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MHD turbulence and relaxation in the SSX wind tunnel

Author: Mike Brown
Requested Type: Consider for Invited
Submitted: 2012-12-07 11:12:36

Co-authors: V. S. Lukin, D. Schaffner

Contact Info:
Swarthmore College
500 College Avenue
Swarthmore, PA   19081
USA

Abstract Text:
SSX is presently operating in the mode of an MHD wind tunnel. We are performing both merging
experiments and single-ended flow experiments. High velocity plumes are generated with coaxial, magnetized plasma guns
and injected into a long flux-conserving cylinder. The cylinder has dimensions $L = 1$~m and $R = 0.08$~m
($R/L = 13$). An extension to increase to $L = 2.5$~m is under construction. The cylinder was chosen to be long enough so that the predicted minimum energy state is a close approximation to the infinite cylinder solution. The plasma is
injected at $v ge 50$~km/s by a plasma gun located at either
end of the cylindrical volume. Typical plasma parameters are $T_i = 25$~eV, $T_e= 10$~eV,
$n_e le 10^{21}$~m$^{-3}$, and $B = 0.25$~T.
Launching a single magnetized plume generates a helical, minimum-energy relaxed plasma state. The relaxed state is rapidly
attained in 1--2 axial Alfv'en times after initiation of the plasma.
Axial magnetic probe arrays provide information about flow speed and axial structure. Arrays of radial magnetic probes are used
for fluctuation and correlation studies. Preliminary results indicate local axial flow speeds of up to $80$~km/s. High velocity merging drives fluctuations across a radius
consistent with a power-law fluctuation spectrum.
The broadband activity rapidly decays as the energy condenses into the
lowest energy mode, which is in agreement with the minimum energy eigenstate of
$nabla times vec{B} = lambda vec{B}$. Fluctuation results from new high
speed, high resolution magnetic radial and axial probes will be presented. The new
radial probe has a higher spatial resolution ($4.5$~mm $le rho_i$) and higher
temporal resolution ($f = 65$~MHz $gg f_{ci}$) than previous probes used in
prior SSX studies. The data acquisition digitizes at 14 bits, providing for a
dynamic range of 4 decades. Hall simulations of both the single-ended MHD wind tunnel flow and the merger of two high-velocity plumes using the HiFi framework will also be presented.

Characterization: 1.1

Comments:
I will recuse myself as EPR chair if necessary or appropriate.