Angular stability measurement of a cryocooled double-crystal monochromator at SSRF

Abstract

Precisely measuring the angular stability is essential to evaluate and improve the performance of the double-crystal monochromator (DCM). Herein, we apply a non-contact high-precision angular measurement method by utilization of the interferometers instead of conventional capacitive sensors, realizing a high resolution of picometer scale; particularly, a large working range of tens of millimeters can be simultaneously achieved, superior to previous method. By this means, we systematically investigate a homemade cryo-cooled DCM at Shanghai Synchrotron Radiation Facility (SSRF). The temporal stability as well as main eigen-frequencies of the DCM mechanics are measured at ambient temperature (25°C) and under liquid nitrogen (LN2) cooling condition, respectively. Fast Fourier Transform (FFT) analysis of these data hints possible vibration sources. Accordingly, flexible tube with an alumina fiber textile inner is adopted and the support of the cooling lines is optimized. Furthermore, relative vibrations with different cryo-cooler settings are compared, revealing a slightly enhanced amplitude with increasing flow rate. Through these optimizations, the DCM achieves a vertical stability of 125 nrad RMS in 1∼100 Hz and 180 nrad RMS in 1∼1000 Hz over the full photon energy, which meets the requirement of the Fast X-ray Imaging beamline at SSRF.