With the development of laser technology, laser plasma research has become a hot spot in recent years, and ultrafast diagnostics with time resolution on the order of nanoseconds or even picoseconds is a pivotal research technique in laser plasma research. On the one hand, the technique serves scientific frontier issues, such as obtaining important data in laboratory astrophysics research, which can be applied to design astrophysics experiments. On the other hand, the technique is oriented to the major national needs, focusing on inertial confinement laser fusion to deepen the understanding of important physical processes in fusion physics. Laboratory astrophysics and inertial confinement laser fusion plasma evolution is on the order of nanoseconds or even picoseconds, and the use of ultrafast diagnostics to probe the physical parameters of the plasma is indispensable for in-depth study of plasma endowment physics.
The laboratory simulation of the interaction between the solar wind and the Earth's magnetosphere is a very mainstream research direction in laboratory astrophysics. We have successfully simulated Earth's bow shock wave by using laser ablation of a solenoid to generate a dipole magnetic field on the scale of Tesla, and by laser ablation of a hydrogen film to simulate solar wind plasma. Ultrafast diagnostic equipment such as optical imaging and B-dot were used to accurately measure and analyze the plasma and magnetic field in the experiment. Our observations reveal that plasma accumulates continuously, forming structures akin to bow shock and magnetopause due to magnetic field obstructions. We delve into the influence of dynamic pressure and magnetic pressure on the tension angle, position, and configuration of shock waves. Furthermore, we conduct the scaling transformation of experimental parameters to validate and enhance the reliability of our simulation experiments. These experimental results are corroborated using MHD simulation programs.
Due to the time-varying nature of solenoid magnetic fields, accurately analyzing their characteristics poses significant challenges. Consequently, we researched the effect of the permanent magnet with low intensity on the shock wave. By using ultrafast optics, magnetic field and X-ray diagnostic techniques, the process of plasma shock wave formation under magnetic field is analyzed comprehensively. The influence of magnetic and non-magnetic obstacles on shock wave formation is compared, and the evolution of plasma and magnetic field configuration is discussed.
We devised a special experiment to explore the magnetic reconnection at the polar tip of the Earth magnetosphere and the bow shock by using a double capacitor coil target. Through optical imaging techniques, we observed a distinct bow shock formation and the convergence of plasma towards the magnetic null point at the polar cusp, thereby validating the astronomical Dungey model. Our analysis revealed that as the distance between the coils increases, the separation between the magnetic nulls and the angle of the bow shock also exhibit an upward trend. Moreover, we detected Kelvin-Helmholtz instability near the coil, indicating a potential connection to plasma dynamics at the magnetopause boundary.
Double Cone Ignition (DCI) represents a novel fusion scheme that has gained prominence in recent years due to its ability to achieve higher energy conversion efficiency with lower laser energy input. In this study, we employ an X-ray streak camera to scrutinize the plasma dynamics in DCI fusion experiments. The results show that a high density plasma with uniform density distribution can be formed at the stagnation time after accurate collision between the high density plasma jet after compression and acceleration in DCI experiment. A high speed implosive plasma jet with a velocity of about 220km/s can be formed by a nanosecond laser with a total energy of 10kJ. Following collision, there was a notable increase in X-ray self-emission from the plasma, indicating a significant rise in plasma density and temperature during the collision phase. We compare the dynamics processes of plasma produced by different ablation materials and find that although there are minor differences in coasting time and other parameters, overall the implosion dynamics processes of plasma from different materials is very similar. The research is crucial for the subsequent fast high-energy electron beam heating process of the DCI.
Utilizing an ultrafast X-ray streak camera, we investigated the impact of various laser waveforms on plasma dynamics in DCI experiments. The experimental results show that the introduction of pre-pulse before the main pulse can reduce the preheating of the spherical shell during the shock wave compression process, and the change of the main pulse to the oblique pulse can increase the ablation depth and the length of the electron heat conduction zone, and reduce the requirement on the compression laser. Therefore, the optimized laser waveform significantly increases the plasma ejection speed from the golden cone mouth under the same laser energy. The plasma temperature and density increase significantly after the collision. This study provides an important reference for laser waveform optimization of DCI fusion schemes.
We successfully captured the collision plasma in DCI experiments using an eight-pinhole array and X-ray streak camera, forming the ultrafast two-dimensional X-ray imaging system (MIXS). With this setup, we obtained the ultrafast two-dimensional X-ray evolution process of the collision plasma, achieving a time resolution of 80 ps and a spatial resolution of 42 μm. However, we observed non-uniformities in the plasma collision process due to disparities in laser energy and irradiation. Despite these achievements, limitations persisted. The short slit of the X-ray streak camera, coupled with constraints on the time window and an unstable aiming scheme, impacted the time and spatial resolution of the MIXS system. Additionally, some deviations in aiming were noted. Crucially, the absence of a precise time reference made it challenging to ascertain the exact commencement and conclusion of the laser pulse.
Based on the shortcomings of the above MIXS system, we upgraded and optimized it, changed longer slit and faster time window for the X-ray streak camera, corrected the nonlinear sweeping speed of the X-ray streak camera by adding the temporal fiducial pulses and determined the accurate arrival time of the laser. The MIXS system was verified by time calibration using the temporal fiducial pulses for the first time in China. At the same time, the aiming scheme was optimized, and a periscope system with easy operation and high stability was designed. After optimization of various measures, we designed the ultrafast two-dimensional X-ray imaging system (MIXS-F) with time resolution of 38ps and space resolution of 18µm. The system has been successfully verified on the Shenguang-II laser facility to confirm its accuracy and effectiveness, which provides an important diagnostic tool for the observation of complex physical phenomena such as the implosion process of laser fusion.
In conclusion, this paper thoroughly investigates the application of ultrafast diagnosis in two laser plasma fields: DCI and laboratory astrophysics. It encompasses a detailed analysis of physical theories alongside the development and optimization of ultrafast diagnosis techniques. Furthermore, the experimental results and analytical conclusions are meticulously verified and compared using MHD numerical simulation programs like FLASH. These work provides more references for the application of ultrafast diagnostics in the field of laser plasma, helps to advance the development of ultrafast diagnostics, improves our understanding of the physical processes involved in the implosion phase of biconical collision ignition, and supports and validates the astronomical study of the interaction between the solar wind and the Earth's magnetosphere.
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