Invesitgation on characterizing the surface topography of KH₂PO₄ optics repaired by different milling modes

Micro ball-end milling has emerged as a promising technique for repairing micro-defects on the surfaces of KH₂PO₄ (KDP) optics used in Inertial Confinement Fusion (ICF) facilities, which are crucial for advancing the clean energy production. However, the widespread recycling of KDP optics through this technique presents significant challenges. A critical issue is that various milling modes—such as pull, push, up, and down milling—can periodically engage in the repair process, leading to different surface topographies that ultimately affect the optical performance of the repaired optics. This study systematically analyzes the surface topographies produced by different milling modes using advanced characterization methods, including power density spectrum (PSD), continuous wavelet transform (CWT), and fractal dimension (FD). Our findings indicate that residual tool marks on micro-milled KDP surfaces are consistently more pronounced perpendicular to the milling feed direction (i.e., along the step direction) than parallel to it across all milling modes. PSD analysis reveals a dominant frequency along the step direction, which is inversely proportional to milling step intervals. The precise orientation of these tool marks for each milling mode can be accurately determined using angular spectrum analysis. Wavelet-based frequency analysis successfully identifies waviness features arising from the dynamic characteristics of the micro-milling system associated with each milling mode, providing insights for optimizing repair processes. Furthermore, the microscopic topographic features of the repaired surfaces were evaluated using FD, demonstrating stronger characterization capabilities than traditional surface roughness metrics. Notably, the circumferential FD of surfaces machined using up-milling modes was smaller than those of the other milling modes, attributed to the occurrence of micro-cracks due to the ploughing effect during up-milling. By examining the theoretical relationship between the amplitude and periodicity of residual tool marks, we recommend milling step intervals of <28 μm for future engineering repairs of KDP optics in ICF facilities.