| Magnetic fields and plasmas prevail in the known universe, and the interactions between them can dominate many processes in Nature and laboratories.. In particular, the interaction between high energy density plasmas and strong magnetic fields plays an important role in the research of astrophysics and controlled nuclear fusion. In the thesis, we are concerned about the interaction between the laser-produced plasma and magnetic field.In hydrodynamics description, the plasma evolution is affected by magnetic field in two ways:direct effect due to the Lorentz force and indirect effect due to the transport coefficients modified by the magnetic field. Therefore, we concentrate the influence of magnetic field on the global and local characteristics of plasma dynamics and the plasma energy transport coefficients.We design and build up a pulsed magnetic field generator that can be synchronized with our present laser system. The jittering between the generator and the laser system is smaller than 10 ns, which is achieved with a laser-induced discharge switch. With this device, we can generate quasi-uniform and quasi-steady magnetic field with a strength of 7 Tesla and a volume of several mm3.In our experiment, a heating laser pulse is focused on a solid or gas target to generate a plasma surrounding with a magnetic field. Various diagnostic methods are applied to diagnose plasma parameters and to explore the evolution of plasma structures. The diagnostics include optical imaging of plasma self-luminescence, optical interferometry, optical spectrum and Thomason scattering, etc.The analysis of plasma dynamic evolution in external magnetic field is divided into global and local perspective. The result shows a spatial separation of the plasmas in the presence of magnetic field. Part of the plasmas has a high flow velocity (107cm/s), which we call high-flow-velocity-plasma (HVP). The other part with a low flow velocity (105～6cm/s) is called low-flow-velocity-plasma (LVP). We find that the structure difference between the two parts of the plasmas is due to the competition between the effect of magnetic freezing and magnetic diffusion:(a) when the magnetic Reynolds number of HVP is larger than 1, the magnetic freezing effect plays a leading role, and HVP pushes the magnetic lines and creates shock envelop and directed flow; (b) when magnetic Reynolds number of LVP is smaller than 1, the magnetic diffusion effect plays the leading role, and LVP moves along the structure of magnetic field created by the HVP. It is worth pointing out that an up-down asymmetry is founded in HVP, which has never been mentioned in previous literatures. We conclude that this asymmetry is due to the charge-separation field and the Hall field at the boundary of the plasmas.In our experiment, instabilities can occur and shock structures can appear when HVP penetrate into external magnetic fields. It is found that the instability of HVP should be ascribed to the interchange instability in the condition of large Larmor radius. The shock structure of HVP should be due to the high flow velocity of the plasmas in presence of strong magnetic fields. There are evidences that LVP has a hollow structure, which could be due to the diffusion of the magnetic field.Gas target is used to study the influence of external magnetic field on plasma energy transport coefficients. The evolution of plasma temperatures with or without external magnetic field is diagnosed by Thomason scattering. It is found that with the external magnetic field the plasma electron temperature is generally 30%～50% higher than the one without magnetic field. As the plasma cools down, its cooling rate is obviously slower in the external magnetic condition. The cooling rate from the experimental data is much larger in comparison with the simulation results, which can be ascribed to the nonlocal thermal conductivity. In addition, we also notice that in our experiment conditions the Joule heating of the magnetic field during the laser heating progress could not be neglected. |
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