December 10, 2023

Caesium Tube

Caesium tube CAD
Assembled ion pump (EADS Sodern)

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

The CT is the core of the PHARAO clock where the interaction between microwave signals and caesium atoms takes place. The atomic molasses is handled and detected with 10 laser beams delivered by the Laser Source (SL) and two microwave signals delivered by the Microwave Source (SH). Here’s how the CT works:

  • The caesium vapour is delivered from the tank where the caesium is stored in its liquid phase. The quantity of caesium atoms is adjusted by the valve aperture and fine-tuned by the thermal control system.
  • The atomic cloud is cooled and launched by the six laser beams in the capture sphere.
  • All atoms are brought to the same energy level and the cloud shape is corrected in the preparation cavity.
  • The cloud is subjected to the microwave signal at the reference transition frequency of the caesium in the Ramsey cavity (interrogation cavity).
  • The brightness of the cloud is measured to determine the proportion of atoms whose energy level has shifted.

Clock performance strongly depends on the tube design. This equipment has to provide very strong vacuum conditions, a very uniform magnetic field and very stable temperature to minimize disturbance of the atomic cloud. The main requirements are:

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

Due to the 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 collisions between caesium and other particles, a very high vacuum is necessary.

Main TC Units

Caesium tank
The tank’s main function is to deliver caesium vapour. The liquid caesium is trapped in a porous titanium matrix of compacted microballs. The atom density is controlled in the cooling zone. The caesium mass to be stored is about three grams and only caesium vapour must escape from the tank. The flux required is about 1012 atoms per second.

titanium porous matrix for storing caesium
Titanium porous matrix for storing caesium
(The caesium tank is made by AER)

One capture collimator
A capture collimator - EADS Sodern
(6 units are mounted on the capture sphere)

Cooling zone
The six laser beams are injected in the cooling volume 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 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 positioning of these collimators.

Microwave Cavity
The cylindrical part of the vacuum tube is designed to accommodate the interrogation cavity. This cavity’s performance depends directly on the quality of machining and welding. Furthermore, the symmetry of the cavity, the uniformity of electrical conductivity and 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’s accuracy level (10-16).

Microwave cavity onto its supporting structure
Microwave cavity on its supporting structure
(Thales TED)

EM Detection module during integration
EM Detection module during integration (EADS Sodern)

The vacuum tube is then equipped with four laser beams in the detection zone to measure the fluorescence of the excited caesium atoms.

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-tight system with steel joints.

One getter unit
Getter unit
Ionic pump developped for PHARAO
Ion pump developed for PHARAO
(EADS Sodern)
Ionic pump mounted
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 shields
Due to physical phenomena during microwave and cold atom interaction, these zones must be protected against external magnetic disturbance. Three concentric µ-metal screens constitute 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 by an internal magnetometer.

EM magnetic shields
EM magnetic shields

Main challenges

The caesium tube uses various very specific technologies for which the main development challenges are:

  • The centre of gravity is very high; as a result, mechanical strain at the junction of each part of the vacuum tube is significant.
  • The interrogation cavity is difficult to outgas due to the low melting point of its tin welds and its thermal expansion factor. Furthermore, this cavity can only be fully verified once PHARAO is built.
  • Background light is difficult to assess and implies specific devices, which must remain compatible with an ultra-high vacuum.
  • The tightness of the windows requires a process that does not damage optical surfaces.
  • Alignment of the laser beam has to be stringent to control the effectiveness of caesium atom capture.
  • Photodiodes are critical due to the required high sensitivity.
  • Ultra-high vacuum is a challenge due to the large number of windows and flanges and to backing restrictions to preserve optical treatment. The ion pump and its high-voltage power supply are designed especially for PHARAO.
  • Magnetic shielding must be uniform despite the numerous sources of disturbance (Earth field rotation, proximity of the MASER, electromagnetic mechanisms within PHARAO).

All of these challenges have been met.

PHARAO Caesium tube on flight Baseplate - © Sodern Ultra vacuum tube - flight model - © Sodern
PHARAO caesium tube on flight baseplate and the ultra-high vacuum tube - Crédit : © Sodern