ANL/APS/TB-4

APS HIGH HEAT LOAD MONOCHROMATOR

by Wah Keat Lee and Dennis Mills

February 1993

Introduction

This document contains the design specifications of the APS high heat load (HHL) monochromator and associated accessories as of February 1993. It should be noted that work is continuing on many parts of the monochromator including the mechanical design, crystal cooling designs, etc. Where appropriate, we have tried to add supporting documentation, references to published papers, and calculations from which we based our decisions.

The underlying philosophy behind performance specifications of this monochromator was to fabricate a device that would be useful to as many APS users as possible, that is, the design should be generic as possible. In other words, we believe that this design will be capable of operating on both bending magnet and ID beamlines (with appropriate changes to the cooling and crystals) with both flat and inclined crystal geometries and with a variety of coolants. It was strongly felt that this monochromator should have good energy scanning capabilities over the classical energy range of about 4 to 20 keV with Si (111) crystals. For this reason, a design incorporating one rotation stage to drive both the first and second crystals was considered most promising [1, 2]. Separate rotary stages for the first and second crystals can sometimes provide more flexibility in their capacities to carry heavy loads (for heavily cooled first crystals or sagittal benders of second crystals), but their tuning capabilities were considered inferior to the single-axis approach.

Much thought was also given to the vacuum requirements of this monochromator. Because this double-crystal monochromator (DCM) is designed to operate in the 4 - 20 keV range, the final decision was to choose a high vacuum device (10^-7 to 10^-8 Torr) but not to required a true UHV device. The impetus for this choice was to facilitate crystal changes, reduce the effort and time for design, and keep the overall cost of the system at a reasonable level. It was realized that some groups may be interested in a true UHV-compatible monochromator design, and hence the APS is currently investigating possible design approaches for a UHV device.

The center of rotation for this DCM is to be the intersection of the surface of the second crystal and the normal to the first crystal. This means that, during an energy scan, the first crystal must move to keep the offset fixed. The decision to select this position for the center of rotation was made in part to eliminate an additional motion that would have to be implemented on the second crystal if the center of rotation was at the first crystal. Furthermore, because we would like white-beam access through the monochromator, a translation stage to move the first crystal out of the incoming beam would be required in any event, and a decision to put the rotation between the crystals was made. Note that the motion of the first crystal is not made through a mechanical linkage with the theta rotation, and so the monochromator can be operated in a "channel cut" mode if one can tolerate a small motion in the beam offset.

In previous designs that were similar in concept to this design, a relative translation between the first and second crystal along the beam direction was required if a small second crystal is used. This translation is not necessary in the inclined crystal geometry because the length of the second crystal will be sufficient to intercept the beam reflected from the first crystal through the entire energy range for that set of crystals. Hence in this initial design, we have eliminated this motion, and, therefore, when operating this monochromator with flat crystals, a long second crystal will be required. This is not a problem when using silicon as the monochromating crystal but could represent a problem if other optical devices are used. We feel that it would not be difficult to incorporate a translation stage for the second crystal into this design, and, in fact, we will probably do so in the future.

The staff of the APS have also been working with a vendor for the fabrication of a liquid gallium pump. It is our hope that, should users desire to use liquid gallium to cool their optical components, a commercially produced pump will be available to them in the near future. The APS has now placed an order for such a pump and expects to receive it sometime in the summer of 1993.

A considerable amount of R&D effort has also gone into the design and fabrication of internally cooled perfect single crystals of silicon. Advanced heat exchangers, such as the pin/post pattern geometry developed by Rockwell Power Systems (RPS) look very attractive for this application. Recent experiments using a water-cooled pin/post pattern heat exchanger at CHESS have very favorable results. The APS is currently having RPS fabricate a similar crystal with the heat exchanger optimized for liquid gallium cooling. We hope to test this device in the near future at existing synchrotron radiation sources.

Although there may be other successful approaches to the problem of mitigating the thermal distortion effects in monochromators (i.e., cryogenic cooling; the use of power filters, such as mirrors and multilayers, before the monochromator; etc.), we feel that this solution is the most expedient at this time because it lends itself to quick adoption to existing monochromator and beamline designs. The staff of the APS is, however, pursuing several of these other approaches with the hope of providing the user community with several alternatives to this difficult problem, so that they may select the solution most appropriate to their particulat needs and beamline configuration.

All the power and pwer density calculations shown in this document are for the standard APS undulator A. The physical parameters describing undulator A are listed in Table 1 [3]. Closed-gap calculations are for a magnetic gap of 11.5 mm. Calculations indicate that, using the inclined crystal geometry and liquid gallium cooling, we will be able to handle the expected heat loading from undulator A at closed gap with minimal loss of beam brilliance. We have not included any recommendation of crystal coolong configurations for bending-magnet radiation or other insertion devices in this document. However, we feel that the thermal problems from these sources will not be nearly as challenging as those from undulator A and should not present any serious problems.

Undulator A Power and Power Densities

The total radiated power (P_t) and peak power density (P_p) from an insertion device can be calculated from the two equations given below:

where E_R is the ring energy in GeV, B_o the peak magnetic field in Tesla, I the position current in milliamperes, L the length of the device in meters, N the number of periods, G(K) a dimensionless parameter (approximately equal to 1 when K is greater than or equal to 1), and S the source-to-component distance in meters. The power density is calculated for a point source (zero emittance) and therefore represents a worst case in determining power loading on an optical component. At closed gap (i.e., 11.5 mm), the total radiated power from undulator A is 3.8 Kwatts and the normal incidence power density at 30 meters (nearly the closest practical source-to-monochromator distance) is 150 watts/mm² . These numbers are the raw values; that is, no windows or filters have been included. In Figure 1, the first harmonic energy as a function of magnet gap for undulator A is shown. Figure 2 shows the P_t and P_p plotted as a function of energy of the first (from 4.2 to 12.6 keV) and third (from 12.6 to 37.8 keV) harmonic energies. This plot reminds one that the highest powers and power densities occur only over selected energy ranges.

High Heat Load Monochromator Design

Introduction and Background Information

The purpose of the DCM is to select from the incident polychromatic x-ray beam a desired x-ray wavelength (i.e., to monochromatize the beam). The DCM is typically the first optical component on which the x-ray beam impinges. As such, the DCM will experience extremely high (x-ray) radiation levels, and materials that degrade under radiation exposure must be avoided. To eliminate ozone production (from the interaction of the radiation with the oxygen in the air) and for maintaining general cleanliness of all optical components, the monochromator must be operated in a vacuum at a pressure of 10^-7 to 10^-8 Torr. Hence, all components used in the design of the DCM must be compatible with these vacuum levels. When the APS is fully operational, the DCM will be expected to function 24 hrs/day for long periods of time (several months). The overall lifetime of the DCM should be in excess of 5 years.

A schematic of a DCM is shown in Figure 3. The first crystal of the DCM actually provides the monochromatizing action, while the second crystal simply redirects the monochromatized beam parallel to the incoming beam. Typical x-ray energies of interest are in the 4 -20 keV range corresponding to Bragg angles from 5 to 30 degrees for a Si (111) crystal (d=3.135Å).

Crystal Geometries

We are currently planning to use "flat" crystals (crystal surface normal in the scattering plane) for wiggler radiation and "inclined" crystals (crystal surface not cut parallel to the atomic planes with the surface normal perpendicular to the scattering plane) for the higher power density undulator radiation. The DCM must therefore be compatible with both these two crystal geometries. Schematics of the flat crystal geometry are shown in Figures 4a and 4b. (In the present design, the second crystal will be long enough that a translation will not be required.) Schematics of the inclined crystal geometry are shown in Figures 5a and 5b. There are several differences between the flat and the inclined geometries. In the flat geometry, the Bragg planes are parallel to the surface, whereas they are not in the inclined geometry. At theta = 0 in the flat geometry, the crystal surface is in the horizontal plane, but, in the inclined geometry, it is at an angle to the horizontal plane. For the APS DCM, this inclination angle will be between 70 and 85 degrees. In general, the size of the inclined crystal will be considerably longer and narrower than that of the flat crystal, with the long dimension in the direction of propagation of the x-ray beam. In addition, an inclined crystal DCM requires several more degrees of freedom than the flat crystal DCM. These differences will be described in detail in later sections.

Technical Requirements

A summary of critical performance specifications for the DCM are listed below.

• fixed exit operation (<50 micron vertical beam motion across scanning range)
• 35 mm offset between incoming and outgoing beams
• tunable over the classical energy range (4-20 keV) with Si (111) crystals
• angular resolution of first crystal: 5 microrads or better
• high vacuum (HV) compatible (10^-7 to 10^-8 Torr range)
• design independent of details of the first crystal geometry and cooling scheme
  

Typical Mode of Operation of the DCM

In the standard mode of operation for this DCM, data are collected when the DCM is motionless. That is, data are not taken "on the fly" (while the monochromator is scanning energy, although this may be entirely possible over small energy ranges). However, we wish to be able to scan the DCM over its entire energy range without losing the diffracted exit beam. We plan to drive Y₁ synchronously with the drive for theta, thus obtaining a fixed output beam offset while scanning energy (theta). As mentioned previously, the angular width of the crystal's transmission function is only several arc seconds, and this is the degree of change that can be tolerated in the relative angle between the first and second crystals as the unit is scanned in theta (and hence Y₁ is moved). It is a requirement that the exit beam that sets the tolerance on the yaw of Y₁ is not lost. Because data collection periods can extend over several days or weeks, mechanical stability is critical. The relative angle between the two crystals should therefore be able to be maintained to well within one tenth the adjustment range of the fine theta_adj over this period of time. The stability required by the user of the DCM is considerably more stringent (arc second stability over the measuring period). A feedback system, to be supplied by APS, may be necessary to maintain the higher degree of stability feeding back to a piezoelectric transducer (PZT) on the fine theta_adj to attain the required stability.

Overall Physical Dimensions

The DCM will reside in the first optical enclosure (FOE) of the synchrotron beamline. The overall layout of the FOE is shown in Figure 6, and the approximate location of the DCM is marked. One of the more critical dimensions is the distance from the beam center line to the shielding wall. We have only 0.5 meter clearance in this direction and would like to have a clearance of at least 0.2 meter from the wall to any part of the DCM. The incoming beam height is 1.400 meters, relative to the floor of the experiment hall, and, with the 35 mm monochromatized beam offset (from the incident white beam), the outgoing beam height will be 1435 mm. The flange-to-flange length (along the beam direction) of the monochromator vacuum tank should be kept to 1.5 meters or less.

Vacuum System

The vacuum system consists of all vacuum components (chamber, flanges, feedthroughs, rotary seals, etc.) associated with the monochromator, including ion pumps, power supplies, and controllers but excluding the roughing system. The vacuum system will be designed to support a UHV environment (i.e., internal/full penetration welds should be used and outgassing material avoided), although the vacuum for the monochromator is targeted for 10^-7 to 10^-8 Torr during operation. The assembled apparatus shall be leak-checked to 10^-9 Torr-l/sec or better and maintain a vacuum of better than 1 x 10^-8 Torr under conditions of no beam.

The vacuum chamber must be fabricated of stainless steel to facilitate UHV compatibility and also to aid in the containment of scattered radiation that can interact with air to produce ozone from the first optical component. The chamber should have a wall thickness of at least 0.25 inch. All flanges less than 13.25 inches should be Conflats of standard sizes. For ease of access, the two side panels of the chamber should be removable, possibly two large 27.125-inch flanges. Those flanges in excess of 13.25 inches (diameter) are to be of a proven wire seal design with readily available preformed gaskets. For ease of access to the tank, we would consider the use of quick release flange couplings. The input flange should be a 6-inch Conflat, while the output flange should be an 8-inch Conflat for connection to the rest of the beamline. The output flange must be configured such that, if the first crystal were to be removed, the white beam would pass through unimpeded. Entrance and exit beam sizes are shown in Figure 7. Infrared (IR) transmitting windows must be appropriately placed on the chamber so that the temperature on the surface of the first crystal in either the flat or inclined geometry can be monitored by an IR camera placed outside the chamber. The IR windows should transmit in the 2 µm to 11 µm wavelength range. Near the exit port, a feedthrough (linear or rotary, depending on final design) should be available for moving a fluorescent screen in and out of the diffracted beam. A quartz window must be available for viewing the fluorescent screen during diagnostic tests. Two sets of coolant feedthroughs should be available, one for use with water, liquid gallium (at temperatures of about 50-80°C), or liquid nitrogen for cooling the first crystal and the other set for water alone, which may be required to cool the second crystal. These feedthroughs should be located near the axis of rotation so that torque on the crystals from the coolant tubes is minimized as the monochromator changes energy (i.e., theta changes). The chamber must have all the necessary electrical feedthroughs used to control or monitor the in-vacuum devices such as PZTs, motors, etc., provided by the vendor. In addition, flanges for feedthroughs for ten thermocouples and actuator (PZTs, motors, etc.) on the crystal mounts should be provided by the vendor. These feedthroughs can be shared in a common flange with feedthroughs required by vendor-supplied actuators or can be mounted in separate flanges. Strain relief shall be provided for connections to all electrical feedthroughs. If separate flanges are used, they should be blanked off for testing and delivery. Figure 8 shows the schematic of the UHV chamber. The mechanical, rotary, coolant, and electrical feedthroughs are not shown.

The vacuum pumping system should maintain a high vacuum (10^-7 - 10^-8 Torr) environment inside the vacuum chamber with beam present. The APS beamline vacuum systems will use Perkin Elmer ion pumps. For compatibility, the same pumps should be used for the monochromator vacuum chamber. The pump should be sized to maintain the desired vacuum of 1 x 10^-8 Torr or better with beam present. If the ion/roughing pump flanges are located at or near the bottom of the vacuum chamber, a baffling system should be included in the design to prevent coolant from entering the pumps in case of an in-vacuum coolant leak. (This might be a lip around the flanges to prevent pools of coolant from running into the pump and a cover so that coolant cannot drop directly into the pumps from above.) There should be a leak valve on the chamber for venting purposes. A gate valve between the chamber and the pumps is to be included to allow the pumps to remain operating while the vacuum chamber is vented to atmosphere. A grounded screen shall be provided between the ion pump and the gate valve. Gate valves are to have metal-sealed bonnets with O-ring sealed gates. The valve should be mounted so that the O-ring seal faces away from the chamber (to protect the seal from radiation). Gates are to maintain a vacuum of 1 x 10^-9 Torr against atmosphere applied in either direction. The vendor shall supply the APS with the complete vacuum pumping system, including the pumps, power supplies, gauges, and controllers (Granville Phillips preferred). All pump controllers and gauge readouts should have computer interface (IEEE 488) capability for remote monitoring purposes.

All materials used, including the translation/rotation stages and mounts described in the Mechanical Design section, that reside in the vacuum chamber must be properly cleaned and must be UHV compatible. Electrochemical polishing of all in-vacuum surfaces is recommended. The vacuum chamber and any mechanical parts inside the vacuum chamber must be bakeable to 100°C. All materials used inside the vacuum chamber must have vapor pressures of less than 10^-8 Torr at 100°C. Residual gas spectra of the assembled monochromator at room temperature and at 100°C shall be provided by the vendor. The parameters used in acquiring the spectra shall also be stated by the vendor.

Mechanical Design

Figures 9 and 10 show schematics of the monochromator motions. The monochromator must be able to adapt to both the flat- and inclined-crystal cases and be flexible enough to take minor changes in the cooled crystal manifold. For ease of visualization, the flat-crystal case (i.e., the case where the crystal surface is cut parallel to the diffracting planes) is considered first. The crystals are positioned such that the surface normal (and hence normal to the atomic planes) of the first crystal is perpendicular to and passes through the theta-rotation axis, while the surface of the second crystal (which is parallel to the first crystal) lies in a plane that contains the theta-rotation axis.

Because the range of the theta rotation is quite large, care must be taken so that the coolant lines flex properly without undue strain during rotation, which could misalign one crystal relative to the other. If the theta-rotation stage is attached to the flange nearest the shielding wall, it would allow the easiest and most convenient access to the crystals via the outer flange (further from the wall). However, since the distance from the beam center line to the wall in that direction is only 0.5 meter, this may not be possible. One alternative is to attach the rotation stage to the outer flange and have the flange sit on a rail system. Then, for access, the whole flange (rotation stages, crystals and all) would translate away from the wall (the UHV chamber would stay put).

To compensate for any angular changes that occur between the first and second crystals (from thermal/mechanical instabilities, for example) rotationsand theta_adj are needed. Of these two rotations, much better control and sensitivity are needed for theta_adj. Two ranges of adjustment are required for the theta_adj, a coarse one for gross alignment of the first and second crystals and a fine one for maintaining parallelism of the atomic planes to an arc second or so. Previous experience has shown that PZT devices work very well in this application for the fine theta_adj because they are easy to incorporate into a feedback loop designed to keep the crystals parallel. (The feedback electronics are not to be supplied by the vendor.) Because of the limited range of PZTs (typically, 10 to 50 microns), the coarse adjustment has traditionally been made via a mechanical adjustment or an independent rotational stage onto which the fine adjustment is mounted. If the coarse adjustment is made by mechanical means, this adjustment can be made by assembling a screwdriver-like device on a linear rotary feedthrough to provide in situ changes with the monochromator at some particular rotation angle theta. However, recent advances in PZT technology now permit much longer linear extensions to be made and perhaps the fine and coarse adjustments can be incorporated into one. We will entertain either type of arrangement in the proposed design.

Except for the main theta-rotation axis, all other rotation axes should pass through the center of the front face of the crystal that is being rotated. For this reason, the rotation stages should ride on the translation stages instead of the reverse.

The vendor shall supply all the necessary rotation or translation devices including encoders (where appropriate) and stepper motors. In addition, the vendor shall supply all the necessary mounting hardware. If the motion devices (for example, stepper motors), require cooling, the vendor shall supply all the necessary water-cooled mounts, tubes, and the appropriate vacuum feedthroughs. The cooling vacuum feedthrough for the first crystal assembly must be independent of all the other coolant feedthroughs. The APS shall provide the vendor with details of the crystal assembly for mounting purposes.

Although not shown explicitly in any of the drawings, the second crystal may need to be cooled in some fashion.

The vendor shall supply a kinematic mounting plate to accept the first and second crystal mounting plates. The positions of the mounting plates relative to the beam centers are shown in Figure 11. The mounting plate should permit repositioning of the crystals and their mount to within twice the stated accuracy of the motions on which they are connected.

Precision machining is expected on all mechanical components, and the use of shims to achieve fine alignment shall be avoided. All internal mechanical assemblies (except the theta rotation stage and shaft) shall be constructed so that disassembly and reassembly can be easily done through the use of location pins and/or machined shoulders.

In Table 2a-c are the specifications for required in-vacuum motions. The roll, pitch, and yaw motions of the translations stages are defined as follows: yaw is a rotation about the x-axis, pitch is a rotation about the y-axis, and tilt or roll is a rotation about the z-axis, where the xyz axes are defined in Figures 9 and 10. It is the tolerance on the yaw that is more critical because alterations in yaw change the Bragg angle.

Table 2a. In-Vacuum Motion Range and Accuracy Specifications

Motion:	theta rotation (remotely controllable)
Function:	Changes the angle between the incoming beam and the atomic planes for both crystals
Range:	90° (360° preferred)
Resolution	1 arc second
Load Capacity:	Weight of all attached stages and maximum of 15 kg of crystals and stainless steel manifold (first and second crystals and mounts)
Motion:	thetaadj rotation coarse (manually controllable, remote operation optional) and fine (remotely controllable)
Function:	Fine tunes theta of the first crystal to compensate for any mechanical/thermal instabilities. (Aligns the Bragg planes to the correct angle.) Should have a coarse and fine adjustment.
Fine Range:	2 arc minutes
Fine Resolution:	0.1 arc second
Load Capacity:	Weight of all attached stages and maximum of 10 kg of crystal and mounts
Coarse Range:	±1.0°
Coarse Resolution:	1 arc minute
Load Capacity:	Weight of all attached stages and maximum of 10 kg of crystal and mounts
Motion:	Y1 translation (remotely controllable)
Function:	Moves first crystal perpendicular to the Bragg planes so that the incoming beam hits the center of the crystal
Range:	10 mm
Resolution:	25 microns or better
Load Capacity:	Weight of all attached stages and maximum of 10 kg of crystal and stainless steel manifold
Yaw:	1 arc second over range of travel
Roll and Pitch:	1 arc minute over range of travel
Motion:	X2 translation (remotely controllable)
Function:	Moves the second crystal horizontally in and out of the x-ray beam (for alignment purposes in the inclined geometry)
Range:	25 mm
Resolution:	0.1 mm
Load Capacity:	Weight of all attached stages and maximum of 5 kg of crystal/mount
Motion:	rotation (remotely controllable)
Function:	Adjusts the tilt of the second crystal to match that of the first
Range:	±5°
Resolution	1.0 arc minutes
Load Capacity:	All attached stages and 3 kG of crystal and mount

Table 2b. Support Stand Motion Range and Accuracy Specifications

Motion:	X1 translation
Function:	Moves the first crystal horizontally in and out of the x-ray beam (for alignment purposes in the inclined geometry)
Range:	20 mm
Resolution:	0.1 mm

The above specifications are required for first crystal alignment. In addition, for general alignment of the entire chamber, the support stand must have Y-translation capability (same specifications as X₁) and roll, pitch, and yaw capabilities.

In addition to these motions, some of the required motions will be incorporated into the crystal mounts. To clarify, the following motions will be supplied by us:

Table 2c. Crystal Mount Motion Range and Accuracy Specifications

Motion:	rho1 rotation (manually adjustable)
Function:	Rotates the first crystal about the reciprocal lattice vector (normal to the Bragg planes) (used in the inclined geometry only)
Range:	±5°
Resolution:	0.05°
Load Capacity:	Maximum of 10 kg of crystal and stainless steel manifold
Motion:	rho2 rotation (remotely controllable)
Function:	Rotates the second crystal about the reciprocal lattice vector (used in inclined geometry only)
Range:	±5°
Resolution	0.05°
Load Capacity:	Maximum of 5 kg of crystal and mounting

(The X₁, X₂, rho1,and rho2 degrees of freedom are needed only in the inclined geometry.)

Crystal Assembly Size and Mass

The crystal assembly comprises the crystal itself, coolant manifold, and the input/output cooling tube connections. The crystal assembly shall be fully developed by the APS. It is described in this document for the purpose of defining its size and mass. In the flat geometry case, the first crystal will be approximately 100 mm by 100 mm by 25 mm thick, while the second crystal will probably be about 250 mm by 100 mm by 10 mm thick. In the inclined geometry case, the first crystal will be approximately 250 mm by 75 mm by 25 mm thick, while the second crystal will probably be about 250 mm by 75 mm by 10 mm. The actual cooling scheme and manifold are still under research and development. For the inclined geometry case, the angle of inclination may vary between 70 to 85 degrees. In both the crystal geometries, the total weight of the first crystal assembly including the manifold, coolant, and mounting plates should be less than10 kg, while the weight of the second crystal assembly including the manifold, coolant, and mounting plates will be less than 5 kg. The design of the monochromator must be independent of the details of the first crystal cooling manifold.

Controls/Interfacing/Cabling

The vendor shall supply the APS with all the necessary devices (motors, PZTs, PZT controllers, encoders, microsteppers, power supplies, cables, etc.) for the monochromator motions. Because the DCM will be computer controlled, all devices and drives should be computer compatible. Cables should be long enough to permit control of the monochromator at a remote location 20 m from the monochromator assembly. The current plan at the APS is to use a Unix-based workstation to communicate with the DCM motion drivers via a VME crate. Cables inside the vacuum chamber must be appropriately shielded to withstand the radiation (e.g., Teflon insulation is unacceptable). The vendor shall not supply the APS with the stepping motor controllers, computer I/O boards, and equipment that are computer/operating system specific.

Diagnostics

Diagnostics for the monochromator are straightforward and will be composed of a remotely controllable fluorescent screen in the vacuum chamber that can be positioned in the path of the diffracted beam (avoiding the direct beam). The screen will be viewed through a quartz viewing port in the vacuum chamber by a television camera and monitor system. (Quartz is chosen because it does not darken under the influence of x-ray radiation as rapidly as plate glass.) Therefore, the requirement is that the vacuum chamber has a remotely controllable linear or rotary feedthrough near the exit port for moving the fluorescent screen in and out of the beam.

Hardware Deliverables

The contractor shall deliver to the APS the complete vacuum/mechanical system of the DCM. The following is a list of the hardware deliverables.

1. A vacuum chamber with (a) all necessary IR windows, (b) one quartz window, (c) first crystal coolant input/output feedthroughs (compatible with water, liquid gallium, or liquid nitrogen), (d) second crystal water input and output feedthroughs, (e) blank-off flanges for any ports or feedthroughs not utilized, (f) linear or rotary feedthroughs for the fluorescent screen, (g) all necessary electrical feedthroughs, (h) all coolant transport tubes inside the vacuum chamber, and (i) any rail/translation system that is necessary for removing the flanges during crystal changes.

2. A vacuum pumping system including (a) ion pump (b) all pumping-system-related power supplies, (c) all pumping system controllers, (d) all vacuum gauges and monitoring devices, and (e) all connections to the vacuum chamber, including adapter flanges (if necessary), and all cables.

3. All the mechanical motion devices described in the Mechanical Description section above including (a) all translation stages, (b) all rotation stages, (c) all PZT or inchworm devices, (d) all gear reducers, (e) all encoders, (f) all the necessary motors, (g) all the power supplies needed, (h) all the adapter and/or mounting pieces to achieve the required motions, and (i) all cables inside and outside of the vacuum chamber necessary to power the devices.

4. The vendor shall not supply any computer- or operating-system-specific piece of equipment, for example, stepper motor controllers, computer I/O boards, etc., the silicon crystals, or the heat removal apparatus. The vendor shall also not supply the vacuum roughing system or the television camera and monitor system.

Liquid Gallium Pump

Liquid Gallium Cooling of X-ray Optics

The increased cooling capacity of liquid gallium over water (for a given flow rate) has been well known for some time [4-6]. The pertinent physical properties of water and liquid gallium are given in Table 3.

Table 3. Physical properties of water (27°C) and liquid gallium (30°C)

Property	Gallium	Water
Density (gm/cm3)	6.09	0.998
Melting point (°C)	29.8	0.0
Boiling point (°C)	2403	100
Thermal conductivity (W/cm-°C)	0.28	0.00613
Specific heat (J/g-°C)	0.373	4.179
Vapor pressure (Torr)	10-10	31.8

Other advantages of liquid gallium, in addition to its superior thermal properties, are its extremely low vapor pressure, high boiling temperature, high surface tension, and high thermal and electrical conductivity as compared to water. Our experience has been that seals that leak slightly when pressurized with water do not leak when pressurized with gallium. We attribute this to gallium's high surface tension. The low vapor pressure allows one to have small leaks in the vacuum system without deterioration of the chamber pressure. A high boiling temperature and good thermal conductivity mean that, should flow accidentally be reduced in the crystal, some heat will still be conducted and local boiling of the coolant will not occur (which could result in a critical heat flux and burn out situation). And finally, the electrical conductivity means that the gallium can be pumped electromechanically, that is, the pump need not have any moving parts [7], and hence vibrations from the pump itself should be virtually eliminated. Flow-induced vibrations arising from sharp turns in tubing and manifold, convolutions in bellows, or abrupt changes in the diameter of coolant carriers can be a serious problem. In collaboration with vibration experts from the Materials and Components Technology (MCT) Division at ANL, we are currently investigating approaches that will minimize this phenomena. Gallium is a rather inert material and elaborate safety precautions are not necessary. (Safety precautions for gallium can be found in Appendix A of this document.) Liquid gallium does react adversely with some materials, in particular aluminum. Fortunately, we have found that there seems to be no deleterious effects on silicon, stainless steel, and Teflon tubing from exposure to liquid gallium at modest temperatures (less than 50°C).

The specifications for the commercially procured liquid gallium pump are given below.

Material Specifications

• The permanent magnets will be constructed of neodymium-iron-boron magnetic material. Specifications are listed in Appendix B.
• The magnetic return path will be made of high grade magnet iron. Specifications are listed in Appendix C.
• All parts of the system that come in contact with gallium will be made of nonmagnetic 304 stainless steel or 321 hydraulic stainless steel tubing. The only exception that is allowed is the use of Teflon as part of a flow gauge or as part of a visual level indicator. The flow gauge may have to be made of stainless steel depending on its location. All nonstainless steel parts must be approved by ANL before construction.
• Aluminum will be avoided in the construction of the system for all parts associated with the pump.
  

Performance Specifications

• The pump must achieve a volume flow rate of 4 gallons per minute (gpm) while operating with a head pressure of 75 pounds per square inch (psi).
• The temperature of the gallium delivered by the system should be automatically controllable between the temperatures of 40° and 70°C and regulated to =/- 3°C with any steady state heat load from 0 to 5 kwatts. The recovery time from large transients (power on/off) should be no more than 5 minutes.
• The system should be able to withstand internal pressures of 200 psi and be able to be operated under vacuum (less than 1 Torr).
• The gallium-to-water heat exchanger should be able to remove 5 kW of power (with a 10 kW option) with water cooling of less than 5 gpm and 80 psi.
• The DC current supply should supply 0-3000 A at 0-3 volts and should have an AC ripple on the output of less than 0.5 percent. The power supply leads must be at least 10 feet in length.
• Local readout for (1) pressure and temperature sensors (2) controls for adjusting the temperature of the gallium and (3) controls for adjusting the flow of the gallium.
• The remote status panel (19" rack mountable) should be operable at a distance of 50 feet from the pump. It should contain (1) readouts for the pressure and temperature sensors, (2) controls for adjusting the temperature of the gallium, and (3) controls for adjusting the flow rate of the gallium. All readouts should be via either RS-232 or GPIB. Arrangement of the panel display of parameters will be reviewed before construction.
  

Power Specifications

• 120/220 VAC
  

General Features

• All connections should be easy to assemble and not produce torque of tubing and/or crystal. Recommend using type Cajon/Swagetok, VCR, Face Seal Fitting with stainless steel insert connectors. ANL permission needed for the use of other connectors.
• Gallium level indicator on the pump.
• Spill containment pan under pump.
• Heaters on pump to keep gallium from freezing during nonoperational periods.
• Bypass valves to allow pump to run while crystals are being exchanged.
• Power supply (current and voltage) and gallium pump (flow, pressure, level, temperature) should be alarmed/interlocked so that an out-of-range value for these levels can be used to shut down the beam.

A schematic drawing of the DC current liquid gallium pump is shown in Figure 12. The APS prototype pump performance (flow rate and pressure as a function of current) is shown in Figures 13 and 14.

High Heat Load Crystals

Inclined Crystal Approach

The solution for dealing with the high heat load problem for the APS undulator A will include the use of inclined crystals. Briefly, the inclined crystal geometry spreads the beam footprint on the surface of the crystal, while maintaining a symmetric reflection. That is, the asymmetry parameter b = k_in n/k_out n = -1. A conventional flat symmetric crystal spreads the beam by a factor of 1/sin theta, while the inclined crystal spreads the beam by a factor of 1/cos beta sin theta, where theta is the Bragg angle and beta is the inclination angle [8-13]. The small angular divergence of undulator A together with its high power density makes the inclined crystal an ideal candidate for dealing with the heat load from this ID. Due to the sizes of the crystals involved, slits will be used only to allow the central cone of the undulator radiation through to the monochromator. Appendix D provides further information about the inclined crystal geometry and some useful relationships for determining beam sizes and shapes.

Modeling and Experimental Results

The APS has completed several experimental and computational studies on the performance of the inclined crystal geometry. Independent simulations using the 8X8 matrix approach and 4th order dispersion surface calculations have shown that the inclined crystal diffraction properties are essentially like those of flat symmetric crystals. Figure 15 compares the reflection curves of an 85° inclined crystal with a flat crystal at 5 keV and 13.84 keV. The minor differences will be mentioned later.

Finite element analysis of the APS undulator A thermal loading on back-cooled inclined silicon monochromators have also been performed. Table 4 summarizes some of the important results. At closed gap and 4.2 keV (worst case power loading), with an inclination angle of 85°, the thermally induced slope error is only 56 microradians. Even better performance is expected with the actual crystals because of more sufficient heat exchange and a smaller distance between the diffraction surface and the coolant.

Experimental studies have confirmed that inclined crystals do behave essentially like flat crystals. In addition, inclined crystals with liquid gallium cooling have successfully handled the heat loads of the ANL/CHESS undulator (379 W total power, 48 W/mm² peak power density [2]) and the X-25 focused wiggler at NSLS (38 W total power, 118 W/mm² peak power density [3]).

Table 4. Summary of FEA Results*

Case	Slit	Beta	Theta	Harmonic (keV)	Length (cm)	Width (cm)	Thickness (cm)	Coolant	() max (°C)	(Uy0) max (µm)	(slope) max (µr)
IC-062	no	85°	8.99°	E3=12.6 (K=2.2)	95	15	1.0	Ga	39	5.5	26
IC-063	no	85°	8.99°	E3=12.6 (K=2.2)	95	15	0.5	Ga	33	5.5	24
IC-064	yes	80°	8.99°	E3=12.6 (K=2.2)	15	3	1.0	Ga	58	1.9	68
IC-065	yes	80°	5.7°	E3=19.9 (K=1.5)	23	3	1.0	Ga	23	1.9	40
IC-066	yes	85°	13.96°	E1=8.2 (K=1.2)	18	5	1.0	Ga	21	3.5	15
IC-067	yes	85°	27.96°	E1=4.2 (K=2.2)	9	5	1.0	Ga	92	0.6	56
IC-068	yes	85°	27.96°	E1=4.2 (K=2.2)	29	15	1.0	Ga	70	1.0	50
IC-069	no	85°	27.96°	E1=4.2 (K=2.2)	29	15	1.0	Ga	142	3.2	120

* Explanation of the data: A "yes" in the slit column is for cases in which a 3.6 mm X 1.8 mm slit is used to let only the central cone of the undulator beam through. Beta is the inclination angle. Length and width refer to the size of the crystal used. For the cases in which no slits were used, the crystal length and width were selected so that the entire undulator beam was intercepted. Thickness is the thickness of the crystal used. is the maximum surface temperature of the crystal. U_y0 is the displacement on the surface of the crystal due to thermal expansion. The main quantity of interest is the maximum slope error on the surface of the crystal, the last column.

Crystal Design

From the simulations and experimental results, we have ascertained that the maximum surface power density that our current cooling schemes (i.e., slotted crystals with liquid gallium coolant at 1-2 gpm) can handle without substantial thermal distortion on the crystal is about 4 or 5 W/mm². (Although we hope to improve on this with a pin-post type of heat exchanger and improved bonding techniques with slotted crystals, which will permit greater flow rates for the liquid gallium - see Fabrication section below.) Our design of inclined crystals for use with the APS undulator A is therefore aimed at achieving surface power densities of less than 5 W/mm². Due to the lengths of the crystals involved, only the central cone of the undulator radiation will be accepted. Slits 3.6 mm wide and 1.8 mm high will be used upstream of the DCM. At 30 m from the source, this is the approximate size of the central radiation cone up to its 5 sigma value. In addition, for ease of alignment, the inclination angle should be kept as small as possible. Also, we would like to have one crystal that is capable of handling the heat load from 8-20 keV. With these thoughts in mind, our plan is to use two sets of crystals to cover the 4-20 keV range for the undulator A radiation. Either the silicon (111) or silicon (220) reflections can be used. Table 5 shows some of the parameters of the crystal for the case of silicon (111), which we plan to use. A set of 78° inclined crystals will cover the energy range of 8-20 keV, while a set of 85° inclined crystals will cover the 4-9 keV range. Figures 16a and 16b show schematically the crystal and beam footprints of a silicon (111) crystal, with 78° inclination, that will operate in the 8-20 keV range. The size of the first crystal will be about 8 inches long by 2 inches wide, while the second crystal will be about 10 inches long by 2 inches wide. The additional length is required on the second crystal for the motion of the beam along the crystal during an energy scan. The crystals have unusual shapes in order to avoid blocking the incoming or outgoing beams.

Table 5. Some parameters for a set of Si (111) crystals.*

Footprint dimensions of inclined geometry
w = 3.600000 mm		h = 1.800000 mm		Cross to 3rd harmonic at 13.000000 keV
Beta = 78.000000		Si (111) reflection		beam offset = 35.000000 mm
energy	theta	diagonal	width	power density	beamtrans
3.000000	41.234436	28.039058	1.822414	25.947597	26.549684
4.000000	29.627304	37.640923	1.830137	16.000107	35.399579
5.000000	23.296222	47.177384	1.833745	10.810045	44.249473
6.000000	19.242726	56.689655	1.835714	7.713812	53.099368
7.000000	16.408827	66.189979	1.836905	5.694563	61.949262
8.000000	14.310588	75.683439	1.837678	4.288185	70.799157
9.000000	12.692227	85.172558	1.838210	3.254133	79.649052
10.000000	11.404976	94.658747	1.838590	2.453818	88.498946
11.000000	10.356123	104.142858	1.838871	1.798137	97.348841
12.000000	9.484768	113.625439	1.839085	1.221148	106.198736
13.000000	8.749200	123.106859	1.839252	4.643352	115.048630
14.000000	8.119873	132.587378	1.839384	4.076152	123.898525
15.000000	7.575256	142.067182	1.839491	3.603348	132.748420
16.000000	7.099282	151.546409	1.839578	3.204413	141.598314
17.000000	6.679714	161.025164	1.839651	2.864190	150.448209
18.000000	6.307068	170.503527	1.839712	2.571271	159.298103
19.000000	5.973875	179.981561	1.839763	2.316919	168.147998
20.000000	5.674175	189.459317	1.839807	2.094343	176.997893

Footprint dimensions of inclined geometry
w = 3.600000 mm		h = 1.800000 mm		Cross to 3rd harmonic at 13.000000 keV
Beta = 85.000000		Si (111) reflection		beam offset = 35.000000 mm
energy	theta	diagonal	width	power density	beamtrans
3.000000	41.234436	64.606066	1.803879	10.877128	26.549684
4.000000	29.627304	86.494674	1.805188	6.707180	35.399579
5.000000	23.296222	108.3000025	1.805795	4.531527	44.249473
6.000000	19.242726	130.072673	1.806125	3.233599	53.099368
7.000000	16.408827	151.828610	1.80632	2.387138	61.949262
8.000000	14.310588	173.574736	1.806453	1.797590	70.799157
9.000000	12.692227	195.314572	1.806542	1.364120	79.649052
10.000000	11.404976	217.050117	1.806605	1.028631	88.498946
11.000000	10.356123	238.782598	1.806652	0.753772	97.348841
12.000000	9.484768	260.512812	1.806688	0.511900	106.198736
13.000000	8.749200	282.241300	1.806716	1.946474	115.048630
14.000000	8.119873	303.968441	1.806738	1.708706	123.898525
15.000000	7.575256	325.694512	1.806756	1.510509	132.748420
16.000000	7.099282	347.419718	1.806770	1.433277	141.598314
17.000000	6.679714	369.144214	1.806782	1.200657	150.448209
18.000000	6.307068	390.868121	1.806792	1.077866	159.298103
19.000000	5.973875	412.591532	1.806801	0.971243	168.147998
20.000000	5.674175	434.314524	1.806808	0.877940	176.997893

* Beta is the inclination angle. A 3.6 mm X 1.8 mm slit at 30 m from the undulator A source has been used to allow only the central cone of radiation through. Theta is the Bragg angle. Diagonal andwidth refer to the size of the beam footprint on the surface of the crystal. Power density is the power density on the surface of the crystal. Beamtrans is the distance between the centers of the footprints for different energies for the second crystal. For example, for the case of the 78-degree inclined crystal, the distance on the surface of the second crystal between the 8 keV and 20 keV footprints is about 177-71 = 106 mm.

Fabrication

The current plan at the APS is to have the high heat load inclined crystals fabricated by Rockwell Power Systems (RPS) incorporating their pin-post type of heat exchanger [14]. The whole crystal assembly will include the pin-post heat exchangers, coolant distribution manifold, and the inlet-outlet manifold (see Fig. 17). Both the silicon/silicon and the silicon/metal bonding will be done with frit glass. RPS has substantial experience in silicon bonding and has provided John Arthur of SSRL with a similar device [14], although in that device, O-ring seals were used. Due to our concern about radiation damage, our plan is to not have any O-rings in the crystal assembly. Instead, the silicon will be bonded directly to the metal manifolds. Due to the compatibility of the thermal expansion properties, the current plan by RPS is to use a Fe-Ni alloy instead of stainless steel for the metal part of the assembly. In-house tests have shown that liquid gallium does not appear to attack the alloy. Details of the actual metal manifold and mounting scheme are still under investigation. For compatibility with our liquid gallium pump, the crystal should withstand 100 psi pressure and have about a 20 psi pressure drop at a 5 gpm flow rate.

Some Subtle Aspects of the Inclined Crystal Geometry

Although the diffraction properties of the inclined crystal geometry are essentially the same as those of the flat crystal geometry, there are indeed some minor differences. They are the following:

1. Sensitivity to rotations about the reciprocal lattice vector H. Figure 18 shows the dependence of the asymmetry parameter b, on rho, the rotation about H. Thus, it may be useful to incorporate that degree of freedom into the monochromator or the crystal mount.
2. A slight change in beam shape through a double crystal monochromator. The amount of change in beam shape depends on the location of the crystal relative to the source and the beam divergence. For the APS undulator A, with the monochromator at 30 m from the source, the change in shape is of the order of 0.1 mm.
3. Theta_in does not equal theta_out. There are very slight differences between theta_in and theta_out of an inclined reflection. Figure 19 shows the difference in the case of an 85° inclined crystal at 13.84 keV.
4. The diffracted beam from one reflection no longer lies in the plane spanned by H and the incoming beam. Figure 20 shows the amount of out-of-plane angular difference in the case of an 85° inclined crystal at 13.84 keV. The result is that a doubly diffracted beam from a double crystal monochromator will be translated horizontally by a small amount. For the APS undulator A beam, with the monochromator at 30 m from the source and a 35 mm vertical beam offset, the amount of horizontal displacement is about 5 microns.
5. For an 85° inclined DCM on the APS undulator A beam diffracting in the vertical plane, the beam from the monochromator will be displaced vertically by about 1 mm. This displacement, for practical purposes, is dependent only on the inclination angle. The motion of the beam for an 85° inclined DCM during an 8-20 keV energy scan moves less than a micron. In addition, there is a very small horizontal displacement of the order of 0.1 microns.

Recently, Robert Blasdell of the APS has incorporated the inclined crystal geometry into the ray-tracing program SHADOW. We shall be pursuing the sensitivity of the inclined crystal geometry to many of the above-mentioned parameters (rho, mismatched inclination angles between the first and second crystals, relative misalignments between the first and second crystals, etc.) over the next several months.

References

1. D. M. Mills and M. T. King, Nucl. Instrum. Meth. 208 (1983) 341.
2. R. Hewitt and M. Sansome, private communication.
3. B. Lai, A. Khounsary, R. Savoy, L. Moog, and E. Gluskin, Undulator A Characteristics and Specifications, Argonne National Laboratory Report ANL/APS/TB-3 (1993).
4. R. K. Smither, G. A. Forster, C. A. Kot, and T. M. Kuzay, Nucl. Instrum. Meth. A266 (1988) 517.
5. A. M. Khounsary, J. J. Chrzas, D. M. Mills, and P. J. Viccaro, Optical Engineering 29 (1990) 1273.
6. R. K. Smither, W.-K. Lee, A. Macrander, D. M. Mills, and C. S. Rogers, Rev. Sci. Instrum. 63 (1992) 1746.
7. DC current gallium pump - patent applied for (S.N. 778,456), filed 10/16/91.
8. A. M. Khounsary, Rev. Sci. Instrum. 63 (1992) 461.
9. A. T. Macrander, W.-K. Lee, R. K. Smither, D. M. Mills, C. S. Rogers, and A. M. Khounsary, Nucl. Instrum. Meth. A319 (1992) 188.
10. W.-K. Lee, A. T. Macrander, D. M. Mills, C. S. Rogers, and R. K. Smither, Nucl. Instrum. Meth. A320 (1992) 381.
11. A. T. Macrander and W.-K. Lee, Nucl. Instrum. Meth. A319 (1992) 155.
12. W.-K. Lee amd A. T. Macrander, Nucl. Instrum. Meth. A319 (1992) 158.
13. A. T. Macrander, D. R. Haeffner, and P. L. Cowan, SPIE 1740 (1992).
14. T. Tonneson and J. Arthur, Proceedings of the 1992 SPIE Conference (in press).