CSEM Scientific and Technical Report 2008

Atomic Beams in Space
R. Ruffieux, S. Lecomte, S. Zivanov, D. L. Boiko
A progress in a series of projects supported by ESA on optically pumped Cesium beam clock technology for space applications is reported. Such
frequency standard will be employed in the atomic clock ensemble at GNSS Galileo satellites of second generation.
The first generation of satellites of the European Global
Navigation Satellite System (GNSS) Galileo will bring an
atomic clock ensemble with two Passive Hydrogen Maser
(PHM) clocks for the long-term time keeping and two
Rubidium Atomic Frequency Standard (RAFS) as clockwork
during a swap between PHMs. Both technologies were
developed by the former Neuchatel Observatory (now Time &
Frequency at CSEM) and are industrialized by SpectraTime.
These two frequency standards operate in cell conditions and
the environment might cause long term drifts. While the
RAFSs are very compact (2.4 liters, 3.4 kg), the PHMs show
the long-term stability by a factor of 5 better, reaching the
relative frequency variations of 10-14 for a measurement
interval τ of 104 s. However, PHMs are bulky (26 liters, 18 kg),
which was not the only argument for the European Space
Agency (ESA) to search for a possible alternative, overcoming
the long term drift of PHMs as well.
A good candidate was an atomic-beam frequency standard.
Magnetically-pumped cesium beam clocks with separated
oscillatory fields are used nowadays to define a second in the
SI metric system, as 9’192’631’770 periods of microwave
oscillations at the resonance with ground-state hyperfine
splitting in 133Cs atoms. An atomic beam clock shows no
frequency drifts, however, the clock with magnetic deflection
of atoms cannot be deployed at a long-term mission satellite.
Figure 1: Optically-pumped Space Cesium Atomic Resonator.
(The GNSS Galileo requires a clock that operates 12 years.)
Since the two hyperfine ground states of 133Cs atoms have 16
Zeeman sublevels, a commercial magnetically-deflected clock
will typically run out of cesium in about 3 years, utilizing only
1/16 part of Cs atoms from the beam and yet lacking the
required frequency stability.
The solution was found to optically pump cesium atoms. A
laser-pumped atomic-beam resonator has better short-term
stability and has a real potential to compete with the PHM,
offering smaller frequency drift and a simpler design.
Since 2004, ESA supports the development of ultra stable
atomic beam clocks, which has been initiated at the former
Neuchatel observatory and then continued at CSEM. This
activity resulted in the Optically-pumped Space Cesium
Atomic Resonator (OSCAR) demonstrator shown in Figure 1
and Figure 2. The project was successfully closed in the past
year, reaching the goal of relative frequency stability in the
range 1÷3x10-12 at 1s [1] . However the OSCAR demonstrator
was quite far from a design compatible with the space
applications, by consuming Cesium too fast.
The next step towards space, the project on feasibility study of
cesium clock technology for Galileo (ESA), was initiated in
2006, when Thales Electron Devices (TED) set up a French-
Swiss consortium (CSEM, SYRTE and Oerlikon Space Zurich
as partners) aiming at a breadboard of the flight-compatible
atomic resonator with the relative frequency stability
1x10-12τ-1/2 and flicker noise floor 10-14 for a measurement
interval 105 s. The objective was to develop an atomic
resonator technology ensuring the clock operation for
12 years. In the past 2008 year, the goal of this project was
achieved [ 2, 3] . Figure 2 shows the breadboard of Opticallypumped
Space Cesium Clock (OSCC) for Galileo.
In the next phase of cesium atomic beam clock development
for space applications, which starts in 2009, the French-Swiss
consortium will build a size-representative elegant breadboard
of the clock (15 liters, 10 kg) based on previous development
at CSEM.
Figure 2: Optically-pumped Space Cesium Clock at CSEM.
[1] Project web page
http://telecom.esa.int/telecom/www/object/index.cfm?fobjectid=2
9392
[2] R. Ruffieux, et al., “Optically-pumped Space Cesium Clock for
Galileo: Results of the Breadboard”, Proc. of the 7th Symposium
on Frequency Standards and Metrology 2008, Pacific Grove, CA,
USA, to be published.
[3] P. Berthoud, et al., “Optically-pumped Space Cesium Clock for
Galileo: First Results of the Breadboard”, IEEE International
Frequency Control Symposium jointly with the 22st European
Time and Frequency Forum, Toulouse (FR), April 2008
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