More information about the HRM can be found here.

Context

The generation of x-ray by electrons passing through undulators produces, in addition of the fundamental energy, radiation at integer multiple of the fundamental energy, called harmonics. Because of symmetry, only the odd orders are allowed, so for example, for a fundamental at 5keV, photons at 15keV, 25keV, etc. are also generated.

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More information about the HRM can be found here.