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

Abstract

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 E × B rotation profiles based on an EAST equilibrium. It is shown that the plasma response is enhanced when the E × B rotation zero-crossing occurs near the rational surfaces. The dependence of the plasma response on the location of the E × B 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 E × B 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 E × B 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.

First demonstration of full ELM suppression in low input torque plasmas to support ITER research plan using n = 4 RMP in EAST

“…The global structure of the kink-peeling like plasma response makes the results not to be very sensitive to local plasma rotation near the plasma edge [39]. Recent modelling results suggest that the absolute value of the flow plays a critical role in determining the level of plasma response rather than the existence of a zero-crossing point [40].…”

The effect of plasma beta on error-field penetration in radio-frequency wave heated plasmas in EAST

“…Figure 6 shows a comparison of the mode structures before and after the m = 2 field penetration for two β N . Figure 6(a) presents the real part of the radial structure of the plasma radial displacements Re(ξ ρ 2/1 ) and RMPs (b ρ /B ζ ) 2/1 [46,61] for β N = 0.43 (red) and β N = 0.51 (blue) at the moment before the m = 2 field penetration. Here, we choose the real part of the displacement to compare the variation of the mode structures for convenience.…”

Extension of ELM suppression window using n = 4 RMPs in EAST

“…For non-resonant harmonics at the rational surfaces, the difference in amplitude between even and odd parity can be up to a factor of 3. The characteristic of the plasma response induced by the even parity RMPs is known as the kink-peeling like response [37] and has been observed in previous ELM suppression experiments on EAST [10,51]. The non-resonant harmonics can induce nonlinear effects like neoclassical toroidal viscosity (NTV), which has a strong effect on plasma toroidal flow and field penetration [52,53].…”