磁场对激光驱动Rayleigh-Taylor不稳定性影响的数值研究

Numerical study of effect of magnetic field on laser-driven Rayleigh-Taylor instability

Rayleigh-Taylor不稳定性(RTI)作为流体和等离子体中基础的物理现象,在天体物理、空间物理以及工程领域扮演着重要角色.尤其在惯性约束核聚变(ICF)研究中,RTI等宏观流体不稳定性是不可回避的物理问题.本文利用开源的辐射磁流体模拟程序FLASH对激光驱动调制靶产生的RTI进行了二维的数值模拟,系统地考察和比较了RTI在无磁场、Biermann自生磁场、不同外加磁场情况下的演化.模拟结果表明,Biermann自生磁场和平行流向的外加磁场在RTI演化过程中基本不会改变RTI的界面动力学,而垂直流向的外加磁场对RTI以及RTI尖钉尾部的Kelvin-Helmholtz涡旋有致稳作用,其中磁压力起主导作用.研究结果为后续开展和ICF相关的靶物理研究提供借鉴,也有助于加深对流体混合过程的理解.

Rayleigh-Taylor instability(RTI) is a fundamental physical phenomenon in fluids and plasmas,and plays a significant role in astrophysics,space physics,and engineering.Especially in inertial confinement fusion(ICF)research,numerous experimental and simulation results have identified RTI as one of the most significant barriers to achieving fusion.Understanding the origin and development of RTI will be conducive to formulating mitigation measures to curb the growth of instability,thereby improving the odds of ICF success.Although there have existed many theoretical and experimental studies of RTI under high energy density,there are few experiments to systematically explore the influence of magnetic fields on the evolution of magnetized RTI.Here,a new experimental scheme is proposed based on the Shenguang-II laser facility on which the nanosecond laser beams are used to drive modulation targets of polystyrene(CH) and low-density foam layers.A shock wave is generated after the laser’s CH modulation layer has been ablated,and propagates through CH to low-density foam.Moreover,Richtmyer-Meshkov instability is triggered off when the shock wave accelerates the target.When the laser pulse ends,the shock wave evolves into a blast wave,causing the system to decelerate,resulting in RTI in the reference system of the interface.In this paper the open-source radiation MHD simulation code(FLASH) is used to simulate the RTI generated by a laser-driven modulation target.The evolution of RTI under no magnetic field,under Biermann self-generated magnetic field,and under different applied magnetic fields are systematically investigated and compared with each other.The simulation results show that the Biermann self-generated magnetic field and the applied magnetic field parallel to flow direction do not change the interface dynamics in the evolution process of RTI.Nevertheless,the applied magnetic field perpendicular to flow direction can stabilize RTI and the Kelvin-Helmholtz vortex at the tail of the RTI spike.Magnetic pressure plays a decisive role.The present results provide a reference for the follow-up study of target physics related to ICF and deepen the understanding of the fluid mixing process.