离子Vlasov-Fokker-Planck模拟中的初级中子能谱
Primary Neutron Spectra in Ion Vlasov-Fokker-Planck Simulations
AI总结：
本研究针对现有流体和PiC方法计算中子能谱的局限，提出在离子Vlasov-Fokker-Planck模拟中通过五维积分计算中子能谱的方法，实现于iFP代码，可更准确反映分布偏离麦克斯韦分布的影响，助力模拟与实验的基准比对。
摘要：
氘-氘与氘-氚聚变反应产生的未散射中子能谱是高能量密度物理实验中的重要诊断手段，因为该能谱对反应离子的速度十分敏感。目前已有在辐射流体动力学（“hydro”）模拟和粒子网格（“PiC”）模拟中计算这些能谱的方法。由于在实验室实验达到的能量下，反应截面随反应离子对的动能迅速增大，因此能谱对离子速度分布函数的高能尾部尤为敏感。这意味着流体方法和PiC方法在特定等离子体状态下可能都不适用：流体方法假设离子速度分布局部为麦克斯韦分布，而PiC方法受统计噪声影响，难以准确模拟能谱的更精细细节。本研究提出了一种互补方法：一种在离子Vlasov-Fokker-Planck模拟中计算中子能谱的方法，该模拟中速度分布函数是完全解析的。该方法在用于模拟激光驱动惯性约束聚变实验的球对称代码iFP中实现。该方法计算量较大，因为需要进行五维数值积分，但无需对分布函数或微分截面做近似。结果表明，离子分布函数偏离麦克斯韦分布会对冲击驱动惯性约束聚变内爆中的中子能谱产生显著影响。该方法应有助于实现模拟与实验更准确的基准比对。
英文摘要：
The energy spectra of unscattered neutrons produced by deuterium-deuterium and deuterium-tritium fusion reactions are an important diagnostic in High Energy Density Physics experiments as the spectra are sensitive to the velocities of reacting ions. Methods exist for calculating these spectra in radiation-hydrodynamic ("hydro") and Particle-in-Cell ("PiC") simulations. The spectra are particularly sensitive to the high energy tail of ion velocity distribution functions since reaction cross sections increase rapidly with the kinetic energy of a reacting ion pair at the energies achieved in laboratory experiments. This means both the hydro and PiC method may not be suitable in certain plasma regimes. The hydro method assumes that the ion velocity distribution is locally Maxwellian, while the PiC method is subject to statistical noise that makes it challenging to accurately simulate finer details of the spectra. In this work, we present a complementary approach: a method for calculating the neutron spectra in ion Vlasov-Fokker-Planck simulations in which the velocity distribution function is fully-resolved. The method is implemented in the spherically-symmetric code iFP which is used to simulate laser-driven Inertial Confinement Fusion experiments. The method is computationally intensive as it requires a five-dimensional numerical integral, but no approximations of the distribution functions or differential cross sections are required. Results show that deviations of the ion distribution functions from Maxwellian can have a noticeable effect on neutron spectra in shock-driven ICF implosions. The method should facilitate more accurate benchmarking of simulations and experiments.
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