  Single-particle x-ray diffractive imaging (SPI) of small (bio-)nanoparticles
(NPs) requires optimized injectors to collect sufficient diffraction patterns
to reconstruct the NP structure with high resolution. Typically,
aerodynamic-lens-stack injectors are used for single NP injection. However,
current injectors were developed for larger NPs ($\gg\!100$ nm) and their
ability to generate high-density NP beams suffers with decreasing NP size.
Here, an aerodynamic-lens-stack injector with variable geometry and the
geometry-optimization procedure are presented. The optimization for 50 nm gold
NP (AuNP) injection using a numerical simulation infrastructure capable of
calculating the carrier gas flow and the particle trajectories through the
injector is introduced. The simulations are experimentally validated using
spherical AuNPs and sucrose NPs. In addition, the optimized injector is
compared to the standard-installation "Uppsala-injector" for AuNPs and results
for these heavy particles show a shift in the particle-beam focus position
rather than a change in beam size, which results in a lower gas background for
the optimized injector. Optimized aerodynamic-lens stack injectors will allow
to increase NP beam density, reduce the gas background, discover the limits of
current injectors, and contribute to structure determination of small NPs using
SPI.
