| The design of laser fusion cryogenic target is a large scientific project with multi-disciplines(physics,chemistry,mathematics,etc.)and multi-research systems(target preparation,experiment,diagnosis,etc.).The cryogenic target system is a very complex and precise device.The preparation of cryogenic target has a strict requirements on the material environment and structure.In order to ignite successfully,a temperature field with a spherical distribution must be established around the target(according to theoretical analysis,the temperature gradient of the target surface cannot be higher than 0.1 m K)to obtain high quality ice layer(the roughness of the inner surface of the ice layer must be less than 1 μm and ice fuel uniformity must exceed 99%).Based on the cryogenic target model of cylindrical hohlraum(National Ignition Facility,NIF)and the cryogenic target structure of spherical hohlraum with 6 laser entrance holes,the computational fluid dynamics(CFD)software Fluent was used in this paper to analyze the effects of thermal mechanical package(TMP)structure,sleeve materials,cooling process of the cold wall and environmental radiation on the surface temperature field of target.The main results can be summarized as follows:Firstly,the influences of TMP structures and sleeve materials on the temperature field of the cryogenic target model of cylindrical hohlraum were obtained.TMP structures included hohlraum structures and sleeve wall thicknesses.The results show that the heat transfer efficiency of the cryogenic target system is higher and the temperature field stability in the hohlraum is better when sleeve material chooses the high pure copper(because of its high specific heat and thermal conductivity).The double-convex ring structure of the hohlraum can increase the contact area between the sleeve and hohlraum,it improves the rate of cooling and volume heat export,which obtains lower surface temperature and temperature difference of the target.With the increase of sleeve wall thickness,the natural convection intensity of gas mixture of helium and hydrogen in the hohlraum decreases and the uniformity of temperature field on the surface of the target is improved.Second,based on the spherical hohlrauml and cylindrical hohlraum,the sleeve materials were SS304,high purity copper and Al5052,the variation of cryogenic target temperature field with different cooling methods was obtained.The results show that the sinusoidal disturbance has a greater influence on the temperature field distribution of the spherical hohlraum model in the process of cooling.For cylindrical hohlraum model,the temperature field in the hohlraum remains more stable when sleeve material are Al5052 and SS304 because of its good inhibition effect on temperature disturbance.Rapid cooling has a great influence on the maximum temperature difference on the surface of the target,and the maximum temperature difference on the surface of the target of the spherical hohlraum model is significantly higher than that of the cylindrical hohlraum model,which seriously damages the temperature uniformity on the surface of the target.With the increase of the temperature difference between the upper and lower cold walls,the maximum temperature difference on the target surface of the cylindrical hohlraum model gradually increased.However,the maximum temperature difference on the target surface of the spherical hohlraum model shows a trend from decline to rise,and the minimum value is obtained when the cold walls temperature difference is 20 m K.Finally,the temperature field of the spherical hohlrauml and cylindrical hohlraum is compared and analyzed under the environment radiation(environment temperatures,film transmissivity and the opening process of the shield).The results show that the spherical hohlrauml has a better temperature stability under external environment temperature change.With the increase of the film transmissivity,more environmental radiation will enter the hohlrauml,which leads to the increase of the natural convection intensity of the gas in the hohlrauml and damages the surface temperature uniformity of the target.The maximum temperature difference on the surface of the target in the spherical hohlrauml model is lower than that in the cylindrical hohlraum model under the same film transmissivity.Since the number of laser entrance holes in the spherical hohlrauml model is more than that of cylindrical hohlrauml,more environmental radiation will enter into the hohlrauml after the shield is opened.As a result,the ice deterioration time of the spherical hohlrauml model is far less than that of the cylindrical hohlrauml model,which makes laser ignition difficult. |
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