the Hall effect make these disks MRI-stable. While much effort has been made to simulate non-ideal MHD MRI, these simulations make simplifying assumptions and are not always in agreement with each other. Here, we present our proposed concept of a swirling weakly-ionized argon-plasma experiment between two concentric cylinders threaded with an axial magnetic field [2]. We derive the equilibrium flow and a dispersion relation for MRI that includes the three non-ideal effects. We solve this dispersion relation numerically for the parameters of our proposed experiment. We find that it should be possible to produce MRI in such an experiment because of the Hall effect, which increases the MRI growth rate when the vertical magnetic field is antiparallel to the rotation axis. As a proof of concept, we also present experimental results for gas flow in an unmagnetized prototype. We find that our prototype has a small, but non-negligible, alpha-parameter that could serve as a baseline for comparison to our proposed magnetized experiment, which could be subject to additional effects from the MRI.