Influence of stochastic coil perturbations on magnetic configurations in Chinese First Quasi-axisymmetric Stellarator

Abstract Error fields resulting from deformations of coils are inevitable during fabrication, assembly and operation in all nuclear fusion devices. The magnetic topologies of stellarators are primarily produced by external coils, which are quite sensitive to these error fields, especially the magnetic island configurations. In future experiments, two types of magnetic island configurations with rotational transform ι = 2/5 and 2/6 will be achieved in the Chinese First Quasi-axisymmetric Stellarator (CFQS). Hence, it is essential to evaluate these two resonant error fields resulting from coil deviations. To identify the major coil deformations that have significant effects, each modular coil is examined individually in the standard 2/5 and 2/6 island chains in the CFQS, respectively. The stochastic perturbations are modeled with Gaussian processes and applied to describe irregular deformations in the coils. Several important results are achieved: (i) as the coil perturbations do not satisfy the stellarator (up–down) symmetry, the stellarator-asymmetry error fields appear, which are one order of magnitude lower than the stellarator-symmetry error fields. (ii) The sensitivity of resonant error fields to the same deformations of each coil is different. The deviations of some coils may lead to weak influences on the magnetic island topologies, in some cases being capable of compensating the resonant error fields. (iii) Certain stochastic coil perturbations have the potential to generate more optimal coils since the error fields are not proportional to the amplitude of coil deformations. These findings suggest that relaxing specific coil tolerances is expected to reduce engineering constraints on coil design and fabrication. Furthermore, this work will also help develop a computational model for the mapping experiments to precisely estimate errors in the CFQS magnetic field.