research papers 22 # 2001 International Union of Crystallography  Printed in Great Britain ± all rights reserved J. Synchrotron Rad. (2001). 8, 22±25 Performance limits of indirectly cryogeni- cally cooled silicon monochromators ± experimental results at the APS Wah-Keat Lee,* Kamel Fezzaa, Patricia Fernandez, Gordon Tajiri and Dennis Mills Advanced Photon Source, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, IL 60439, USA. E-mail: wklee@aps.anl.gov The results of high-heat-load tests of indirectly cryogenically cooled silicon monochromators are presented. The measure- ments show that, provided that the total power absorbed by the crystal is less than 150 W, indirect cryogenically cooled silicon monochromators will perform well, with thermal- induced slope errors of less than 2 arcsec. At the Advanced Photon Source, this corresponds to the undulator closed-gap (11 mm) condition at 100 mA with white-beam slit sizes slightly larger than the full width at half-maximum of the radiation central cones. The dependence of the slope errors on the thermomechanical properties of silicon are discussed and clearly demonstrated. Keywords: X-ray optics; high-heat-load optics; cryogenic cooling; silicon monochromators. 1. Introduction Direct cryogenically cooled silicon monochromators are used extensively at the undulator beamlines at the Advanced Photon Source (APS). `Direct cooling' refers to the cases whereby the liquid-nitrogen coolant ¯ows inside the silicon monochromator itself, and is in contrast to `indirect cooling' whereby the liquid nitrogen ¯ows inside a cooling block (e.g. a piece of copper) and the silicon monochromator is brought into close thermal contact with the cooling block. The performance limits of direct cryogenically cooled silicon monochromators have been investigated (Lee et al. , 2000). The results clearly show that direct cryogenically cooled silicon monochromators can successfully handle heat loads that are much higher than those encountered by typical users at current third-generation synchrotron facilities. It is of interest to investigate the possibility of using indirectly cryo- genically cooled monochromators because an indirectly cooled system has several advantages. First, the fabrication of directly cooled crystals is much more complicated, time- consuming and costly. Cutting coolant channels into silicon is a relatively slow process. On the other hand, contact-cooled crystals are usually simple rectangular blocks of silicon. Second, directly cooled crystals require a silicon-to-metal- manifold vacuum seal. At the APS, the seals are made with indium foils and metal C-cross-sectional O-rings. Although these seals have been successfully employed, temperature cycling can cause a loss of seal integrity. Furthermore, the process of installing the seal itself is not reliable: it usually takes several attempts before a good vacuum seal is achieved. An indirectly cooled crystal will circumvent the vacuum seal problem because only metal-to-metal seals are required, which are easily accomplished. Indirectly cryogenically cooled monochromators have been tested (Marot et al., 1992) and are being used (Quintana, 2000), but to date no quantitative measurements pertaining to their performance limits have been published. 2. Experimental setup The tests were performed at the SRI-CAT sector 1-ID undu- lator beamline. The monochromator design is shown in Fig. 1. It is a simple rectangular block of silicon clamped between two cryogenically cooled copper blocks. The copper blocks have 16 3 mm-diameter coolant channels each, as shown in the ®gure. The estimated clamping pressure on the silicon surfaces was 0.29 MPa. The thermal contact between the silicon and the copper was achieved by using 80% gallium±20% indium eutectic. The silicon contact surfaces were polished (chemical- mechanical slurry), while the copper surfaces were ¯y-cut but not polished. The copper blocks were nickel-plated to inhibit the indium±gallium from attacking the copper. The crystal was mounted at room temperature before installing into the monochromator tank and cooled. Although the eutectic freezes at cryogenic temperatures, it has the advantage that it wets the silicon and the nickel-plated copper surfaces very well at room temperature. One feature of the design is that the cooling blocks extend above the crystal diffraction surface. This was done as a result of ®nite-element modelling, which shows that such a design helps dissipate the heat from the crystal surface. The silicon crystal diffracting planes were (111), and the liquid-nitrogen ¯ow rate was 6.4 l min 1 . The setup for the experiment is shown in Fig. 2. At the APS, with a standard undulator of period 3.3 cm and length 2.4 m, the full-width-at-half-maximum (FWHM) size of the central cone of the undulator radiation at the monochromator posi- tion (28 m from the source) is 1.5 mm (H)  0.5 mm (V). The maximum power density at closed gap (11 mm, undulator de¯ection parameter K = 2.6) at that location is Figure 1 (a) Top and (b) side view of the indirectly cooled crystal, showing the silicon crystal and the cooling blocks.