Characterization of a cold cesium source for PARCS: Primary Atomic Reference Clock in Space

The PARCS (Primary Atomic Reference Clock in Space) project is a joint NIST JPL venture aimed at placing a Cs atomic clock aboard the International Space Station. This orbiting clock will achieve high accuracies in part due to the long Ramsey times afforded by the microgravity environment. The clock will allow a wide range of precision tests of fundamental physics including relativity theory. Our group at NIST is performing experimental studies of a "prototype" cold Cs source based on launching atoms from optical molasses. The results of our work will be applied to the design and construction of a robust, space qualified device. Additionally, this work is important for the development of future Cs fountains. We describe our apparatus, the present state of our experimental work, and planned improvements.

shown in Figure 1. The system consists of a vacuum The PARCS (Primary Atomic Reference Clock in chamber and accompanying lasers and optics. The Space) project is a joint NIST JPL venture aimed at placing a Cesium (Cs) atomic clock aboard the International Space Station. This orbiting clock will achieve high accuracies in part due to the long Ramsey times afforded by the microgravity environment. The clock will allow a wide range of precision tests of fundamental physics including relativity theory. Our group at NIST is performing experimental studies of a "prototype" cold Cs source based on launching atoms from an optical molasses. The results of our work will be applied to the design and construction of a robust, space qualified device. Additionally, this work is important for the development of future Cs fountains.
Here we describe our apparatus, the present state of our Region experimental work, and planned improvements.
An overview of the PARCS space clock project has been presented elsewhere [l].
Similarly, the French PHARAO space clock addresses many of the same issues and problems [2]. The PARCS design goals are to achieve a stability of a,(z) -3 ~1 0 ' '~ 7-" with a Ramsey time on the order of 10 s and an accuracy of 1~1 0 "~. This requires 2x lo7 Cs atoms in the m=O state with a temperature of 2 pK to be launched into the PARCS microwave cavity. The design of the PARCS Cs source will have to confront problems that are not present in current Cs fountain clocks. For example, consider the long Ramsey times ( -10 s ) afforded by the microgravity environment. In this regime, atom loss due to background collisions and thermal expansion of the launched ball of atoms becomes significant. Although the PARCS will employ shutters between the source and the microwave interrogation region, atoms with a small launchvelocity will still encounter elevated background pressures before leaving the source. This paper describes laboratory studies of a "prototypen cold Cs atomic source and studies of parameters that are required to achieve these goals. Additionally, we are concerned with developing design guidelines for producing a robust and reliable space qualified device. Cs atoms cooled in the optical molasses are launched upwards into the drift tube by detuning the upward and downward beams to create a moving molasses. The atoms travel up, turn around, and fall back through the source region and down below into the detection region. The detection region consists of a sheet of light in a standing wave tuned to the F =4 -F' = 5 transition, a spherical collection mirror, and a photodetector.
The temperature and density of the Cs cloud is measured by tossing the atoms to various heights and recording the fluorescence of the atoms as they pass through the detection beam. Presently we have achieved launch velocities in excess of 4 m/s and we are pursuing several avenues to increase the number of atoms returning to the detection region in order to improve our temperature and density measurements. Preliminary launch data are shown in Figure 2. Here, Cs from the molasses has been launched to several heights and the returned signal measured. These results indicate an initial atom ball of radius s0.5 cm and a temperature of less than 10 pK . The signal from the atoms dropped out of the molasses indicates that there are 2 lo7 atoms in the molasses.

Fluorescence Signal Detected as a Function of Several Toss Heights
Time ( sec ) Figure 2. Plot of the Fluorescence signal of Cs launched from a molasses to several heights.
to a conventional (0,0,1) molasses beam geometry because of the lack of retro reflected beams which automatically maintain beam to beam power balance.
Our system is sensitive to beam to beam power imbalance and it is likewise difiicult to accurately adjust in our current configuration. Thus, we are presently developing an active laser beam intensity servo. We anticipate that this improvement will allow us to achieve higher launches with much larger numbers of atoms returning to the detection region.
We have begun to investigate various parameters and their effect on the temperature and density of our cold atom source and will present these results. These parameters include, cooling beam power, alignment of the beams in the (l,l,I) geometry, polarization requirements on the (1,1,1) beams, power imbalance tolerance, molasses load times, launch and post cool parameters, Cs oven performance and magnetic field uniformity required in the source region.
Once the wld atom source parameters have been satisfactorily characterized, this apparatus will be useful for investigating other PARCS related issues. For example, we plan to attach the PARCS prototype microwave cavity above the source in order to evaluate the performance of the cavity.