Simulation study on gas flow in curved capillary used in laser wakefield acceleration
Authors: Zhao Yue-Qi , Cui Pei-Lin , Li Jian-Long , Li Bo-Yuan , Zhu Xin-Zhe , Chen Min , Liu Zhen-Yu ,   School of Mechanical Engineering, Shanghai Jiao Tong University, Shanghai 200240, China,   School of Physics and Astronomy, Shanghai Jiao Tong University, Shanghai 200240, China
Published: July 2023

<sec>Based on the standard <i>k</i>-<i>ε</i> 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 gradually changing curvature is numerically simulated. Compared with other methods of 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 designing 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.</sec><sec>1) Comparing with the single-sided straight-bent capillary structure, the gas density fluctuation between the left gas inlet and the right gas inlet 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.</sec><sec>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 a more uniform plasma density distribution with different lengths can be obtained by controlling the position of the gas inlet.</sec><sec>3) In the “traight + 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 diameters of the electron injection channel, 100 μm and 150 μm are more suitable for the application in the “direct + bend” cascade acceleration capillary structure design.</sec><sec>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 diameters, 100 μm and 150 μm, are 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.</sec>