August 8, 2016

Caesium Tube

The design of the Caesium Tube (TC) is derived from the design of the ground clocks. EADS Sodern has been entrusted with developing the Caesium Tube.

The Caesium tube is the core of the PHARAO clock where the interaction between microwave signals and the caesium atoms takes place. The atomic molasses is handled and detected with 10 laser beams delivered by the Laser Source and two microwave signals delivered by the Microwave Source. The principle of the caesium tube can be explained with the following sequence:

The caesium vapour is delivered from the caesium reservoir storing the caesium in liquid phase. The quantity of caesium atoms is tuned using the valve aperture level, and the fine-tuning using the thermal regulation.
The atomic cloud is cooled and launched by the 6 laser beams in the sphere of capture.
All atoms are put in the same energy level and the cloud shape is corrected in the preparation cavity.
The cloud is submitted to the microwave signal at the frequency of the caesium reference transition in the Ramsey Cavity (interrogation cavity).
The brightness of the cloud is measured in order to determine the proportion of atoms with shifted energy levels.

The clock performances strongly depend on the tube design. This equipment has to provide very low vacuum conditions, very homogeneous magnetic field and very stable temperature to minimize perturbation of the atomic cloud. The main requirements are the following:

Vacuum 2.10^-8 Pa
Magnetic field < 1 nT
Temperature controlled to ± 0.1° (for the interaction zone)
Alignment: 1 mrad

Due to physical phenomena involved during microwave signal and cold atom interaction, all zones of the vacuum tube must be protected against thermal and magnetic disturbances.
To minimize collision between Caesium and other particles, a very high vacuum is necessary.

Main TC Units

Caesium Tank Its main function is to deliver caesium vapour. The liquid caesium is trapped in porous titanium matrix made of compacted micro balls. The atom density is controlled in the cooling zone. The caesium mass to be stored is about 3 grams and only vapour of caesium must escape from the tank. The flux to be delivered is about 10¹² atoms per second.	Titanium porous matrix for storing caesium (The caesium tank is made by AER)
One capture collimator (EADS Sodern) (6 units are mounted on the capture sphere)	Cooling Zone The six laser beams are injected in the cooling volume in order to trap the caesium cloud. The spherical cloud of atoms is created at the intersection of the laser beams. In this zone the caesium atoms are captured, cooled and then launched. A photodiode monitors the fluorescence light emitted by the captured atoms and a camera observes the optical molasses (the camera is used only on ground). The design is driven by the dimensions and the positioning of those collimators.
Microwave Cavity The cylindrical part of the vacuum tube is designed to receive the interrogation cavity. The performance of this cavity is directly linked to the quality of machining and welding. Furthermore, the symmetry of the cavity, the homogeneity of the electrical conductivity and the roughness are also very important criteria. The flight model of the cavity was tested in an atomic fountain by SYRTE. It was verified that there is no magnetic field singularity and no evidence of end-to-end phase shift at PHARAO accuracy level (10^-16).	Microwave cavity onto its supporting structure (Thales TED)
EM Detection module during integration (EADS Sodern)	Detection The vacuum tube is then equipped with 4 laser beams in the detection zone in order to measure the fluorescence of the excited caesium atoms.
Vacuum Atom interaction phenomena require a vacuum lower than 2.10^-8 Pa. The vacuum tube is made of several pieces screwed together with a vacuum tightness system with aluminium joints.	Getter unit

Ionic pump developed for PHARAO
(EADS Sodern)

Ionic pump mounted
(EADS Sodern)

The pumping system is located at the end of the vacuum tube. Pumping is ensured by a 3 l/s ion pump and completed with a set of getters distributed around the interrogation cavity.

Magnetic
Due to physical phenomena involved during microwave and cold atom interaction, these zones must be protected against external magnetic disturbance. Three concentric µ-metal screens make the magnetic shielding (about 50% of the total caesium tube mass). Coils provide the necessary constant weak internal magnetic field, plus an active compensation coil with a servo loop monitored with an internal magnetometer.

EM magnetic shields

Main challenges

The caesium tube uses various very specific technologies; the main challenges of this development are:

The centre of gravity is very high; as a consequence, mechanical efforts at the junction of each part of the vacuum tube are significant and difficult to withstand.
The interrogation cavity is difficult to maintain due to its thermal expansion factor. Furthermore, this cavity can only be verified at PHARAO level.
Background light is difficult to assess and implies specific treatment, which must remain compatible with high vacuum.
The tightness of the windows requests a process which shall respect optical surfaces.
Alignment of the laser beam has to be stringent to control the effectiveness of caesium atom capture.
Photodiodes are critical with regard to the required high sensitivity.
Ultra-high vacuum is a challenge due to the high number of windows and flanges and to backing restriction for optical treatment preservation. The ion pomp and its high voltage power supply are designed especially for PHARAO.
Magnetic field should be homogeneous despite the numerous causes of perturbation (Earth field rotation, vicinity of the MASER, electromagnetic mechanisms within PHARAO).

All those challenges have been met.