Plasma response to resonant magnetic perturbations near rotation zero-crossing in low torque plasmas

The plasma response to resonant magnetic perturbations (RMPs) near the pedestal top is crucial for accessing edge localized modes (ELMs) suppression in tokamaks. Since the radial location of the rotation zero-crossing plays a key role in determining the threshold for field penetration of RMPs, the plasma response may be different in low input torque plasmas. In this work, the linear MHD code MARS-F is applied to reveal the dependence of the plasma response to RMPs on the location of the rotation zero-crossing by a scan of ExB rotation profiles based on an EAST equilibrium. It is shown that the plasma response is enhanced when the ExB rotation zero-crossing occurs near the rational surfaces. The dependence of the plasma response on the location of the ExB rotation zero-crossing is well fitted by a double Gaussian, indicating two effects in this enhancement. One is induced by a rotation screening effect shown as a wide base (with a width around 10-20 krad/s), and the other is related to a resistive singular layer effect characterized by a localized peak (with a width around 3-4 krad/s). The peak of each resonant harmonic in the plasma response appears always at the rotation zero-crossing. The width of the peak scales with the resistive singular layer width. The plasma displacement suggests that the response is tearing-like when the zero-crossing is within the singular layer, while it is kink-like when the zero-crossing is far from the layer. The reduction in the magnetic islands width is only by a factor of two, when the absolute value of the local ExB rotation increases from 0 to 10-20 krad/s. This is further confirmed in modeling of the plasma response in an EAST ELM suppression discharge. Although there is a zero-crossing in the ExB rotation but none in the electron perpendicular rotation, no significant difference in the plasma response is obtained using these two rotation profiles. This suggests that the rotation near the pedestal top should not be far away from zero but it may not be necessary to have a zero-crossing for accessing ELM suppression. Published under an exclusive license by AIP Publishing.