纳秒激光驱动非相对论无碰撞静电冲击波反射离子能谱测量的Geant4模拟

间接驱动惯性约束聚变真空或者近真空黑腔实验中，纳秒激光烧蚀产生的腔壁等离子体可以在靶丸烧蚀等离子体（或低密度填充气体）中驱动无碰撞静电冲击波，冲击波电场会以二倍冲击波速度反射离子。为了测量纳秒激光驱动非相对论无碰撞静电冲击波产生的10 keV量级的反射离子能谱，设计了低能汤姆逊离子谱仪。利用Geant4建模，对离子测量过程进行了全过程蒙特卡罗模拟，用以评估靶室残余气体和喷气气体对低能离子测量的影响。模拟结果显示，靶室残余气体会造成10 keV量级D离子信号在谱仪电场和磁场方向展宽。电场方向的展宽会增加不同荷质比离子谱线发生交叠的风险，而磁场方向的展宽会导致离子能谱展宽。喷气气体会造成离子信号向低能区移动并拖尾，导致测量的离子谱偏离真实的反射离子能谱。

Geant4 simulations of measurement of energy spectra of reflected ions generated by nanosecond-laser-drive non-relativistic collisionless electrostatic shocks

In indirect-drive inertial confinement fusion experiments with vacuum or low-gas-fill hohlraums, collisionless electrostatic shocks can be launched in the hohlraum wall/alblator (or the low-density fill-gas) interpenetration region, which reflect ions at twice the shock velocity. A low-energy Thomson ion spectrometer was designed to measure the energy spectra of the reflected ions on the order of 10 keV generated by nanosecond-laser-driven non-relativistic collisionless electrostatic shocks. Monte Carlo simulations of ion measurement were carried out with Geant4 modeling to evaluate the influence of residual gas in the vacuum chamber and gas jet on the measurement of the low-energy ions. Simulation results show that the residual gas in the vacuum chamber causes the signal of D ions on the order of 10 keV to broaden in both the electric and magnetic deflection of the spectrometer. The broadening of the electric deflection will increase the risk of overlapping of ion spectral lines of different charge-to-mass ratios, while the broadening of the magnetic deflection will lead to the broadening of the energy spectra of the ions. The gas jet causes the ion signal to move and tail into the lower energy region, causing the measured ion spectra to deviate from the actual energy spectra of the reflected ions.