Very nice results obtained at FERMI, demonstrating optimal focusing of FEL pulses using the KAOS active optics system using X-EUV wavefront sensor or HASO EUV from Imagine Optic. This sensor is used to align both mirrors of a custom, active Kirkpatrick-Baez focusing system, as well as to drive actuators through a feedback loop down to minimal wavefront residual error. Optimal focus spot down to 1,8 x 2,4 µm at 4,14nm – very close to the limits of the optical system – is demonstrated, and it is proved experimentally that the WF measurement from HASO EUV is able to predict the focus spot profile with very good agreement with the 2D distribution from an ablation sheet.

Check out details in the very good paper from L. Raimondi et al. J. Synchrotron Rad. (2019). 26