A closed loop feedback system for synchrotron radiation double crystal monochromators

We report on a dc closed loop feedback system for a fixed exit double crystal monochromator. The feedback control is part of a complete system which includes beam position and current monitors. The monochromator consists of two identical crystals mechanically coupled so that the two diffraction planes are almost parallel to each other. The feedback system controls a piezoelectric transducer to maintain the new parallelism, and it can be set under computer control to be at any point in the rocking curve at all times. The system has been thoroughly tested and found to be adequate for EXAFS measurements performed on beamline X18B at the National Synchrotron Light Source at Brookhaven National Laboratory. (orig.)