An investigation on laser-induced damage resistance of KDP optics repaired by different micro-milling strategies

Although micro-milling has been regarded as the most promising method to repair the micro-defects on KH2PO4 (KDP) optics surfaces, residual tool marks are inevitably generated on the repaired surfaces and would pose a direct impact on the laser-induced damage (LID) threshold of repaired KDP optics. In this work, a residual tool marks model was developed to characterize the repaired surface morphologies processed by different milling strategies (e.g. layer-milling and spiral-milling strategies). Then the predicted tool marks were utilized to built a light-filed intensity modulation model to reveal the relationship between the residual tool marks and its related optical performance. The practical of LID tests well verified the simulation results. It was revealed that the milling strategies indeed have a significant influence on the ultimate LID resistance of KDP optics after repair through impacting the generation of residual tool marks. The spiral-milling method with a smaller path interval can produce a laser-friendly repaired surfaces, which have a equivalent laser-induced resistance with the original fly cutitng surface, while the layer-milling method has a higher machining efficiency. Therefore, an optimized repair procedure, adopting layer-milling path as rough milling and spiral-milling as fine milling, was proposed for pratical engineering application in ICF facilities in the future.