Abstract Details

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you_sausage_instability_leading_to_spheromak_formation.pdf2006-02-21 14:08:45Setthivo You

Sausage Instability Leading to Spheromak Formation

Author: Setthivo You
Submitted: 2005-12-20 19:15:40

Co-authors: P. M. Bellan

Contact Info:
Caltech
1200 E. California Blvd
Pasadena, CA   91125
USA

Abstract Text:
The ideal MHD sausage instability is generally ignored since it is
well understood and straightforward to stabilize in the laboratory.
Indeed it is considered detrimental to the magnetic confinement of
plasmas. However, in the steps leading to spheromak formation, we
observe the ideal sausage instability to be a necessary ingredient.

As opposed to conventional spheromak experiments that use coaxial
guns, the Caltech experiment presents an unobstructed view of the
complete spheromak formation process. The planar, instead of
traditionally cylindrical, coaxial electrodes produces spheromak
configurations even in the absence of a flux conserver. High-speed
photography, magnetic probe arrays, spectroscopy and interferometry
help resolve the sequence in this relaxation process.

To date, the actual topological transformation of a jet-like plasma
column into a spheromak has not yet been elucidated. However, some
of the steps have been identified (see adjoining presentation). A
plasma column expands like a jet, ingesting plasma from nozzles at
the electrodes [1] while collimating due to a "gobble" MHD mechanism
[2]. The current-carrying column eventually becomes long enough to
kink paramagnetically and amplify the vacuum poloidal field. As the
structure detaches, a spheromak configuration is observed [3]. This
detachment is now found to be consistent with the ideal MHD sausage
instability. An approximate stability space is derived from ideal
MHD linear analysis, assuming the column has collimated due to the
"gobble" mechanism. The space plots k vs lambda, where k is
essentially the aspect-ratio of the column and lambda the ratio of
current to magnetic flux carried by the column. Measurements of k
and lambda throughout the duration of the discharge trace a path in
this operating space. The results show that detachment of the plasma
column occurs in the sausage-unstable region.

[1] S.You, G.S.Yun, P.M.Bellan, "Dynamic and stagnating plasma flow
leading to magnetic-flux-tube collimation", Phys. Rev. Lett., 95,
45002 (2005)

[2] P.M.Bellan, "Why current-carrying magnetic flux tubes gobble up
plasma and become thin as a result", Phys. Plasmas, 10 (2003) 1999

[3] S.C.Hsu, P.M.Bellan, "Experimental Identification of the MHD
Kink Instability as a Mechanism for Coaxial Gun Spheromak
Formation", Phys. Rev. Lett., 90, 215002 (2003)

Characterization: E3

Comments:
If possible, please place next to P. M. Bellan contribution.

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