FERMI@Elettra is a VUV/Soft X-ray Free Electron Laser (FEL) user facility under commissioning in Trieste, Italy. It provides a spatially coherent transform-limited photon beam in the sub-ps regime with high fluence and tunable wavelength. One of the FERMI beamlines, TIMEX, will be dedicated to the study of matter under extreme and metastable conditions, created and probed by the FEL radiation. Moreover, an active optics dedicated to perform the beam shaping at focus is needed in order to provide the necessary flat-top intensity distribution for heating the sample uniformly. In this work the principles of the beam shaping applied to the TIMEX beamline will be discussed as well as the adopted solution. Ray tracing simulations will be shown for theoretical mirror profiles as well as the metrological measurements with an interferometer and the Long Trace Profiler (LTP).
FERMI@Elettra is a Free Electron Laser (FEL) under commissioning at Sincrotrone Trieste. It will
provide an almost fully coherent and transform limited radiation with a very high brilliance in the
VUV/Soft X-ray range. This article describes the working principles of the Variable Line Spacing
diffraction gratings applied to the photon energy spectrometer as well as the design concept, ray tracing
and efficiency simulations. Metrological results at various spatial frequencies of the optics involved
and the first characterization results with FEL radiation will be shown.
The optical metrology laboratory of Elettra is equipped with some state of the art instruments for the characterization of high precision optical components for the UV and x-ray energy range. Among them, the most important is the Long Trace Profiler, which is capable of very accurate measurement of the shape of long aspheric mirrors. It is a direct slope measurement device, able to measure slope errors below the mrad level, once properly operated and calibrated. Our device is an LTP II model dating back to 1992. Nevertheless, it has been deeply in house modified during these years. Recently we have assembled a second optic head (OH) that could be used in spite of (or together with) the original one. This second OH works without folding mirrors and uses a set of short focal distance Fourier Transform lenses. The absence of folding mirrors reduces the source of systematic errors and the use of short focal distance lenses increases the angular acceptance of the instrument. This fact is particularly useful when short radius of curvature mirrors as well as high groove density variation gratings have to be measured. Some other modifications have been made to help the stitching procedure or to change the measurement set-up. These will be described in details.
The interest of the scientific community in the use of synchrotron radiation has become higher and higher with the improvement of instrumentation and with publication of better and better results. For this reason, the concept of standard beamline could not be used anymore, and a lot of solutions must be considered to satisfy the requests of the different users. A very important part of these "solutions" involves mirrors, gratings and crystals adopted to carry out the light from the source to the experimental chamber. In the last years, for instance, we have seen an increased interest for the mechanically deformable mirrors, as well as normal incidence mirrors (for IR or UV photons). From the point of view of the optical metrology, this implies the use of different methods and different instruments to guarantee the quality of the optics and consequently of the delivered photons. In this work, we compare the performance of two "state of the art" instruments, aimed at the non-contact measuring of optical surface profiles. The first one is an in house modified version of the Long Trace Profiler (LTP) developed for grazing incidence optics by P.Z. Takacs and Al. The second is a Fizeau like interferometer (a WYKO RTI 4100), primarily used for 2D mapping of surfaces. The aim of this paper is to outline when, according to our experience, an instrument is preferable with respect to the other, what are the limits of both and what kind of improvement could be made. Some examples will be reported. Spatial frequency, calibration and systematic errors will be compared and outlined.
A diffraction plane grating with single-layer coating able to reach photon energy up to 3 keV (possibly 4 keV) will be adopted at the TwinMic beamline at ELETTRA. The TwinMic beamline will exploit the unique capabilities of the novel twin X-ray microscope, which combines scanning and full-field imaging microscopes in a single multipurpose end-station. The needed moderate energy resolving power will be provided by a variable included angle plane grating monochromator working in a collimated light mode (also known as collimated SX700). This configuration allows freely selection of the incidence and diffraction angles at the grating, therefore permitting, for instance, to optimize its efficiency. This monochromator uses two mechanically ruled gratings to cover a very wide working energy range. The first grating goes from 150 eV to 1000 eV while the second goes from 600 eV to 4 keV. The two gratings were ruled using the CARL ZEISS Grating Ruling Engine GTM6, which is operated under interferometric control. The high-energy plane grating, with a line density of 600 lines/mm, has a triangular profile with a blaze angle of 0.4° and an apex angle of 178°. The grating profile is ruled on a silicon substrate and is covered with a 30 nm thick gold film. The small blaze angle permits one to work in blaze condition at very grazing incidence angles and therefore allows reaching high photon energies not accessible by means of conventional gratings.
The Long Trace Profiler (LTP) has proved to be one of the major metrological aids for the characterization of synchrotron radiation optics. Currently the optical components installed at the beamlines face higher and higher demands, requiring a precise calibration and control of the measuring conditions. One important parameter to be considered while scanning is the temperature drifts afflicting the measuring sessions. We will review our experiences about the influence of this parameter on the LTP ability in measuring very accurate optical surfaces. It is possible to discriminate at least four major sources of perturbations due to temperature changes: air turbulence, deformation in the optical train inside the LTP optics head, deformation of the optical surface under test (SUT) and deformation of the holders of the SUT itself. Some addresses on the curing of these perturbations can be obtained.
Two bendable elliptical cylinder mirrors arranged in a Kirkpatrick-Baez (KB) geometry are installed at the Nanospectroscopy beamline at ELETTRA for refocusing soft x-rays provided by an APPLE II type undulator. This achromatic focusing device delivers the beam to a micrometer-scale, high photon density spot, which is the source for a Spectroscopic Photoemission and Low Energy Electron Microscope (SPELEEM). A similar second pair of KB mirrors will refocus the monochromated light in a second experimental station for a different imaging microscope. These four mirrors, developed by S.E.S.O., are manufactured from Glidcop™ in a U shaped design with 380mm length. They are electroless nickel plated for polishing and are bent into an elliptical shape applying two unequal end moments. They have been tested in the optical metrology laboratory of ELETTRA using an in-house modified version of the Long Trace Profiler (LTP): the surface slope variation as a function of the bender actuators has been measured to characterize the behavior of the bender mechanism and the accuracy of the elliptical profiles that can be achieved. Both metrological optical data and x-rays performances show the achievement of a microradian accuracy for the different profiles in which each mirror can be bent and the possibility to vary the focal distance by about 30-40% around the nominal value.
The optics metrology laboratory of Sincrotrone Trieste is operating some non-contact interferometers since 1992, in order to characterise slope errors, figure deviations and surface roughness for synchrotron radiation optics (SR) up to 1.4 metres in length, prior to their installation at the beamlines. During these years, prompted by the increasing needs of experimentalists, the requirements for FEL and SR optical components have become more and more severe. We will review here the history of our measurements during the last nine years, comparing the match between the given specifications and measured optical quality of the delivered items. We will also illustrate which has been the evolution of the main optical concepts, that has ultimately boosted the suppliers to develop machining and testing methods to a novel level of accuracy.
With the advent of third generation synchrotron radiation sources, more flux was available for the experimentalist. At the same time, the request in term of spot dimension and energy resolution rapidly increased. For this reason, opticians try to design beamlines with higher and higher performances. To this end the shape of every optical component of a beamline is specified to have very tight constrains, because every small figure error produces a sudden reduction in terms of the overall performance. Nevertheless, the necessity to positioning and cooling the components implies the presence of a safe clamping system which unavoidably would modifies the shape of the component, causing possible reduction of resolving power or increasing the spot dimension. Thus it is not sufficient to measure accurately the slope or the profile of a mirror in laboratory before the mounting, but it is useful to test it also after this procedure. We, at ELETTRA, have measured by means of a modified version of the LTP II (Long Trace Profiler) several mirrors and gratings before and after their clamping, in order to estimate the effect of the holder on the final performances of the beamlines. Since our LTP II measures directly the local slope of the surface under test with a repeatability better than 0.02 arcsec on a 1 meter long optical surface, it is very easy to single out any small distortion of the tangential profile introduced by the mounting system. Different kinds of supports for both small and large optical elements, were taken into consideration and the effect of the deformation induced by them on the beamline performance was simulated and will be presented here together with the results of each measurements. The results give us a way to select properly the kind of clamping and invite the opticians to try to take into consideration also this effect before designing a complex beamline.
The diffraction gratings are widely used to monochromatize and even focus the soft X-ray radiation produced by the high brilliance third generation synchrotron radiation sources. Nevertheless, the final performance of an instrument that uses a diffraction grating is sensitive to any figure error and to any undesired groove density variation along the surface of the grating itself. Therefore, typical requirements are 0.1 - 0.2 arcsec (even less) on the residual slope errors (after proper shape subtraction), while the groove density is required to be constant along the surface with a percentage error below 0.1%. Vice-versa, sometimes groove density variation is required along the surface to correct spherical aberrations or to change the focal properties of a grating. Since the gratings, in the soft X-ray region, work in grazing incidence mode, the ideal instrument to measure it is a mono-dimensional profilometer. At ELETTRA, the Italian third generation synchrotron radiation source, we have an in house modified version of the Long Trace Profiler.
The Long Trace Profiler has become the instrument of choice for mirror surface figure test and slope error measurement of synchrotron radiation and x-ray astronomy optics. In order to achieve highly accurate measurements with the LTP, systematic errors need to be reduced by precise angle calibration and accurate focal plane position adjustment. A self-scanning method is presented to adjust the focal plane position of the detector with high precision by use of a pentaprism scanning technique. The focal plane position can be set to better than 0.25 mm for a 1250 mm focal length FT lens using this technique. The use of a 0.03 arc second resolution theodolite coupled with the sensitivity of the LTP detector system can be used to calibrate angular linearity error very precisely. Some suggestions are introduced for reducing the system error. With these precision calibration techniques, accuracy in the measurement of figure and slope error on meter-long mirrors is now at a level of about 1 (mu) rad rms within whole testing range of the LTP.
Modern third generation storage rings, require state-of-the- art grazing incidence x-ray optics, in order monochromate the Synchrotron Radiation (SR) source photons, and focus them into the experimental stations. Slope error tolerances in the order of 0.5 (mu) Rad RMS, and surface roughness well below 5 angstrom RMS, are frequently specified for mirrors and gratings exceeding 300 mm in length. Non-contact scanning instruments were developed, in order to characterize SR optical surfaces, of spherical and aspherical shape. Among these, the Long Trace Profiler (LTP), a double pencil slope measuring interferometer, has proved to be particularly reliable, and was adopted by several SR optics metrology laboratories. The ELETTRA soft x-rays and optics metrology laboratory, has operated an LTP since 1992. We review the basic operating principles of this instrument, and some major instrumental and environmental improvements, that were developed in order to detect slope errors lower than 1 (mu) Rad RMS on optical surfaces up to one meter in length. A comparison among measurements made on the same reference flat, by different interferometers can give some helpful indications in order to optimize the quality of measurement.
A Variable included Angle Spherical Grating Monochromator was installed on beam line at ELE1TRA in Italy. This unique monochromator is capable of high resolving power. For certain combinations of
energy range and grating, resolving powers in excess of 20,000 are theoretically possible. To achieve these high resolving powers, precise mechanical alignment of all internal components is a necessity. Parallelism of rotation axes and optical surfaces must be better than 10 arc seconds, and position errors in locating components are required to be in the range of 10 microns. During the final alignment of this monochromator on the beam line, it was found to have errors 50 times larger than the specifications. Realignment of the internal components was necessary to improve the instrument
performance. A penta-prism method was suggested and an autocollimator test method was used to correct parallelism between components in the monochromator while on the beam line. However, it is recommended to prealign and check the monochromator accurately in the optical metrology laboratory prior to the final beam line alignment. This two-stage procedure will ultimately save time and allow the beam line to achieve the highest possible performance in the shortest amount of time.
The first in-situ distortion profile measurement of a high heat load mirror by use of the penta-prism LTP is presented. A maximum height distortion of 0.47 micron in tangential direction over a length of 180 mm was measured for an internally water-cooled mirror of a undulator beam line at ELETTRA while exposed to a total emitted power of 600 W (undulator gap 30 mm and current 180 mA). The experiment has an accuracy and repeatability of 0.04 micron. The test schematic and the test equipment are presented. Two measuring methods to scan a penta-prism being installed either outside or inside the vacuum chamber are introduced. Advantages and some possible applications of adopting the penta-prism LTP to make the in-situ profile test are explained.
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