Spinning an unmagnetized plasma for magnetorotational instability studies in the Plasma Couette Experiment.

摘要

A new concept for creating a large, steady-state, fast flowing, hot plasma which is weakly magnetized has been demonstrated experimentally, marking an important first step towards laboratory studies of a wide variety of phenomenon important in plasma astrophysics. In particular, the magnetorotational instability (MRI) mechanism is of great interest for its role in generating the turbulence necessary for efficient outward transport of angular momentum in accretion disks. The instability has been the subject of extensive analytical and numerical investigations for several decades, yet experimental verification of the MRI remains elusive.;In the Plasma Couette Experiment, plasma is confined by a cylindrical ``bucket'' assembly of permanent magnets, arranged in rings of alternating polarity, to form an axisymmetric cusp magnetic field. The field is localized to the boundaries, leaving a large, unmagnetized plasma in the bulk. Plasma is produced with 2.45 GHz microwave heating, reaching Te < 10 eV, Ti < 1 eV, and n = 1010--10 11 cm-3. The plasma is stirred using J x B forces in the magnetized edge region, where current is driven by toroidally localized, electrostatically biased hot cathodes. Torque can be applied at the inner or outer boundaries, resulting in a controlled, differentially rotating flow.;Mach probe measurements show that the azimuthal flow viscously couples momentum from the magnetized edge into the unmagnetized bulk. Collisional ion viscosity must overcome the drag due to ion-neutral collisions for the plasma to rotate. A self-consistent, rotation-induced radial electric field is also measured. Maximum speed limits have been observed for various gas species (He ~ 12 km/s, Ne ~ 4 km/s, Ar ~ 3.2 km/s, Xe ~ 1.4 km/s), consistent with a critical ionization velocity limit reported to occur in partially ionized plasmas. The experiment 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 which incorporates dissipation, the Hall term, and momentum loss through ion-neutral collisions.