Abstract
FERMI is a seeded free electron laser (FEL) based on the high gain harmonic generation (HGHG) scheme and has been generating intense and fully coherent extreme ultra-violet and soft x-ray pulses for several years. The high-degree performance of the FEL leans on a high brightness electron beam, with small transverse emittance ($\ensuremath{\sim}1\text{ }\text{ }\ensuremath{\mu}\mathrm{m}$) and a peak current of about 700-800 A. The main constraint on lasing at high harmonics of the seed is low electron time-slice energy spread. The optimization of the photoinjector and of some linac parameters has allowed a reduction of the relative slice energy spread to the level of few times of ${10}^{\ensuremath{-}5}$. With these new conditions, the FEL can be operated without the need of a laser heater to suppress micro bunching instabilities and this ``cold'' beam has allowed generation of extreme UV pulses with pulse energy exceeding a mJ, and with peak power of about 10 GW. We describe the electron beam characterization and the FEL performance improvement, including the extension of the range of harmonics of the seed which can be amplified, up to the twenty-fifth harmonic, i.e., 10 nm.
Highlights
Free electron lasers (FELs) can exceed by about 10 orders of magnitude the peak brightness of existing synchrotron sources
The harmonic conversion efficiency of high gain harmonic generation (HGHG) decreases at high harmonics [6,7] and the quality of the pulses is increasingly affected by the electron beam defects, such as phase space distortions [24,25] or microbunching instability
We report recent results obtained under conditions allowing transport of the electron beam up to the FERMI FEL-1 undulator without significant gain of the microbunching instability
Summary
Free electron lasers (FELs) can exceed by about 10 orders of magnitude the peak brightness of existing synchrotron sources. The harmonic conversion efficiency of HGHG decreases at high harmonics (shorter wavelengths) [6,7] and the quality of the pulses is increasingly affected by the electron beam defects, such as phase space distortions [24,25] or microbunching instability The latter is induced by the collective self-amplification of nonuniformities in the electron longitudinal distribution [26,27]. Among the various seeding schemes recently proposed [7,30,31], echo enabled harmonic generation [7] allows improvement of the high harmonic conversion efficiency and a reduction of the sensitivity to electron beam phase space modulations [24,25,28,29,30,31,32]. We focus on the FEL performance obtained with this “cold” beam, both in terms of spectral purity and intensity at high harmonics
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