No abstract

As a typical plasma-based optical element that can sustain ultra-high light intensity, plasma density grating driven by intense laser pulses have been extensively studied for wide applications. Here, we show that the plasma density grating driven by two intersecting driver laser pulses is not only nonuniform in space but also varies over time. Consequently, the probe laser pulse that passes through such a dynamic plasma density grating will be depolarized, i.e., its polarization becomes spatially and temporally variable. More importantly, the laser depolarization may spontaneously take place for crossed laser beams if their polarization angles are arranged properly. The laser depolarization by a dynamic plasma density grating may find the application in mitigating parametric instabilities in laser-driven inertial confinement fusion.

Based on the standard k-epsilon model, a gas flow calculation model in a curved capillary is established, and the flow process of helium working medium in a curved capillary with gradual curvature changes is numerically simulated. Compared with other methods for studying micro-scale gas flow, this simulation obtains the gas density distribution in the curved capillary more conveniently, and has the same variation trend as the experimental measurement of the plasma electron density distribution, and can predict the gas flow distribution in the tube more accurately. The situation provides a theoretical basis for the structure design of the discharge capillary experiment. Based on this model, the gas flow process in the capillary of the one-sided direct flushing, double-sided hedging and "straight + curved" cascade acceleration structures are numerically simulated. The results and conclusions are summarized as follows.(1) Compared with the single-sided straight-bent capillary structure, the gas density fluctuation between the left and right gas inlets of the double-sided hedging-bend capillary is smaller, the gas flow is more stable, and a relatively stable plasma density channel can be generated.(2) In the double-sided hedged curved capillary, a relatively uniform gas density distribution is formed between the two inlets of the capillary under the same inflation back pressure; further research results show that different lengths can be obtained by controlling the position of the gas inlet A more uniform plasma density distribution.(3) In the "straight + curved" cascaded accelerating capillary structure, the diameter of the electron injection channel will affect the gas density distribution in the bend. When the diameter of the electron injection channel is small, the absolute pressure in the capillary is low. The larger pressure difference between them will lead to a higher gas flow rate in the elbow, which will increase the fluctuation of the gas density in the elbow; the final research shows that the diameter of the electron injection channel of 100 µm and 150 µm is more suitable for the application in the "direct +bend" cascade acceleration capillary structure design.In summary, the calculation model of gas flow in the curved capillary constructed in this paper can accurately predict the gas flow distribution in the tube. The double-sided hedged curved capillary can generate a relatively stable plasma density channel, and the electron injection channel diameter of 100 µm and 150 µm is more suitable for application in the "straight + curved" cascade accelerating capillary structure design. The research results obtained are expected to provide theoretical guidance and technical support for the laser wake cascade acceleration experiment based on the curved capillary with gradually changing curvature.