Abstract Details

finn.pdf2006-02-21 09:53:33John Finn

Feedback stabilization of tearing modes in RFPs with a resistive wall above the ideal wall limit

Author: John M. Finn
Submitted: 2005-12-22 09:27:49


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Los Alamos, NM   87545

Abstract Text:
The stability of tearing modes in RFPs in the presence of a resistive
wall and feedback is studied. I has been found in tokamaks that
feedback based on sensing the tangential component of the magnetic
field at the wall is more effective than that based on the normal
component of the field. We find, surprisingly, that proportional
feedback based on the tangential component is capable of stabilization
above the limit for tearing stability with an ideal (perfectly conducting) wall. This
stabilization typically occurs in a small range of parameters, but if
feedback is based on tangential and normal components, stability can
be achieved in a large range of parameters above the ideal wall
tearing limit. It appears that stabilization in this range is not
possible above the ideal plasma threshold, but this is not a problem because the
unstable modes in the mean-field RFP profiles are typically resistive
modes. The relationship with the 'virtual shell' concept of Bishop
will be discussed. This observation leads to the possibility that
single-helicity or quasi-single-helicity states can be obtained by
controlling all unstable m=1 modes except for the primary
single-helicity mode. Another possibility involves controlling the
amplitude of the m=0, n=1 mode, which is typically stable but is
driven by coupling of m=1 modes with different n, and whose coupling affects the m=1 spectrum.

Characterization: A2


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