Error field analysis for modular coils of China First Quasi-axisymmetric Stellarator (CFQS)

The presence of error fields has been shown to degrade plasma confinement and drive instabilities. Error fields are predominantly attributed to deviations in the coil geometry. To determine how coil misplacements drive error fields, previous investigations scanned through all the possible rigid displacements of coils to provide insights prior to construction.

Here we employ a Hessian matrix approach [C. Zhu et al., PPCF 60, 054016 (2018)] for determining error field sensitivity to coil deviations. A primary cost function used for designing stellarator coils was adopted to evaluate the deviation of the generated magnetic field from the desired magnetic field. The sensitivities of error fields to coil displacements are then determined by the eigenvalues of the Hessian matrix, calculated by the FOCUS code.

The new method is applied to analyze the possible errors fields in the CFQS experiment, which will be constructed in SWJTU in China, in collaboration with NIFS, Japan. The results provide information required to avoid dangerous coil misalignments, and that each modular coil has different tolerance. Relaxing unnecessary tolerances could lead to significant cost and schedule savings.