Speaker
Prof.
Cary Forest
(University of Wisconsin, Madison)
Description
The leak width of plasma in the WiPAL multi-dipole ring cusp$^1$ has been directly measured using a novel array of probes embedded in the insulating plasma limiters.
The large plasma volume ($\approx$10 m$^3$), small loss area associated with strong rare earth permanent magnets, and large heating power ($\le$ 200 kW) allowed for
a broad range of electron temperatures, ion temperatures, plasma densities, ionization fractions, and ion masses, all of which were accurately measured.
Plasma parameters measured at the surface of ceramic limiter tiles covering the magnets
and along radial chords in the cusp field indicate density and temperature are nearly constant on magnetic field lines and that the
mirror forces play little role in confining the plasma other than to constrict the loss area.
Particle balance modeling is used to determine the cross field diffusion from the measured parallel losses at the limiters.
The experimentally determined cross field diffusion (which determines the leak width) is consistent with ambipolar diffusion across five orders of magnitude. This ambipolar diffusion for a given field line
is set primarily by the electron-neutral collisions in the region where the magnetic field is the weakest, even though these plasmas can have ionization fractions near 1.
The implications of the plasma losses on charge, particle, and energy balance will be then discussed. The primary heating source is currently a set of 8, LaB$_6$ cathodes injecting 100s of amps of fast electron current at $\sim$400 volts. Plasma potential is set by ambipolarity, but is strongly dependent upon the size of the electron collecting anodes within the plasma. Electron and ion energy confinement is strongly coupled to the plasma potential.
In addition to describing these confinement results, a brief overview of the physics goals for the WiPAL facility will be described. Its mission involves creating a weakly monetized, but hot and conducting plasma suitable for studying dynamos, magnetic reconnection, shocks, and other astrophysical plasma processes.
$^1$C.B. Forest, et al, Journal of Plasma Physics 18, 345810501 (2015).
Primary author
Prof.
Cary Forest
(University of Wisconsin, Madison)
Co-authors
Dr
C.M. Cooper
(ORAU)
D. Endrizzi
(University of Wisconsin, Madison)
Mr
D.M. Weisberg
(University of Wisconsin, Madison)
Mr
E. Peterson
(University of Wisconsin, Madison)
Mr
J. Milhone
(University of Wisconsin, Madison)
Mr
K. Flanagan
(University of Wisconsin, Madison)
