Research Progress on Rapid-Growth Technique for KDP-Type Crystals(Invited)
Significance The main methods of controlled fusion are magnetic and inertial confinement.Between them,inertial-confinement fusion(ICF)uses a higher-power laser beam or higher-energy particle-beam irradiation to focus energy on a deuterium-tritium fuel target pellet with a diameter of only a few millimeters,which rapidly yields a temperature and pressure similar to the core of a star or nuclear explosion,followed by nuclear fusion.Basov.N.G.first proposed the concept of ICF in 1963,and then Chinese nuclear physicist Wang Ganchang proposed laser-driven ICF,which propelled investigations into ICF in China.The emergence and development of high-power lasers have enabled ICF.Currently,the world's major stakeholders are investing significant resources into developing high-power laser devices,e.g.,the world's largest laser device NOVA developed in the United States in 1985,the construction of a megajoule laser device at the United States National Ignition Facility(NIF),France,and the development of a series laser device by Shenguang,China.Short-wavelength laser beams have greater energy and couple more efficiently to the target pellet compared with long-wavelength laser beams.Therefore,for large ICF devices,high-quality electro-optical and nonlinear optical materials are required to convert the 1064 nm laser output wavelength of neodymium glass to 355 nm.Additionally,the materials must exhibit a large aperture,a high laser-damage threshold,large nonlinear optical and electro-optical coefficients,a wide transmission band,and low refractive-index inhomogeneity.KDP-class crystals are excellent nonlinear optical crystal materials with high resistance to laser damage,wide transmittance bands,high electro-optical coefficients,good optical uniformity,and the ability to grow into large crystals.Therefore,large KDP-class crystals are the only type of crystal that can be used for electro-optical switches and frequency-conversion devices in large-caliber high-power laser-driven devices.Progress To address the low efficiency of the conventional rapid growth of point-seed crystals at the cylindrical cone interface,an innovative process for the rapid growth of long-seeded crystals was proposed(Fig.2).Compared with the conventional growth method,the rapid-growth method reduces the crystal growth time from 3 years to half a year.Additionally,the cutting efficiency is doubled and the column-cone interface is absent.The rapid-growth method revamps the entire process route from batching,point crystal,growth,cutting,to annealing.Additionally,a full set of production equipment and processes required for crystal growth,cutting,rough grinding,and annealing is independently developed,thus providing a solid foundation for the entire chain of DKDP components.Dynamic-light-scattering technology was developed to characterize the particle size of solutions prepared using DKDP crystals,which supports the rapid increase in the probability of the laser-damage threshold and provides technical support for obtaining high-quality DKDP crystals(Fig.3).This technology involves the independent development of large-caliber DKDP crystal wire-cutting equipment,requires fewer cutting process parameters,and achieves large-caliber crystal cutting flatness(0.2 mm),thus providing technical support for large-sized crystal cutting(Fig.4).A precision annealing equipment for large-diameter DKDP crystals was independently developed(Fig.5),and the damage threshold of 1-2 J/cm2 increased after annealing(Fig.6).Based on the free-growth technology of long-seeded crystals,320 mm long-seeded crystals were used to rapidly achieve 520 mm X 521 mm X 540 mm large-diameter DKDP crystals at 120 d of growth.The internal transparency of the crystal blank was favorable and satisfied the cutting requirements of large-diameter second-type mixing elements and did not present a conical column interface.The successful growth of the crystals validates the rapid-growth technique for long-seeded DKDP crystals.Owing to continuous research efforts,the growth success rate of DKDP crystals has exceeded 80％,and their optical performance has improved continuously,among which the core index of the crystal anti-laser damage ability has increased significantly and the anti-laser damage ability of the fundamental-frequency KDP component has reached 30 J/cm2(1ω,3 ns).The zero-probability laser-damage resistance of triple-frequency small-aperture DKDP crystals has reached 18J/cm2(3w,3 ns),and after subnanosecond pretreatment,it exceeded 20 J/cm2(3ω,3 ns),which is the highest level for DKDP crystals with 70％deuteration rate in China.Conclusions and Prospects To develop high-power laser drivers,four requirements are to be fulfilled:technology,quality,production capacity,and target cost,Additionally,two goals are to be achieved.First,the technology should be improved continuously and the quantitative and deterministic control of crystal growth should be realized.Second,the development of a full-link device for the growth of DKDP crystals should be promoted;the extremely limited growth,cutting,and annealing capabilities should be addressed;and large-caliber DKDP components should be mass produced.
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