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Author: Linda E. Sugiyama
Requested Type: Pre-Selected Invited
Submitted: 2018-03-01 04:08:33


Contact Info:
Bldg. 26-557, 77 Massachusetts
Cambridge, MA   02319

Abstract Text:
Important instabilities in magnetically confined plasmas develop an accelerating, faster-than-exponential growth rate that leads to a rapid ``crash'' of the background plasma, expelling substantial
amounts of plasma and energy and reducing the destabilizing gradients. Examples include the 1/1 internal kink (sawtooth) over a central q<1 region and large Edge Localized Modes (ELMs) with a steep near-edge pressure gradient. They develop a highly localized radial bulge (or several) of the unstable surface. Strong compression against the surrounding plasma increasingly aligns the magnetic field lines, which already have a magnetic shear close to that of the mode resonant surface, and strengthens the transverse pressure gradient. Above a threshold amplitude, a mostly ideal, nonlinear interchange can rapidly push plasma across this field without changing the orientation of the magnetic field vectors or field lines. A protruding ``magnetic pipe'' produced by reconnection is unnecessary, although reconnection may ultimately occur. Time scales and plasma redistribution are similar to experimental observations. The bulge develops through the nonlinear interaction of the mode harmonics, which create successively higher toroidal and poloidal harmonics that further localize the instability, a process that feeds back on itself. The mechanism exists in MHD and can be demonstrated analytically and numerically. The fast stage of the sawtooth crash is independent of resistivity [1], despite strongly resistive growth rates at small amplitude. It does not follow the Kadomtsev model, because density is not tied to the field lines. For sawteeth[1] and ELMs[2], no magnetic flux tubes connect the expelled plasma to its original location, beyond the immediate vicinity of the resonant surface, unlike some theories of explosive growth.
*Work partially supported by U.S. DOE DE-SC0007883.

[1] L. Sugiyama, Phys. Plasmas 21, 022410 (2014).
[2] L. Sugiyama, Phys. Plasmas 17, 062505 (2010).