Development of the new Long Trace Profilometer at LCLS for bendable X-ray mirror metrology

The Linac Coherent Light Source (LCLS) is undergoing an upgrade to a double source setup to provide eight experimental hutches (five existing and three new) with either high-repetition or high-intensity pulses and highly coherent X-ray beams. The photon transportation and distribution to each hutch relies on, among other elements, bendable mirrors. Given the coherence of the LCLS source, and to avoid introducing wavefront distortions beyond workable limits, the mirrors need to have extremely smooth surfaces, with a figure compliant with the nominal profile (usually elliptical). The effectiveness and the accuracy of the bending system and of the actuators over the entire length of the mirror (up to 1.2 m) need to be assessed by an appropriate metrology system. Long Trace Profilometry (LTP) is a suitable technique to characterize a slightly-curved surface mirror profile with very high sensitivity, provided that the optomechanical system implementation enables sensitivity and accuracy values compatible with the mentioned surface quality requirements. In this paper, we show the status and performance of the LTP under development at LCLS. The LTP essentially consists of an advanced optical head that endows a laser beam with sharp interferential features to increase its resolution and detects the optical lever of the beam reflected by the sample, plus a high-precision gantry system (Q-Sys) for accurate scanning of the mirror under test, under impact of its bending mechanics and cooling system. The measured results are compared to the simulated performance of the LTP, and we show the way of the oncoming improvement of the instrument.