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Plasma Confinement in the Madison Plasma Dynamo Ring Cusp Experiment

Author: Cary B Forest
Requested Type: Consider for Invited
Submitted: 2014-05-30 13:27:25

Co-authors: D. Weisberg, I. Khalzov, C. Cooper, K. Flanagan, J. Mihone, E. Peterson

Contact Info:
University of Wisconsin, Madison
1720 Vilas Ave
Madsion, WI   53706
USA

Abstract Text:
the Madison Plasma Dynamo Experiment (the MPDX) a 3 meter diameter spherical vacuum vessel that uses an array of 3000 4 kG samarium cobalt permanent magnets in a 36 ring ring cusp arrangement, and ~200 kWs of LaB6 hot cathode power to ionize and heat plasma; this is now generating a steady-state, hot (20 eV) plasma with densities of 10^18 m-3. The cathodes also can stir the unmagnetized core plasma at speeds of up to 12 km/s through $Jtimes B$ torque at the magnetized edge of He plasmas.
Laminar counter-rotating two-vortex flows, are predicted to have a
critical threshold of spontaneous creation of magnetic field that depends upon the magnetic Reynolds number $text{Rm} = mu_0 sigma L V$, a dimensionless triple product that must exceed 250 for self-excitation. Hence the electron confinement plays a critical role in governing the dynamo onset. Two dimensional VPIC simulations of density, temperature and wall losses to the dipole magnet rings are compared directly to measurements of 2D plasma profiles and measured confinement times.
The fluid Reynolds number, depends largely upon density, the ion temperature and the effective charge, scaling $text{Re} propto L V Z^4 n/T_i^{5/2}$; hence ion heating and confinement are also important. To study the hydrodynamic turbulence requires high density and low ion temperature (and/or multiply charged ions). So far, MPDX has achieved $Rmapprox 300$ and $Reapprox 200$ which is nearly optimal for observing a laminar dynamo. Fast turbulent dynamos seem feasible give the confinement scaling projections
based upon the experimental parameters.

Work supported by NSF and DOE.

Characterization: 4.0

Comments:

Workshop on Exploratory Topics in Plasma and Fusion Research (EPR) and US-Japan Compact Torus (CT) Workshop
August 5-8, 2014
Madison, Wisconsin

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