The Evolution of KAOS, a Multipurpose Active Optics System for EUV/Soft X-rays

Introduction KAOS, the Kirkpatrick-Baez Active Optical System, is the flagship optical system of FERMI, the first—and presently only—fully seeded Free Electron Laser facility in the world. KAOS has been entirely developed in-house and, after progressive revisions and upgrades, it presently empowers three out of six beamlines at FERMI (DiProI, LDM, MagneDyn). It also serves two beamlines at FLASH, Hamburg (FL23 and FL24). Although it is grounded on the well-established concept of Kirkpatrick-Baez (KB) mirrors, the challenges it addressed and the needs it was built for ultimately produced a unique system with unique features. Its success over time is the result of a simple and clean mechanical design coupled with the consolidated use of in-series Hartmann wavefront sensors, mounted downstream of the experimental end-stations. Wavefront sensing proved itself a valuable tool to assess the focusing capabilities of KAOS at the early stage of development. It has now overcome this initial duty, becoming used for the optimization of the curvature to face a plethora of needs, such as minimizing aberrations, shaping the beam, accommodating a varying source position, and providing extra diagnostics to the users. Ultimately, KAOS has grown up around, and thanks to, wavefront sensing; if KAOS were a sports car, wavefront sensing would be the pilot. This article aims to tell how KAOS was born and grew up, and will show how wavefront sensing made it work for the best.