Micro plasma-assisted localized deliquescence polishing for the remediation of surface micro-defects in KDP crystal

• A hybrid strategy integrates micro-cold plasma modification with site-specific deliquescence polishing. • Micro-defect surface roughness is remarkably reduced from the micrometer scale to the sub-10 nm level. • Heterogeneous deliquescence caused by localized solution stagnation is effectively eliminated. • Experiments elucidate how process parameters regulate deliquescence kinetics and morphology evolution. • Plasma-induced super-hydrophilicity expands the processing window for Gaussian profile restoration. Potassium dihydrogen phosphate (KDP) crystals serve as critical components for non-linear optical frequency conversion and electro-optic switching in inertial confinement fusion (ICF) laser drivers, where surface integrity dictates the power-handling capability of high-energy laser systems. However, the inherent material properties of KDP—low hardness, high brittleness, hygroscopicity, and thermal sensitivity—render it highly susceptible to micro-defects during ultra-precision machining. Under high-fluence laser irradiation, these topographical imperfections trigger localized thermal deposition, eventually inducing laser-induced damage. This study presents a hybrid remediation strategy integrating micro-cold plasma-assisted surface modification with site-specific deliquescence polishing via micro-capillary delivery. The approach utilizes an atmospheric-pressure cold plasma jet to transform the defect region into a super-hydrophilic state, followed by the precise transport of deliquescent solutions to the defect site through a micro-capillary, enabling stress-free and tool-mark-free smoothing at the nanoscale. The mechanisms of plasma modification and its influence on deliquescence kinetics were systematically characterized using laser confocal microscopy, scanning electron microscopy, white-light interferometry, and Raman spectroscopy. The results demonstrate that this process effectively eliminates micro-defects while reducing surface roughness to below 5 nm, offering a robust technical pathway for the high-performance fabrication and in-service maintenance of large-aperture KDP optics.