Initial Tests of a Rubidium Space Cold Atom Clock

We report the initial test results of a rubidium (^87Rb) space cold atom clock (SCAC). The space-qualified ^87Rb SCAC is composed of the physical package, the optical bench, the microwave synthesizer and the control electronics. After the system is integrated, about 10^8 ^87Rb cold atoms are captured by magneto-optical trap. The linewidth of the Ramsey fringe is about 10 Hz for the free evolution time of 50 ms on the ground, and the signal-to-noise ratio is measured to be larger than 300. We demonstrate a good medium-term fractional frequency stability of 1.5\times10^-14@1000 s in the closed-loop operation on the ground. The main effects of the noise on the stability are also presented, and the optimized operating parameter is analyzed for the operation of SCAC in the microgravity environment.