The seed laser system of the FERMI free-electron laser: design, performance and near future upgrades

P Cinquegrana,A Demidovich,M B Danailov,G Kurdi,I Nikolov,P Sigalotti,P Susnjar

doi:10.1017/hpl.2021.49

P Cinquegrana, A Demidovich + Show 5 more

Open Access

https://doi.org/10.1017/hpl.2021.49

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Abstract

Abstract 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 high-gain 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.

Highlights

FERMI was the first FEL facility where external seeding was implemented
The laser system providing the external UV seed pulses required by the scheme has been designed nearly 15 years ago and is entirely based on Ti:Sapphire pumped IR OPA technology, at that time the only laser configuration allowing to reach the needed parameters
The final part of the paper contains an outlook of the new seed laser system design needed for the implementation of the Enabled Harmonic Generation (EEHG) seeding scheme at FERMI

Summary

INTRODUCTION

FERMI was the first FEL facility where external seeding was implemented It is based on the HGHG seeding scheme [1]. The laser system providing the external UV seed pulses required by the scheme has been designed nearly 15 years ago and is entirely based on Ti:Sapphire pumped IR OPA technology, at that time the only laser configuration allowing to reach the needed parameters. The FERMI’s presently unique pump-probe scheme based on a seed-laser derived pulse delivered to the end-stations for pump-probe experiments allowed a very tight synchronization of the FEL pulse to the seed [4]. This scheme provided nearly jitter-free pump/probe pulse-pairs and has been an additional driver for constant improvement and increase of the complexity of the seed laser system.

II.1. Main Requirements to the Seed pulses in HGHG scheme

II.2. Laser System and Layout

OUTLOOK

CONCLUSIONS