ERMI is the seeded Free Electron Laser (FEL) user facility at Elettra laboratory in Trieste, operating in the VUV to soft X-rays spectral range. In order to extend the FEL spectral range to shorter wavelengths, an upgrade plan for increasing the Linac energy from 1.5 GeV to 2.0 GeV is actually going on. After successful testing of the short prototype of new high gradient S-band accelerating structure up to an accelerating gradient of 40 MV/m, a full length 3.0 m HG structure has been built in collaboration with Paul Scherrer Institute (PSI). In the first step, two such new structures would be installed in place of S0a and one deflector at K15 increasing the beam energy to 1.7 GeV. In the next phase 14 new HG structures would replace the present Backward Travelling Wave sections reaching to the final goal of 2.0 GeV. Currently first 3.0 m HG structure is under conditioning and high power testing at Cavity Test Facility of Elettra. In this paper we report the low power measurement results as well as conditioning results of 3.0 m HG structure.

ABSTRACT: FERMI is implementing a development plan to keep the facility in a world-leading position on the base of the requests coming from the user community and the advises from the Scientific Advisory Council and the Machine Advisory Committee. The ultimate goal of this plan consists in doubling the maximum photon energy available and in reducing the pulse duration below the characteristic lifetime of the atomic core levels in the source spectral range. An upgrade of FERMI aimed at reaching the oxygen K-edge requires a profound modification of the FEL configurations and of the main components of the machine, including the linac and the undulator lines. One of the most promising approaches for this upgrade is to implement the echo-enabled harmonic generation (EEHG) scheme, relying on two external lasers to precisely control the spectrotemporal properties of the FEL pulse. The conversion to EEHG of the first stage of the double-stage harmonic cascade presently in use on FEL-2, would allow to reach harmonics as high as 120, enabling to generate coherent pulses down to 2 nm. The main aspects of the upgrade strategy will be discussed in this contribution.

In order to meet the user request of extending the FERMI FEL spectral range over the whole water window, we are developing an upgrade strategy that is based on the implementation of the Echo Enabled Harmonic Generation (EEHG) scheme. The FERMI upgrade strategy is structured as follow: during a first phase, the single cascade FEL-1 branch will be adapted to operate either in EEHG or in HGHG. This upgrade can be achieved with relatively low cost and impact on FERMI operations and will improve the spectral range, spectral quality and scheme flexibility of FEL-1. Furthermore, it will provide a versatile test bench opening the possibility to explore in details the EEHG scheme potentialities and address many of the possible issues related to the second and more critical phase of the upgrade project: the upgrade of FEL-2. These two phases will proceed in parallel to the LINAC upgrade to increase the nominal energy. Solutions aiming at a peak beam energy of 1.8 and 2.0 GeV are under study. In this contribution we will focus on the upgrade of the FEL-1 branch that has already started and is foreseen to provide light to users with the new configuration by spring 2023.