Assessing adaptive optics for fast polarization switching of synchrotron light for x-ray magnetic circular dichroism

X-ray magnetic circular dichroism (XMCD), an experimental technique that utilizes circularly polarized light at synchrotron light sources to probe the magnetic properties of materials, is performed by taking the difference of two x-ray absorption spectroscopy (XAS) spectra with x-rays of opposite circular polarizations. Each XAS spectrum measurement requires an energy scan by rotating a diffraction grating, which takes on the order of minutes to complete. Variations in the light source beam conditions or sample material environment can occur in the time between each XAS measurement and can lead to systematic shifts and fluctuations in the collected spectra and ultimately affect the XMCD measurement. We describe a concept using adaptive optics with active feedback to enable the use of fast polarization switching with lock-in amplification to reduce these effects and increase the sensitivity of XMCD measurements, and the development of such an adaptive optics system at the Advanced Light Source that consists of a Hettrick-Underwood monochromator with a deformable pre-mirror, a variable-line-spacing planar grating, and a photon energy sensor at the exit slit for feedback control.