Frequency-Mixing Lasing Mode at European XFEL

Gianluca Geloni,Winfried Decking,Evgeny Schneidmiller,Sergey Tomin,Dirk Noelle,Theophilos Maltezopoulos,Mikhail Yurkov,Marc Guetg,Jan Grünert,Svitozar Serkez,Frank Brinker

doi:10.3390/app11188495

Abstract

We demonstrate generation of X-ray Free-Electron Laser (XFEL) pulses in frequency mixing mode at the SASE3 line of the European XFEL. The majority of the SASE3 FEL segments are tuned at two frequencies ω1 and ω2 following an alternate pattern. Leveraging on non-linearities generated through longitudinal dispersion in the system, we obtain electron bunching at a frequency difference ωFM=ω2−ω1. FEL amplification at ωFM follows in a few last radiator segments. We report on the generation of frequency mixing at photon energies between 500 eV and 1100 eV with pulse energies, depending on the length of the radiator, in the mJ level. This method allows generating low photon energies in cases where the FEL runs at high electron energy and the target photon energy cannot be reached in the main undulator, with the simple addition of a short, custom-made afterburner.

Highlights

Self-Amplified Spontaneous Emission (SASE) X-ray free-electron lasers (XFELs) generate beam energy and density modulation as well as output radiation by exploiting a narrow-bandwith FEL instability centered around a single resonant frequency
We demonstrate generation of X-ray Free-Electron Laser (XFEL) pulses in frequency mixing mode at the SASE3 line of the European XFEL
We report on several experimental results recently obtained at the SASE3 line of the European XFEL

Summary

Introduction

Self-Amplified Spontaneous Emission (SASE) X-ray free-electron lasers (XFELs) generate beam energy and density modulation as well as output radiation by exploiting a narrow-bandwith FEL instability centered around a single resonant frequency. If one tunes different undulator segments at two well-separated frequencies ω1 and ω2, both frequencies would be separately amplified in the linear regime, further yielding frequency mixing signals in the bunching at frequencies ω2 ± ω1, within the reach in resonance frequency of a final radiator. The bunching at these two frequencies can be further optimized with the help of a downstream element adding longitudinal dispersion, which can be provided by a few detuned undulator segments. Besides constituting a novel method for the generation of XFEL radiation, as discussed in Section 4, frequency mixing may constitute a useful mode of operation, allowing to obtain target photon energies too low to be obtained in the main undulator, with the simple addition of a short afterburner with larger-than-baseline K parameter reach

Theoretical Considerations about Frequency Mixing

Setup and Results

Outlook and Conclusions