Wide Operational Windows of Edge-Localized Mode Suppression by Resonant Magnetic Perturbations in the DIII-D Tokamak

Edge-localized mode (ELM) suppression by resonant magnetic perturbations (RMPs) generally occurs over very narrow ranges of the plasma current (or magnetic safety factor q_{95}) in the DIII-D tokamak. However, wide q_{95} ranges of ELM suppression are needed for the safety and operational flexibility of ITER and future reactors. In DIII-D ITER similar shape plasmas with n=3 RMPs, the range of q_{95} for ELM suppression is found to increase with decreasing electron density. Nonlinear two-fluid MHD simulations reproduce the observed q_{95} 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 q_{95} 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 q_{95}. For sufficiently low density (penetration threshold) multiple magnetic islands form near the top of the pedestal giving rise to continuous q_{95} windows of ELM suppression. The model predicts that wide q_{95} 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.

Imaging divertor strike point splitting in RMP ELM suppression experiments in the DIII-D tokamak

Fast visible imaging of the lower divertor from above is used to study the structure and dynamics of lobes induced by resonant magnetic perturbations (RMPs) in Edge-Localized Mode (ELM) suppression experiments in DIII-D. The best compromise between the amount of light and sharp imaging was obtained using emission at 601 nm from Fulcher band molecular deuterium. Multiple spatially resolved peaks in the D₂ emission, taken as a proxy for the particle flux, are readily resolved during RMPs, in contrast to the heat flux measured by infrared cameras, which shows little spatial structure in ITER-like conditions. The 25 mm objective lens provides high spatial resolution (2-4 mm/pixel) from the centerpost to the outer shelf over 40° toroidally that overlaps the field of view of the IRTV that measures the divertor heat flux, allowing direct comparison in non-axisymmetric discharges. The image is coupled to a Phantom 7.3 camera using a Schott wound fiber bundle, providing high temporal resolution that allows the lobe dynamics to be resolved between ELMs and across ELM suppression onset. These measurements are used to study the heat and particle flux in 3D magnetic fields and to validate models for the plasma response to RMPs.

Pedestal bifurcation and resonant field penetration at the threshold of edge-localized mode suppression in the DIII-D Tokamak

Rapid bifurcations in the plasma response to slowly varying n=2 magnetic fields are observed as the plasma transitions into and out of edge-localized mode (ELM) suppression. The rapid transition to ELM suppression is characterized by an increase in the toroidal rotation and a reduction in the electron pressure gradient at the top of the pedestal that reduces the perpendicular electron flow there to near zero. These events occur simultaneously with an increase in the inner-wall magnetic response. These observations are consistent with strong resonant field penetration of n=2 fields at the onset of ELM suppression, based on extended MHD simulations using measured plasma profiles. Spontaneous transitions into (and out of) ELM suppression with a static applied n=2 field indicate competing mechanisms of screening and penetration of resonant fields near threshold conditions. Magnetic measurements reveal evidence for the unlocking and rotation of tearinglike structures as the plasma transitions out of ELM suppression.