Attenuator

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. Beamline 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) beamlines and must be capable of indefinite operation while 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. More recent attenuator assemblies e.g. for LCLS-II at SLAC are designed to remove 200W.

Mechanical design
The solid attenuator assembly allows the insertion of a combination of absorbers with different thicknesses and 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.