Linear optics control of sideband instability for improved free-electron laser spectral brightness

G Perosa,M Trovò,P Sigalotti,E M Allaria,N Bruchon,M Veronese,N Mirian,A Demidovich,M B Danailov,L Badano,P Rebernik,P Cinquegrana,L Giannessi,G M Penco,S Spampinati,E Roussel,G De Ninno,S Di Mitri

doi:10.1103/physrevaccelbeams.23.110703

Abstract

Extension of stable longitudinal coherence from vacuum ultraviolet to x rays is highly sought after in the free-electron laser (FEL) community, but it is often prevented by bandwidth broadening originated in the electron beam microbunching instability. We demonstrate that a proper tuning of the linear optics before the beam enters the undulator mitigates the microbunching-induced sideband instability. The experiment was conducted at the Fermi FEL operated in echo-enabled harmonic generation mode, where the spectral brightness at 7 nm wavelength was doubled. The FEL performance is compared to nonoptimized optics solutions and characterized in terms of peak intensity and spectral bandwidth shot-to-shot stability. The technique has straightforward implementation, because it uses quadrupole magnets routinely adopted for beam transport, and it applies to any FEL architecture, so paving the way to the production of high-intensity Fourier-transform limited x-ray pulses in existing and planned FEL facilities.

Highlights

UV and x-ray free-electron lasers (FELs) have become invaluable tools for the exploration of matter [1,2,3,4]
The technique has straightforward implementation, because it uses quadrupole magnets routinely adopted for beam transport, and it applies to any FEL architecture, so paving the way to the production of high-intensity Fourier-transform limited x-ray pulses in existing and planned FEL facilities
We have reported a characterization of the Fermi echoenabled harmonic generation (EEHG) FEL emission as a function of the linear momentum compaction of the spreader line

Summary

INTRODUCTION

UV and x-ray free-electron lasers (FELs) have become invaluable tools for the exploration of matter [1,2,3,4]. On top of exquisite synchronization and high intensities, temporal coherence of the FEL pulses, which commonly translates into narrow spectral bandwidth, is an essential requirement to FWM experiments [11]. This is improved in x-ray self-seeding FELs, but at the expense of large shot-to-shot intensity fluctuations [14,15,16]. The optimum laser heater intensity often consists in a balance of FEL spectral purity and high pulse energy. Strategies for the mitigation of the microbunching instability which do not rely on an increase of the beam energy spread would allow high-intensity FEL pulses and a stable, very narrow bandwidth. The study was conducted at the Fermi FEL operated in EEHG mode and lasing at the fundamental wavelength of 7.3 nm [22]

EXPERIMENT

Findings

DISCUSSION

CONCLUSIONS