The Primary Slits are the first optical element and serves to adapt the shape and dimension of the X-ray polychromatic (White beam) beam delivered by the 2-Pole Wiggler (D25) to the mirrors and monochromator requirements. They are water cooled due to the high power of the X-ray beam at this position. Although the X-ray beam delivered by the EBS machine will be less divergent in the vertical direction, due to the fact that the source will be ~3 m further the beam dimension at this position will be similar than before the upgrade.

The dimension of the slits gap will be typically few mm (between 1 and 4 mm) in the vertical direction and by ~60 mm in the horizontal direction. Such a gap collect a vertical aperture between 35 to ~145 mrad that match perfectly the vertical X-ray beam FHWM produced at 2-Pole Wiggler, covering the foreseen beamline energy range. The primary horizontal slits fully collect the X-ray beam horizontally aperture of  1.6 mrad. Both, the vertical and horizontal slits fulfil performance requirement in terms of aperture range, resolution and accuracy.