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nset and saturation of the kink instabilities in a current carrying, line-tied plasma surrounded by a resistive shell

Author: Cary B Forest
Requested Type: Consider for Invited
Submitted: 2006-12-20 15:51:43

Co-authors: Will Bergerson, Gennady Fiksel, David Hannum, Roch Kendrick and John Sarff

Contact Info:
University of Wisconsin, Madison
1150 University Ave
Madison, WI   53706

Abstract Text:
The MHD stability of a current carrying, line-tied plasma have been studied in a linear screw pinch with and without a surrounding conducting walls. The experiment utilizes an array of 19 plasma guns to generate a controllable current profile. The main diagnostics for characterizing the MHD activity are: a 2D array of 80 radial magnetic field pickup coils surrounding the plasma column, a segmented anode, which serves to measure current distribution inside the plasma, and an array of 40 poloidal and axial magnetic field coils inside the conducting shell. Several phenomena have been observed direclty analogous to MHD observed in periodic toroidal plasmas. First, an internal kink instability is observed to grow when the safety factor $q = frac{4pi^2 r^2 B_z}{mu_0 I_p(r) L}$ drops below 1 inside the plasma. The mode has some characteristics of an ideal mode. It existence is independent of the presence of a wall, and it saturates as a rotating, helical equilibrium with line-tied ends. Linear stability has been studied under similar conditions using the NIMROD code, which indicate that resistivity may play some role in determining the onset conditions. Second, for sufficiently low values of $q$, reconnection phenomena resembling sawteeth are observed, which periodically flatten the current profile and alter the magnetic topology. This too appears independent of the outer wall conditions and can be classified as internal in character. Finally, preliminary analysis of plasmas with a resistive wall and an edge $q(a)<1$ show signs of a resistive wall mode. The growth rate is on the order of the wall time, and increases as $q(a)$ drops. This final mode provides a target for future experiments using a spinning conducting shell which will test the hypothesis that moving metal walls can stabilize the resistive wall mode.

Characterization: A2,A4


University of Maryland

Innovative Confinement Concepts Workshop
February 12-14, 2007
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