34-ID is a tandem configuration based on APS Undulator A using a horizontally-deflecting mirror to split the beam into pink and white branches. The pink beam supplies a Coherent X-ray Diffraction (CXD) experiment with a general-purpose diffractometer. The white beam supplies a Microfocus Diffraction (MFD) experiment via an optional monochromator in the third optics enclosure. Both experiments are brilliance-limited and can accept only a fraction of the divergence from the source. For this reason, the beam-splitting mirror, which divides the undulator beam, does not compromise the flux in either experiment. Apart from the design constraints associated with the liquid-nitrogen cooling of the mirror as the first optical element, there are special personnel-safety issues associated with tandem operation, which are addressed in this document.
The scientific goals of 34-ID were described in the Conceptual Design Report (CDR) and will not be repeated here. A wide range of experiments in the fields of CXD and MFD were described in the CDR. These and the scientific subjects they lead us to pursue further are expected to occupy all the resources of 34-ID. It is not planned to provide any experimental facilities of a ‘general purpose’ nature, since these are already in place on 33-ID; the Individual Investigator (II) program will be similarly restricted in scope. The Proposal Evaluation Board (PEB) has reviewed and approved our scientific objectives. In response to questions from the PEB, we have evaluated whether the split beam concept can lead to a degradation of the performance of the CXD experiment due to not utilizing the central rays. On 33-ID we investigated monochromatic CXD patterns from Cu3Au thin-film samples using different parts of the incident beam over a ±0.4mm range and did not observe any qualitative or quantitative (speckle contrast) change in the quality of the CXD images produced. We concluded that there is no observable variation in the coherence with position within the undulator beam’s central maximum.
34-ID utilizes tandem experimental hutches operating simultaneously with separate shutters. There is an optics hutch in front of each experiment. Immediately outside the concrete shield wall is a first optics enclosure containing the beam-splitting mirror, located at 29m, which is as close to the source as possible. In order to accumulate sufficient beam separation for the CXD experiment to be carried out in close proximity to a white beam transport, the experimental hutches are positioned starting around the 46m mark. This hutch layout is shown in Figure 1.
Figure 1 Hutch layout for 34-ID.
The First Optics Enclosure (FOE), designated 34-ID-A, is shown in Figure 2. The hutch is relatively empty of components because most of the beam conditioning functions are deferred until after the separation point of the two beams in the SOE. Because it incorporates the concrete shield wall, this is a relatively cheap hutch to build, so we are leaving space for possible future developments. Beyond the FOE, in the narrow region of floor space constrained by ring-access doors, the two beams are carried in a shielded pipe, pumped from either end.
Figure 2. 34-ID-A, First Optics Enclosure layout
The Second Optics Enclosure (SOE), designated 34-ID-B, is shown in Figure 3. This service hutch is situated at the first convenient access point where the floor space widens after the ring-access door. The beam separation here is 150mm, sufficient to be divided into two physically separated pipes. The first component following the pump is the septum mask (2.6), responsible for thermal shielding of the joint between the two beams analogous to the front-end ‘crotch’. The septum mask doubles as a beam-diagnostic station. A viewpoint is situated so that the face of the mask can be seen through a window in the hutch. Phosphor particles placed on its cooled Glydcop® surface (or perhaps the sapphire particles within it) will allow the beam position to be observed during steering of the main mirror.
In the white MFD branch are installed further thermal masks that reduce the maximum power in the white beam to a level that can be accepted by the optics and shutters of the TOE. The mask is followed by a bremsstrahlung collimator (2.10) responsible for shielding the beam so that it can never reach the inside of the narrow beam transport through the CXD hutch that immediately follows. In the pink CXD beam are a position monitor used for steering the mirror, a space for an optional Be window and a space reserved for the future addition of a monochromator. Finally there is the main photon shutter (2.7) for the branch that is interlocked to allow access to the CXD hutch while maintaining the illumination of the optics.
Figure 3. 34-ID-B, Second Optics Enclosure layout.
This first experimental hutch, designated 34-ID-C, shown in Figure 4, is where the CXD experiments take place. Space is constrained by the presence of the white-beam transport passing 200mm from the sample position. A special diffractometer (2.12), optimized to work in the limited space, operates in one of two positions, forward and back. In the back position, it allows for a 1m optical table on which are placed beam conditioning optics that operate in air or rough vacuum. In the forward position, the diffractometer connects through a bellows and seal to orient and position samples inside a UHV chamber for vacuum-based experiments such as the investigation of surfaces. Although it is not planned for the initial stages of operation, we expect eventually to be able to operate with samples in this UHV chamber connected right through to the storage ring vacuum. This would avoid the known phase perturbation problems in coherent diffraction believed to be due to inhomogeneities in window materials. In this mode of operation, all optics, beam defining slits, monitors and diagnostics would be UHV compatible. Only the detectors, which are not phase-sensitive, would remain outside UHV.
There is room in the hutch for optics to be inserted before the sample. The most important component is the second (vertical) member of a Kirkpatrick-Baez pair of focusing mirrors and the beam-defining slit, placed just millimeters in front of the sample. We have developed a ‘roller-blade’ design for such a slit which is water cooled, UHV-compatible and positioned with sub-micron accuracy. This is discussed in section 2.13 below.
The detectors to be provided are mainly commercially-available charged-coupled devices (CCDs). For the highest time resolution, these would be operated in the manner of ‘streak’ cameras with a one-dimensional readout. The ability to rotate the detector about the exit beam direction is therefore a basic design requirement for the CXD diffractometer, so that the readout direction can be aligned precisely with the reciprocal-space direction of interest. This feature is also needed for polarization analysis of the diffracted beam as in the magnetic- and Templeton-scattering experiments that are planned. Because of limitations of available pixel size in commercial CCD arrays to 7µm or even 22µm, horizontal and vertical space will be retained for at least a 3m long detector arm.
Figure 4. 34-ID-C, Coherent Diffraction Experimental Hutch.
The Third Optics Enclosure (TOE), designated 34-ID-D, is shown in Figure 5. This is used exclusively by the MFD experiments in the hutch that follows. The first component following the pump of the white-beam transport is a special small displacement x-ray ‘micromonochromator’ (2.15) with a choice of x-ray crystals or multilayers. The small displacement monochromator is designed to allow rapid cycling between white beam and monochromatic beam with negligible change in the phase space incident on the focusing optics. This special monochromator facilitates x-ray microbeam diffraction by allowing both white beam Laue and tunable monochromatic experiments to be carried out on the same spot of the sample. The monochromator is followed in the TOE by the main white beam photon shutter (2.16) and a safety shutter which allows access to the MFD experimental hutch while CXD experiments are still in progress.
Figure 5. 34-ID-D, Third Optics Enclosure layout
The Microfocus Diffraction (MFD) hutch, designated 34-ID-E, is shown in Figure 6. The first component is a precision beam position monitor, capable of accepting both white and monochromatic beams. This is responsible for tracking and correcting beam stability. It is followed by the focusing system (2.17), a crossed elliptical Kirkpatrick-Baez mirror pair with sub-micron resolution. Near the center of the hutch, the sample is held on a precision sample manipulator (2.17), surrounded by an array of area detectors and spectrometers. Reproducible sample stage steps of 0.05 µm are specified.
When the special monochromator in the TOE is (rapidly) removed from the beam, white beam passes onto the x-ray optics with identical phase space properties so that the focal spot remains fixed. For rapid white-beam Laue experiments the white beam is collected about 1 mm below the centerline of the beam and therefore trades brilliance for larger energy bandpass. For white beam experiments requiring the highest possible x-ray brilliance, the end station can be moved down 1mm.
Figure 6. 34-ID-E, Microfocus Diffraction Hutch layout.
The first bremsstrahlung collimator is an in-vacuum tungsten collimator, which is modified from a standard component designated K3-22. Its 6mm aperture (modified from the original 12mm) is critically important in keeping bremsstrahlung under control (see Section 6.2 below), since it is difficult to introduce any further collimators into the region where two beams are present, i.e. between the mirror (2.4) and the septum mask (2.6). As designed, the K3-22 is not sufficiently wide in the horizontal direction, so it has been extended by means of two lead blocks straddling the beam pipe immediately following. There is also an ion pump installed on the table supporting the lead blocks.
This is the standard component designated L5-90. Four independently-controlled grazing-incidence blades further reduce the primary undulator beam dimension. One slightly unconventional aspect of this implementation is that the lower and inside blades are controlled by the CXD experiment, while the upper and outside blades are controlled by the MFD experiment. For the horizontally-cutting blades, this situation is demanded by the partition of real estate within the beam cross-section; since the vertically-cutting blades are packaged with the horizontally-cutting ones, this allocation minimizes the possibility of interference.
The horizontal mirror in the FOE is the most critical component of the entire design. It is exposed to the full power density of the undulator beam, even though the maximum aperture is limited by the thermal fixed mask (2.1) to 3×2mm2, and reduced considerably further by the L5 slits under normal operation. The mirror is normally positioned so that it cuts less than half way into the beam, so as not to infringe on the white beam passing over it into the MFD experiment. The mirror operates at a fixed incidence angle of 5mrad, as determined by the position of fixed apertures further downstream. Because the maximum horizontal cross section of beam intercepted by the mirror in normal operation is 0.75mm, the mirror is only 200mm long. It is 10 to 20mm thick and 30mm tall to allow for the possibility of multiple coatings that can be interchanged dynamically. The reflected beam is used exclusively by the CXD experiment, which therefore has full control over its positioning.
Because of their critical role, several important features have been designed into the positioning system, which is illustrated in Figure 7. The mirror and all its instrumentation are suspended from a single 8-inch flange and can be completely removed from its tank by swinging around a single pivot point. This allows for rapid turnaround if the mirror needs maintenance or replacement. Full control of the two Eulerian angles is provided for precise steering of the reflected beam in two dimensions. The principal angular motion (vertical q -axis) is neutrally vacuum loaded and driven by a micrometer at the end of a 1m-long sine-arm to ensure sufficient precision and repeatability. The tilt motion is provided by a Huber rotation table and x-z positioning by translation stages, all passed through a single side-bellows on the mirror tank.
Figure 7. Mirror positioning system.
We have included a piezoelectrically-driven fine adjustment of the principal mirror angle, which will operate at audio frequency. The piezo pushes against the far end of the mirror support bar, which acts as a torsion-spring restraint. This is partly to compensate for anticipated stability problems associated with a mirror mounted at a single point, and partly because of favorable experience at the ALS with high frequency feedback as a way to correct fundamental instabilities in their electron beam orbit. The piezo is driven in an AC closed loop, for bandwidth narrowing purposes to avoid 1/f noise, in conjunction with a beam position monitor located in the SOE. It is also conceived that this piezo may be used as a fast shutter synchronized with pump-probe time-resolved experiments, since CCD detectors are difficult to gate electronically.
To minimize the slope error induced by beam-heating (‘thermal bump’), the mirror itself will be made from a single crystal of silicon cooled to liquid nitrogen (LN2) temperature. This temperature is slightly below the inversion temperature of the thermal expansion coefficient, which is already smaller than most other potential mirror materials. LN2 temperature also provides substantially higher thermal conductivity than room temperature. Evaporated heavy metal coatings will be used to obtain reflectivity up to 15keV in the X-ray spectrum and provide adequate rejection of undulator harmonics.
We estimate the steady-state dissipation of LN2 to be very small because of the typical 0.5mm(vert)´ 0.7mm(horiz) entrance aperture of the CXD experiment, which passes just 100W at closed gap [1]. We have nevertheless designed for a temporary worst case full load of 2000W, corresponding to boiling 70lb/hr of LN2. An unpressurized gravitational feed is sufficient to meet these needs and allows the cooling to be applied through a single feedthrough as shown in Figure 7. Commercial ‘triax’ hoses [2] and bayonet connectors will be used to drop 3m of pressure head from a phase separator installed on the FOE hutch roof. Gravitationally driven flow through such a system [2] will deliver 350lb/hr of LN2. In our case, flow is limited by the gas return rate through the phase separator, which we estimate to be about 30lb/hr. Our current design will therefore run at full power only for short periods and not in steady state, but the gas exhaust can be improved if necessary. The mirror itself will be edge-cooled, again to minimize the ‘thermal bump’, by means of copper clamps through a eutectic alloy to the copper supporting bar, which is in direct thermal contact with the LN2 flow. The last 10mm of the mirror are beveled away from the beam at a 1ş angle so that the cutting edge, a likely hot spot, is adequately cooled. A cross-sectional view showing details of the mirror cooling is given in Figure 8.
Figure 8. Cross-sectional view of the liquid-nitrogen cooling path for the primary mirror.
The relatively low cost of a 200mm´30mm´15mm Si mirror will allow us to maintain a selection of optional coatings and prefigured spare parts. Some of the CXD experiments will also require modest focusing as the first element of a Kirkpatrick-Baez pair; this is achieved with a fixed spherical figure. Since it requires shutting down two beamlines, emphasis has been placed on a rapid turnaround procedure. Gate valves immediately before and after the mirror are provided for vacuum isolation, as well as a directly coupled turbopump to allow rapid return to service.
The narrow region of floor space in the vicinity of the ring access door is bridged by a white beam transport. Pedestrian and equipment access to 35-BM front end will require ducking under this transport, which will be designed to obstruct as little as possible. In order to carry both the pink and white beams inside the same pipe, an 8" ID is required at the far end. The shielded pipe will be fabricated with a 12mm lead wall in 3m lengths, joined by lead-shielded flanges resting on stands. It is unnecessary to install pumps within the span; ion pumps just inside the hutches at each of the two ends will be sufficient to maintain UHV.
We are considering an alternative possibility of using unshielded pipe contained inside a rectangular passageway, lined with 12mm of lead. This would simplify the alignment procedure, and allow for the option of insertion of additional beamline components at a future time.
The transition from a single pipe to individual pipes for the two beam branches is the function of this custom-designed mask. Neither beam touches the mask when the mirror is correctly aligned. However, this is the component that must dissipate the full power of the pink beam whenever the mirror is mis-steered. Particularly at small incidence angles, this power can reach rather high levels. The mask face is contoured to a convex surface as shown in Figure 9. The taper angle steepens with distance from the centerline of the white beam, allowing for the progressively larger mirror incidence angles, and correspondingly softer reflected X-ray spectrum. The taper angles calculated for the septum mask are based on the acceptable power density incident on a Glydcop® surface, as taken from an L5-83 mask designed to be located at 25m, just inside a generic ID-line FOE. This calculation is then modified by factors that take into account distance from the source, power transmitted by the mirror and the small area of the mask illuminated by the beam.
Figure 9. Septum mask design. The face is tapered to accept the maximum possible heat load at each possible deflection angle of the primary mirror.
Due to the distance of the septum mask from the source, the power density of the white beam at the septum mask at 44m is reduced by a factor of (25/44)2 = 0.3 relative to masks positioned at 25m in the FOE. Only a portion of this total power will remain in the spectrum when it reaches the mask after reflecting from the mirror, as shown by Figure 10. Both of these effects reduce the power density on the surface of the mask and thus allow a steeper inclined surface to safely encounter the reflected beam. A third increase in the safe power load, and hence mask surface angle, is gained by comparing the effective cooling rates for large incident beam areas versus small incident beam areas. Masks designed for use in the front end must be able to maintain cooling even with a large amount of their total surface exposed to the white beam. Because the mirror is only 200mm long, the septum mask will never encounter a reflected beam with a horizontal width greater than 1mm. We made finite element calculations comparing the two geometries, which showed that the cooling of a 1mm incident beam by water-cooled surface 8mm deep in a copper block was more effective than a wide beam by a factor of 2.5. Taken together these three factors allow mask angles as shown in Figure 9, with normal incidence possible for mirror 2q angles greater than 4.5 mrad.
Figure 10. Fraction of the total power expected in the pink beam as a function of mirror 2q -angle. The calculation is based on the published spectrum of undulator A at closed gap (10.5mm) [3]. The spectrum was integrated up to an angle-dependent cutoff energy, assuming perfect reflectivity of a Pt mirror coating up to its critical angle.
Our mask design uses APS-standard explosion bonding between the Glydcop® and stainless-steel vacuum parts. We have confirmed the availability of Glydcop® in 9" diameter blanks. We employ integral water cooling with internal frits and no water-to-vacuum joints, following the APS designs for other masks already in service.
The septum mask also serves as a beam diagnostic station for the characterization of the pink beam. A downstream-oriented viewport in line with a hutch window will allow visual examination of the beam on the face of the mask. Phosphor coating of the Glydcop® will probably be unnecessary because its sapphire precipitate particles will fluoresce. The diagnostic station will facilitate the steering of the pink beam into the pink branch aperture.
The position of the pink-beam in its branch will be monitored with a beam-position monitor (BPM). The analog horizontal position signal will be fed back to the piezo positioner of the mirror to preserve the alignment. We plan to use the current APS design fabricated by patterning of metallic electrodes onto two diamond blades placed above and below the pink beam.
An optional Be window is located at this position also. The window will be needed during commissioning and early operation, but is intended to be removed to reach a windowless configuration. Space is reserved here for the addition of a monochromator at a later date. The window will probably be required again when the monochromator is used.
The pink-beam shutter is also a standard component designated P9-30. Although there is some ambiguity in the power handling capabilities of the P9 series as presented on the APS Design Exchange, the model P9-30 is nominally rated at 600W total power. We estimate the worst case power level expected in the pink beam reflected from the beam-splitting mirror (2.4) to be 165W, comprising 610W determined by the maximum aperture [1] multiplied by the spectral softening factor of 0.27 at 2q =10mrad from Figure 10. These 165W are spread over 5.8mm2 on the face of the shutter, giving a power density of just 28W/mm2. We estimate this will give a temperature rise of 324°C, using the calculation method described in section 2.16.
In addition to blocking the pink X-ray beam, the P9 also contains 2×35mm of tungsten that will adequately cover the beam-pipe penetration behind it in the hutch back wall to permit safe access to the CXD hutch. The P9 has a fully redundant mechanism, whereby either half can attenuate the pink beam to the practically safe level of 2.5mrem/hr, and the combination of both halves is perfectly safe. As designed, the P9-30 contains additional masks to minimize the leakage of radiation scattered from the shutter blades.
The white beam thermal mask is responsible for protecting the bremsstrahlung collimator (2.10) and white beam transport (2.11) which immediately follow it. It also serves to limit the maximum total power in the white beam reaching the TOE components so that they do not need to be cooled so aggressively. The synchrotron light ray tracing shows that possible source points cannot reach the septum mask (2.6), but can still be far off-axis at this location.
The standard component L5-83 reaches out to this maximum excursion of the white beam and cuts it back down to 4.5mm´4.5mm. We will modify this mask to provide a beam of 1.5mm vertical by 1.0mm horizontal, which limits the maximum transmitted power to 175W [1]. The L5-83 is designed to work at 25m but we are using it at 44m, so the slope of its inclined faces can be increased by a factor of 3. We are investigating the possibility of using a metal insert to obtain the small opening.
The second bremsstrahlung collimator is a 6mm in-vacuum tungsten design. It is a modification of the standard component designated K3-22 and is identical to the first bremsstrahlung collimator (2.2). It is protected from synchrotron radiation by the previous mask (2.9). Such a small diameter is necessary to be sure that the bremsstrahlung rays cannot touch the critical second white beam transport (2.11) that traverses the CXD hutch. The collimator is insufficiently wide on its outside and will be extended by a block of lead mounted directly behind it.
This custom component is a critical part of the overall design as it allows access to the CXD hutch while the MFD experiment is running. It consists of a 12mm ID pipe covered with 16mm of lead. It will be manufactured in three sections with lead covering of the joints. It is important to keep its outer size to the minimum possible, as CXD experiments will be carried out just 200mm from the white beam centerline. A critical safety feature of this component is its support structure, which will be made of a steel angle girder spanning the length of the hutch. This girder will be anchored sufficiently that no accidental collision of the diffractometer can result in motion of the beam transport.
The second white beam transport will be pumped at both ends, but will not remain at UHV-specification pressures along its entire length because of outgassing and very limited conductance. We estimate it will reach 10–5 Torr in the middle, but this will not cause noticeable absorption of the beam. Neither does this present a bremsstrahlung or secondary scattering hazard, since the lead wall thickness is sufficient to absorb all scattering from a tube filled with air at atmospheric pressure. We do not intend to employ vacuum interlocks anywhere in the PSS system.
This component will be specially designed to provide full functional capabilities in the partially obstructed space of the CXD hutch. The second white beam transport (2.11) spans the hutch and is rigidly held 200mm from the CXD centerline. We plan to employ the ‘double kappa’ diffractometer design illustrated in Figure 11 to achieve this. This 6-axis layout takes maximum advantage of the accessibility to the center from a single side, without running the risk of collisions with the second white beam transport.
The sample moves on three axes in the familiar kappa configuration [4], while the detector also has three degrees of freedom. This permits several modes of operation including both horizontal and vertical scattering planes, or anything in between. This gives maximum flexibility for working with heavy sample environments, or those with a gravitational preference, such as liquid surfaces. There is a large working distance between mounting circle and the sample. This will allow space for displex cryostats or coupling into a stationary UHV sample environment.
Continuous rotation of the detector arm is provided about its own axis. This allows for slits to follow the optimum orientation of the resolution function, particularly when the CCD detector is used in a one-dimensional mode. It also allows for the incorporation of polarization analysis in a routine way, which is important for magnetic scattering or Templeton scattering experiments.
Figure 11. Schematic diagram showing the motions of a double kappa diffractometer. The sample moves on three axes in the normal kappa geometry. The detector has three degrees of freedom also.
In addition to the provision of the diffractometer for samples in air or locally-mounted environments is an integral UHV chamber for in situ sample preparation. The UHV chamber, shown in Figure 12, can be physically connected to the beamline and hence the vacuum system of the APS positron storage ring. This has been designed so that it will be feasible to operate under windowless conditions, needed to avoid the coherent beam phase contamination believed to be associated with Be windows. The procedures for windowless operation are discussed in section 5.4 below.
The beam-defining roller-blade slits (2.14, Figure 13), marked ‘V-slit’ and ‘H-slit’, are micropositioned just in front of the sample for precise definition of the coherent beam, as required for CXD experiments. A wide Be window allows a range of exit beam directions to reach the detector. The window will be electron-beam welded to the UHV vacuum wall following standard procedures.
The diffractometer couples to the UHV sample chamber through a bellows and rotating seal, following the standard practice for surface diffraction experiments [5]. Standard sample preparation and diagnostic tools are provided to allow in-situ surface preparation. A sample load-lock will also be included for rapid turnaround between experiments in this chamber. At the present time the details of the chamber design have not been finalized.
Figure 12. Schematic design of the in-line UHV chamber for surface diffraction experiments. The view is schematic since the design is not yet finalized, but is useful in the context of the overall beamline functionality.
These components are critical for the success of the CXD experiments because a precise aperture is needed to define the illumination profile of the sample [6]. Because of the limited lateral coherence of the beam, the largest slit opening would be around 10µm. However, because of Fresnel diffraction by the slit itself, the smallest conceivable slit opening cannot be less than around 1µm [6]. We have previously utilized a ‘roller blade’ design successfully in experiments [7] and have developed a water-cooled UHV design for 34-ID.
Two highly polished cylinders of heavy metal, such as molybdenum, separated by a spacer of about 100µm, are rolled past each other on an axial flex-pivot bearing. The beam is then symmetrically eclipsed as narrowly as desired and the blades can even cross over. A flex-pivot translation stage with a levered-down micrometer allows the slit to be positioned very accurately. Because of Fresnel diffraction around the slit edges, both slits must be placed as close as possible to the sample. This presents a significant challenge for the UHV experiments proposed, but is achieved by designing the slit with a slender profile and mounting it on a single flange very close to the sample position. The UHV roller-blade slit is shown in Figure 13. Water-cooling is essential for mechanical stability in an intense beam, so this has been included as shown.
Figure 13. UHV roller-blade slit design. A double-ended flex pivot allows the rollers to rotate about their common edge of contact. All positioning and manipulations of the slit are decoupled from the vacuum chamber by the bellows.
The micromonochromator is designed to pass either a white beam, or a monochromatic beam through the same exit slit. To achieve this, the beam offset in the monochromator is kept to 1 mm, which is within the dimension of the white beam entering the TOE. To reduce the thermal load on the second slit, the first slit restricts the white beam both in both the white and monochromatic modes. For a non-dispersive monochromator, the beam offset O is almost exactly twice the gap, G, between the crystal faces for Bragg angles, q , less than 20°.
O = 2G cos q
The highest q angle of operation for the micromonochromator is 14°. At this angle, a gap of 500 microns will result in a beam displacement of 970 microns. At the lowest angle of operation (5°) the beam displacement for the same 500 micron gap is 996 microns. The required gap is therefore 500 microns (0.020").
Figure 14. Schematic layout of the micromonochromator. The top panel shows the white-beam mode, while the bottom panel shows the monochromatic mode. No offset between these is seen by the experiment.
|
Optical element |
Vertical position compared with undulator center |
Longitudinal position with respect to main flex-pivot center |
|
Entrance slit |
+1.3 and -0.3 |
>6 m upstream (not critical) |
|
First slit |
0 |
~20 cm upstream (not critical) |
|
First crystal top downstream corner |
0 |
2 mm downstream |
|
Second crystal bottom upstream corner |
+0.5mm |
1 mm upstream |
|
Second slit |
+1.0 mm |
~20 cm downstream |
Table 1. Summary of the key alignment features of the micromonochromator.
Figure 15. End view of the micromonochromator.
In most APS beamlines the white-beam shutter is located in the front end inside the shield wall. 34-ID’s front end has such a shutter, but this must remain open during access to the MFD hutch, in order to continue operation of the CXD experiment. We therefore have added a duplicate safety shutter (S1) in the TOE. When the S1 is closed, access is permitted to the MFD hutch, even though the front end may still be operating. For thermal protection, the S1 is preceded by a P9 photon shutter, which must be closed before the S1 itself can close. The P9 has the required redundancy of operation, with two independent halves that can close separately.
Both the S1 and P9 shutters will be ordered as standard components. The maximum total power passing the white beam thermal mask (2.9) is 175W. As with the pink-beam photon shutter (2.8), this total power falls comfortably within the 600W rating of the P9-30 design.
Between the aperture at 44m and the shutter located at 57m, the beam spreads to fill an area of 4mm2, reducing the maximum power density to 44W/mm2 on the face of the shutter, which is also an important consideration. We carried out a three-dimensional finite-element calculation for 175W impinging on a 4mm2 area under normal incidence on a 10mm thick plate of copper, water cooled on the back. We found a temperature rise of 430°C at the illuminated point, which we consider to be safely below the 1083°C melting point of copper, and within the temperature range over which Glydcop® retains its strength.
The focusing system and microfocus diffractometer will be installed on an optical table in the MFD hutch. The short working distance and high stability requirements demand that these components be intimately linked together. A variety of detectors, including CCD arrays and energy dispersive detectors, can be installed on the extendible arm of the diffractometer. This allows tracking of the directions of diffracted beams to determine their point of origin inside the sample.
X-ray microbeams will be produced by both Fresnel Zone plates and by Kirkpatrick-Baez (KB) optics. The Kirkpatrick-Baez mirrors will be fabricated from cylindrically polished Si blanks approximately 40-120mm in length. Inexpensive cylindrical Si blanks in these dimensions can be obtained with sub-angstrom roughness and sub-microradian cylindrical figures. An elliptical figure is obtained by differentially coating the mirrors. We have used this technique to produce 1m m beams, and with further calibration of the deposition process anticipate achieving beams in the 0.25-0.5m m-diameter range. The free working distance for 0.5m m-diameter beams is about 30mm.
Our cost estimates have not been revised significantly since the submission of the CDR, except that there is no longer a provision for a monochromator in the CXD branch. Such a monochromator, which would broaden the scientific scope of the coherent diffraction experiments, is now considered as a future addition.
Funding for construction the 34-ID beamline described here is now completely in place. NSF has supported the CXD portion of the project with a grant of $1.3M over the period 9/97 to 8/00. Renewal funding from the State of Illinois (HECA) for the construction and operation of beamlines by a consortium of universities has now been approved. UNICAT has allocated an amount of $1.35M from the HECA grant, spread uniformly over the period 9/98 to 8/01, for construction of the common part of the 34-ID facilities to be used by both CXD and MFD. The construction of the MFD portion will be paid with $1.44M from the DOE’s ongoing support of UNICAT, allocated to 34-ID in equal annual installments during the period 10/97 to 9/01. Oak Ridge National Laboratory (ORNL) has already contributed $0.5M to the development of the X-ray microprobe instrument. The total budget of $4.5M available before the end of 2001 is consistent with our previous spending and construction schedule estimates in the CDR.
The operations of 34-ID will be managed under the UNICAT umbrella. UNICAT intends to use the LOM space adjacent to 34-ID and will make their request known to the APS separately.
A unique feature of the 34-ID design is the separate independent operation of two experiments sharing the same undulator beam. This complicates the Personnel Safety System slightly, but not in any way that obscures its logic: access to each independent hutch is permitted only when the appropriate safety shutter directly in front of that hutch is closed. Each safety shutter is protected by its own photon shutter in the usual way. The bremsstrahlung and photon shielding of each possible configuration independently meets the APS specification.
The beamline design is also complicated by the additional possibility of radiation scattered from optical elements of one beamline reaching the hutch of the other experiment, perhaps through indirect paths. While we have anticipated and blocked all of these paths wherever they are obvious, it is expected that a low level of mode-dependent ‘crosstalk’ between the experiments will occur. This will require additional monitoring in the early days of operation.
White beams will be propagated in the open air in both the CXD and MFD hutches. In both cases the small apertures will keep the total flux down to a level comparable with monochromatic beams elsewhere, so excessive ozone should not be generated. If ozone levels considered to be dangerous by APS-published standards are encountered on a routine basis, they will be mitigated in the recommended way.
Although it is not planned for initial operation, windowless operation right up to the sample in the pink-beam CXD branch is anticipated in the longer term. Consequently, the CXD experimental configuration has been designed so that it should not preclude this mode of operation. It is for this reason that we have included sufficient details of the UHV instrument in the component descriptions above.
Removable Be windows have been included in the CXD branch at the locations listed in Table 2. Before windowless operation is achieved, it is intended to place beam-conditioning optics, such as slits and the vertical-focusing mirror, on an optical table in the CXD hutch. These will operate in air, helium or roughing vacuum.
|
Distance from source |
Location of Be window |
|
25m |
As part of the front-end mask assembly (2.1) directly behind the shield wall in the FOE |
|
44m |
Behind the septum mask (2.6), before the position where a monochromator may later be added in the SOE |
|
47m |
Where the beam emerges into the CXD hutch |
|
50m |
Entering the UHV sample environment (2.13) |
|
51m |
Leaving the UHV sample environment (2.13) |
Table 2. Location of Be windows in 34-ID.
Under windowless operations, all of these except the last would be removed from the beamline. All of the beam-conditioning optics would be operating in UHV at this time and will be designed to APS vacuum standards. Vacuum protection of the ring and front end will be achieved by differential pumping and will be monitored by a Vacscan 100AMU residual gas analyzer located downstream of an interlocked isolation valve in the FOE. The beamline’s vacuum will only be opened to the front end’s after the spectrum is deemed clean, and will be monitored continuously throughout operation.
Shielding is provided along the 34-ID beamline to insure that the exposure of APS personnel and users to ionizing radiation is below the recommended rate of 0.25 mrem/hr. To this end, care has been taken to adopt standard APS designs wherever possible. One point of departure from APS standards is the transport of white beam through the CXD hutch by means of a lead-lined pipe. This exception will be discussed below.
The optical enclosures and hutches follow the guidelines set out in APS document TB-7 [8]. The white beam hutches are 34-ID-A (FOE), 34-ID-B (SOE), 34-ID-D (TOE) and 34-ID-E (MFD Hutch). Each will each require a lead wall thickness of 50mm for the back walls, 19 mm on the front and side walls, and 12mm for the roof. An additional 50 mm of lead will cover a circular region of the back wall of each hutch centered on the white beam and extending to a radius of 0.6m to account for low angle secondary bremsstrahlung scattering.
Enclosure 34-ID-C (CXD hutch) must shield only the reflected pink beam and thus requires 19mm of lead on the back wall, 16mm on the side walls and 12mm on the roof. Because no bremsstrahlung is present in this hutch, it needs only a synchrotron radiation beam stop.
Ray tracings of both bremsstrahlung and synchrotron radiation have been prepared to confirm that each type of radiation is appropriately shielded. Synchrotron radiation masks are positioned to restrain the beam from touching any beamline components which are not appropriately cooled, and bremsstrahlung collimators limit the divergence of extremely hard radiation so that it can safely pass through the experimental hutches and be dumped at a beam stop. Of special importance to the design of 34-ID are the series of masks used to separate the two beams, and the collimation of the white beam as it passes through the CXD hutch.
Figure 16 shows the bremsstrahlung ray diagram for 34-ID. Modified standard K3-22 collimators and lead blocks allow the bremsstrahlung to pass safely through the white beam transports. No bremsstrahlung ever reaches points inside the CXD hutch (34-ID-C). The S1 shutter in the TOE allows safe entry into the MFD hutch (34-ID-E), assuring the independent operation of experiments in the CXD and MFD hutches.
Figure 16. Horizontal and Vertical Bremsstrahlung Ray Tracings (one half of the S1 shutter is displayed in the open position for reference)
Figure 17 shows vertical and horizontal synchrotron ray tracings for 34-ID. Masks are employed to protect all the bremsstrahlung shields used in Figure 16 from radiation. Masks are also employed to limit the total power propagating down the beamline for the protection of optical components and shutters. All masks are manufactured to APS vacuum requirements using water-cooled Glydcop® surfaces with appropriate grazing beam incidence where required.
Figure 17. Horizontal and Vertical Synchrotron Ray Tracings
Table 3 gives the numerical data for the radiation beam dimensions at various points along the beamline. Table 4 provides the same for the mask and collimator apertures and dimensions.
|
Optical Element |
Position |
Synchrotron |
Bremsstrahlung |
|||
|
Horizontal |
Vertical |
Horizontal |
Vertical |
|||
|
L5-83 |
25215 |
± 17.5 |
± 5.0 |
+52.2, -42.4 |
± 13.5 |
|
|
M4-20 |
26525 |
± 3.2 |
± 2.5 |
+64.6, -45.3 |
± 14.7 |
|
|
K3-22 |
27084 |
± 2.05 |
± 1.1 |
+71.6, -47.0 |
± 15.4 |
|
|
Lead Block 1 |
27433 |
± 2.4 |
± 1.2 |
+75.1 |
± 3.1 |
|
|
Septum |
43696 |
± 18.6 |
± 4.3 |
+83.9, -31.4 |
± 18.2 |
|
|
L5-83 |
44103 |
± 19.1 |
± 4.4 |
+86.3, -32.3 |
± 18.6 |
|
|
K3-22 |
44646 |
± 0.55 |
± 1.5 |
+88.8, -33.1 |
± 19.1 |
|
|
Lead Block 2 |
44889 |
± 0.57 |
± 1.6 |
+89.8, -3.0 |
± 3.0 |
|
|
2nd Trans. Pipe |
46800 |
± 1.8 in, ± 2.8 out |
f 7.4 in, f 13.0 out |
|||
|
Synch stop |
66000 |
± 1.2 |
± 4.2 |
f 20.3 |
||
|
Brem stop |
66500 |
-- |
-- |
f 20.5 |
||
Table 3. Transverse dimensions (mm) of bremsstrahlung and unfiltered synchrotron radiation at locations of shielding components.
|
Optical Element |
Length |
Aperture |
Outside Dimension |
||||
|
Horizontal |
Vertical |
Vertical |
Clockwise |
Counter- clockwise |
|||
|
L5-83 |
275 |
± 20.05 in± 2.25 out |
± 12.01 in± 2.25 out |
± 53.3 |
53.3 |
53.3 |
|
|
M4-20 |
133.5 |
± 4.69 in± 1.5 out |
± 4.69 in± 1.0 out |
± 31.75 |
31.75 |
31.75 |
|
|
K3-22 |
198.4 |
f 6.0 |
± 49.5 |
49.5 |
49.5 |
||
|
Lead Block 1 |
300 |
± 38 |
-- |
± 50 |
187.5 |
187.5 |
|
|
Septum |
70 |
+18.7,-20.0 |
± 49.1 |
± 58.7 |
174.2 |
22.6 |
|
|
L5-83 |
275 |
± 20.05 in+1.0,-0 out |
± 12.01 in± 0.75 out |
± 53.3 |
53.3 |
53.3 |
|
|
K3-22 |
198.4 |
f 6.0 |
± 49.5 |
49.5 |
49.5 |
||
|
Lead Block 2 |
300 |
+38 |
-- |
± 50 |
-- |
123 |
|
|
2nd Trans. Pipe |
8400 |
f 16.0 and f 25.4 |
-- |
-- |
-- |
||
|
Synch stop |
250 |
-- |
-- |
± 21 |
57.1 |
57.1 |
|
|
Brem stop |
300 |
-- |
-- |
± 18.5 |
47.3 |
47.3 |
|
Table 4. Mask and Collimator Dimensions (mm)
No special considerations need to be made for special insertion devices. In its starting configuration, 34-ID will use a standard APS Undulator A. The entire thermal design has been based on published parameters for this undulator with a 33mm period and a 7.5mm gap [1]. The addition of a second undulator to augment the independence of CXD and MFD operations is currently under discussion. Such a future undulator would have a shorter period, probably 27mm, for improved brilliance. This takes advantage of the availability of smaller undulator gap values than were available at the time of designing undulator A. The shielding requirements of the entire design would be reviewed before such a step would be taken.
All beamline components will be compatible with APS UHV standards. This will facilitate future operation in a windowless mode for Coherent X-ray Diffraction experiments.
Two white beam transports are used in the 34-ID beamline. The first is a large (8 inch) diameter lead covered pipe that will pass both the white (MFD) beam and pink (CXD) beams together from the FOE to the SOE. The second is much smaller in diameter (16 mm) and carries the white beam through the CXD hutch. Both will be constructed by inserting standard, stainless steel vacuum pipe into a lead pipe of the same length, then welding vacuum flanges on each end.
|
Transport Segment |
Starting Position |
Length |
Inner Diameter |
Lead Thickness |
|
1a |
35.707 m |
2.359 m |
150 mm |
12 mm |
|
1b |
38.066 m |
2.450 m |
200 mm |
12 mm |
|
1c |
40.516 m |
2.450 m |
200 mm |
12 mm |
|
2a |
46.815 m |
2.500 m |
16 mm |
12 mm |
|
2b |
49.700 m |
3.500 m |
16 mm |
12 mm |
|
2c |
53.200 m |
2.130 m |
25.4 mm |
12 mm |
Table 5. Dimensions of White Beam Transports on 34-ID.
Radiation concerns for white beam transport fall into three categories: contact with direct synchrotron or bremsstrahlung radiation, scattering from solid targets upstream of the transport, and scattering from air or residual gas within the transport.
Due to careful masking, neither transport can be touched by either the direct synchrotron or bremsstrahlung beams. Scatter from optical components upstream of either transport is expected to be minimal due to the large distances ( 6m in FOE, 3m in SOE) between potential sources of scattered radiation and the penetration of the transport through the hutch walls.
Under normal vacuum conditions, the scattering from residual gas is negligible. The PSS will not take the beamline pressure into account, however, so the transports must be safe to operate even in the case of total vacuum breach. A layer of lead 12 mm thick is sufficient to reduce scattering from 1 atm. of air within the transport to a safe level (according to TB-7 [8], page26).
Figure 18. Schematic of White Beam Transport Joint
The long spans traversed by the white beam transports in 34-ID necessitate two joints in each of the shielded pipes. Collars will be fitted around each vacuum flange joint to assure that all possible paths for radiation to escape are sufficiently covered with lead. Figure 18 shows two parallel rays as they exit the white beam transport in the vicinity of a joint. For a gap of width W, the lead collar must extend a distance W beyond the joint to insure that the ray emerging through the joint is fully shielded. There are additional concerns for the 16 mm pipe due to its frailty and proximity to the large detector arm of the CXD diffractometer. A special support girder will be attached to the pipe to keep it in place. The support structures will be sufficiently rigid to prevent any deflection of the transport pipe if it is bumped by personnel working in the CXD hutch while the beam is on in the MFD hutch. Also an optical limit switch will be mounted on the pipe to detect any dangerous encroachment of the CXD diffractometer.
The Personnel Safety System (PSS) will protect APS workers and beamline users from accidental exposure to radiation inside the optical enclosures and experimental hutches of 34-ID. All radiation enclosures in 34-ID will be searchable and will use standard APS search boxes and door hardware. Since the CXD and MFD beams are independently shuttered and shielded, safe access is possible in either experimental hutch regardless of the operational status of the other.
The logic that will be employed in programming the PSS to distinguish safe and unsafe conditions for entry to the various beamline enclosures is outlined below. The various enclosures are described in terms of safe and secure. Secure means that the enclosure is locked with no one inside and is ready for radiation. Safe means that there is no radiation present and the enclosure is safe for entry by users or staff.
34-ID
|
States |
Requirements |
|
LINE OK |
ID-A, ID-B, ID-C, ID-D, ID-E, and CXD white beam transport all OK. If not LINE OK, close FE white beam stop |
|
User Actions |
Requirements |
|
open FE white beam stop ENABLE |
RING OFF or USER ENABLED and LINE OK and ID-A SECURED, ID-B SECURED and ID-D SECURED |
|
open micro-focus white beam stop ENABLE |
RING OFF or USER ENABLED and LINE OK and ID-D SECURED and ID-E SECURED and CXD white beam transport OK |
|
open CXD pink stop ENABLE |
RING OFF or USER ENABLED and LINE OK and ID-B SECURED and ID-C SECURED |
34-ID-A (FOE)
|
States |
Requirements |
|
SAFE |
RING OFF or FE white beam stop CLOSED |
|
SECURED |
hutch searched and door closed |
|
OK |
SAFE or SECURED |
|
User Actions |
Requirements |
|
open door |
ID-A SAFE |
|
open micro-focus white beam stop |
see above |
34-ID-B (SOE)
|
States |
Requirements |
|
SAFE |
RING OFF or FE white beam stop CLOSED |
|
SECURED |
hutch searched and door closed |
|
OK |
SAFE or SECURED |
|
User Actions |
Requirements |
|
open door |
ID-B SAFE |
|
open pink beam stop |
see above |
34-ID-C (CXD Hutch)
|
States |
Requirements |
|
SAFE |
RING OFF or FE white beam stop CLOSED or pink beam stop closed and (micro-focus white beam stop closed or CXD beam transport OK) |
|
SECURED |
hutch searched and door closed |
|
OK |
SAFE or SECURED |
|
User Actions |
Requirements |
|
open door |
ID-C SAFE |
CXD White Beam Transport
|
States |
Requirements |
|
SAFE |
RING OFF or FE white beam stop CLOSED |
|
SECURED |
Transport surveyed into place and approved |
|
OK |
SAFE or SECURED |
|
User Actions |
Requirements |
|
remove transport |
APS Approval |
34-ID-D (TOE)
|
States |
Requirements |
|
SAFE |
RING OFF or FE white beam stop CLOSED |
|
SECURED |
hutch searched and door closed |
|
OK |
SAFE or SECURED |
|
User Actions |
Requirements |
|
open door |
ID-D SAFE |
34-ID-E (MFD Hutch)
|
States |
Requirements |
|
SAFE |
RING OFF or FE white beam stop CLOSED or MFD white beam stop closed |
|
SECURED |
hutch searched and door closed |
|
OK |
SAFE or SECURED |
|
User Actions |
Requirements |
|
open door |
ID-E SAFE |
An Equipment Protection System (EPS) will be employed to monitor beamline conditions that could cause damage to equipment in 34-ID or, once windowless operation is achieved, a compromise of storage ring vacuum. If an unacceptable condition is detected or if the EPS itself becomes inoperable, a signal will be generated instructing the APS to close the front end shutters. General monitoring will be done on beamline vacuum, electrical power to critical components, and water flow to all cooled masks and beryllium windows.
The beam splitting mirror is a special case that will require temperature monitoring. In the event of a serious mis-steering of the mirror, the small mask that shadows the upstream end of the mirror may be driven into the white beam to an extent that could cause it to melt. It may also be possible to turn the mirror to such an angle that the liquid nitrogen cooling will not keep pace with the thermal load. To avoid damage in either of these cases, EPS linked thermocouples will be mounted on the back of the mirror mount to detect any significant or sudden change in temperature.
Toxic-gas handling and extraction has been installed by UNICAT on 33-ID and 33-BM. The capacity of their system can be shared with 34-ID, provided all vents are not required to run at the same time. The bridging distance from 34-ID-C to the ducting is very short, about 5m. To allow for possible future toxic-gas handling for experiments in 34-ID-C, the ducting and termination will be included in the hutch specification, but not connected until such time as the experiments require it.
Many of the components listed above are standard APS designs with few or no modifications. These will be purchased directly from APS-approved vendors, following the University of Illinois bidding procedure. Our R&D efforts will therefore be concentrated on the few critical components that are specific to our beamline design. These efforts are summarized in the following paragraphs.
The mirror positioning system (2.4) and the septum mask (2.6) will be designed in house, fabricated by qualified machine shops, and assembled by ourselves. We will consult with vendors about the best way to design the LN2-handling system [2]. It is anticipated that further development of the methods employed will be needed when we gain experience of their performance. For these critical components, it is essential that we retain the technical knowledge and experience base ourselves. Our design engineer will therefore remain associated with the project into the operation phase.
The CXD experimental chamber and UHV-compatible precision slits (2.13) are closely associated with parts of the planned experimental program on 34-ID. Students and postdoctoral associates will therefore be involved with certain aspects the design and subsequent improvement as part of their academic experience. As with all experiments, it is expected that the capabilities will evolve in time, as new discoveries in the field of CXD are made. For this reason, only preliminary plans for the instrumentation have been provided in this document.
The CXD diffractometer (2.12) will be developed in the form of a technical document providing an exact specification. This will be furnished to reputable diffractometer vendors for further development and, eventually, procurement. It is anticipated that the instrument will be delivered as a complete package with full functionality at the level of interfacing directly to both the CXD hutch and to our beamline controls.
Prototypes of the micromonochromator (2.15), the KB Focusing System (2.17) and the MFD diffractometer have already been developed and tested on the MHATT-CAT beamline. Minor modifications to the interfacing with the 34-ID hutches and controls are expected. It is planned to redesign the micromonochromator to incorporate various improvements. This development will be ongoing with the start of MFD experiments.
[1]1 All maximum power levels have been calculated for Undulator A (33mm period) at closed gap (7.5mm) using the largest power output characteristic of 230kW/(mrad)2, published in "Planning Radiation Source Requirements", G. Shenoy (Dec 1997)
[2]2 Vacuum Barrier Corp. Woburn, MA
[3]3 R. J. Dejus, B. Lai, E. Moog, E. Gluskin, Technical Bulletin ANL/APS/TB17
[4]4 I. K. Robinson, H. Graafsma, Å. Kvick and J. Linderholm, Review of Scientific Instruments 66 1765-7 (1995)
[5]5 P. H. Fuoss and I. K. Robinson, Nuclear Instruments and Methods 222 171 (1984)
[6]6 I. A.Vartanyants, J. A. Pitney, J. L. Libbert and I. K. Robinson, Physical Review B 55 13193-13202 (1997)
[7]7 J. L. Libbert, J. A. Pitney and I. K. Robinson, Journal of Synchrotron Radiation 4 125-127 (1997)
[8]8 N. Ipe, D. R. Haeffner, E. E. Alp, S.C. Davey, R. J. Dejus, U. Hahn, B. Lai, K. Randall, D. Shu, Technical Bulletin ANL/APS/TB7
