Magnetic shielding of the cold atom space clock PHARAO

• Overview of the space atomic clock PHARAO cesium tube is given.
• Mumetal space atomic clock shielding is examined (an attenuation of 19,000 is achieved in the critical region).
• Appearance of hysteresis and attenuation dependency on amplitude are analyzed.
• Good magnetic homogeneity using static field coils was achieved.
• Clock resonance signals are used to examine the internal homogeneity.

The space clock PHARAO is an atomic clock based on laser cooled cesium atoms. In order to attenuate magnetic field fluctuation in orbit, PHARAO clock uses three concentric Mumetal magnetic shields combined with several coils to improve the field homogeneity. We have characterized the attenuation and magnetic field homogeneity of the shields used to build the flight model. The average value of attenuation inside the three shields is around 18,000 when the external field is similar to the orbit field (30 µT) and the field homogeneity is lower than 10 nT. These values have not changed after vibrations and thermal tests for the space qualification. Permeability variation of the shields as a function of the intercepted flux has been analyzed.