"Coherent extreme ultraviolet (XUV) light sources open up new opportunities for science and technology. Promising examples are attosecond metrology, spectroscopic and structural analysis of matter on a nanometer scale, high resolution XUV-microscopy and lithography. The most promising technique for table-top sources is femtosecond laser-driven high-harmonic generation (HHG) in gases. Unfortunately, their XUV photon flux is not sufficient for most applications. This is caused by the low average power of the kHz repetition rate driving lasers (<10 W) and the poor conversion efficiency (<10-6). Following the traditional path of increasing the power, numerous research teams are engineering larger and more complex femtosecond high-power amplifier systems, which are supposed to provide several kilowatts of average power in the next decade. However, it is questionable if such systems can easily serve as tool for further scientific studies with XUV light.The goal of this proposal is the realization of a simpler and more efficient source of high-flux XUV radiation. Instead of amplifying a laser beam to several kW of power and dumping it after the HHG interaction, the generation of high harmonics is placed directly inside the intra-cavity multi-kilowatt beam of a femtosecond laser. Thus, the unconverted light is “recycled”, and the laser medium only needs to compensate for the low losses of the resonator. Achieving passive femtosecond pulse formation at these record-high power levels will require eliminating any destabilizing effects inside the resonator. This appears to be only feasible with ultrafast thin disk lasers, because all key components are used in reflection.Exploiting the scientific opportunities of the resulting table-top multi-MHz coherent XUV light source in various interdisciplinary applications is the second major part of this project. The developed XUV source will be transportable, which will enable the fast implementation of joint measurements."