Design of infrared and soft X–ray beamlines
Radovan Vašina
Apertec s.r.o., Dukelská tøída 47, Brno, CZ-614 00
rvasina@apertec.cz
Keywords: beamline, monochromator, optics, mirror, grating, soft X–ray
The synchrotron beamlines are complex devices that represent the state of the art in many instrumentation branches, namely in optics, optical technology, mechatronics, precision mechanics, vacuum physics as well as surveying techniques. Although the development of this instrumentation is very fast and successful, there are still problems to be solved.
The beamline is a scientific instrument that transfers, collimates, monochromatizes and focuses a beam of elementary particles, in our case photons, into an experimental station. The beam in the experimental station excites (usually) a secondary radiation that is then collected and analyzed.
The synchrotron radiation beamlines belong nowadays to standard scientific instruments even though each beamline is unique. A big variety of beamline designs have been developed within about half a century. However, there are common issues that must be addressed in any case: optics together with the source choice, vacuum, mechatronics and thermal problems.
The source of light can be characterized with source size σx, σy, source divergence σx’, σy’, photon energy E distribution and photon flux F. One uses the derived parameters as emittance (or éténdue), brilliance and brightness. The emittance should not increase during the passage in the optical system, brilliance and brightness should not decrease in the beamline [1].
The most common source of synchrotron radiation is a bending magnet. The bending magnet is an inherent component of a synchrotron storage ring. It emits radiation with a continuous spectrum from infrared to hard X–rays. The bending magnet radiation is linearly polarized in the orbit plane and elliptically above and below it. The bending magnet has its virtues and usage for lithographic, infrared and broad band photoemission beamlines.
Another type of the source is a wiggler. The wiggler is an insertion magnetic device that influences the electron trajectory in a straight section of the synchrotron storage “ring”. A periodical array of magnets deflects laterally deflect the electron beam trajectory producing broad band photon spectrum with higher intensity than a bending magnet.
An undulator is an insertion device that differs from the wiggler in a number of magnetic poles and their magnetic field, namely with a bigger number of periods (in the range of 100) and smaller magnetic field than a wiggler. The qualitative change in the radiation is caused by interference of light; the electron beam, that “generates” radiation, is slower than the photon beam that travels with the speed of light.
The current development of undulators goes in the direction of undulators producing variably polarized light. One of the most common examples is the elliptically polarizing undulator Apple II [2].
Almost all the optics used for broad band radiation is reflective. Thanks to rapid technology development, one can use aspherical shapes besides plane and spherical ones. The figure of merit of the optical surfaces for the synchrotron radiation is the parameter slope error. The smallest attainable slope errors are about 0.1 microradians for plane and spherical elements, and about 5 microradians for the aspherical ones.
Surface roughness of the optical surface is another important factor that influences its specular reflectivity. Surface roughness down to 0.5 nm (rms) is now standard achievable.
The most common materials for the synchrotron radiation optics are silicon, GlidCop, Zerodur (glass) and fused silica.
The specular reflectivity of the optical elements for photon energies higher than 5 eV is poor and depends on incidence angle as well as on the surface layer material [3]. Usually, a so–called “grazing” incidence angle (from 89° to 80° to the normal) is often used.
At grazing incidence, effects of the meridional (along the beam on the optical surface in the plane of incidence) and sagittal (perpendicular to the beam on the optical surface) slope error can differ very much. It is desirable to use sagittal focusing, since the impact of the slope error is in this case diminished by a “forgiveness factor”.
Gratings represent the most crucial optical elements for monochromators. The designer should optimize many parameters of the grating in order to match the grating efficiency to the requested tuning range of the monochromator. The properties to be optimized are line density, profile of the grooves and reflective coating.
The vacuum in beamlines is required in order to protect the synchrotron storage ring, the optical elements of the beamline against contamination and the environment in the experimental station. The standard attainable vacuum is ultra high vacuum (UHV) ranging at 10-9 to 10-10 mbar. Ion getter pumps, titanium sublimation and non–evaporable getter and cryopumps are usually used for at beamlines. Oil–free roughing pumps are nowadays standard. There is still a problem with carbon that is present in the stainless steel vacuum vessels. Carbon diffuses into the UHV and optical elements under the intense synchrotron radiation get contaminated with an unwanted effect of decreasing reflectivity in certain photon energy regions. The aluminum vacuum vessel technology, which is carbon–free, is unfortunately not yet mature enough to be used regularly.
The mechanical design of the optical element manipulators and respective vacuum vessels must provide stability and precision that is better than the quality of the optical elements expressed in the term of slope errors that is in the sub–microradian range. The angular movement of the optical elements is for the beamline of the primary concern; therefore the requirements on angular accuracy, repeatability are much more stringent than these for translations.
The design of the mechanical parts should push eigen–frequencies up using stiff and light parts in the internal mechanics. The girder and supports have to suppress unwanted surrounding exciting oscillations. For this function, a synthetic granite block or hollow steel girders filled with sand or foam are used.
There has been a big development in actuator and encoding technology. 5–phase stepper motors can divide one revolution into 125000 (micro) steps [4], closing the gap between a DC servo and a stepper motor in terms of positioning, but keeping the advantages of the steppers. There are now UHV compatible angular encoders that work in sub–microradian range [5]. We think that the optimum mechanical concept for a precision manipulator in UHV is a suitable 5–phase stepper that actuates through a bellow the mechanics in UHV encoded with an UHV encoder. The encoder gives the “real” angular position of the optical element and thanks to the close–loop operation; it is possible to reach sub–microradian repeatability.
The third generation sources as undulators and wigglers emit high–intense beams. Due to the small specular reflectivity, a portion of radiation is always absorbed in the illuminated optical element. The absorbed energy causes heating of the optical surface. Because of non–zero thermal expansion coefficients of the most materials used for substrates, deformation of the optical surface occurs. The heat should be dissipated through active or passive cooling. The most demanding application use internally cooled or cryogenically cooled silicon substrates. The thermal expansion coefficient of silicon at the temperature of liquid nitrogen is very close to zero.
Finite element analysis is often needed to optimize the cooling schemes and to estimate the magnitude of the thermal induced slope error.
The most common beamline elements are slits, beam defining apertures, mirror units, monochromators and diagnostics units.
The slit opening usually ranges from 0.001 to 1 mm. The design of the slits has to assure that the slit blades open symmetrically and cannot be destroyed by closing to zero opening.
Monochromators represent the heart of soft X–ray beamlines. There are several boundary conditions that influence the design of the synchrotron radiation beamlines:
1. The position of the light source as well as the experimental station is fixed in the space, what concerns the position as well as the direction.
2. The vertical size of the source and opening angle of the radiation in the case of bending magnet and wiggler is much smaller than in the horizontal plane. It is therefore desirable to keep the dispersion plane vertical.
3. The reflectivity of all optical materials in the soft X–ray range is poor. Grazing incidence is required.
The historically first grating monochromator is Rowland monochromator with a spherical grating. The spherical grating disperses and focuses the radiation into the exit slit. The grating curvature radius of the Rowland monochromator is equal to the diameter of the circle where the entrance and exit slit should be placed. This means that the positions of the entrance and exit slits varies with the change of the photon energy. The Rowland monochromator, in its original form is not suitable for the synchrotron radiation application.
This disadvantage of the Rowland monochromator is removed on the cost of adding one more optical element. A plane pre–mirror is inserted in front of the spherical grating. The pre–mirror can change the included angle of the grating and thus keep the input and output arms of the monochromator at constant lengths during scanning of a certain range of photon energy. To cover a bigger span of photon energies, one usually has to exchange a grating in the operation. This type of monochromator is usually called Variable included angle spherical grating monochromator (VASGM) [6].
A plane grating monochromator (PGM) represents a development in the direction of bigger flexibility of a monochromator. One keeps the feature from VASGM, i.e. the plane pre–mirror, and uses the plane grating as a dispersing element. The focusing function of the monochromator is taken over by another added optical element (sphere, toroid, ellipsoid, plane ellipse…) positioned after the plane grating.
The plane grating of a PGM can be then used in a very broad photon energy range (over several octaves). If the beam impinging on the plane grating is not collimated, there are certain functions that determine the incidence and diffraction angles over the photon energy range in order to get correct focusing. However, these angles are not necessarily coincident with the optimum angles for the grating.
This drawback has been overcome with Collimated plane grating monochromators (CPGM) which now represent the most usual monochromator type for undulators [7]. Thanks to the fact that the brilliance of the undulator, i.e. size and divergence of the source, are small enough, one can collimate the radiation with the first mirror, and direct it into a PGM. In this case, one has to fulfill only one condition, namely the grating diffraction equation, which has two unknowns: incidence and diffraction angle. The additional needed condition stems from the grating efficiency map.
The bending magnet emits a broad spectrum of photon energies. The infrared beamlines use the infrared (IR) portion of the synchrotron radiation [8]. The bending magnet emits the IR radiation with a big divergence (tens of miliradians). One has recently discovered that there is another kind of radiation that is created between the edges of the precedent and the respective bending magnet. This (IR) radiation is called edge radiation and is much more intense than the “classical” bending magnet IR radiation.
The IR beamline should collect as much as possible of the IR radiation. This is realized by an extraction mirror that separates the IR and visible light from VUV, soft X-ray and hard X–ray components of the radiation. The most common way is to use a plane mirror that deflects the beam by 90° (usually upwards). The reflectivity for X–rays at a 45° mirror is poor and therefore much of the radiation is then absorbed in the extraction mirror. The absorbed power is for a bending magnet of the third generation storage ring in the range of units of kilowatts. One uses a slotted mirror lets the hard X–ray radiation fan go through or a Beryllium mirror that is partly transparent for the hard X–ray radiation.
The IR beamlines are usually divided into two parts, the first one, close to the storage ring, with UHV environment, and then the second one with HV or poor vacuum conditions. The separation windows, which are transparent for IR radiation, are made from diamond, z–cut quartz or Calcium Fluoride according to the IR range needed.
The instrumentation of the IR and soft X–ray beamlines is developing rapidly with the pushing demands from the experimentalist scientific community and steadily increasing amount of synchrotrons over the world.
References
1. W. B. Peatman,Gratings, mirrors and slits, Gordon and Breach Science Publishers, 1997, ISBN 60-5699-028-4.
2. S. Sasaki, Conceptual design of a hybrid-type elliptically polarizing undulator, Rev. Sci. Instrum. 73, 1451 (2002).
3. http://henke.lbl.gov/optical_constants/
4. http://www.renishaw.com/en/6473.aspx
5. http://catalog.orientalmotor.com/category/stepping-motors-motor-driver-packages--1073?
6. H. A. Padmore and T. Warwick, New developments in soft X-ray monochromators for third generation synchrotron radiation sources, Journal of Electron Spectroscopy and Related Phenomena 75, 9, (1995)
7. Rolf Follath, The versatility of collimated plane grating monochromators, Nucl. Instr. and Methods A, 467-468, 418, (2001).
8. P. Roy et al, Infrared synchrotron radiation: from the production to the spectroscopic and microscopic applications, Nucl. Instr. and Methods A, 467-468, 426, (2001).