Simulation of Flow, Temperature and Molecule Transport in a Buffer-Gas Cell for Cryogenic Buffer-Gas Cooling
Background and MotivationThis project aims at improved understanding and optimization of cryogenic buffer-gas cooling. With this advanced technique, cold molecules are produced via collisions with cold buffer-gas atoms. Such cold molecules then allow for studying fundamental physics. A buffer-gas cell (see Fig. 1) represents the setting within which the cooling of the buffer gas proceeds. For this purpose, continuous flows of hot molecules are mixed and thermalize with continuous flows of buffer gas at cryogenic temperatures of 5 K or 20 K, respectively. Typically, helium or neon are used as buffer gas, and the flux of buffer-gas is one to two orders of magnitude larger than the molecule flux. The cell is equipped with various small holes: on the one hand, there is an inlet and an outlet hole for molecules and buffer gas, which are arranged at the respective ends of the centerline of the cell, and on the other hand, there are additional side inlets for buffer gas only, as shown in Fig. 1. The outlet hole allows for the gas mixture to leave the cell. There, a continuous effusive or supersonic beam of buffer-gas atoms and molecules is formed. The molecules are then separated from the buffer gas and transported to other experiments for further cooling. It is of utmost importance that the largest possible flux of cold molecules is provided for the subsequent experiments. For this purpose, the goal of this project is the optimization of the buffer-gas cell by means of simulations of the flow and transport dynamics inside the cell, such that it is eventually arrived at a design of the cell which allows for maximum extraction of molecules. Moreover, the simulations will provide unprecedented insights into the flow and transport dynamics in the cell.
Various Severe Challenges
When simulating the flow in the buffer-gas cell, it is crucial to take the variable density of the buffer gas into account, which varies due to high temperature gradients within in the cell. Furthermore, the transport of the molecules, which is coupled to the buffer-gas flow, must be resolved. Thus, in comparison to, e. g., much more widely used incompressible flows, such a variable-density flow with molecule transport poses severe additional challenges:
- Large temperature variations have to be taken into account of – in this case – up to 200 K.
- These temperature variations are coupled to correlative density variations of – here – more than 1.000 %(!).
- The variable-density flow is coupled to the transport of the thermalizing hot molecules.
Due to the strong mutual physical coupling of flow, temperature transport and molecule transport, only a fully-coupled computational method for variable-density flow at low Mach number with coupled mass-transport solver can provide a realistic representation of the flow dynamics in the cryogenic buffer-gas cell and thus enable predictive simulations. A recent preceding attempt to simulate such a cell configuration utilized a non-fully-coupled numerical method within a commercial software package, which was obviously not capable of adequately covering all of the aforementioned challenges.