Laboratory Studies of the Magneto-Rotational Instability in Plasma

University Of Wisconsin-Madison, Madison WI

Investigators

Abstract

This exciting project will lead to the first ever experimental demonstration of an effect long studied theoretically and numerically, which is crucial for understanding astrophysical systems powered by material flowing through a complex flattened structure and onto a central compact object. Accretion through a swirling disk concerns many astronomical phenomena on scales from distant quasars to nearby star systems. This novel and innovative experiment will also provide many educational opportunities. This project will be the first ever study of the magnetorotational instability (MRI) in a plasma experiment. Although the MRI is a major component of current explanations of accretion, studies have so far been theoretical, numerical, and with a limited set of liquid metal experiments. This study will use the Plasma Couette Experiment (PCX), which uses a new way both to confine and to stir hot, fast-flowing plasmas under the right conditions to observe the MRI, which explains how matter in an accretion disk transports angular momentum outward, and thus falls into the central object. A weak magnetic field from somewhere outside the disk leads to a quickly growing magnetic instability that itself leads to turbulence, which enhances the effective viscosity. Numerical simulations have shown an effect of exactly the required sort, and recent experiments with liquid metals have studied the onset conditions. However, plasma experiments have several key advantages over liquid metals: (1) plasmas are the state of matter that makes up most naturally occurring accretion disks; (2) plasmas can be configured with more appropriate fluid parameters; and (3) plasma viscosity can be varied independently of the conductivity, mimicking conditions both in the outer regions of most disks, and in the regime near the center. The PCX is now being upgraded to stronger confining magnets, higher input power, and larger plasma volume. These improvements should more effectively show the onset of the MRI, and even multi-mode MRI turbulence. This award will support (1) a search for signs of the instability and its comparison with numerical simulations; (2) measurement of the effective momentum transport to compare with theoretical predictions; and (3) identification of physics effects which are plasma specific and beyond the standard model. Among the outreach activities, a basic plasma experiment will be added to the undergraduate physics capstone lab class. The popular "Exotic Plasmas of the Universe" trading cards will be updated, and plasma astrophysics themes will be part of the university's "Wonders of Physics" show.