Solid Attenuator Assembly SAA

The Solid Attenuator Assembly is originally designed to precisely attenuate the XFEL beam intensity in several different circumstances: In cases where the full intensity would be harmful, or would create thermal distortions on downstream x-ray optical devices, and to adjust the beam intensity delivered to the sample for fluence or intensity-dependent experiments.

In the first case, the attenuation will typically be adjusted once per experiment, whereas in the second case, the user will make frequent changes throughout the duration of the experiment. Beam line staff will control the SAA remotely from the Instrument control room defining the attenuation configuration, and the system’s fine alignment.

Each SAA will be integrated into Ultra-High Vacuum (UHV) beam lines and must be capable of indefinite operation whilst illuminated by the XFEL beam. The high heat load of the XFEL beam requires that the SAA is water cooled and the system can easily remove >50W absorbed power.

The four water cooled absorber arms.

Mechanical design
The solid attenuator assembly allows the insertion of a combination of absorbers with different thickness and of different materials. Four arms (or rods) holding six different absorbers each can be inserted into the beam, giving a total of 24 different absorbers. Any absorber on one rod can be inserted into the beam with any combination of the absorbers on the three other rods giving the total number of combinations of 64=1296.  An example of the distribution of the absorbers is illustrated below.

Position   Material Arm 1 Arm 2 Arm 3 Arm 4
1        Diamond 0.1 mm 0.2 mm 0.4 mm 0.8 mm
2 Diamond 0.4 mm 0.8 mm 1.6 mm 3.2 mm
3 Silicon 0.2 mm 0.4 mm 0.8 mm 1.6 mm
4 Silicon 0.8 mm 1.6 mm 3.2 mm 6.4 mm
5 B4C 0.5 mm 1.0 mm 2.0 mm 4.0 mm
6 B4C 2.0 mm 4.0 mm 8.0 mm 16.0 mm

Key Facts
Number of absorbers 6 per arm, 4 arms = 24 absorbers
Absorber thickness 0.1 mm to 16 mm 
Cooling capacity 50 W
Positioning accuracy 10 µm
Repeatability 2-3 µm
Common x-translation stroke 20 mm
Vacuum level UHV (1·10-9 mbar)