A focused laser can cause local optical breakdown of a gas, which leads to rapid deposition of energy into a high-temperature plasma kernel that expands and induces a complex flow. For some conditions, hot gas is rapidly ejected along the laser axis up to distances several times the kernel size, with a particularly curious feature: relatively small changes in, for example, initial pressure can cause the direction of this ejection to reverse. Detailed axisymmetric simulations of a model energy kernel in an inert gas provide a hydrodynamic description of this phenomenon, reproducing key observations in corresponding experiments, including the vortex-ring-like features that constitute the ejection. These simulations are analysed to show how changes in the early-time kernel can lead to ejection or its reversal via alteration in the relative strength and position of the vorticity produced. A corresponding semi-infinite geometry is used to isolate two mechanisms: vorticity production by the generated shock and by baroclinic torque at the kernel boundary. Dependence on the initial kernel asymmetry is quantified, as it ultimately determines whether the vorticity, upon its subsequent evolution, develops into the ring-like structure that ejects. Even simple elongation of the energy kernel alone can reverse the direction.

中文翻译：

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由激光诱导的光学击穿引起的流体动力学喷射

聚焦激光会导致气体局部光击穿，从而导致能量快速沉积到高温等离子体内核中，该内核膨胀并引发复杂的流动。在某些情况下，热气体会沿着激光轴快速喷射到几倍于内核大小的距离，具有一个特别奇怪的特征：例如，初始压力的相对较小的变化会导致喷射方向反转。惰性气体中模型能量核的详细轴对称模拟提供了这种现象的流体动力学描述，重现了相应实验中的关键观察结果，包括构成喷射的涡环状特征。分析这些模拟以显示早期内核的变化如何通过改变产生的涡流的相对强度和位置而导致喷射或逆转。相应的半无限几何用于隔离两种机制：由产生的冲击和内核边界处的斜压扭矩产生的涡量。对初始内核不对称性的依赖是量化的，因为它最终决定了涡度在其随后的演变中是否会发展成喷射出的环状结构。即使是能量内核的简单伸长也可以反转方向。对初始内核不对称性的依赖是量化的，因为它最终决定了涡度在其随后的演变中是否会发展成喷射出的环状结构。即使是能量内核的简单伸长也可以反转方向。对初始内核不对称性的依赖是量化的，因为它最终决定了涡度在其随后的演变中是否会发展成喷射出的环状结构。即使是能量内核的简单伸长也可以逆转方向。