Speaker
Seiya Nishimura
Description
See the full Abstract at http://ocs.ciemat.es/EPS2018ABS/pdf/P1.1038.pdf
Neoclassical tearing mode induced by error field penetration
S. Nishimura1 , R. Numata2
1 Faculty of Science and Engineering, Hosei University, Koganei, Japan
2 Graduate School of Simulation Studies, University of Hyogo, Kobe, Japan
In tokamaks, magnetic islands are generated by both the neoclassical tearing mode and the
forced magnetic reconnection due to intrinsic or externally applied error fields. In addition,
interaction between the neoclassical tearing mode and error fields is of great importance in
tokamaks. It is known that the rotating neoclassical tearing mode is decelerated and locked by
error fields, once error field amplitude exceeds a critical value. Even when the neoclassical tear-
ing mode is stable, penetration of error fields directly produces non-rotating magnetic islands
that continue growing due to plasma responses, which are so-called born-locked modes. Those
locked modes are often observed in precursor phases of tokamak disruptions[1].
Much work has been done to understand interaction between the unstable neoclassical tear-
ing mode and the error fields. For example, in our previous work, a low dimensional model
of rotating magnetic islands is introduced to understand simulation results based on reduced
magnetohydrodynamic (MHD) equations[2]. While, a relation between the stable neoclassical
tearing mode and the error fields has not been fully discussed so far.
In this study, a simulation code solving reduced MHD equations in two-dimensional slab
geometry is developed to study the effects of error fields on the stable neoclassical tearing
mode. The reduced MHD equations are composed of a vorticity equation, a generalized Ohm’s
law and a pressure evolution equation. Perturbed bootstrap currents are phenomenologically
introduced in the generalized Ohm’s law. The error fields are introduced through edge boundary
conditions of magnetic flux perturbations. Nonlinear simulations are conducted in parameter
regimes, where the neoclassical tearing mode is stable for any initial magnetic island width.
In simulations, the error field penetration is observed, when plasma flow velocity and diamag-
netic drift velocity almost cancel out. Once the error field penetration occurs, magnetic islands
grow to sizes much larger than those expected from given error field amplitude. This indicates
that the error field penetration destabilizes the stable neoclassical tearing mode. Our results are
suggestive to a mechanism of spontaneous growth of the born-locked modes.
References
[1] R. J. La Haye, R. Fitzpatrick, T. C. Hender, A. W. Morris, J. T. Scoville, and T. N. Todd, Phys. Fluids B 4,
2098 (1992).
[2] S. Nishimura, M. Yagi, K. Itoh, S.-I. Itoh, and S. Benkadda, Nucl. Fusion 50, 054007 (2010).
