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Turbulence and selective decay in a long cylindrical geometry in SSX*

Author: Tim Gray
Requested Type: Consider for Invited
Submitted: 2011-06-09 14:01:04

Co-authors: M. Brown, V. Lukin, D. Dandurand, M. Fisher, K. Flanagan, D. Weinhold, ,

Contact Info:
Swarthmore College
500 College Ave. Dept. of Phys
Swarthmore, PA   19081
USA

Abstract Text:
A helical, minimum-energy relaxed plasma state has been observed in a long
cylindrical volume. The cylinder has dimensions $L = 1$~m and $R = 0.08$~m
($R/L = 13$). The cylinder is 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 coaxial magnetized plasma gun located at one
end of the cylindrical volume. Typical plasma parameters are $T_i = 25$~eV,
$n_e le 10^{21}$~m$^{-3}$, and $B = 0.25$~T. The relaxed state is rapidly
attained in 1--2 axial Alfv'{e}n times after initiation of the plasma.
Magnetic data is favorably compared with an analytical model. Magnetic data
exhibits broadband fluctuations of the measured axial modes during the formation
period. The broadband activity rapidly decays as the energy condenses into the
lowest energy mode, which is in agreement to the minimum energy eigenstate of
$nabla times vec{B} = lambda vec{B}$. Fluctuation results from a new high
speed, high resolution magnetic radial probe will also be presented. The new
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. Merging studies with plasma plumes injected from
both ends of the cylinder are planned.

Characterization: D7

Comments:

University of Washington

Workshop on Innovation in Fusion Science (ICC2011) and
US-Japan Workshop on Compact Torus Plasma
August 16-19, 2011
Seattle, Washington

ICC 2011