This project will study the MagnetoRotational Instability (MRI), carrying out the first-ever experiments with a newly constructed plasma couette flow device which uses a novel geometry to confine and stir a hot plasma. Accretion is a fundamental process by which virtually all astrophysical bodies form, but for a long time it was unclear what causes the disk matter to lose orbital energy, what it is that transports angular momentum outward and makes the disk material fall onto the central object. The current working hypothesis is that a weak magnetic field leads to a quickly growing magnetic instability, rendering the disk turbulent and tremendously increasing the effective viscosity, and this is supported by numerical simulation. Recent exciting experiments use liquid metals to study the MRI, but a plasma experiment allows larger magnetic Reynolds number and the variation of viscosity independent of conductivity, uses the state of matter most likely to occur in natural accretion, investigates the transition from weakly to strongly magnetized with only modest field strengths, and uses measurement techniques well-developed in plasma physics. Such an experiment has never been performed, but simple extrapolations from existing multi-dipole confinement experiments show it is feasible. This work will characterize the parameters of the ring cusp plasma with electrostatic stirring, search for signs of the MRI to compare to theory, and identify plasma-specific effects beyond the standard magneto-hydro-dynamical model.
A central part of research includes the training and mentoring of undergraduate and graduate students, who are involved in all aspects of the project and will be sent regularly to scientific meetings to present their results. Outreach activities will include public lectures and press releases.
This project has resulted in ground-breaking new experimental plasma device that can now be used, for the first time ever, for the study of Magnetorotational Instability (the MRI) in a plasma. The MRI is thought to govern accretion, a fundamental process in astrophysics by which virtually all astrophysical bodies are formed. Due to gravity, the interstellar gas and plasma collapse into rotating disks around the central, point-like accreting objects such as protostars, collapsed stars in binary systems, and super- massive black holes in active galactic nuclei. Accretion disks power many of the most luminous astrophysical objects, including X-ray binaries, probably quasars, and perhaps even gamma-ray bursts. Accretion around young stars, though less luminous, is of great interest for its role in planet formation. For a long time it was unclear what causes the disk matter to lose its orbital energy and transport angular momentum outward to fall on the central object; the dissipation mechanism has been an area of active debate for many decades as the molecular viscosity is negligibly small, and the disks are predicted to be remarkably stable without plasma effects. The current working hypothesis, put forth in the early 1990’s is that a weak magnetic field, generated somewhere outside the disk and frozen into the highly conducting plasma that makes up the accretion disk, might lead to a quickly growing magnetic instability, rendering the disks turbulent and tremendously increasing the effective viscosity. The Madison Plasma Couette Experiment (the MPCX) is a novel confinement geometry being used for confining and stirring a hot plasma that is similar in critical ways to accretion disk plasmas. The device uses a high order multipole magnetic field on the surface of the plasma to confine a hot, fast flowing plasma in a large magnetic field free volume. The PCX consists of a 1 meter diameter, 1 meter tall cylindrical vacuum vessel and uses a magnetic field at the surface of the plasma to confine a hot, fast flowing plasma in a large magnetic field free volume. Microwaves are used to ionize and heat the plasma, and then, plasma flow is driven by "stirring electrodes" in the plasma edge where cross field currents are used to impart a torque on the plasma. This grant has supported a graduate student and experimental operations on this device. The plasma properties (electrical conductivity and viscosity) have been measured and the novel stirring technique has been demonstrated. Plasma flows up to 7 km/s have been observed in the device, more than enough for proceeding to a full MRI study.
