A simple optical system
Argonne researchers have discovered a simple optical system capable of correcting the strong defocus caused by deformation of mirrors from an undulator. The research has been published in the journal Synchrotron Radiation.
Photon-hungry experiments which do not require high-energy-resolution X-rays, e.g. time-resolved imaging and tomography, high-pressure emission spectroscopy, use a pink-beam, the radiation from an undulator filtered by the reflection from a grazing-incidence mirror coated with the appropriate reflecting material.
As is well known, the power emitted by an undulator varies significantly as the resonant photon energy is tuned by changing the k value of the insertion device. This in turn means that the power absorbed by the optics changes and, consequently, the optics shape. Therefore, changing the energy emitted by the undulator will have an effect on the spot size at the experiment.
The radiation from an undulator reflected from one or more optical elements (usually termed pink-beam) is used in photon-hungry experiments. The optical elements serve as a high-energy cutoff and for focusing purposes. One of the issues with this configuration is maintaining the focal spot dimension as the energy of the undulator is varied, since this changes the heat load absorbed by the first optical element. Finite-element analyses of the power absorbed by a side water-cooled mirror exposed to the radiation emitted by an undulator at the Advanced Photon Source (APS) and at the APS after the proposed upgrade (APSU) reveals that the mirror deformation is very close to a convex cylinder creating a virtual source closer to the mirror than the undulator source.
Here a simple optical system is described based on a Kirkpatrick-Baez pair which keeps the focus size to less than 2 µm (in the APSU case) with a working distance of 350 mm despite the heat-load-induced change in source distance. Detailed ray tracings at several photon energies for both the APS and APSU show that slightly decreasing the angle of incidence on the mirrors corrects the change in the `virtual’ position of the source. The system delivers more than 70% of the first undulator harmonic with very low higher-orders contamination for energies between 5 and 10 keV.
Ruben Reininger, Zunping Liu, Gilles Doumy and Linda Young, “A Simple Optical System Delivering A Tunable Micrometer Pink Beam That Can Compensate for Heat-induced Deformations,” Synchrotron Radiation, DOI: 10.1107/S1600577515006566. Published Online June 9, 2015.