Speaker
Aleksandra V. Dudkovskaia
Description
See the full Abstract at http://ocs.ciemat.es/EPS2018ABS/pdf/P1.1041.pdf
A reduced drift magnetic island theory of neoclassical tearing modes for
low collisionality tokamak plasmas
A.V. Dudkovskaia1, J.W. Connor2, D. Dickinson1, P. Hill1, K. Imada1, H.R. Wilson1,2
1
York Plasma Institute, Department of Physics, University of York, Heslington, York YO10
5DD, UK
2
CCFE, Culham Science Centre, Abington Oxon OX14 3DB, UK
Successful operation of next generation fusion devices, such as ITER, directly depends on
understanding the physics of the NTM onset and its control techniques. This, in turn, requires
a more quantitative theory of the NTM threshold physics.
In our new theoretical approach, we solve the drift kinetic equation for both the ion and the
electron plasma components, employing an expansion in the small ratio of island width to
tokamak minor radius to obtain orbit-averaged equations. From these we derive the
streamlines, S, along which the distribution function is constant, if collisions are neglected.
This S function describes drift islands of the same geometry as the real magnetic islands but
shifted radially by a value comparable to the poloidal Larmor radius in the absence of the
electrostatic potential. The radial shift of the islands is in opposite directions for the two
streams and . Adding a low level of collisions provides the S dependence of
the distribution function, showing it is flattened inside the S islands (not the magnetic islands,
Fig. 1) due to a competition between the ion parallel flow and plasma drifts. Hence, the
density and temperature profile flattening becomes incomplete for magnetic islands
comparable to the ion banana orbit width, reducing the bootstrap current drive and hence
suppressing the drive for NTMs with widths . These results provide a new
understanding of how finite ion orbit width effects influence the NTM threshold.
Fig. 1. The distribution function flattening across the O-points of the drift islands (the magnetic
island is centred at ): (a) and (b) for the two streams and . (c) The sum over
the two streams, representative of density, showing almost complete flattening across the magnetic island at
for , but a substantial gradient for .
