摘要
质子背光成像技术是一种诊断等离子体电磁场的重要诊断手段.当质子穿过等离子体的电磁场,质子受洛伦兹力影响在成像板上重新分布.如何从质子成像结果中重构电磁场是一个非常重要的研究课题.本文以激光驱动电容线圈靶为例介绍和对比了粒子追踪法和流量分析法这两种通过质子成像结果重构磁场的方法.激光驱动电容线圈靶通过激光打靶在电容靶两侧产生电势,然后很强的电流流过线圈,最后产生高达千特斯拉的感应磁场,在激光等离子体实验中这是一种重要的产生磁场的手段.本工作中先使用粒子追踪法在不同强度的理论磁场环境下得到质子成像结果,然后使用流量分析法从这些理论质子成像结果重构磁场,最后对比理论磁场和重构磁场以获得两种方法的优缺点.粒子追踪法可以重现实验中质子源、等离子体磁场和成像板的布局结构,但是依赖于精确的理论磁场的计算和庞大的计算量来模拟质子的轨迹,并需要不断修正理论磁场来获得最接近实验结果的模拟结果.流量分析法可以直接从实验的质子成像结果重构磁场结构.但是,流量分析法只适用于磁场较小的情况,当磁场较大时其重构的磁感应强度会误差较大.可以使用一个无量纲参量μ来衡量质子穿过作用区域单位长度后在成像板上的偏折距离,流量分析法适用于μ<<1的情况.并且靶的结构在质子成像上形成的阴影区域也会造成磁场重构时磁场结构的失真.
Proton radiography is a widely used method to diagnose the electromagnetic field of plasma.When protons pass through the electromagnetic field of plasma,they are deflected by Lorentz force and redistributed on the recorder.How to reconstruct electromagnetic field from the experimental result is an open problem.In this paper,we take the laser-driven capacitor-coil target for example to introduce and compare particle tracing and flux analysis,which are two widely used methods in proton radiography experiment to reconstruct the magnetic field.The capacitor-coil target is an important method to generate strong magnetic field in laser plasma experiment,where the strong current flows in the coil and its producing magnetic field may be larger than kilotesla.Firstly,the theoretical magnetic fields of capacitor-coil target are calculated with current being 10 kA and 50 kA.Secondly,the Geant4 is used to simulate the proton radiographs,where protons with 7.5 MeV pass through the target and the theoretical magnetic field is recorded.Thirdly,the theoretical proton radiographs are analyzed by the flux analysis method,and two magnetic fields are reconstructed.Finally,the theoretical magnetic fields are compared with the reconstructed ones,and the advantages and disadvantages of these two methods are analyzed.Particle tracing rebuilds the geometry distribution of proton source,plasma magnetic field and recorder in experiment,and it needs few assumptions.However,it strongly relies on accurate calculation of theoretical magnetic field and proton trajectory,and it requires to change the magnetic field over and over to achieve a closest result to the experimental proton radiograph.Meanwhile,particle tracing method consumes a lot of computation sources.The flux analysis directly reconstructs the magnetic field from experimental proton radiograph.However,it is only applicable to the case of weak magnetic field,and the error becomes larger for the case of stronger magnetic field.A dimensionless parameterμis used to estimate the deflection of proton in the magnetic field,which measures the amount of deflection per unit length in the interaction region.The flux analysis method is applicable to the μ≪1 regime.Additionally,the target may absorb the proton when the energy of proton is low and produces shadow on the proton radiograph,which leads to some difference between the original magnetic field and the reconstructed result.
作者
韩波
梁雅琼
Han Bo;Liang Ya-Qiong(Department of Astronomy,Beijing Normal University,Beijing 100875,China;School of Logistic,Beijing Wuzi University,Beijing 101149,China)
出处
《物理学报》
SCIE
EI
CAS
CSCD
北大核心
2020年第17期158-166,共9页
Acta Physica Sinica
基金
国家自然科学基金(批准号:11903006,U1930108)
北京市自然科学基金(批准号:1172006)资助的课题~~
关键词
质子背光成像
激光等离子体
磁场产生
诊断技术
proton radiography
laser plasma
generation of magnetic field
diagnostic technique
作者简介
通信作者:梁雅琼,E-mail:yaqliang@hotmail.com。
