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icc_slides.pdf2011-08-23 19:04:31Auna Moser

Experimental observation of instability cascade from MHD to ion skin depth scale resulting in magnetic reconnection

Author: Auna L. Moser
Requested Type: Consider for Invited
Submitted: 2011-06-07 16:36:51

Co-authors: P. M. Bellan

Contact Info:
Caltech
1200 E. California Blvd.
Pasadena, CA   91104
USA

Abstract Text:
Magnetic reconnection is of critical importance in many fusion plasmas, including tokamaks, spheromaks and RFPs. Reconnection necessarily involves processes beyond the scope of ideal MHD; finite resistivity is known to provide a possible non-ideal MHD process, but is quantitatively inadequate to explain observed reconnection rates in many important situations. Shibata and Tanuma[1] proposed that the required high magnetic reconnection rate results from a cascade of instabilities through progressively smaller scales until a microscopic scale length is reached where non-MHD processes enable magnetic reconnection. An experiment at Caltech produces a magnetically driven plasma jet that undergoes an ideal MHD, current-driven kink instability; the addition of a longer duration power supply revealed that the kink instability amplitude could grow either linearly or exponentially. In the case of exponential growth, a segment of the kinked jet thins to form a bright filament. The laterally outward acceleration of the kinked jet segment provides an effective gravity, leading to development of a Rayleigh-Taylor instability on the trailing edge of the thin filament. At the time we observe this distinct, spatially periodic Rayleigh-Taylor instability, the plasma has reached the ion skin depth scale (i.e. non-MHD scale), the current density becomes so large that the electron drift velocity reaches the order of the Alfven velocity, and the instability initiates observed magnetic reconnection of the jet. The instability cascade from macroscopic kink instability to microscopic Rayleigh-Taylor instability transitions the plasma from an ideal MHD scale to an ion skin depth scale where non-MHD processes enable magnetic reconnection.

[1] Shibata, K. & Tanuma, S. Plasmoid-induced-reconnection and fractal reconnection, Earth Planets Space, 53, 473-482 (2001).

Characterization: D6

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