Research On Temperature Distribution Characteristics Of Large Aperture Frequency Doubling Crystal In Final Optics Assembly

In order to obtain short-wavelength laser, large aperture frequency doubling crystals are used to achieve laser frequency doubling in Inertial Confinement Fusion(ICF) devices. To avoid the narrow angular acceptance and walk-off effect of angle phase matching, non-critical phase matching(NCPM) begins to receive more attention. In the NCPM, the frequency conversion efficiency exhibits high sensit ivity of temperature variation. Working under the best match temperature is necessary to guarantee a high frequency conversion efficiency. A precise temperature control of frequency doubling crystals is difficult to be achieved because the crystals used in ICF devices are usually with large aperture and thin thickness in dimension, and are fragile, deliquescent and with low coefficient of heat conduction. In order to ensure the stability and uniformity of the crystal temperature in full aperture, both simulations and experiments were developed to obtain the temperature distribution characters in different heating methods. The results provide a basis for the selection of design and operating parameters.The temperature control methods which were used in frequency doubling crystals have been summarized in the thesis. Based on the temperature control device for large-aperture frequency doubling crystal, the overall heat transfer process has been analyzed. Accordingly, liquid-solid conjugate heat transfer models were established and solved by the finite volume method(FVM). Heat transfer characteristics of different thermal control methods, which were applicable for normal pressure or vacuum condition internal the device, were compared and selected for practical application.Heating or cooling the crystal frame using constant temperature circulation water was chosen in vacuum condition. The influence of the velocity of water flow, the aperture of crystals and the target temperature were analyzed through simula tions, respectively. The heat transfer characters, like the heating time needed and temperature distribution of crystal in steady state, of those factors were set as the emphasis in this research. An experimental platform for crystal temperature measurement was built based on the fourth harmonic generation module of FOA. Thermal infrared imager was used to obtain the temperature distribution of the crystal at vacuum condition. The results obtained by simulations and experiments in similar conditions were compared for validation.The heating method, running constant temperature water in channels of both crystal frame and walls of the cavity, is used to control the temperature of crystals at normal pressure. Main factors like the convective heat transfer coeff icient of exterior surfaces, internal water channel density, target temperature and velocity of the circulation water were investigated. Temperature difference within ±0.1°C is set as a critical point to obtain the design parameter threshold. When the internal of the device is at normal pressure, the timely temperature of the monitoring points were obtained by high-precision temperature sensors in experiments. The relationship between the steady state temperature of crystals and the temperature of water inlet was investigated through changing the target temperature.