**Acknowledgements**

The author appreciates kind collaboration of many researchers in HiLASE Centre in Czech Republic, Waseda University, and Gigaphoton Inc., in Japan. Dr. M. Smrz is especially acknowledged for his excellent work in the 100 kHz thin-disc picosecond laser, Dr. H. Turcicova and Dr. O.Novak for their high average power wavelength conversion, and Dr. P. Sikocinski for his aberration measurement in cryogenic Yb:YAG. Dr. Sakaue is highly appreciated for his experimental and theoretical work in the Laser Compton and FEL seeding research and Dr. T. Yanagida for his pioneering work of tin droplet atomization by a picosec‐ ond laser. Experimental work of this chapter is cofinanced by the state budget of the Czech Republic (project HiLASE: Superlasers for real world: LO1602). This work is also supported by the Czech Science Foundation (GACR) under project GA16-12960S.
