Optimizing the geometry of aerodynamic lens injectors for single-particle coherent diffractive imaging of gold nanoparticles

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.