磁化套筒惯性聚变中轴向磁场演化特征与Nernst效应影响

Evolution characteristic of axial magnetic field and Nernst effect in magnetized liner inertial fusion

轴向磁场是磁化套筒惯性聚变(magnetized liner inertial fusion,MagLIF)有别于其他惯性约束聚变构型的主要标志之一.本文在建立集成化物理模型并编写一维模拟程序的基础上,通过对ZR装置驱动能力下典型MagLIF负载参数的模拟,系统研究并获得MagLIF各个阶段轴向磁场演化与分布特征,发现预加热引起的压力不平衡导致燃料中磁通保有量并未呈现随时间单调递减的关系,而是反复震荡甚至出现局部短时间内反而增加的演化曲线.通过在磁场演化方程中引入控制项来讨论Nernst效应的影响,计算结果表明随着初始磁场强度降低(30,20,10 T),Nernst效应越发明显,磁通损失增大(28%,44%,73%),a粒子能量沉积比例则大幅降低(44%,27%,4%),因此初始磁场强度不宜太低;预加热结束后应使燃料中温度径向分布尽量均匀、平缓,有助于减少Nernst效应的影响.所取得的研究结果有助于加深对MagLIF中磁通压缩和磁扩散过程的物理图像认知和理解,对未来实验负载参数设计也有重要的指导作用.

Axial magnetic field is one of the main parameters of magnetized liner inertial fusion(MagLIF),which is greatly different from other traditional inertial confinement fusion configurations.The introduce of axial magnetic field dramatically increases energy deposit efficiency of alpha particles,when initial Bz increases from 0 to 30 T,the ratio of deposited alpha energy rises from 7% to 53%.In the MagLIF process,the evolvement of magnetic flux in fuel can be roughly divided into three main stages:undisturbed,oscillation,and equilibrium.The distributions and evolution characteristic of axial magnetic field are both determined by the liner conductivity,fuel conductivity,and the fluid dynamics.The pressure imbalance between fuel and liner,caused by laser injection,is the source of fluid oscillation,which is an intrinsic disadvantage of laser preheating method.This fluid oscillation does not lead the magnetic flux to decrease monotonically in the fuel during implosion process,but oscillate repeatedly,even increase in a short time.Nernst effect plays a negative role in MagLIF process.As initial axial magnetic field decreases from 30 to 20 to 10 T,the Nernst effect causes magnetic flux loss to increase from 28% to 44% to 73% correspondingly,and the deposited alpha energy ratio drops from 44% to 27% to 4% respectively.So the initial magnetic field is supposed to be moderately high.The radial distribution of temperature in fuel should be as uniform as possible after preheating,which is helpful in reducing the influence of Nernst effect.Compared with Nernst effect,the end loss effect is much responsible for rapid drawdown of fusion yield.A large number of physical images are acquired and summarized through this work,which are helpful in understanding the process of magnetic flux compression and diffusion in MagLIF process.The simulation can act as a powerful tool and the simulation results can serve as a useful guidance for the future experimental designs.