Quasisymmetric correction to non-resonant error fields in ITER

Reliable error field correction (EFC) is essential for next-step tokamaks due to unfavorable scale projections. The leading-order EFC must be deployed to minimize the overlap with the dominant resonant mode as already planned for ITER, while the next-order correction typically aims to reduce non-resonant residual modes to prevent excessive rotational damping from neoclassical toroidal viscosity (NTV). Here we propose a systematic non-resonant EFC that extends a validated scheme to generate a quasisymmetric magnetic perturbation (QSMP) [1], based on a torque response matrix obtained from a self-consistent perturbed equilibrium. By including intrinsic errors as extra matrix elements, the optimal correction can be identified simply by the eigenvector with the smallest eigenvalue, normalized by the fixed error. This expanded matrix contains all necessary information for non-resonant NTV correction: diagonal elements represent driving forces, off-diagonals reflect coupling, and engineering constraints can be incorporated via matrix optimization. Applications to ITER show that this QS correction can achieve an order-of-magnitude further reduction in NTV compared to solely resonant EFC, effectively mitigating various non-resonant errors in primary magnets. Additional outcomes from QS corrections across different ITER scenarios will also be discussed, supporting comprehensive EFC development. [1] J.-K. Park et al., Phys. Rev. Lett. 126, 125001 (2021).