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Effect of nozzle curvature on supersonic gas jets used in laser-plasma acceleration

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Supersonic gas jets produced by converging-diverging 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 the 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 for mm-scale axisymmetric nozzles that, at mm-scale distances from the nozzle exit, curvature significantly impacts shock formation and the resulting gas jet density field and, therefore, is an essential parameter in LPA gas jet design. We show that bell nozzles are able to produce focused regions of gas with higher densities. We find that the trumpet nozzle, similar to straight and bell nozzles, can produce flat-top profiles if optimized correctly and can produce flatter profiles at the cost of slightly wider edges. An optimization procedure for the trumpet nozzle is derived and compared to the straight nozzle optimization process. We present results for different nozzle designs from computational fluid dynamics simulations performed with the program ANSYS Fluent and verify them experimentally using neutral density interferometry.

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