  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.
