Fermi@elettra status report

The FERMI@Elettra seeded Free Electron Laser (FEL) is based on two separate FEL lines, FEL-1 and FEL-2. FEL-1 is a single stage cascaded FEL delivering light in the 65-20nm wavelength range, while FEL-2 is a double stage cascaded FEL where the additional stage extends the frequency up-conversion process to the spectral range of 20-4nm. The FEL-1 beam line has been in operation since the end of 2010, with user experiments carried on in 20112013 and user beam time allocated until the first half of 2014. Fermi FEL-2 is the a seeded FEL operating with a double stage cascade in the "fresh bunch injection" mode [1]. The two stages are two high gain harmonic generation FELs where the first stage is seeded by the 3rd harmonic of a Ti:Sa laser system, which is up converted to the 4th-12th harmonic. The output of the first stage is then used to seed the second stage. A final wavelength of 10.8 nm was obtained (the 24th harmonic of the seed wavelength) during the first commissioning in October 2012. The experiment demonstrated that the FEL is capable of producing single mode narrow bandwidth pulses with energy of several tens of microjoules. The commissioning of FEL-2 continued in March and June 2013, where the wavelength of operation was extended down to 4nm and below, demonstrating that an externally seeded FEL is capable of reaching the soft X-ray range of the spectrum. INTRODUCTION FERMI@Elettra free electron laser (FEL) is a fourth generation light source at the research centre Elettra – Sincrotrone Trieste, Italy that functions as a user facility producing photons in the ultraviolet and soft X-ray wavelength regions. The scientific case, based on three experimental programs, namely Diffraction and Projection Imaging (DiProI), Elastic and Inelastic Scattering (EIS), Low Density Matter (LDM), calls for stable, high peak brightness, nearly fully coherent (both transversely and longitudinally), narrow bandwidth photon pulses, together with wavelength tunability and variable polarization [2-4]. FERMI is driven by a single-bunch, S-band high brightness electron linac. The linac is presently capable of reaching a final energy up to 1.4 GeV, in conditions of FEL operation (i.e., including energy losses due to the required off crest operation of two linac sections for compression and the X-band cavity for phase space linearization). The linac energy was increased from the previously available 1.2 GeV by an extensive RF conditioning plan program during May 2013. At the same time the machine repetition rate was increased to 50 Hz, with the linac has operating at this rate during all the conditioning of the RF plants. However, the rep rate was reduced back to 10 Hz for the FEL commissioning shifts, in order both to increase the linac reliability for the FEL operation at the higher energy, and to reduce the (expected) cathode aging that was observed on the new 50 Hz gun, delivered by Radia Beam and installed during the winter 2013. In the future the linac energy will be extended further to 1.5 GeV in order to increase the FEL gain in the shortest wavelenght range (at and below 5 nm). ____________________________________________ *Corresponding author: luca.giannessi@elettra.eu. This work has been supported in part by the Italian Ministry of University and Research under grants FIRBRBAP045JF2 and FIRB-RBAP06AWK3 WEPSO22 Proceedings of FEL2013, New York, NY, USA ISBN 978-3-95450-126-7 546 C op yr ig ht c ○ 20 13 C C -B Y3. 0 an d by th e re sp ec tiv e au th or s