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The development of an in-vacuum long trace profiler (IVLTP) which can provide ultra high precision real time measurements on the surface slope of optical elements is reported. The unique features of this instrument include a specifically designed ultra high repeatability linear slide, a pentamirror composed of two plane mirrors of ultra low slope error, and an ultra high vacuum (UHV) compatible complementary metal oxide semiconductor (CMOS) image sensor. The mechanical system is made of Invar 36 alloy to reduce the room temperature drift effect during data acquisitions. Each IVLTP scan completes in 120 seconds, covering a 168 mm of surface length and generating more than 5000 slope data points. The root mean square (RMS) of the difference between two consecutive scans was found to be 0.0081 μrad on average and 0.0045 μrad RMS at its best. The main application of this IVLTP is to provide the real time feedback signals needed for controlling the 25-actuator surface benders of the active mirrors and active gratings installed in the ultra high resolution soft X-ray beamlines at the Taiwan Photon Source (TPS).

To achieve an ultrahigh-resolution with soft x-ray beamlines at the Taiwan Photon Source (TPS), the slope error of a highly precise grating must be at the level of 0.1 μrad (RMS) under thermal loading with various curvatures. Some optics generally operate under a high power density from an undulator magnet on a beamline. The thermal load introduces a thermal bump in the optics profile and degrades the beamline performance, such as the energy resolution and beam size. Hence, active X-ray mirrors are emerging as primary solutions to meet the performance requirements of TPS. First, active X-ray optics were assembled in the laboratory and bent to the desired surface using an in-house long-trace profiler (LTP). When the optics were installed in the beamlines, LTP in situ (ISLTP) was used to fine-tune the mirrors under various operating conditions. This paper introduces the design and construction of ISLTP with a precision of 0.1 μrad (RMS) developed to measure the mirror profile in soft x-ray beamlines. The ISLTP can provide feedback perturbations owing to localised heat loading from the X-ray beam and thermal changes in the ambient environment, and its performance and in situ measurements are presented.

We have constructed a new soft X-ray beamline, namely the Taiwan Photon Source (TPS) 45A NSRRC-MPI beamline, to facilitate submicron angle-resolved photoemission spectroscopy (ARPES) experiments with ultra-high energy resolution. The beamline uses an elliptically polarized undulator with 46 mm magnet period (EPU46) that provides photon energies from 280 to l500 eV with horizontal and vertical linear polarization, as well as left and right circular polarization. The vertical focusing mirror (VFM) and the active grating monochromator (AGM) utilize the novel 25-actuator optical surface bender developed for ultra-high resolution soft X-ray spectroscopies. With the surface slope error being reduced down to 0.03 f! ad root-mean-square (rms) by the bender as verified by the long trace profiler (LTP) measurements, the ray-tracing simulation shows that an energy resolution of 5 meV can be achieved at 750 eV photon energy and the beam spot size can each 0.5 μm × 0.4 μm at the sample position. By adjusting only the 6 actuators to control the surface major profile, our preliminary results show that the VFM is able to focus 70% of the photon flux from EPU46 through an entrance slit set at 1.9 ¼m opening. As deduced from the measured ARPES data at the Fermi-level of Au, the energy resolving power of the monochromator was found to reach 34,000 full-width-half-maximum (FWHM) at 850 eV photon energy. By using the in-vacuum LTP measurement system currently under development and all 25 actuators of the surface bender, we anticipate to further reduce greatly the intrinsic and thermal-induced slope errors of the VFM and AGM. It is feasible that the overall ARPES energy resolving power can reach the l40,000 target in the soft X-ray spectral region.

To facilitate ultra-high energy resonant inelastic X-ray scattering (RIXS) and angle-resolved photoemission (ARPES) in the soft X-ray spectral region, a high precision 25-actuator optical surface bender has been designed, constructed and tested. The mechanical design of the bender is based on a novel high precision, high resolution flexure hinge adjustment system. Among the 25 actuators, 6 actuators are for controlling the surface major profile by setting positions, while the remaining 19 actuators are for fine tuning the profile by setting forces. The long trace profiler measurements show that, before bending, the intrinsic surface slope error of the flat mirror or grating is around 0.3 µrad root-mean-square (rms). By using all 25 actuators of the bender, a surface slope error as low as 0.023 µrad rms has been achieved with the surface profile set to a desired higher degree polynomial. The bender can also be used to eliminate the slope error caused by thermal deformation set off by the synchrotron radiation heat load. Based on the ray tracing simulation, gratings with an overall surface slope error better than 0.03 µrad rms are sufficient to reach an energy resolving power of 100,000 and 140,000 for RIXS and ARPES, respectively.

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.