Pursuing the plasma dynamo and MRI in the laboratory: Hydrodynamic studies of unmagnetized plasmas at large magnetic Reynolds number

A new method for studying flow-driven MHD instabilities in the laboratory has been developed, using a highly conductive, low viscosity, spherical plasma. The confinement, heating, and stirring of this unmagnetized plasma has been demonstrated experimentally, laying the foundations for the laboratory studies of a diverse collection of astrophysically-relevant instabilities. Specifically, plasma flows conducive to studies of the dynamo effect and the magnetorotational instability (MRI) are measured using a wide array of plasma diagnostics, and compare favorably to hydrodynamic numerical models.;The Madison plasma dynamo experiment (MPDX) uses a cylindrically symmetric spherical boundary ring cusp geometry built from strong permanent magnets to confine a large (R=1.5 m), warm (Te < 20eV), dense, unmagnetized plasma. Detailed probe measurements of plasma transport into the edge cusp have demonstrated that particle confinement follows an ambipolar diffusion model, wherein unmagnetized ions are the more mobile plasma species and total plasma transport is limited by the slow cross-field diffusion of magnetized electrons. Emissive discharge heating is shown to be an efficient method of plasma heating, but limitations caused by instabilities in the anode-plasma sheath are found to prohibit the desired access to the full dimensionless parameter space in Re and Rm.;The plasma is stirred via J x B torques using current drawn from emissive LaB6 cathodes located at the magnetized plasma edge, which also ionize and heat the plasma via sizable discharge current injection. Combination Langmuir/Mach probes measure maximum velocities of 6 km/s and 3 km/s in helium and argon plasmas, respectively, and ion viscosity is shown to be an efficient mechanism for transporting momentum from the magnetized edge into the unmagnetized core. Momentum loss to neutral charge-exchange collisions serves as the main source of drag on the bulk plasma velocity, and ionization fraction (He ∼ 0.6, Ar ∼ 0.95) is shown to be a limiting factor in momentum penetration. High Alfven Mach number flows have also been generated by drawing current across a global axial magnetic field, resulting in a velocity geometry conducive to MRI experiments. The experiment has achieved magnetic Reynolds numbers of Rm < 250 and fluid Reynolds numbers of Re < 200 (significantly higher than previous flow experiments in cusp-confined plasmas), setting the stage for future research of flow-driven MHD instabilities.