Results and Discussions The measured S-parameters are in good agreement with the simulation data in 100 kHz-150 MHz band (Fig. 4). The maximum deviation between the simulation and measurement results is less than 1 dB, the impedance bandwidth of the antenna ranges from 100 kHz to 127.1 MHz, and the relative bandwidth reaches 199.69%, which meets the demand of an ultra-wideband antenna. A cross-validation experiment is performed using a ring probe antenna for an RF field radiation test (Fig. 5), which proves the credibility of the RF antenna simulation results from the perspective of signal transmission. The electronic evaluations imply that the simulation results are credible and the antenna performance meets the design requirements. By exponentially fitting the evolution of the number of atoms at a low density (after 10 s) (Fig. 6), the atomic lifetime is found to be 1/γO.D.=(41.5±3.5)s and the derived background vacuum degree in the atomic region is found to be 2.1×10-8 Pa according to Equation (6), which is even better than that (5×10-8 Pa) of our previous ultra-high vacuum chamber with no RF antenna inside, indicating that our built-in antenna has little influence on the vacuum environment. The phase space density (PSD) rises while the temperature and atomic cloud size decrease with increasing power, and all of them begin to gradually level off after the power reaches 50% of full-load power (Figs. 7 and 8). When the excitation power is higher than 50% of full-load power, the difference in the final PSD is less than 1.27×10-6, and temperature is not more than 50 μK, implying that the evaporative cooling efficiency tends to saturate. These results indicate that the RF magnetic field strength meets the evaporative cooling requirements, the antenna excitation meets the task needs, and the design achieves a closed loop.
As one of the means to acquire degenerate quantum gases of ultracold atoms, radio frequency evaporative cooling is crucial for the realization of Bose-Fermi sympathetic cooling. To obtain ultra-cold quantum degenerate gases on a space station, we design a unique radio-frequency (RF) antenna built in a vacuum chamber. The universal ultra-cold atomic physics experimental system, which consists of cooling, detection, an optical trap, a magnetic trap, an optical lattice, a Feshbach magnetic field, an RF antenna and other devices integrated in the chamber, meets the stringent requirements of manned aerospace engineering in terms of size, weight, power consumption, reliability, and electromagnetic compatibility. In this study, we use finite element simulations to design and evaluate the antenna and experimentally verify its various performance indicators on a ground-based experimental platform. In addition to reducing the RF power requirement by 90%, this design can maintain an ultra-high vacuum degree and perform well in terms of electromagnetic compatibility, meeting the requirements of manned aerospace engineering.
This paper presents a standard engineering design flow of RF antenna system. First, a circular arc antenna prototype is set up to evaluate the design specifications. The prototype design is then modified to a single-turn multi-segment circular arc structure to match the actual cooling beams. The well-designed model is imported into the finite element analysis software, HFSS, to obtain the simulation results of the antenna’s S-parameters and the emitted RF field in the region of the cold atom cloud under different excitation conditions. After the antenna is assembled into the system, the electronic parameters of the antenna are measured by a vector network analyzer and compared with the simulation data to verify the reliability of the simulation results. Next, the influence of the antenna on the background vacuum of the scientific chamber is evaluated according to an atomic lifetime measurement experiment in the optical dipole trap. Finally, RF-induced evaporative cooling experiments are conducted to judge whether the antenna design meets the actual experimental requirements.
In the present study, a RF antenna design inside a vacuum chamber for Chinese Space Station applications is proposed. In addition to allowing the antenna to be closer to the atomic action area, this design also uses the absorption of the titanium chamber to reduce the crosstalk of RF signals and meet the electromagnetic compatibility design requirements. We model and simulate the antenna, with the results indicating that the RF intensity at the center of the magnetic field is approximately 60 mG under an excitation of 3 W signal, and the direct current component of impedance bandwidth of the RF antenna is 0-127.1 MHz. After assembly, we measure the vacuum pressure by the atomic lifetime method and use a vector network analyzer to test the S-parameters of the antenna. The antenna has little impact on the vacuum system, and the measured electronic performances are in good agreement with the simulation results. Finally, we use the antenna to perform RF-induced evaporative cooling in a quadrupole magnetic trap. Under an excitation of more than 1 W RF signals, the atomic temperature is less than 50 μK, and the PSD is increased by an order of magnitude. The experimental results show that the RF signal strength is fully saturated. The RF antenna design realizes a closed loop. This design could have applications beyond space, including gravimetric measurements, magnetometers, and optical lattice clocks.
