Entrance Optics (M1) The first optical elements play a key role in the beamline’s stability and performance. CASSIOPEE entrance optics is made of three silicon mirrors (a M1a plane mirror, and two M1b and M1c spherical mirrors) attached to the same rotating table whose rotation axis is vertical, in the plane of M1a surface. Ideally, the surfaces of the three mirrors should be parallel (see figure).
These three mirrors (and especially M1a) have to absorb the power radiated by the ring and the insertion devices, without suffering any deformation. In the worst condition, this absorbed power can be as high as 250W. These three mirrors are hence cooled at a temperature around 100 K, where the silicon dilatation coefficient is zero, to avoid any thermal bump at the impact point of the photon beam. This is insured by a 9 bars liquid nitrogen closed loop circulation.
The entrance optics has two working positions:
-A low energy position, in the 8 eV-100 eV photon energy range, when HU256 is used. In this case, the rotating table is set for an incident angle of 5.02° on M1a. The photon beam is then reflected towards M1b, whose radius is calculated to focus the HU256 source point onto the monochromator’s gratings.
-A high energy position, in the 100 eV-1500 eV photon energy range, when HU60 is used. The rotating table is then set for an incident angle of 2.44° on M1a. The photon beam is then reflected towards M1c, whose radius is calculated to focus the HU60 source point onto the monochromator’s gratings.
When going from one position to the other, an in-vacuum motor can also be used to rotate only M1a, in order to adjust the parallelism between M1a and the other concerned mirror.
These two reflections shifted the useful VUV radiation axis from 62mm in the horizontal plane. Hence, a tungsten rod can be placed along the undulators axis to block the gamma rays emitted by the ring (which go through the silicon mirror).
