  Edge-Localized-Mode (ELM) suppression by resonant magnetic perturbations
(RMPs) generally occurs over very narrow ranges of the plasma current (or
magnetic safety factor q95) in the DIII-D tokamak. However, wide q95 ranges of
ELM suppression are needed for the safety and operational flexibility of ITER
and future reactors. In DIII-D ITER Similar Shape (ISS) plasmas with n=3 RMPs,
the range of q95 for ELM suppression is found to increase with decreasing
electron density. Nonlinear two-fluid MHD simulations reproduce the observed
q95 windows of ELM suppression and the dependence on plasma density, based on
the conditions for resonant field penetration at the top of the pedestal. When
the RMP amplitude is close to the threshold for resonant field penetration,
only narrow isolated magnetic islands form near the top of the pedestal,
leading to narrow q95 windows of ELM suppression. However, as the threshold for
field penetration decreases with decreasing density, resonant field penetration
can take place over a wider range of q95. For sufficiently low density
(penetration threshold) multiple magnetic islands form near the top of the
pedestal giving rise to continuous q95 windows of ELM suppression. The model
predicts that wide q95 windows of ELM suppression can be achieved at
substantially higher pedestal pressure in DIII-D by shifting to higher toroidal
mode number (n=4) RMPs.
