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arXiv:2508.04364 · New Journal of Physics

Hydrodynamic effects in cryogenic buffer gas cells: design insights from hybrid simulations

Nick Vogeley, Bernd Bauerhenne, Daqing Wang

Published Nov 1, 2025• 0 citations

Abstract

Abstract Cryogenic buffer gas beam(CBGB) sources have become an essential tool for experiments requiring cold molecular beams with low forward velocities. Although recent experimental advances have led to significant progress in source optimization, numerical studies remain limited due to the challenges posed by the large parameter ranges required to describe both the dense buffer gas and the dilute seed molecules. In this work, we report a numerical evaluation of CBGB cells operating in the hydrodynamic extraction regime. While most prior studies focused on box-like or cylindrical cells, we investigated hydrodynamic effects including vortex formation in a dome-shaped cell and assessed whether these could be utilized to enhance the performance in molecule cooling and extraction. To achieve this, we performed steady-state slip-flow simulations for helium buffer gas and employed a direct-simulation Monte Carlo diffusion routine to track particle trajectories. We compared the performance of the source across different buffer gas throughputs and injection angles and identified parameter regimes where vortex formation enhances molecule extraction. From the simulations, we extracted experimental observables, which allow these effects to be verified through velocity or time-of-flight measurements on the molecular beam.

DOI: 10.1088/1367-2630/ae1ce6

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Employing a two-stage cryogenic buffer gas cell, we produce a cold, hydrodynamically extracted beam of calcium monohydride molecules with a near effusive velocity distribution. Beam dynamics, thermalization and slowing are studied using laser spectroscopy. The key to this hybrid, effusive-like beam source is a "slowing cell" placed immediately after a hydrodynamic, cryogenic source [Patterson et al., J. Chem. Phys., 2007, 126, 154307]. The resulting CaH beams are created in two regimes. In one regime, a modestly boosted beam has a forward velocity of v(f) = 65 m s(-1), a narrow velocity spread, and a flux of 10(9) molecules per pulse. In the other regime, our slowest beam has a forward velocity of v(f) = 40 m s(-1), a longitudinal temperature of 3.6 K, and a flux of 5 × 10(8) molecules per pulse.