Abstract
The grazing-incidence operating mode of large-aperture X-ray mirrors imposes bidirectional anisotropic requirements on surface figure accuracy: extremely stringent in the meridional direction, while relatively relaxed in the sagittal direction. The strict meridional accuracy requirement necessitates the use of small-beam deterministic figuring processes, making machining efficiency a critical bottleneck to be overcome. To address this issue, this paper proposes a spatially decoupled figuring strategy based on a bidirectionally anisotropic elliptical diaphragm. By exploiting the disparity in accuracy requirements between the meridional and sagittal directions, a non-axisymmetric ion-beam removal function is designed. First, a multi-objective collaborative optimization model is established, and the diaphragm dimensions are optimized using the Non-dominated Sorting Genetic Algorithm II (NSGA-II) with an elitist strategy. An optimal structural parameter of 4 mm (meridional) × 7 mm (sagittal) is determined. The geometric decoupling characteristics and stability of the bidirectionally anisotropic removal function are then verified through multiphysics-field simulations and fixed-point calibration experiments. Finally, figuring simulations and experimental validation are conducted on a Φ100 mm monocrystalline silicon substrate. Compared with the conventional high-precision scheme using a Φ4 mm circular diaphragm, the proposed strategy increases the volumetric removal rate to 304%, reduces the machining time to 32.3%, while achieving a meridional slope error converged to 0.28 µrad and a surface figure height RMS error converged to 1.850 nm. The figuring capability is comparable to that of the high-precision scheme. This work not only theoretically elucidates the formation mechanism of bidirectionally anisotropic removal functions, but also provides an engineering-feasible technological pathway for the fabrication of large-aperture, high-precision X-ray mirrors.