Strong magnetic field amplification in quasi-Keplerian high-β plasma flows

Quasi-Keplerian flows in the Big Red Ball (BRB) develop a peaked axial magnetic field approximately an order of magnitude larger than the applied field. The method of stirring, volumetric flow drive (VFD), consists of forcing current injected by biased cathodes to cross a weak (v_ii > >Ω_ci) externally applied field using a Helmholtz coil set. Flows can reach a Mach number of M 0.3 with β 100 and δ_i / L 1 . The magnetic field amplification is observed in spherical NIMROD simulations only when including the Hall term in Ohm's law and when the injected current is directed radially outwards at the equator. The magnetic flux increase results from conversion of toroidal flux from the injected current into poloidal flux by the Hall effect. In the experiment there are also significant pressure gradients due to losses along open field lines in the core that cause poloidal flux to peak in the core via the diamagnetic effect. These observations suggest mechanisms important to magnetic induction beyond MHD in unmagnetized plasmas.

This work was supported by the DOE Office of Fusion Energy Science and the NSF.