Xray optics for ESRF beamlines: Problems and projects (invited)Andreas K. Freund Citation: Review of Scientific Instruments 63, 413 (1992); doi: 10.1063/1.1142718 View online: http://dx.doi.org/10.1063/1.1142718 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/63/1?ver=pdfcov Published by the AIP Publishing

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High Brilliance Beamlines and Cool Optics

X-ray optics for ESRF beamlines: Problems and projects (invited) Andreas K. Freund European Synchrotron Radiation Facility, B. P. 220, 38043 Grenobe Csdex, France

(Presented on 15 July 1991)

Third generation, dedicated synchrotron radiation sources will produce x-ray beams of unprecedented quality and power. This calls for substantial research and development of adequate instrumentation. In particular, the unprecedented properties of the new sources must be followed up by a complete review of the optical devices which constitute the first part of the beamline after the beryllium window. This article gives a short account of where and what the problems are in x-ray optics for the European Synchrotron Radiation Facility and what is presently going on and being envisaged to solve these. This review is divided into problems concerning conservation of the low source emittance such as the optical quality of mirror surfaces and high crystalline perfection of monochromators, and the important problem of how to preserve the very high brilliance resulting in unprecedented x-ray total power and power density generated by wigglers and undulators. Finally, a brief presentation of the ESRF optics laboratory is given.

I. INTRODUCTION: THE UNPRECEDENTED

The 6 GeV storage ring of the European Synchrotron Radiation Facility (ESRF) with its various types of syn- chrotron radiation sources such as wigglers, bending mag- nets, and undulators will provide the scientific community with x-ray beams of unprecedented properties. Gaining about ten orders of magnitude in brilliance with respect to a rotating anode x-ray tube is often difficult to imagine in concrete terms, even for an experienced experimentalist. The various problems occurring at the existing, less pow- erful synchrotron radiation laboratories are already diffi- cult to solve, as most of us are aware. Of course, extrapo- lation from existing experience will help to prepare ESRF instrumentation, but the “unprecedented” expresses a cer- tain risk factor: not all effects produced by the considerable increase of source strength will be fully anticipated for the instrumentation-and even much less for the experimental results. This was recently evidenced at CHESS, where the effect of Compton scattering from a beryllium/copper beam stop was underestimated and an unexpected though “expectable” x-ray heating up of the vacuum tube was observed.’

SPRING 8 project in Japan (8 GeV), where according to the increased electron energy, still higher x-ray power will be produced. Even if efficient cooling loops will permit to avoid melting of slit and window materials these can be heavily deformed and even be destroyed by the strong ther- mal stresses. Fortunately, it has been shown that beryllium windows, beam shapers, and filters should withstand the high power loadings when designed with care and using state of the art technologies.

But then comes the next challenge: how to maintain the low emittance and, still more complex, the high bril- liance of the source; in other words, how to minimize the unavoidable loss of the useful photons all through the var- ious components of the beam line optics until the sample (not mentioning detectors and their specific problems).2 Model calculations-both rough estimates and sophisti- cated finite element analysis-show that the loss can often be very high, see, for example, recent experiments on liquid gallium cooled silicon single crystals,4 where a factor 2 was already lost when using an undulator beam of 29 W/mm*, i.e., five times less than the power density expected at the ESRF. So the crucial question is: up to which level of x-ray power can the optics conserve brilliance?

Obviously, the final usefulness of this huge flux of ra- In the following the discussion of the performance of diation strongly depends upon the capability to manipulate optical elements will be divided into two types of problems: and condition x-ray beams of high brilliance.* At existing those related to emittance and those related to brilliance. facilities the predictions of substantial intensity or flux in- Whereas the first concern the size of the volume in the creases were often not confirmed when they were con- six-dimensional phase space occupied by the photons, the fronted with the hard reality of radiation damage, thermo- second correspond to the phase space density: Ideal optics mechanical problems, and stability occurring as soon as conserves both emittance and brilliance when transforming some piece of material is put into the beam. And at the the shape of the phase space volume in order to match the ESRF the beams are expected to be even more powerful3 beam parameters to the experiments. A degradation of than at second generation synchrotron sources: multipole emittance is equivalent to an increase of phase space vol- wigglers will generate a total power up to 6 kW and un- ume, for instance, of beam divergence at constant (virtual) dulators will emit beams with a power density of up to 160 source size or magnification. Thus an increase of emittance W/mm*, not to speak of other forthcoming sources such as will always entail a decrease of brilliance. Strictly speaking, the Advanced Photon Source in the U.S. (7 GeV) and the emittance and brilliance are conserved if it is possible to

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obtain the original source characteristics by inversing the x-ray beam path through the optics. For example, the ther- mal deformation of a mirror induced by cooling does not decrease brilliance because it is theoretically possible to put a mirror with a very special surface shape in the reflected beam so that the original beam quality is recovered. In the following the terms emittance and brilliance should be un- derstood as being x-ray beam parameters in a more prac- tical sense.

II. CONSERVATION OF EMITTANCE

Before discussing in detail the heat load problems aris- ing from the high x-ray power, the following question must be answered: considering the small source size and beam divergence and neglecting any heat load effects for the mo- ment, are the optical devices capable of transmitting the consequently low source emittance downstream to the ex- periment? There are both fundamental and technological limits. The former ones, for instance, are determined by aberrations occurring in focusing schemes, the latter de- pend on the quality of single crystals and on surface and interface imperfections of mirrors and multilayer struc- tures, respectively.

As an example for aberrations let us consider the spherical aberrations that frequently occur because the ideal aspherical surfaces are often replaced by cylinders or toroids that are easier and cheaper to manufacture. Until now these aberrations have been described analytically only for point sources, maybe because the effects were com- parable or small than other effects, for instance, because the source size was already relatively big. However, in the case of the ESRF storage ring the source size is small and it is quite useful to dispose of an analytical formula for predicting the effect of finite source size instead of running a complex ray-tracing simulation for calculating the amount of focal blur caused by spherical aberrations. In- terestingly, it turned out that there is an optimum magni- fication for each type of source, which can be quite differ- ent from unity being usually taken as the value giving the smallest amount of aberrations5 For a cylindrical mirror and a bending magnet having a source size of 0.3 mm the optimum magnification is, for example, 4 instead of 1. Therefore, a careful analysis of all kinds of effects must be carried out anew for different types of sources and appli- cations in order to optimize the geometrical configuration of a beamline.

Equally important are the technological limits, not only the quality of single crystals, mirrors and multilayered materials, and the stability of the materials when exposed to very high radiation doses, but also strain-free mechani- cal mounting, precise spatial and angular positioning,and overall stability of the optical elements. A good starting point for the definition of the optics tolerances is given by the stability of the source parameters expected from the machine, i.e., f 10% of the source size and of the beam divergence. The distances from the source to the first op- tical element and from the latter to the sample (focus) are of the order of 25 m, the source size is about 200 pm (FWHM) and the beam divergence of high p undulators is

414 Rev. Sci. instrum., Vol. 63, No. 1, January 1992

400 I 3 bi

Ge (220) E=1381 keV

q 300- 0.025”~121ntsd

01 3 , I I -0.4

RCIC!%G ANGh (AR&C) 0.4

FIG. 1. Diffraction pattern of two Ge single crystals (220 reflection) in the Laue case recorded with gamma rays of 1381 keV energy.

as small as 20 ,urad. Thus the tolerances of, say, a focusing mirror correspond to a rms slope error of the order of 1 prad. In order to limit the losses due to diffuse scattering3 to about 10% the microroughness of mirror surfaces must be inferior to a few tenths of nm. The same is true for multilayer structures. As to single crystals, the variations of lattice plane orientations and distances in the diffracting crystal volume should not exceed 1 ,urad and 10 - 6, in the extreme case 10 - *, respectively. Whereas the penetration depth of x-rays in mirrors and multilayers is very small compared to the beam height or width, it can become a non-negligible source of beam degradation for single crys- tals, in particular at high x-ray energies and for strong demagnifications.

With some research and development effort we are, in general, capable to meet the above requirements using present-day technologies (involving some investment of capital and personnel, however), and the performances of real optical elements are not too far from what is theoret- ically possible. For example, extremely perfect silicon sin- gle crystals were developed for different applications by the semiconductor industry that match in many respects the experimental requirements: what would synchrotron radi- ation be without silicon? More recently, highly perfect ger- manium single crystals have been successfully grown (Metal Hoboken Overpelt, Belgium) that were tested on a very accurate double crystal gamma-ray spectrometer in- stalled by the Quantum Metrology Group of the National Laboratory of Science and Technology (NIST, Gaithers- burg, DC) at the high flux reactor of the Institut Laue- Langevin in Grenoble. A transmission (Laue case) rocking curve is shown in Fig. 1 that was taken with 1.381 MeV gamma rays from a neutron activated Ti source. For very precise crystal characterization high energy radiation is quite useful because rocking curves are very narrow and the bulk of big crystal ingots can be studied in a destruc- tion-free way since absorption is very small.6 When fitting the measured data shown in Fig. 1 to the theoretical pat- tern for two ideal crystals it turned out that the excess width produced by crystalline imperfections was as small as 0.007 arcsec or 33 nrad,i.e., small enough for most cur- rent applications.

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FIG. 2. Bending device for opti- cal el lements (equipped with a Si singlf : crystal) based on a mono- lithic design using flexur ‘e hinges.

A very promising type of optical element is provided by layered synthetic materials that permit to obtain a wider bandpass and thus more flux than perfect single crystals and that have very good focusing properties. Recent tests of samples fabricated by OVONICS (Troy, MI) and Phil- ips Research Laboratories ( Lime%Brevannes, France) proved to give high reflectivities and to survive in high power density x-ray beams.t3 However, further improve- ments are needed and the size of the multilayers must be increased requiring very specific R&D work. Therefore, a major investment was made to build a multilayer fabrica- tion facility at the ESRF. First results are expected in the beginning of 1992.

Much case has to be taken when preparing, handling, and mounting all these elements,and often new technical solutions have to be found for specific cases. Let us take as an example the sagittal focusing by single crystals. In the past tunable double crystal fixed exit monochromators were built and are commercially available where the first crystal is flat and cooled and the second is sagittally curved by a device permitting to vary the radius of curvature. For the ESRF a new design of such a device was carried out (Grenoble Modular Instruments) and a prototype will be tested in the beginning of 1992. With bent crystals hori- zontal focusing of monochromatic radiation can be achieved for various energies at constant focal distance, which is best chosen with a magnification of one-third to give the lowest amount of aberrations. There is, however, a technical problem that arises from the elastic behavior of a bent plate. When bending a plate in one direction to create

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a concave shape, a convex curvature called anticlastic bending is observed in a direction perpendicular to the synclastic bending. For small deformations, i.e., the height of the arc being small with respect to the thickness of the plate, the ratio of the two radii of curvature is equal to Poisson’s ratio. If the two crystals are of equal nature and diffract from the same set of lattice planes, this leads to a reduction of transmission through the double crystal ar- rangement.

Two solutions were found to avoid this problem. The first uses narrow slots grooved into the front side of the plate, the second uses stiffening ribs on the back of the plate. Calculations show that for the ESRF beamlines both solutions are not satisfactory. Therefore, we have devel- oped a new type of crystal bender where thin crystal plates are kept straight along their edges by clamping them onto two flat supports, which apply opposite but otherwise equal moments to the edges. The machining both of the crystal and of its support must be very precise and during the change of curvature the rotation of the supports must be accompanied by a small translation toward the crystal center. Such a bender was built and already successfully tested in meridional focusing geometry.7 A photograph of the first device of this type is shown in Fig. 2. Both the studies of the Ge crystal perfection and the development of the bender were made in collaboration with NIST.

There are several other ESRF projects in optics that are not related to heat load problems, for instance, the R&D program on Bragg-Fresnel optical elements (BFO), which consist of circular or elliptical microstructures

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(Fresnel zones) machined into single crystals and multi- layered materials by anisotropic or plasma etching. These elements should provide aberration-free microfocusing (down to 1 pm and maybe less) and produce high energy resolution and focusing in backscattering. This develop- ment is performed in collaboration with the Institute of Microelectronics Technology in Chernogolovka (USSR).

III. CONSERVATION OF BRILLIANCE

As already mentioned, important problems arise from the high power and power density produced by third gen- eration storage ring sources. At 30 m from the source, power densities up to 150 W/mm* normal to the beam are generated that must be handled by the first optical ele- ment (s). Fortunately, this load can be decreased by win- dows and filters and, furthermore, by the angle of incidence so that the final power density normal to the crystal (or mirror) surface will be by more than about one (or two) orders of magnitude less. Then cooling is not too difficult a problem but thermal deformation due to gradients parallel and perpendicular to the surface of the optical element gives rise to a convex shape. The corresponding “thermal slope error” exceeds by far what is acceptable (see “con- servation of emittance” mentioned above) and it is there- fore absolutely vital to find a solution to this problem.

Many workshops, meetings, and conferences were held on this topic* and at least as many possible solutions were proposed (a special Satellite Meeting of this Conference was devoted to this subject as well). As main ESRF projects we decided, on one hand, to further develop and to concretize one of these solutions, namely cyrogenic cooling of single crystals and, on the other hand, to add a new one that was inspired from astronomy: adaptive mirror optics. These projects are complementary to techniques studied by other groups, such as microchannel cooling technology, active single crystal optics, and liquid gallium cooling.

Cryogenic cooling of silicon (germanium) crystals down to about 125 K (50 K) can drastically reduce ther- mal deformation because at these temperatures thermal expansion vanishes and thermal conductivity increases by a factor 3. First cryocooling experiments were performed in June 1990 at the Harwi wiggler at the Hamburger Syn- chrotronstrahlungslabor (in collaboration with HASY- LAB) and confirmed the theoretical predictions for, how- ever, relatively low power density.’ Another series of tests was carried out at and in collaboration with the National Synchrotron Light Source at Brookhaven National Labo- ratory (beamlines X17 and X25) in January 199 1 .l” The results are shown in Figs. 3 and 4. The solid line in Fig. 3 is the temperature dependence of the ratio O/K, where a is the thermal expansion coefficient and K is the thermal con- ductivity. The full circles are the full widths at half-height of rocking curves recorded with two perfect Si single crys- tals (333 reflection, E = 18.54 keV)., where the first one was positioned in an x-ray beam of 200 W/mm* power density (normal to the beam) and cryocooled with liquid nitrogen. Figure 4 shows two rocking curves, one taken at room temperature, the other at 99 K. The residual “ther- mal slope error” at low temperature is of the order of 1

416 Rev. Sci. Instrum., Vol. 63, No. 1, January 1992

15-

temperature (K)

FIG. 3. Temperature dependence of the materials parameters CX/K for Si and of the measured rocking curve width showing the expected decrease of thermal deformation at low temperature.

arcsec, i.e., totally acceptable and comparable to the effect of mounting strain. The total power, however, was only 75 W, and the cryocooling technique has to be tested further at higher power. Although substantial technical problems still remain to be solved (thermal transfer at low temper- atures versus strain-free mounting, coolant connections, in- tegration into double crystal fixed exit monochromators, and into beamlines) we are now much more optimistic than one year ago and feel that cryogenic coohng is a suitable technique, at least for undulators. A yet unknown phenomenon is the formation of radiation-induced defects, which, at room temperature, instantly annihilate, but maybe not at Iow temperature.

In contrast to this opportunity to avoid thermal defor- mations we are presently trying to exploit a technique that will try not to prevent but to cure the “thermal disease:” adaptive optics. This method is not new and our problem “only” consists in adapting the adaptive technology used by astronomers to synchrotron x-ray mirrors. The term “adaptive” is currently defined as being the capability of an optical system to readjust its configuration (e.g., shape of

5- ’ I - * 1 * ’ Si (333) rocking curves

. E=1&5keV

IO 40 70 rocking angle (arcsec)

FIG. 4. Si 333 rocking curves at room and at low temperature in a beam of 200 W/mm* power density. The intensity scales are not the same.

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-50 -30 -10 10 30 50

MIRROR COORDINATES (Cm)

FIG. 5. The thermally induced deviation of the mirror surface from the ideal surface (upper curve), the mechanically generated deformation (lower curve), and the sum of both giving the resulting, corrected shape of the adaptive mirror consisting of three segments.

the optical surface) in real time by reading out the data of a device that analyzes permanently the configuration (shape) and by injecting them into a feedback control loop. Therefore, without analyzer and feedback loop such a system is not truly adaptive. In our case the principle looks quite simple: The mirror body is supported by a number of actuators while its surface is observed by an optical device; both are connected via a feedback loop and any (almost any!) desired shape of the surface can be created at will. Therefore, by steering the actuators in the right way, it should be possible to correct for any thermal deformation detected by optical inspection of the mirror surface.

The reality is much less simple, of course. The actua- tors will be piezoelectric stacks that have to be attached to the back of the mirror without any print-through to the front, they must withstand high radiation doses, the mirror must be in ultrahigh vacuum, etc. In order to decrease the cost of mirror body fabrication and to increase flexibility (very long mirrors can be assembled of several smaller pieces) we added segmentation: Our one meter long pro- totype mirror will consist of three segments, i.e., one 50 cm long central element plus two 25 cm long side pieces. All three will be made of special graphite coated with a 100 pm thick Sic layer that will receive the final figure and finish. The whole mirror will rest on two sets of 11 piezos: this number came out of an optimization calculation that was carried out for the forces of the actuators.” A Shack Hart- mann analyzer will be used as surface inspection device. According to a complete thermomechanical analysis (see Fig. 5)and ray-tracing (SHADOW) the adaptive tech- nique should be able to reduce the uncorrected thermal slope error of 363 prad down to a few prad. This mirror prototype was specified according to the needs of the ma- terials science beamline and is presently under construction.‘*

Again, there are several other projects relative to the heat load problem but they cannot be described within the scope of this presentation. Some ideas and experimental results were presented in this conference, for example, on

417 Rev. Sci. Instrum., Vol. 63, No. 1, January 1992

the performance of layered synthetic materials including cryocooling, l3 on the use and the efficiency of Si and Be mosaic crystals’4*6 and on the performance of thin Si crys- tal foils under high x-ray power density.

IV. OPTICS LABORATORY AND STAFF

ESRF has both the critical size and the organizational structure that are needed for establishing general support groups. In the past two years the Optics Group has built up a well-equipped x-ray optics laboratory. The capability of evaluating optical elements experimentally by various com- plementary techniques (not only by x-rays) is in fact very important in order to validate new concepts emerging from the goal to achieve a steadily ongoing improvement in the field of x-ray instrumentation. For the fabrication of opti- cal elements, these techniques are also very important as a feedback information.

Whereas mirrors will be entirely subcontracted out- side, single crystal monochromators are produced starting with big ingots bought from industry or from specialized laboratories, and layered synthetic materials will be fully “made in ESRF.” In fact, the most impressive apparatus of the laboratory will be the multilayer fabrication facility comprising sputtering and plasma CVD techniques and in situ control by x-ray diffractometry and laser ellipsometry. This apparatus is presently under construction.

Already operating are two commercially available op- tical test devices: a microroughness interferometer with a 0.1 nm (rms) resolution (WYKO model Topo 2D/3D) and a Fizeau interferometer (WYKO model 6000) for very accurate figure testing. They are used to characterize the surfaces of mirrors and multilayer substrates and to study the performance of bending devices. A long-trace profiler is presently under construction in collaboration with Brookhaven National Laboratory which will permit us to evaluate the figure of up to 2 m long mirrors. Adja- cent to this optical metrology laboratory is an x-ray triple axis diffractometer with high angular and positional reso- lution: better than 0.2 prad and 2 pm over 1 m, respec- tively, that can accommodate quite big optical elements (see Fig. 6). This instrument was built in collaboration with the firm Grenoble Modular Instruments. Besides a 3 kW generator with sealed x-ray tubes a high energy gen- erator is used that provides up to 400 keV radiation for destruction-free characterization of big single crystals. All these testing facilities are inside a clean room (class 1000) and placed in a mechanically and thermally stable environ- ment. Another well-functioning laboratory contains crystal preparation equipment: a crystal orientation unit with Laue camera and a high precision (a few arcsec) attach- ment, a CNC diamond saw (Meyer Burger), grinding and polishing facilities, and a precise double crystal x-ray dif- fractometer.

An optics test beamline is planned to be installed on a bending magnet port that will permit a great variety of experiments related to hard x-ray instrumentation, from high heat load tests (power density in excess of 400 W/mm*) to the evaluation of Bragg-Fresnel focusing op- tics and to the pretesting of entire beamline scenarios. We

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FIG. 6. The three axis diffractometer.

hope that this beamline will work at the very day when first photons will be available.

At present the Optics Group consists of three scien- tists, three Ph.D. students, five technicians, and two train- ees. We receive regularly guests from external laboratories.

ACKNOWLEDGMENTS

I am indebted to my colleagues of the ESRF for many suggestions and fruitful discussions and to the technical staff, in particular, J. Courteau and J. C. Laidet, for ail their help with the technical installations of the various laboratories. As to external collaboration I am particularly grateful to R. D. Deslattes, P. Dhez, W. Graeff, J. B. Hast- ings, and D. P. Siddons for their generous advice when defining the scope of our activities in x-ray optics and for their continued interest, stimulating discussions and en- couragement. Many thanks go to St. Joksch, A. Suvorov, and P. Zaumseil for their contributions to the three axis diffractometer and to P. Z. Takacs for kind advice relative to mirror characterization and for his contribution to the long trace profiler.

’ B. W. Batterman (private communication). 2A. K. Freund, in Proceedings of the Brookhaven Symposium on Syn- chrotron Radiation in Structural Biology (Plenum, New York, 1989), pp. 255-292.

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‘A. K. Freund, F. de Bergevin, G. Marot, C. Riekel, J. Susini, L. Zhang, and E. Ziegler, Opt. Eng. 29, 928 ( 1990).

4R. K. Smither, G. A. Forster, D. H. Bilderback, M. Bedzyk, K. Finkel- stein, C. Henderson, J. White, L. E. Berman, P. Stefan, and T. Over- sluizen, Rev. Sci. Instrum. 60, 1485 (1989).

‘G. Fiirstner, M. Krisch, and J. Susini, these proceedings. ‘A. K. Freund, Proc. SPIE 1345, 234 (1990). ‘M. Krisch, R. Hustache, and A. K. Freund (in preparation). %ee, e.g., Proceedings of the High Heat Load X-Ray Optics Workshop, edited by R. K. Smither and A. K. Freund, Report No. ANL/APS/ TM-6, Argonne National Laboratory ( 1990). A short summary of the Satellite Meeting at Chester (22 July 1991) is given by A. K. Freund and D. M. Mills, these proceedings.

9St. Joksch, G. Marot, A. K. Freund, and M. Krisch, Nucl. Instrum. Methods Phys. Res. A 306, 386 (1991).

“G. Marot, A. K. Freund, M. Rossat, H. Kawata, St. Joksch, E. Ziegler, L. E. Berman, D. Chapman, J. B. Hastings, and M. Iarocci, these proceedings.

“J. Susini, G. Fiirstner, L. Zhang, C. Boyer, P. Jagourel and R. Ravelet, these proceedings.

‘*J. Susini, L. Zhang, G. Marot, P. Jagourel, and R. Ravelet, these pro- ceedings.

13E. Ziegler, G. Marot, A. K. Freund, St. Joksch, H. Kawata, L. E. Berman, J. B. Hastings, and M. Iarocci, these proceedings.

14P. Zaumseil, IJ. Winter, St. Joksch, and A. K. Freund, these proceed- ings.

I5 J. Ah-Nielsen and A. K. Freund, these proceedings. “A. K. Freund, St. Joksch, H. Kawata, G. Marot, E. Ziegler, L. E.

Berman, and J. B. Hastings, these proceedings. ‘?A. K. Freund, G. Marot, H. Kawata, St. Joksch, L. E. Berman, and J.

B. Berman, these proceedings.

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