喷嘴曲率对用于Laser -等离子体加速的超音速气体射流的影响
Effect of Nozzle Curvature on Supersonic Gas Jets Used in Laser-Plasma Acceleration
摘要：
缩放喷管产生的超音速气体射流是激光等离子体加速实验中常用的靶。这些目标的一个主要兴趣点是相互作用区域的气体密度，在该区域中，激光电离气体羽流以产生等离子体，从而提供加速结构。在这个相互作用区域调整密度分布是LPA优化的关键。在该相互作用线处期望“平顶”密度分布以控制激光传播和高能电子加速，而短的高密度分布通常优选用于加速较低能量的紧密聚焦的激光-等离子体相互作用。一个特殊的设计参数是喷嘴的扩张部分的曲率。我们研究了三种不同曲率的喷嘴设计：凹“钟”，直锥形和凸“喇叭”喷嘴。我们证明，在毫米级的距离从喷嘴出口，喇叭和直喷嘴，如果优化，产生“平顶”密度分布，而钟形喷嘴创建具有较高密度的气体的集中区域。导出了喇叭形喷管的优化方法，并与直喷管的优化方法进行了比较。我们发现，喇叭形喷嘴，通过其曲率提供一个额外的控制参数，是更通用的创建平顶轮廓和其优化过程是更精细的直喷嘴和直喷嘴优化过程相比。我们提出了不同的喷嘴设计的计算流体动力学（CFD）与程序ANSYS Fluent进行模拟的结果，并使用中性密度干涉法进行实验验证。
英文摘要：
Supersonic gas jets produced by converging-diverging (C-D) nozzles are commonly used as targets for laser-plasma acceleration (LPA) experiments. A major point of interest for these targets is the gas density at the region of interaction where the laser ionizes the gas plume to create a plasma, providing the acceleration structure. Tuning the density profiles at this interaction region is crucial to LPA optimization. A "flat-top" density profile is desired at this line of interaction to control laser propagation and high energy electron acceleration, while a short high-density profile is often preferred for acceleration of lower-energy tightly-focused laser-plasma interactions. A particular design parameter of interest is the curvature of the nozzle's diverging section. We examine three nozzle designs with different curvatures: the concave "bell", straight conical and convex "trumpet" nozzles. We demonstrate that, at mm-scale distances from the nozzle exit, the trumpet and straight nozzles, if optimized, produce "flat-top" density profiles whereas the bell nozzle creates focused regions of gas with higher densities. An optimization procedure for the trumpet nozzle is derived and compared to the straight nozzle optimization process. We find that the trumpet nozzle, by providing an extra parameter of control through its curvature, is more versatile for creating flat-top profiles and its optimization procedure is more refined compared to the straight nozzle and the straight nozzle optimization process. We present results for different nozzle designs from computational fluid dynamics (CFD) simulations performed with the program ANSYS Fluent and verify them experimentally using neutral density interferometry.
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