Effect of Nozzle Curvature on Supersonic Gas Jets Used in Laser-Plasma Acceleration

TL;DR

This study investigates how the curvature of nozzles affects the gas density profiles in supersonic jets used for laser-plasma acceleration, demonstrating that curved nozzles can produce more versatile and optimized density profiles.

Contribution

The paper introduces a comparative analysis of different nozzle curvatures, highlighting the advantages of trumpet nozzles in achieving flat-top density profiles for laser-plasma applications.

Findings

Trumpet nozzles produce more versatile flat-top density profiles.

Optimization of trumpet nozzles is more refined and effective.

CFD simulations and experiments confirm the impact of nozzle curvature.

Abstract

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.