Magnetic equilibrium of the CN-H1 heliac: a comparative study of design, as-built, and historically built configurations

The relocation of the Australian H-1 National Facility (H-1 NF) to China and its reconstruction into the CN-H1 device necessitated a rigorous reassessment of its magnetic configuration to address systematic deviations introduced during the engineering process.This paper presents a comprehensive comparative study of the magnetic equilibrium across three distinct definitions of the device: the idealized H-1 Design, the historically built as-built H-1 NF, and the newly reconstructed CN-H1. High-precision laser tracker metrology was employed to generate a high-fidelity coil model for CN-H1, incorporating the remanufactured poloidal and helical windings. Calculations of the vacuum magnetic field reveal that, compared to the H-1 NF, the CN-H1 exhibits a systematic inward radial shift of the magnetic axis, a slight increase in the rotational transform, while maintaining a comparable level of stellarator symmetry breaking. Three-dimensional MHD equilibria were reconstructed using the VMEC code and validated against field-line tracing. To streamline this process, a new open-source pipeline, coils2vmec , was developed to automatically generate robust VMEC input files directly from coil geometry inputs. Comparative analysis across representative configurations demonstrates that the defining characteristics of the flexible heliac—including high rotational transform, shear tunability, and magnetic well depth—are preserved in CN-H1. Magnetic spectra in Boozer coordinates of CN-H1 are nearly indistinguishable from the H-1 NF historically built baseline, despite coilpositioning errors in the millimeter range. These results validate the engineering fidelity of the CN-H1 reconstruction and establish a precise equilibrium basis for future finite-beta plasma experiments.