  We present in this paper an update on the design of BEaTriX (Beam Expander
Testing X-ray facility), an X-ray apparatus to be realized at INAF/OAB and that
will generate an expanded, uniform and parallel beam of soft X-rays. BEaTriX
will be used to perform the functional tests of X-ray focusing modules of large
X-ray optics such as those for the ATHENA X-ray observatory, using the Silicon
Pore Optics (SPO) as a baseline technology, and Slumped Glass Optics (SGO) as a
possible alternative. Performing the tests in X-rays provides the advantage of
an in-situ, at-wavelength quality control of the optical modules produced in
series by the industry, performing a selection of the modules with the best
angular resolution, and, in the case of SPOs, there is also the interesting
possibility to align the parabolic and the hyperbolic stacks directly under
X-rays, to minimize the aberrations. However, a parallel beam with divergence
below 2 arcsec is necessary in order to measure mirror elements that are
expected to reach an angular resolution of about 4 arcsec, since the ATHENA
requirement for the entire telescope is 5 arcsec. Such a low divergence over
the typical aperture of modular optics would require an X-ray source to be
located in a several kilometers long vacuum tube. In contrast, BEaTriX will be
compact enough (5 m x 14 m) to be housed in a small laboratory, will produce an
expanded X-ray beam 60 mm x 200 mm broad, characterized by a very low
divergence (1.5 arcsec HEW), strong polarization, high uniformity, and X-ray
energy selectable between 1.5 keV and 4.5 keV. In this work we describe the
BEaTriX layout and show a performance simulation for the X-ray energy of 4.5
keV.
