# Hybrid Electrostatic-Atomic Accelerometer for Future Space Gravity Missions
## 3 Results and Discussions
### Hybrid Instrument Design
#### 3.4.1 Atom Interferometer  
Global Architecture  
For the atomic accelerometer, we considered primarily an atomic source constituted by a molasses source of \({}^{87}\)Rb that benefits from a more mature technology development and that should lead to a more favorable SWaP for a satellite-based mission than an accelerome  
Figure 25: HybridSTAR accelerometer noise performance obtained with the hybrid instrument simulator. The EA’s proof-mass is rotated around the \(Y\) axis for satellite rotation compensation. The noise along the radial axis \(Z\) is slightly degraded due to this functionality.  
ter based on a BEC source [72; 73] or evaporatively cooled source [25; 38]. Nevertheless, the performance in terms of accelerometry with a BEC are expected to be better [74]. Indeed, a BEC source benefits from a lower velocity dispersion and a better control/stability of the position and initial velocity of the atomic cloud. A parallel to a BEC-based instrument is given at the end of the design presentation.  
For the CAI, we considered the standard configuration of a three-light-pulse Mach-Zehnder interferometer and a Raman double diffraction scheme [75], well adapted for microgravity environments, with a laser retro-reflected on the EA's proof-mass.  
The vacuum chamber (see See Figure in Section 3.4.3) is based on a glass cell setup connected to a titanium base where the vacuum is maintained thanks to non-evaporable getters and an ionic pump. Two rubidium dispensers are also integrated in the titanium base, which allow the atomic cloud to be loaded from a background vapor.  
The detection of the atomic population is realized thanks to four photodiodes surrounding the atomic cloud and a laser beam selecting spatially the atoms in a region of typically 1 cm \(\times\) 1 cm \(\times\) 1 cm, providing an effective velocity selection of the atoms and decreasing artificially the temperature of the detected atoms to \(\approx\)100 nK for typical foreseen interrogation times. Two layers of mu-metal magnetic shield are foreseen to lower the impact of external magnetic fields, leading to a theoretical shielding factor of \(\approx\)2000 in the interferometer region.  
The measurement cycle begins with the cooling and trapping of the atomic cloud in a 3D magneto-optical trap (MOT). The atoms are loaded for 1 s. A molasses phase of typically 20 ms follows, which decreases further the temperature of the atoms down to \(\approx\)1 \(\mu\)K. A preparation phase aiming to put the atoms in a specific state follows. This phase is a combination of microwave transitions and/or Raman velocity selective transitions and a blast pulse to populate a magnetically insensitive atomic state. Then, \(2.5\times 10^{8}\) atoms are expected to participate in the interferometer. The interferometer phase begins with a Raman double-diffraction pulse, followed by a blast pulse, which removes unwanted populated states that would participate in a contrast decrease at the interferometer output. A second Raman pulse occurs, followed also by a blast pulse. The interferometer sequence finishes by a third Raman pulse, which recombines the two interferometer paths. The population in the two hyperfine atomic states, which reflects the interferometer phase, is measured with an independent detection beam, which selects, in conjunction with four photodiodes, the center of the cloud and, therefore, the colder atoms. The number of detected atoms should be \(\approx\)\(3\times 10^{5}\) for a typical interrogation time \(T=1\) s (see Section of Performance Aspects).