报告题目：Generation of Astrophysical Magnetized Collisionless Shock in Laser-Produced Plasmas

　　报告人：Zhenyu Wang (Princeton University)

　　报告摘要：Magnetized collisionless shocks commonly occur in the heliosphere and interstellar medium, and have recently become the subject of laboratory investigations at high energy density (HED) facilities. We describe a campaign of laser experiments designed to generate high Mach number magnetized collisionless shocks on OMEGA-EP facility. In the experiment, a laser-produced high-velocity plasma collides with a magnetized, pre-ablated plasma. Proton radiography shows a moving region of proton deficit followed by a sharp enhancement of proton density. These features are produced by gradients in the propagating compressed magnetic field. We use a particle tracing code and analytical arguments to model the proton radiography signal and determine the speed of the compressed magnetic field and put constraints on the compression ratio in the experiment. We compare the data to the results of PIC simulations of plasma collision in realistic geometry, and describe the signatures of the formation of magnetized shocks detected in the laboratory, including the early stage electrostatic-dominated transition, and a later stage magnetic reflection with the formation of magnetic overshoots. We point out the importance of the establishment of the contact discontinuity between the driver and background flow, and its dependence on the magnetization of the background plasma. We explain the geometrical effects on the radiography introduced by density gradients in expanding plasma and by the curvature of the imposed magnetic field from Helmholtz coils. We conclude that our experiments have reproducibly achieved magnetized shocks with Alfvenic Mach number 3 to 9 in laboratory conditions. This experiment creates a platform for further study of physical processes in collisionless magnetized shocks.

　　报告时间：2018年9月26日，星期三, 上午10:00

　　报告地点：紫金山天文台仙林园区2号楼512室

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