Dynamic control of a narrow-line red magneto-optical trap for strontium atoms under microgravity

The microgravity environment modifies the resonance condition that is formed with the participation of gravity in the 689 nm narrow-line magneto-optical trap (red MOT) of a ground-based strontium optical clock. Consequently, a cooling scheme optimized under standard 1g ground-commissioning conditions cannot be directly transferred to an in-orbit space optical clock. In this work, we establish a one-dimensional semiclassical Monte Carlo stochastic dynamics model along the ground-gravity direction to study the cooling dynamics, spatial distribution, and position compensation of a 88Sr narrow-line MOT under microgravity. The simulations show that directly applying ground-optimized parameters in microgravity leads to heating, cloud broadening, and displacement from the lattice position calibrated on the ground. Reducing the detuning can produce a smaller spatial distribution, but this behavior is mainly phase-space truncation and cannot maintain a sufficient effective atom number. We propose a dynamic-control scheme in which the laser detuning and intensity are continuously ramped, achieving an atomic temperature below 3 μK under microgravity while retaining more than 98% of the atoms in the MOT, outperforming the 1g ground-gravity reference case. On this basis, magnetic-gradient screening is used to obtain a cloud size matched to the optical-lattice mode. After an additional compensation magnetic field is introduced, the deviation between the narrow-line MOT center and the lattice position is ∼2.4 μm, far smaller than the transverse scale of the lattice beam. These results provide a physical picture and theoretical guidance for in-orbit commissioning of the narrow-line MOT in a strontium optical clock on the Chinese Space Station.