Study Of Shock Interactions During Compression Process In Laser Inertial Confinement Fusion

Fusion energy has long been a coveted source of clean energy for scientists.In laser inertial confinement fusion,precise diagnosis of a series of complex physical processes not only provides experimental insolation performance of fusion targets at different stages,but also plays an important role in the calibration of theoretical models,numerical simulation programs and the design of reference targets.The generation and propagation of shock waves and their interaction in the target are one of the most important studies in the compression and acceleration processes of laser inertial confinement fusion.Experimentally,the diagnosis of the changes in density,pressure and temperature induced by the shock front moving at high speed under different driving laser pulse shape loading can provide important information on the implosion performance of the target under these special loading paths.This dissertation focuses on the diagnostic construction and experimental research of shock interactions in the compression process of laser-driven inertial confinement fusion.We have developed a dual-sensitivity velocity interferometer system for any reflector based on optical Doppler shift,a streaked optical pyrometer based on Planck grey-body radiation,and a three-dimensional self-emission measurement technique combining compressed ultrafast photography with streaked optical pyrometer.Using these diagnostics,precise measurements were made on the compression and acceleration processes of the multiple shocks under the loading of shaped laser pulses in experiments of the double cone ignition scheme.Combined with radiation hydrodynamics simulations,the dependence of different implosion processes on the driven laser pulse shapes and the shocks they generate were studied.The front surface of a target is irradiated by a nanosecond laser,and energy is transferred from the critical surface to the ablation front via electronic thermal conduction,generating shock waves.The measurement of the time and spatial resolution of the shock velocity and self-emission intensity,combined with the high-pressure equation of states,can provide important physical parameters such as density,pressure,and temperature during the implosion process.In this dissertation,a dual-sensitivity velocity interferometer system for any reflector and a streaked optical pyrometer are developed on the SG-Ⅱ Upgrade facility to diagnose the velocity and temperature of the shock front.Due to the design of a shared imaging system,the two diagnostic devices are highly integrated in the optical path and have high spatial resolution(< 7 μm),enabling the efficient acquisition of multiple physical information in one shot.In contrast to previous designs,the illumination system for the diagnosis uses the polarization properties of the probe laser and combines the polarized beam splitter and 1/4-wave plate to increase the return signal intensity by 300%,greatly improving the signal-to-noise ratio of the system.The addition of a servo mirror systems in front of the interferometers and Dove prisms in front of the optical streak cameras provides convenience for the coupling of the signal light and the adjustment the imaging orientation of the rear target.For the streaked optical pyrometer,we use square pulse ablating Al planar targets to perform an absolute calibration of the dependence relationship between the system counting intensity and self-emission brightness temperature.In order to break through the limitation of one-dimensional diagnostic spatial resolution of optical streak cameras,we combine compressed ultrafast photography with streaked optical pyrometer to develop a self-emission diagnostic technology with two-dimensional spatial resolution and one-dimensional temporal resolution.And we have achieved a 300 ps temporal resolution and 60 frame measurements.Using the diagnostic systems described above,this dissertation presents a detailed investigation of the multiple shock interactions within the target and the implosion process of a shell target in the gold cone directly driven by ramp pulses.Firstly,in order to clarify the physical mechanism underlying the generation and interaction of multiple shocks during the compression process,the first single-ramp pulse of a theoretically designed double-ramp pulse is isolated,and the compression process that generated the shock wave was studied.In experiments,the phenomenon of three shocks being generated by a 2.5 ns single-ramp pulse is verified using an Al step target,and the velocities and coalescence processes of the three shocks are directly measured using a polystyrene target,obtaining an initial compression density of 3.46 g/cm~3 and the highest shock front pressure of 7.65 Mbar.Combining with radiation-hydrodynamics simulation,a dual-slope compression pulse shape suitable for the standard target of the double cone ignition scheme is optimized,significantly improving the density and velocity for the initial compressed target.In addition,experimental and simulation studies of the compression and acceleration of the shell target irradiated by a single-ramp pulse combined with a high-intensity main pulse show that the plasma fuel ejected from the gold cone tip has a velocity of 126.8 km/s and a density of 14.92 g/cm~3.In double cone ignition scheme,the physical process after compressing and accelerating the target is the head-on collision process of two plasmas.In order to obtain a better collision effect,it is necessary to optimize the density,temperature and velocity of the plasma ejected from the single cone,so as to help improve the energy conversion efficiency of the collision process.Therefore,it is necessary to understand that different laser pulse shapes with distinct features and their generation of multiple shocks have a direct influence on the control of the implosion parameters for the targets.We diagnose and compare the velocities and compression ratios ablated by two density-prior shaping pulses,one optimized by manual adjustment and the other by machine learning.The results show that the machine learning optimized pulse shape has advantages in generating and controlling multiple shocks,and can obtain higher compression ratio compared to the manually optimized pulse shape,even when reducing the driving energy.Subsequently,in a temperature-prior shaping pulses loading experiment,we separated out the 3-picket compressional front for shock-timing modulation experiments.We achieve a maximum shock velocity of 47km/s by merging three shocks near the same spatial and temporal position within the planar targets.Accordingly,we perform an implosion experiment on gold cone targets using the optimized 3-picket pulse following a square main pulse.The diagnostic results combined with the radiation-hydrodynamic simulation results show that the characteristic time for plasma jet from the gold cone is 5.0 ns,the temperature is 42 eV,and the average velocity of the plasma jet is 182 km/s.During the total implosion process,the maximum IFAR of the shell target is 29.5,and the adiabat is 1.34,achieving the low-entropy implosion.In this dissertation,experimental results closely related to the double cone ignition scheme are obtained,which provide references for the next steps in shaping laser pulse optimization,reference target design and calibration of numerical simulation codes.In addition,the multiple shock generation and kinetic behaviour under direct drive with single ramp pulse and 3-picket pulse can be extended to the study of high-pressure phase transitions,high-pressure equation of state and other applications.