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arXiv:2011.11887 · Physical Review Research

Simulation of cryogenic buffer gas beams

Yuiki Takahashi, David Shlivko, Gabriel Woolls, Nicholas R. Hutzler

Published Apr 6, 2021• 14 citations
arXivHTMLPDF

Abstract

The cryogenic buffer gas beam (CBGB) is an important tool in the study of cold and ultracold molecules.While there are known techniques to enhance desired beam properties of a CBGB, such as high flux, low velocity, or reduced divergence, they have generally not undergone detailed numerical optimization.Numerical simulation of buffer gas beams is challenging, because the relevant dynamics occur in regions where the density varies by orders of magnitude, rendering typical numerical methods unreliable or intractable.Here we simulate CBGBs with a hybrid approach that combines gas dynamics methods with particle tracing.The simulations capture important properties, such as velocitiy and divergence, across an assortment of designs, including two-stage slowing cells and de Laval nozzles.This approach should therefore be a useful tool for optimizing CBGB designs across a wide range of applications.

DOI
10.1103/physrevresearch.3.023018

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The cryogenic buffer gas beam (CBGB) is an important tool in the study of cold and ultracold molecules.While there are known techniques to enhance desired beam properties of a CBGB, such as high flux, low velocity, or reduced divergence, they have generally not undergone detailed numerical optimization.Numerical simulation of buffer gas beams is challenging, because the relevant dynamics occur in regions where the density varies by orders of magnitude, rendering typical numerical methods unreliable or intractable.Here we simulate CBGBs with a hybrid approach that combines gas dynamics methods with particle tracing.The simulations capture important properties, such as velocitiy and divergence, across an assortment of designs, including two-stage slowing cells and de Laval nozzles.This approach should therefore be a useful tool for optimizing CBGB designs across a wide range of applications.
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.
Molecular magneto-optical traps (MOTs) are loaded from cryogenic buffer gas beam (CBGB) sources, which produce a large flux of cold molecules. However, only a small fraction of this flux is slowed and captured. Here, we investigate and optimize one of these CBGB sources through a combination of experiment and simulation to maximize the number of molecules loaded into a MOT of CaF molecules. We experimentally determine the range of molecule velocities from the source that are successfully slowed and captured in the MOT, and perform a systematic study of buffer gas source geometries to identify designs that maximize MOT loading. Our observations agree well with Monte Carlo simulations, which provide physical insight into the dynamics within the source and explain how source geometry influences the subset of molecules that are ultimately trapped. These results provide practical design guidelines for optimizing cryogenic buffer gas beam sources used in laser cooling experiments.
We study the properties of a cryogenic buffer gas source that uses a low-temperature two-stage buffer gas cell to produce very slow beams of ytterbium monofluoride molecules. The molecules are produced by laser ablation inside the cell and extracted into a beam by a flow of cold helium. We measure the flux and velocity distribution of the beam as a function of ablation energy, helium flow rate, cell temperature, and the size of the gap between the first and second stages of the cell. We also compare the velocity distributions from one-stage and two-stage cells. The one-stage cell emits a beam with a speed of about 82 m s − 1 and a translational temperature of 0.63 K. The slowest beams are obtained using the two-stage cell at the lowest achievable cell temperature of 1.8 K. This beam has a peak velocity of 56 m s − 1 and a flux of 9 × 10 9 ground-state molecules per steradian per pulse, with a substantial fraction at speeds below 40 m s − 1 . These slow molecules can be decelerated further by radiation pressure slowing and then captured in a magneto-optical trap. Published by the American Physical Society 2024
Cryogenic buffer gas cells have been a workhorse for the cooling of molecules in the last few decades. The straightforward sympathetic cooling principle makes them applicable to a huge variety of different species. Notwithstanding this success, detailed simulations of buffer gas cells are rare, and have never been compared to experimental data in the regime of low to intermediate buffer gas densities. Here, we present a numerical approach based on a trajectory analysis, with molecules performing a random walk in the cell due to collisions with a homogeneous buffer gas. This method can reproduce experimental flux and velocity distributions of molecules emerging from the buffer gas cell for varying buffer gas densities. This includes the strong decrease in molecule output from the cell for increasing buffer gas density and the so-called boosting effect, when molecules are accelerated by buffer-gas atoms after leaving the cell. The simulations provide various insights which could substantially improve buffer-gas cell design.
We demonstrate and characterize a high-flux beam source for cold, slow atoms or molecules. The desired species is vaporized using laser ablation, then cooled by thermalization in a cryogenic cell of buffer gas. The beam is formed by particles exiting a hole in the buffer gas cell. We characterize the properties of the beam (flux, forward velocity, temperature) for both an atom (Na) and a molecule (PbO) under varying buffer gas density, and discuss conditions for optimizing these beam parameters. Our source compares favorably to existing techniques of beam formation, for a variety of applications.