XCHV: Four-jaw Collimators

Most of the collimators installed in the secondary beamlines are 4-jaw collimators of the XCHV type. As the name indicates, these collimators contain 4 independently motorised iron blocks of 1 meter length each. From the outside such a device looks as below:

Looking along the beam, the blocks are arranged according to the following sketch:

Two blocks control the horizontal gap, the other two the vertical gap.

According to their position along the beam and depending on the optics chosen, collimators may have different roles:

• Acceptance collimators: to control the beam flux
• Momentum slits: to control the momentum band. Normally the flux transmitted is proportional to the momentum band.
• Cleaning collimators: to clean up the debris produced by scattering off upstream elements

Note that 4-jaw collimators do not allow properly defined small slits. As soon as the gap becomes similar to alignment and machining tolerances (few hundreds of microns) the gaps in between blocks become comparable in size to the central gap. Therefore gaps below one or a few millimeters are not recommended:

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