摘要
An important trend in extreme ultraviolet and soft X-ray free-electron laser(FEL)development in recent years has been the use of seeding by an external laser,aimed to improve the coherence and stability of the generated pulses.The highgain harmonic generation seeding technique was first implemented at FERMI and provided FEL radiation with high coherence as well as intensity and wavelength stability comparable to table-top ultrafast lasers.At FERMI,the seed laser has another very important function:it is the source of external laser pulses used in pump–probe experiments allowing one to achieve a record-low timing jitter.This paper describes the design,performance and operational modes of the FERMI seed laser in both single-and double-cascade schemes.In addition,the planned upgrade of the system to meet the challenges of the upgrade to echo-enabled harmonic generation mode is presented.
