The goal of the interdisciplinary seminar series Computation & Data at HSU is to bring together researchers and foster exchange on the development of algorithms, methods and software. The seminar series is typically scheduled for the last Wednesday every month, 16:00-17:00, with 1 presentation per hybrid session (digital and at HSU). Immediately after the seminar series, the HPC Café take place.
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X-ray free-electron lasers (XFELs), operated at DESY, generate extremely intense and ultrashort X-ray pulses that enable imaging of matter at atomic and nanometer length scales. One important application of XFELs is single-particle diffractive imaging (SPI), a technique that aims to determine the three-dimensional structure of individual nanoparticles, viruses, and biomolecules without the need for crystallization. In SPI experiments, particles are injected into the XFEL beam in free flight, and diffraction patterns are recorded before X-ray pulse destroys the sample, a concept known as diffraction before destruction.
A central challenge in SPI is sample delivery: The injector must provide a dense, well-collimated beam of isolated particles with minimal background gas load. Achieving this is non-trivial because the carrier-gas flow inside injector systems spans a wide range of regimes, from continuum flow at high pressure to highly rarefied conditions near the interaction region. Accurate modeling of such systems therefore requires methods that go beyond classical computational fluid dynamics. In this talk, a multiscale simulation framework for nanoparticle injector design is presented, focusing on aerodynamic lens systems (ALS) and their combination with cryogenically cooled buffer-gas cells (BGC). The approach couples continuum Computational Fluid Dynamics (CFD) methods with the Direct Simulation Monte Carlo (DSMC) method, allowing molecular-scale effects to be resolved only where necessary while retaining computational efficiency elsewhere. The coupled simulation methodology is installed on HSUper and most of the simulations are carried out on this HPC cluster.
Special attention is given to particle-gas interaction modeling: Several drag-force formulations are evaluated, and a relaxation-based correction is introduced to improve particle-trajectory predictions in transitional and rarefied flow regimes. The framework is validated against experimental measurements from ALS, BGC, and combined BGC-ALS injectors operated at XFEL facilities. Results show that the hybrid CFD-DSMC approach significantly improves agreement with experiments compared to conventional CFD. In particular, cryogenic BGC-ALS configurations enhance the focusing of protein-sized nanoparticles by reducing thermal velocities and Brownian motion, leading to improved beam collimation and injector performance. Overall, this work demonstrates how multiscale simulations can guide the design and optimization of injector systems for XFEL-based SPI, enabling higher hit rates and more reliable structural imaging of nanoscale objects.
