Simulations of NTM Stabilization using Current Drive Offset Relative to the ISLAND Center
J. Woodby, E. Schuster, G. Bateman, A.H. Kritz and A. Pankin
Division of Plasma Physics (DPP) Annual Meeting of the American Physical Society (APS)
Orlando, Florida, November 12-16, 2007
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Abstract
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High plasma pressure can cause simply nested magnetic flux surfaces to
tear and reconnect, leading to the formation of magnetic islands. The
neoclassical tearing mode (NTM) instability drives magnetic islands to
grow to saturated widths, at which the islands can persist stably. The
presence of magnetic islands leads to a local flattening of the
pressure profile and locally hollow current profile within the island.
The flattening of the pressure profile is undesirable in that it can
result in degradation of plasma confinement. One common method of
stabilizing NTMs and shrinking the magnetic islands is to replace the
diminished currents within the islands using direct current injection
via electron cyclotron current drive. Maximum stabilization is achieved
when current is driven at the island center, the location of which is
not accurately known in experiments. In this study, the current drive
is expressed mathematically as a Gaussian current drive density in
Hamada coordinates. The effect of current drive offset relative to the
island center is investigated in preparation for feedback control.
