Synchrotron Radiation (SR) mirrors are ultra precision optical components with very high requirements to shape accuracy and smoothness. According to the special functions mirrors with different shapes are used. The dimensions of such mirrors extend from some tenfold of millimetres to a length of more than one meter. Commonly such mirrors are made of single crystal silicon, Zerodur(R), ULE(R) glass and in rare cases of silicon carbide, special steel or Glidcop(R). Some considerations lead to the result that also tungsten is an interesting alternative material for SR-mirrors. The paper presents the design, some results of the ultra precision machining and some functional parameters of the SR-mirror prototype.
The performance of x-ray beamlines at 3rd generation synchrotron radiation sources and Free Electron Lasers (FELs) is limited by the quality of the state of the art optical elements. Proposed FEL beamlines require optical components which are of better quality than is available from the optical manufacturing technology of today. As a result of a joint research project (Nanometer Optik Komponenten - NOK) coordinated by BESSY, involving both metrologists and manufacturers it is possible now to manufacture optical components beyond the former limit of 0.1 arcsec rms slope error [1, 2]. To achieve the surface finishing of optical components with a slope error in the range of 0.04 arcsec rms (for flat or spherical surfaces up to 300 mm in length) by polishing and finally by ion beam figuring technology it is essential that the optical surface be mapped and the mapping data used as input for the multiple ion beam figuring stages. Metrology tools of at least five times superior accuracy to that required of the component have been developed in the course of the project. The Nanometer Optical Component measuring Machine (NOM) was developed at BESSY for line and area measurements of the figure of optical components used at grazing incidence in synchrotron radiation beamlines. Surfaces up to 730 cm2 have been measured with the NOM a measuring uncertainty in the range of 0.01 arcsec rms and a correspondingly high reproducibility . Three dimensional measurements were used to correct polishing errors some nanometers high and only millimeters in lateral size by ion beam treatment. The design of the NOM, measurement results and results of NOM supported surface finishing by ion beam figuring will be discussed in detail. The improvement of beamline performance by the use of such high quality optical elements is demonstrated.
Modern synchrotron radiation sources of the 3rd generation like BESSY II, Spring-8 and others with their high brilliance beam characteristics need very high quality optics to exploit the full power of this radiation. For the grazing incidence reflecting type of that optics (flat, spherical or aspherical) besides roughness the slope deviation error is the most important spec, which has to be improved to meet the present and future performance requirements. Together with partners from industry we investigate and develop on the one hand surface figuring and polishing techniques for final finishing by using mainly ion beam milling technology and on the other hand we improve and make use of the combination of the surface shape measurements by means of interferometry, long trace and auto-collimation profilometry. We aim to achieve the following slope deviation errors on silicon optical elements: flat surface 310 mm long 0.03 arcsec rms, flat surface 100 mm long 0.02 arcsec rms and elliptical cylinder surface 210 mm long 0.1 arcsec rms. This is a five to ten-fold improvement compared to the present state of the art in production. To achieve the demanding specification it is necessary to measure and to deterministically machine the surface over a wide range of spatial wavelength down to the sub-millimeter range. In depth scale the sub-nanometer shape error level has to be achieved. The roughness of about 0.2 nm rms has not to be increased during the shape finishing.
Traditional optical manufacturing methods employing both conventional and modern interferometric techniques, enable one to measure surface deviations to high accuracy, e.g. up to (lambda) 100 for flats (6 nm P-V). In synchrotron radiation applications the slope error is an important criterion for the quality of optical surfaces. In order to predict the performance of a synchrotron radiation mirror the slope errors of the surface must be known. Up to now, the highest achievable accuracy in the production of synchrotron radiation mirrors and in the measuring methods did not fall significantly below the 0.1 arcsec rms limit (spherical and flat surfaces). A long-trace profiler (LTP) is ideally suited for this task since it directly measures slope deviations with high precision. On the other hand, using an LTP becomes very sensitive to random and systematic errors at the limit of 0.1 arcsec. The main influence is the variation of the surrounding temperature in creating temporal and local temperature gradients at the instrument. At BESSY both temperature and vibrations are monitored at the most sensitive points of the LTP. In 1996 BESSY started a collaboration with a neighboring optical workshop combining traditional manufacturing technology with quasi- in-process high precision LTP measurements. As result of this mutual polishing and LTP measuring process, flat surfaces have been repeatedly produced with slope errors of 0.05 arcsec rms, e.g. 1 nm rms and 3 nm P-V (approximately equals (lambda) /200).