间接驱动内爆靶丸示踪元素Ar发射X光谱线的理论模拟研究∗

摘要

惯性约束聚变内爆物理研究中，示踪元素X光谱线诊断方法是推测内爆压缩温度、密度以及燃料混合的有效办法，因此，对示踪元素X光发射的规律及其与内爆过程的关系的研究非常必要，有助于通过谱线发射特征诊断内爆状态。以SGIII原型装置的实验条件下的内爆过程为例，对内爆靶丸示踪元素Ar发射X光谱线进行了理论模拟。研究了谱线自吸收效应、Ar掺杂浓度、等离子体空间分布不均匀等对Ar发射的X光谱线分布的影响。还对Ar发射X光谱线强度的时间演化及其与内爆过程的关系进行了研究。结果表明，增加掺杂浓度，谱线强度增强，但是谱线自吸收效应的影响也明显增强。示踪元素Ar发射的X光谱线强度的峰值时刻与中子产生速率的峰值时刻接近(前者延迟约15 ps)。高温、高密度及合适的电离度是谱线发射的3个条件，在X光谱线发射的峰值时刻，由于燃料芯部Ar等离子体过电离， Ar等离子体发射的X光谱线的空间峰值区域靠近燃料边界区域，占燃料总体积56%的薄壳(厚度∼4µm)，其发射的X光谱线强度约为总强度的72%。因此，对发射谱线分布拟合得到的空间平均的等离子体温度、密度主要反映这一区域的等离子体状态。%As the X-ray spectrum of tracer in inertial confinement fusion implosion target is usually used to infer electron temperature, density, and the mixture of fuel and shell, it is necessary to study the relation between the characteristics of X-ray emission spectrum and the implosion process, which is helpful for inferring the implosion status. Under the condition of SGIII prototype, approximately 0.5% atomic percent of Ar atoms are doped in an indirectly driven implosion target, X-ray spectrum of Ar is numerically simulated. In this article, the influences of line re-absorption effect, tracer concentration, and profile of fuel plasma state on the emission spectrum are studied. The relation between the temporal evolution of the emission spectrum and the implosion process is also investigated. It is found that as the tracer concentration increases up to ∼ 1%, the X-ray intensity is enhanced, but the influence of line re-absorption becomes severe. Temporal evolution shows that the peak time of Ar X-ray intensity is almost the same as that of neutron production (the former delays about 15 ps, which usually cannot be resolved). As is well known, the strong line emission occurs in the plasma with high temperature, high electron density, and proper ionization. The detailed analysis shows that at the peak emission time, as the core Ar plasma is over ionized, the main X-ray line emission region is located near the boundary region of the fuel, and this thin shell, whose thickness is about 4 µm and whose volume accounts for 56%of the total fuel plasma volume, emits the X-ray whose intensity is about 72%of the total line intensity. Therefore, the space-averaged plasma temperature and density, which are obtained by fitting the emission spectrum, mainly reflect the plasma state in this region.