Wavefront sensing of XUV beams

For the purpose of the wavefront profile measurement of XUV beams emitting at 21.2 nm and 30 nm, we designed the PDI (Point Diffraction Interferometer) wavefront sensor. PDI is a self-referencing monolithic device consisting of a thin neutral filter and a very small pinhole located near the axis of the XUV beam focal spot. The small pinhole works as a diffraction aperture generating a reference spherical wave, and working as well as a spatial filter. The material of the thin foil is partially transparent for the XUV radiation, and it determines the visibility of the interference fringes. The interference pattern is recorded by an XUV detector placed behind the foil. From the information encoded in the pattern it is possible sequentially to reconstruct the beam wavefront profile. We will discuss the design and optimization of the PDI wavefront sensor setup.