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Journal of Physics B Atomic Molecular and Optical Physics

Ground experiment verification and on-orbit prediction of the two-stage cooling at pK level in the Chinese space station

Y. Xie, Bo Fan, Hui Li, Angang Liang, Mingshan Huang, Biao Wu, Bin Wang, Xuzong Chen, Liang Liu

Published Aug 31, 2022• 6 citations
Publisher

Abstract

Abstract In order to infer the expected temperature of the two-stage cooling experiment onboard the Chinese space station (CSS) through ground experiments, we apply the direct simulation Monte Carlo algorithm to construct a simulation model suitable for both the standard gravity and the microgravity conditions. The simulations are verified by the ground experimental results, from which we can deduce that the on-orbit experiments can reach the order of as low as 100 pK. This algorithm calculates the corresponding relationship between the temperature in the ground experiments and that in the microgravity conditions, keeping the other experimental parameters unchanged, which provides an important basis of design for the on-orbit atomic cooling experiment onboard the CSS which is due to be launched at the end of 2022.

DOI
10.1088/1361-6455/ac8e3d

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Physical Review A•Apr 16, 2024•2 citations
Abstract In order to infer the expected temperature of the two-stage cooling experiment onboard the Chinese space station (CSS) through ground experiments, we apply the direct simulation Monte Carlo algorithm to construct a simulation model suitable for both the standard gravity and the microgravity conditions. The simulations are verified by the ground experimental results, from which we can deduce that the on-orbit experiments can reach the order of as low as 100 pK. This algorithm calculates the corresponding relationship between the temperature in the ground experiments and that in the microgravity conditions, keeping the other experimental parameters unchanged, which provides an important basis of design for the on-orbit atomic cooling experiment onboard the CSS which is due to be launched at the end of 2022.
The Cold Atom Physics Rack (CAPR) of Chinese space station will be launched at the end of 2022. The important goal of CAPR is to achieve BEC at 100 pk. In order to obtain ultracold atoms in microgravity of space station, we propose a two-stage cooling scheme using all-optical trap with different waist beams. The cold atom cloud obtained by this scheme is composed of condensate and thermal atoms around condensate. The design of our two-stage cooling scheme will effectively reduce the temperature of the thermal atom cloud and the effective temperature generated by the interaction energy of the condensate. The atomic temperature of 5 nk is obtained from the ground test experiment, and the corresponding temperature under the microgravity condition of the space station is theoretically predicted to be less than 100 pk. Taking the advantages of ultracold temperature and long-time detection, many scientific experiments will be arranged. In this paper, the ground test experiments based on ground principle prototype and pre-prototype for CAPR are also introduced.
The China Space Station (CSS) provides an ideal experimental platform for researching and applying ultracold atoms in microgravity. On 31 October 2022, the Cold Atom Physics Research Rack (CAPR) was launched to the CSS together with the Mengtian lab module, designed by the Shanghai Institute of Optics and Fine Mechanics of the Chinese Academy of Sciences (SIOM) and Peking University. The main goal of the CAPR is to build an ultracold atomic physics experiment platform using 87Rb Bose Einstein Condensate (BEC) in the CSS. In microgravity, ultracold atoms can be cooled to picokelvin (pK) via Two-Stage Crossed Beams Cooling (TSCBC), which is three orders of magnitude lower than on earth and can inspire the novel physical phenomena. The preparation of the BEC and the deep cooling are introduced in this paper, as well as a series of advancements in ultracold atoms in microgravity. The CAPR is an experimental platform that relies on evaporative cooling in a crossed optical dipole trap. Furthermore, the CAPR’s design scheme and ground verification experiment are also introduced. So far, the CAPR has continued to conduct research on ultracold atoms in microgravity, with more than two years of uninterrupted operation in orbit. Up to now, preliminary experimental results have been achieved, realizing the primary purpose of the CAPR.
To obtain cold atom samples with temperatures lower than 100 pK in the cold atom physics rack experiment of the Chinese Space Station, we propose to use the momentum filtering method for deep cooling of atoms.This paper introduces the experimental results of the momentum filtering method verified by our ground testing system.In the experiment, we designed a specific experimental sequence of standing-wave light pulses to control the temperature, atomic number, and size of the atomic cloud.The results show that the momentum filter can effectively and conveniently reduce the temperature of the atomic cloud and the energy of Bose-Einstein condensation, and can be flexibly combined with other cooling methods to enhance the cooling effect.This work provides a method for the atomic cooling scheme of the ultra-cold atomic system on the ground and on the space station, and shows a way of deep cooling atoms.
Abstract In this paper, we numerically studied the effects of mechanical vibration and magnetic fields on evaporative cooling process carried in space station by direct simulation Monte Carlo method. Simulated with the vibration data of international space station, we found that the cooling process would suffer great atomic losses until the accelerations reduced tenfold at least. In addition, if we enlarge the s -wave scattering length five times by feshbach resonance, the phase space density increased to 50 compared to 3 of no magnetic fields situation after 5 s evaporative cooling. We also simulated the two stages crossed beam evaporative cooling process under both physical impacts and obtain 4 × 10 5 85 Rb atoms with a temperature of 8 pK. These results are of significance to the cold atom experiments carried out on space station in the future.
Applying the direct simulation Monte Carlo (DSMC) method developed for ultracold Bose-Fermi mixture gases research, we study the sympathetic cooling process of 6Li and 133Cs atoms in a crossed optical dipole trap. The obstacles to producing 6Li Fermi degenerate gas via direct sympathetic cooling with 133Cs are also analyzed, by which we find that the side-effect of the gravity is one of the main obstacles. Based on the dynamic nature of 6Li and 133Cs atoms, we suggest a two-stage cooling process with two pairs of crossed beams in microgravity environment. According to our simulations, the temperature of 6Li atoms can be cooled to T = 29.5 pK and T/TF = 0.59 with several thousand atoms, which propose a novel way to get ultracold fermion atoms with quantum degeneracy near pico-Kelvin.
In this paper, to investigate how atoms evaporate and cool in microgravity environments of the space station, we have developed the dynamic equation for evaporative cooling of trapped atoms and obtained the analytical expression of atomic temperature with respect to trapping laser parameters and gravitational acceleration. In our model, the evaporation of atoms is equivalent to applying a damping force to the trapped atoms, and the evaporative cooling process of trapped atoms is comprehended as the damped oscillation of trapped atoms in the optical dipole traps. By introducing the gravity in our model, we obtained the analytical model of temperature variation with gravity after cooling, and the theoretical results agree well with the evaporation experiment of rubidium-87 atoms on the ground. Our theoretical results show that, compared with the atomic evaporative cooling experiment on the ground, the microgravity environment of the space station can achieve cooler atomic gases when the losses of atoms, such as the one-body loss caused by background-gas collisions and the three-body recombination loss caused by interatomic inelastic collisions, can be ignored.