Controlling an active bimorph deformable mirror with sub-nanometre resolution

Abstract

There is growing interest at synchrotron light and X-ray free electron laser facilities to explore and improve the dynamic performance of piezoelectric bimorph deformable X-ray mirrors. Many beamlines, especially those dedicated to Macromolecular Crystallography, need to measure hundreds of samples per day. Shorter acquisition time requires rapid changes in the focus of the X-ray beam to condense the maximum photon density onto the sample. This is necessary to match the X-ray beam to the dimensions of the sample, or to probe variable sized regions of larger samples. Fine control of the X-ray beam becomes crucial for ensuring the highest quality of scientific data and increased throughput. Previous work at Diamond Light Source successfully changed the X-ray beam focus and stabilised it in under 10 seconds using piezoelectric bimorph deformable mirrors. Further updates to the controls software of the programmable HV-ADAPTOS high-voltage power supply (from CAEN / S.RI. Tech) now make it possible to control individual electrodes at 1 Hz using custom voltage profiles. This allows localized compensation of piezo creep, thus improving X-ray beam shape, significantly reducing stabilisation time, and eliminating curvature drift. For ex-situ validation, dynamic changes in the surface of the bimorph mirror need to monitored in real-time with sufficient spatial sensitivity. In this paper, we show that the active optical surface of a bimorph mirror (from Thales-SESO) can be accurately changed with sub-nanometre height sensitivity by dynamically monitoring the mirror’s surface using an array of high-speed (up to 200 kHz) Zygo ZPS™ absolute interferometric displacement sensors mounted in an independent metrology frame.