"Stirring Unmagnetized Plasma: A First Step for Laboratory Studies of Astrophysical Accretion Disks and Dynamos"

 

 

ABSTRACT:

​A technique for spinning an unmagnetized plasma has been demonstrated experimentally. This now makes it possible to create flow-driven MHD instabilities, such as the generation of magnetic fields through self-excited dynamos, or the magnetorotational instability (MRI), the mechanism of interest for its role in creating efficient angular momentum transport in accretion disks. In a prototype experiment at UW-Madison, called the Plasma Couette Experiment (PCX), plasma is confined in a cylindrical, axisymmetric, multicusp magnetic field, with Te< 10 eV, Ti< 1 eV, and n< 10^11 cm-3 . Azimuthal flows (up to 12 km/s) are driven by JxB torque using biased, heated filaments in the magnetized edge. Measurements show that momentum couples viscously from the magnetized edge to the unmagnetized core, and that collisional ion viscosity must overcome the drag due to ion-neutral collisions for the plasma to rotate.

 

Since the plasma is partially ionized, flow speeds seem to be limited by the Alfv é n critical ionization velocity. PCX has achieved magnetic Reynolds numbers of Rm~65 and magnetic Prandtl numbers of Pm~0.2-10, which are approaching regimes shown to excite the MRI in local linear analysis and global Hall-MHD numerical simulations. The stirring technique developed on PCX is now being implemented in the larger Madison Plasma Dynamo Experiment, which has been designed to produce Rm>1000 plasmas ideal for dynamo studies. The current status of both of these UW-Madison experiments will be presented.