Laboratory Search for Magnetorotational Instability

Princeton University, Princeton NJ

Investigators

Abstract

One of the more common, but less understood, phenomena in astrophysics is the dissipation of orbital energy of charged plasma in accreting disks. This general phenomenon occurs in situations as diverse as protoplanetary disks and active galactic nuclei. The dissipation is thought to arise due to one or two distinct phenomena. First, a non-linear hydrodynamic shear flow instability can arise from multiple layers of material in a disk moving at different speeds, similar to the simpler Kelvin-Helmholtz instability. Second, a linear instability can arise in the charged and magnetized flow of a plasma. This latter phenomenon is called a magnetorotational instability (MRI). The team has already conducted a series of experiments that seem to exclude hydrodynamic shear flow instability as the reason for the dissipation of orbital energy. They now seek to conduct a series of experiments using charged liquid metal to explore MRI. The team will also train a postdoc and a graduate student. Two experiments have been set up at Princeton to examine dynamic instabilities that might lead to turbulent flow in astrophysical accretion disks. The Hydrodynamics Turbulence Experiment (HTX) studies instabilities such as shear flow instability; the Liquid-Metal MRI experiment is meant to study magnetorotational instability. So far, HTX has provided results that seem to preclude shear flow instability as a major contributor to the dissipation of orbital energy in accretion disks. However, the MRI hypothesis remains unproven. The team will add new diagnostics to the Liquid-Metal MRI experiment, to include ultrasound doppler velocimetry to measure the flow field, since the liquid metal is opaque. Motors will be upgraded to allow operation at higher rotation speeds (Rm > 4.5) where the team’s simulations indicate that MRI should arise. The experimental campaign will be accompanied by numerical simulations to better understand the laboratory results. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.