Development of a long trace profiler in situ at National Synchrotron Radiation Research Center

Abstract

To achieve an ultrahigh resolution of a beamline for soft X-rays at the Taiwan Photon Source (TPS), the profile of a highly precise grating is required at various curvatures. The slope error could be decreased to 0.1 µrad (rms) at a thermal load with a specially designed bender having 25 actuators. In the meantime, a long-trace profiler (LTP) was developed in situ to monitor the grating profile under a thermal load; it consists of a moving optical head, an air-bearing slide, an adjustable stand, and a glass viewport on the vacuum chamber. In the design of this system, a test chamber with an interior mirror was designed to simulate the chamber in the beamline. To prevent an error induced from a commercial viewport, a precision glass viewport (150CF, flatness 1/150 λ rms at 632.8 nm) was designed. The error induced from the slope error of the glass surface and the vacuum deformation was also simulated. The performance of the optical head of the LTP in situ (ISLTP) has been tested in the metrology laboratory. The sources of error of this LTP including the linearity and the glass viewport were corrected after the measurement. For the beamline measurement, an optical head was mounted outside the vacuum chamber; the measuring beam passed through the glass viewport to measure the grating profile in vacuum. The measurement of the LTP after correction of the above errors yielded a precision about 0.2 µrad (rms). In a preliminary test, an ISLTP was used to measure the grating profile at soft X-ray beamline TPS45A. The measured profile was for the bending mechanism to optimize the slope profile. From the measured energy spectrum, the slope error of the grating was estimated with software for optical simulation to be about 0.3 µrad (rms), consistent with our estimate of the ISLTP. In the future, it will be used to monitor the thermal bump under a large thermal load. In addition, an ISLTP was used to monitor the properties of optical elements—the twist and radius in the beamline during the installation phase.

Investigation on lateral resolution of surface slope profilers

“…The LTP-II optical sensor is based on the pencil beam interferometer, initially suggested and patented by K. Von Bieren in 1985 [24][25][26] and firstly applied in long trace profilers for precision characterization of x-ray mirror surface slope topography in 1986 [27,28]. Having a number of important modifications to the optical schematic [29][30][31], LTP type profilers remain one of only two classes of surface slope measuring tools that are broadly in use at metrology laboratories of x-ray facilities [14][15][16][17][32][33][34][35][36][37][38][39][40][41][42][43].…”

Developing bendable air bearing slide supporting long trace profiler to compensate gravity effect during measuring process

“…Furthermore, with the assistance of a long trace profiler (LTP) [2], the 25-actuator surface benders obtained the necessary feedback signals during installation, enabling them to achieve the desired surface profiles and eliminate deformations caused by optical polishing and installation stress. In addition, in situ LTP achieves the required surface profiles in real-time slope profile feedback at the beamline, effectively helping the 25-actuator surface eliminate thermal load-induced deformations [3] [4].…”