research papers J. Synchrotron Rad. (2007). 14, 339–344 doi:10.1107/S0909049507016317 339 Journal of Synchrotron Radiation ISSN 0909-0495 Received 8 March 2007 Accepted 2 April 2007 # 2007 International Union of Crystallography Printed in Singapore – all rights reserved Windowless transition between atmospheric pressure and high vacuum via differential pumping for synchrotron radiation applications T. Gog,* D. M. Casa, I. Kuzmenko, R. J. Krakora and T. B. Bolin CMC-XOR, Advanced Photon Source, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, IL, USA. E-mail: gog@anl.gov A differential pump assembly is introduced which can provide a windowless transition between the full atmospheric pressure of an in-air sample environment and the high-vacuum region of a synchrotron radiation beamline, while providing a clear aperture of approximately 1 mm to pass through the X-ray beam from a modern third-generation synchrotron radiation source. This novel pump assembly is meant to be used as a substitute for an exit vacuum window on synchrotron beamlines, where the existence of such a window would negatively impact the coherent nature of the X-ray beam or would introduce parasitic scattering, distorting weak scattering signals from samples under study. It is found that the length of beam pipe necessary to reduce atmospheric pressure to below 10 mbar is only about 130 mm, making the expected photon transmission for hard X-rays through this pipe competitive with that of a regular Be beamline window. This result is due to turbulent flow dominating the first pumping stage, providing a mechanism of strong gas conductance limitation, which is further enhanced by introducing artificial surface roughness in the pipe. Successive reduction of pressure through the transitional flow regime into the high-vacuum region is accomplished over a length of several meters, using beam pipes of increasing diameter. While the pump assembly has not been tested with X-rays, possible applications are discussed in the context of coherent and small- angle scattering. Keywords: differential pump; X-ray window; coherent scattering; small-angle scattering. 1. Introduction Advances in recent years in the field of third-generation synchrotron radiation sources have provided the researcher with ever brighter, smaller and dramatically more coherent X-ray beams. This has lead to the advent of new X-ray analytical techniques such as speckle, coherent diffraction, X-ray photon correlation spectroscopy, and other coherent techniques (Sutton, 2002) that were technically not feasible just a decade ago. While X-ray sources have undergone a vast improvement, a remaining challenge is to preserve the coherent characteristics of the X-ray beam as it traverses the various optical elements of the beamline on its way to the experimental station. One class of known offenders are vacuum windows, which usually provide the interface between the vacuum environment of the beamline or accelerator and the outside world. These windows are often not optically flat on the scale of an X-ray wavelength and thus can distort the wavefront of an X-ray beam passing through, increase its divergence, and thereby substantially degrade its coherent quality (Suzuki et al., 1998; Snigirev et al., 1996). Another area which can benefit from the elimination of an exit window is small-angle X-ray scattering. Here, parasitic scattering from material windows often obscures signals from weakly scattering samples at low angles and thereby deter- mines the smallest accessible scattering vector (Bo¨ secke & Diat, 1997; Henderson, 1995; Jaski & Cookson, 2007). Given advances in vacuum technology, combined with the small sizes of modern synchrotron beams, it appears feasible to eliminate vacuum windows for these special applications altogether and negotiate the gradient between the atmo- spheric pressure of an in-air sample environment and the high- vacuum region of the beamline or accelerator solely through differential pumping. This concept as such is not new. Indeed, a successful attempt has been made to pass micrometer-sized X-ray beams through differentially pumped glass capillaries (Nebiki et al., 2006), and various facilities, for example soft X-ray photochemistry beamlines around the world, employ differentially pumped reaction chambers with allowable pressures of up to 1 mbar (Ohashi et al., 2001). A windowless transition has also been achieved by means of a ‘plasma window’ (Hershcovitch, 1998; Hershcovitch et al., 2003;