Center for Magnetic Self Organization
in Laboratory and Astrophysical Plasmas
Stirring Unmagnetized Plasma
Mechanical stirring of fluids is common in experimental physics and everyday life, and strongly magnetized plasmas can be stirred by electromagnetic torques. But generating flow in an unmagnetized plasma has been elusive.
Left panel: plots of the angular velocities of Ar and He plasmas, and the viscosities responsible for transmitting torques.Right panel: a vertical slice through the cylindrical Plasma Couette Experiment (PCX), showing how the alternating cathodes and anodes at the wall produce a strong magnetic field that drops nearly to zero in the volume.
This past year, CMSO researchers experimentally demonstrated a new concept for spinning unmagnetized plasma, marking an important first step towards laboratory studies of a wide variety of phenomenon in plasma astrophysics. The ability to produce a hot, fast flowing, magnetic field-free plasma (in contrast to highly magnetized plasmas used for fusion energy research) may help us better understand the dynamo, a process thought to be responsible for the creation of magnetic fields in stars in galaxies. Flows can be adjusted in the experiment to mimic those of accretion disks, making it possible to study, for the first time, the magneto-rotational instability in a plasma, a mechanism of interest for its roles in providing the fuel for high energy emission from compact objects, and aiding in the formation of stars and planets.
In the 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. The plasma is stirred using JxB torques, where current is driven by electrostatically biased electrodes in the magnetized edge region. Measurements show that the azimuthal flow viscously couples momentum from the magnetized edge (where the plasma viscosity is small) into the unmagnetized bulk (where the viscosity is large) so that the bulk rotates like a solid body. Flow speeds can be adjusted by simply increasing the bias voltage of the electrodes, and the addition of electrodes at the inner boundary will allow studies of shear flow profiles, including the Keplerian-like flows in accretion disks, where v_φ /sqrt(r). Flows as high as 6 km/s have been observed in subsequent experiments. See C. Collins et al. Phys. Rev. Lett. 108, 115001 (2012).