New insights into error field assessment from mode coupling effects based on RMP spectrum scanning experiments in EAST

Numerical simulations concerning the spectrum of resonant magnetic perturbation (RMP) on field penetration threshold in EAST are presented, since the underlying physics is not well understood by the linear response model. The quasi-linear full MHD model with toroidal geometry is adapted to investigate error field penetration, taking into account both resonant electromagnetic force and non-resonant neoclassical toroidal viscosity (NTV) force as plasma braking mechanisms, with the assumption that the local electric drift frequency to be the same as the mode frequency in model. The nonlinear results reproduce the observed dependence of the penetration threshold current on the toroidal phase difference (ΔφUL) between the upper and lower $n=1$ RMP coils. The results indicate that the penetration of boundary resonant component can accelerate the onset of $2/1$ field penetration. As a result, the observed dependence of the $2/1$ penetration threshold on ΔφUL aligns more closely with the spectrum of boundary resonant component, rather than the local $2/1$ resonant field strength predicted by linear response model. Moreover, even in the absence of NTV torque, the simulations confirm that a strongly penetrated $3/1$ component in toroidal geometry can trigger $2/1$ penetration. The inclusion of NTV torque, however, further reduces the $2/1$ threshold current by enhancing rotational coupling across rational surfaces. The physical mechanism by which mode coupling, derived from adjacent resonant components, significantly influences the penetration threshold of the $2/1$ component in toroidal plasma, may provide a new metric for future error field optimization and assessment.