Beamline fast and automatic attenuation system for X-Ray detectors at Synchrotron Soleil

Attenuators are commonly used on beamlines to control incident photon flux. Attenuators are mainly controlled by software. In some experimental cases using various diffraction techniques, this architecture is not fast enough to manage high flux variation. The fast attenuation system inserts and extracts filters quickly, allowing very fast beam attenuation at the maximum rate allowed by the filter mechanism and the beam detector response. To build the solution, we used an off-the-shelf CPCI General Purpose board (GPIO) from TEWS that is based on a SPARTAN-3 Xilinx FPGA: We have developed a daughter board and an embedded VHDL program. The logic is dedicated to maintaining incident detector photon flux within an acceptable range for optimized measurements and protecting the X ray detector against over-exposure. This system is part of a continuous scan process. Some low level process logic is also embedded in order to optimize data exchange. During continuous scanning, this process allows each experimental data item collected to be associated with its corresponding photon flux value. This system is in operation on the SIXS beamline and will be soon installed on the DIFFABS beamline. This paper describes the principle and the results obtained with this solution and the possible improvements and perspectives (interfacing more complex detectors such as XPad).