As a critical component of inertial confinement fusion (ICF), the research on the rapid-growth technology for large-sized deuterated potassium dihydrogen phosphate (DKDP) crystals has attracted significant attention worldwide. The control of growth and application costs for DKDP crystals as well as the regulation of the deuteration degree of the crystals and their performance are among the fundamental issues currently faced in improving load capacity.

1) Neutrons interact with atomic nuclei through a non-monotonically varying scattering intensity function, making them suitable for determining light element positions in crystal lattices and distinguishing adjacent element positions; 2) Neutrons can differentiate between isotopes of the same element, enabling hydrogen-deuterium labeling which shows particular advantages in studying organic molecular materials and related fields; 3) Possessing magnetic moments, neutrons interact with atomic magnetic moments to generate unique magnetic diffraction, through which one can determine magnetic moment magnitudes and orientations of magnetic atoms in crystal lattices, serving as a crucial approach for studying magnetic structures; 4) Their significantly high penetration capability makes them particularly suitable for structural studies requiring specialized thick containers under extreme conditions like high/low temperatures and high pressure. The main limitations of neutron diffraction lie in requiring specialized intense neutron sources and the typical need for larger samples and longer data collection time due to insufficient source intensity. Currently, China possesses three major neutron science facilities: China Spallation Neutron Source (CSNS), China Mianyang Research Reactor (CMRR), and China Advanced Research Reactor (CARR).

The current critical issues in enhancing load capacity and the hydrogen (H)-deuterium (D) network system of DKDP crystals are highly compatible with the research strengths of neutron physics methodologies. This paper, based on recent research results from both domestic and international sources, focuses on the application of neutron physics methods in the rapid growth technology of DKDP crystals and the study of laser damage. It also introduces our research progress in this field, points out research directions that should be continuously focused on, and looks forward to the application prospects of large-sized and high-quality DKDP crystals in the future.

Higher performance standards have been imposed on critical optical components within laser transmission systems (Fig. 1). As ICF progresses toward higher energy levels in the future, the growth processes and defect control of DKDP crystals becomes research priorities, particularly exploring the relationship between process parameters and performance from microscopic perspectives, which forms the foundational scientific basis for advancing load capacity. The DKDP crystal structure is primarily dominated by ionic bonding, where each phosphorus (P) atom is coordinated by four oxygen (O) atoms arranged approximately at the vertices of a regular tetrahedron, forming PO₄ groups, interconnected via H/D atoms (Fig.2). The unit cell parameters a and b increase with deuterium content, while parameter c shows no significant variation, resulting in relatively complete crystal structures for both low-deuterium and high-deuterium DKDP crystals (Fig. 3). Neutron diffraction methodology provides enhanced precision in quantifying H/D ratios and spatial localization within DKDP crystals. Experimental findings reveal that as the H—O and D—O bonds align parallel to the [100] and [010] crystallographic directions, deuterium content directly influences the lengths of O—O and P—O bonds in the structure (Table 1). The lighter mass of hydrogen facilitates quantum tunneling phenomena in O—H—…—O bonds, resulting in a more symmetric proton distribution between two oxygen atoms. This symmetry can lead to the formation of “quantum depolarization defects,” where PO₄ groups fail to contribute to macroscopic polarization, thereby reducing the crystal spontaneous polarization strength and phase transition entropy. In contrast, D, with its heavier mass, exhibits a lower probability of quantum tunneling. D tends to deviate from the hydrogen bond center, forming an asymmetric distribution, which enhances the crystal spontaneous polarization strength and phase transition entropy. These microscopic characteristics manifest distinct differences in the optical field response of high-power laser systems, which is identified by the simulation (Fig. 4).

1) Cone-column interface. Crystals grown via rapid growth methods exhibit distinct cone-column interfacial boundaries, which can induce phase jumps under optical field conditions, compromising homogeneity, and limiting their application in ICF engineering. Implementing controlled growth environments with structured constraints can improve crystal quality and growth efficiency (Fig. 5). 2) Nonuniform D distribution. Studies have investigated flow field states on crystal surfaces under varying growth conditions through hydrodynamic simulations, proposing optimized flow field strategies. Neutron imaging enables precise detection of H and D distributions in DKDP crystals, facilitating exploration of D-content variations in growth solutions and their impact on the growth process. Coupled with finite element simulations, this approach identifies correlations between growth parameters and flow field dynamics, thereby optimizing growth protocols to mitigate lattice mismatch, local overcooling, and deuterium inhomogeneity, ultimately enhancing crystal quality and laser damage resistance. 3) Residual stress in crystals. Three types of residual stresses arise during crystal growth. Neutron diffraction techniques allow noninvasive measurement of residual stress in DKDP crystals, revealing internal stress levels. Research indicates that macroscopic stress does not increase with deuterium content. Maximum lattice mismatch and microscopic strain occur when the mass fractions of H and D in the crystal are both 50%, with defects identified as the source of macroscopic residual stress. Additionally, developing real-time monitoring systems to track stress field dynamics and relaxation processes can provide critical feedback for refining key growth parameters.

For significantly enhancing device load capacity, it is crucial of optimizing the entire operational workflow of DKDP components. As the initial stage of the workflow, crystal growth process optimization and fundamental research on ultraviolet damage response can systematically address engineering challenges. This paper focuses on the application of neutron physics methods in DKDP crystal growth and laser damage studies, where notable progress has been made. However, future work still faces multiple challenges. With the rapid development of large scientific facilities like CSNS, future neutron diffraction technology will advance towards higher resolution, faster dynamic response, and multi-physical field coupling analysis. Combined with X-ray and synchrotron radiation methods, these techniques can provide atomic-scale scientific insights for the regulation of DKDP crystal growth. Supported by theoretical simulations, closed-loop optimization of processes, structures, and properties is expected to achieve controllable preparation of large-size, low-stress DKDP crystals. Through collaborative efforts from Chinese researchers, DKDP crystals are anticipated to continue playing critical roles in laser fusion, high-energy laser systems, and emerging optoelectronic applications.