A Fourier-space four-field resonant response model for tokamak plasmas with applications to pedestal physics

The linear drift-magnetohydrodynamical response of a resonant layer in a tokamak plasma to an externally generated, static, resonant magnetic perturbation (RMP) is shown to be governed by four, coupled, first-order, ordinary differential equations in Fourier space that can be conveniently solved by means of a Riccati transformation. The resulting Fourier-space, four-field, resonant plasma response model is successfully benchmarked against the equivalent configuration-space model of Lee et al. Nucl. Fusion 64, 106058 (2024). The Fourier-space model is then used to estimate the response of an ITER pedestal to an RMP. The strong magnetic shear present just inside the magnetic separatrix suppresses the response of the plasma to such an extent it is indistinguishable from that of a vacuum. Consequently, rational surfaces that lie in the strong-shear region can be neglected when calculating the response of the plasma. Furthermore, the vacuum response of the plasma in the strong-shear region is associated with the density pump-out phenomenon. The shielding of driven magnetic reconnection by the plasma is found to also break down at rational surfaces associated with comparatively low poloidal mode numbers that are situated close to the top of the pedestal. This breakdown is associated with the RMP-induced suppression of edge-localized modes.