This project begins the first ever study of the magnetorotational instability (MRI) in a plasma experiment. The MRI is potentially very important in plasma astrophysics, but it has only been studied theoretically, numerically, and in a limited fashion in liquid metal experiments as a substitute for plasma. The MRI studies planned here will be carried out in the Plasma Couette Experiment (PCX), a novel experiment that both confines and stirs a hot plasma, with the parameters necessary for observing the MRI. Accretion is a fundamental process in astrophysics whereby interstellar gas and plasma collapse into rotating disks around a central, point-like accreting object. There must be a mechanism that causes disk matter to lose orbital energy, transport angular momentum outward, and fall into the central object. The MRI is a leading candidate for this mechanism, based on theory and numerical simulations. This study builds on the recent excitement caused by liquid metal experiments to investigate the onset conditions for the MRI. A plasma experiment has several advantages over a liquid metal experiment, including that most naturally occurring accretion disks are made up of plasmas, that it allows larger and more realistic magnetic Reynolds numbers, and that it permits independent adjustment of the viscosity and the conductivity. The PCX has recently demonstrated high speed, MRI-stable Couette flows, and this work will 1) search for signs of the MHD instability, 2) compare results with numerical simulations; 3) measure the effective momentum transport of the saturated instability; and 4) identify physical effects which are plasma specific and beyond the standard magnetohydrodynamics model.
The PCX experiments will have broad impact on all flow-driven MHD problems and will continue a strong connection between laboratory plasma physicists and astrophysics facilitated by the NSF Center for Magnetic Self-Organization in Laboratory and Astrophysical Plasmas. This award supports a single graduate student to carry out the proposed experiments, with partial support for technical staff and student supervision.
