Elettra 2.0 will be a fourth generation storage ring light source replacing the existing Elettra synchrotron. This article illustrates design strategies, physical investigations and technical choices to meet multiple and sometimes conflicting requirements. These include to make Elettra 2.0 a fully transversely coherent source up to 0.5 keV-photon energy, diversify the type of experiments through a very broad range of photon energies, from infrared to hard x-rays, maximize the number of photon beamlines in excess of 2-times the machine periodicity, and be able to produce picosecond-long light pulses at MHz repetition rate without interference to the standard multi-bunch operation. Most recent advancements in beam physics, technical systems and installation plan are reported with some detail.

Stable and reliable beam position measurement is of paramount importance for the present and future light sources. Stabilization with a pilot-tone technique was developed by Elettra Sincrotrone Trieste and supported in the commercial BPM electronics Libera Spark. Both system components (the pilot-tone front-end and BPM electronics) are controlled through a common software interface which is compatible with TANGO, EPICS and LabVIEW/MATLAB clients. The system provides a reliable self-diagnostics, cable and button diagnostics and drifts compensation. This paper presents results from beam measurements under different environmental and beam conditions.

Elettra Sincrotrone Trieste Research Center (Elettra) is one of the Italian Institutions, together with Istituto Nazionale di Fisica Nucleare (INFN) and Consiglio Nazionale delle Ricerche (CNR), committed to the realization of the Italian in-kind contributions for the European Spallation Source. Elettra contributions are concentrated on the proton accelerator and more specifically they concern the construction of the conventional iron-dominated electromagnets and related power converters to be installed in the superconducting part of the linac and in the High energy Beam Transport (HEBT), the RF power stations for the superconducting spoke cavity linac section and the wire scanner acquisition system for the beam diagnostics. This paper provides a description of the contributions and an overview of the status of the construction activities.

The next generation light sources will require Beam Position Monitoring systems capable of performing high resolution measurements as well as assuring long-term measurement stability. One possible solution to stabilize the position measurements long-term drifts is using a pilot-tone signal which is transferred together with the BPM signal and measured by the BPM electronics. To investigate this solution, Elettra Sincrotrone Trieste developed a pilot-tone injector which was used together with the commercial BPM readout electronics Libera Spark to validate the concept with several measurements related to the typical figures of merit of the BPM systems: position resolution, long-term drift and dependence from beam current and fill pattern. In addition, the behaviour of the system was studied under different environmental conditions (changes in temperature and humidity). After the first measurements with beam at Elettra Sincrotrone Trieste, the test-setup was provided also to other laboratories and the measurement results are presented in this paper.

FERMI is the seeded Free Electron Laser (FEL) user facility at the Elettra laboratory in Trieste, operating in the VUV to EUV and soft X-rays spectral range; the radiation produced by the seeded FEL is characterized by wavelength stability, low temporal jitter and longitudinal coherence in the range 100-4 nm. During 2018 a dedicated experiment has shown the potential of the Echo Enabled Harmonic Generation (EEHG) scheme [1] to cover most of this spectral range with a single stage cascade [2]. Such a scheme, combined to an increment of the beam energy and of the accelerator performances, could extend the FERMI operating range toward the oxygen k-edge. With this perspective, we present the development plans under consideration for the next 3 to 5 years. These include an upgrade of the linac and of the existing FEL lines, consisting in the conversion of FEL-1 first, and FEL-2 successively, into EEHG seeded FELs.

Long-term stability is one of the most important properties of the BPM readout system. Recent developments on pilot tone capable front end have been tested with an established BPM readout electronics. The goal was to demonstrate the effectiveness of the pilot tone compensation to varying external conditions. Simulated cable attenuation change and temperature variation of the readout electronics were confirmed to have no major effect to position data readout. The output signals from Elettra front end (carrier frequency and pilot tone frequency) were processed by a Libera Spark with the integrated standard front end which contains several filtering, attenuation and amplification stages. Tests were repeated with a modified instrument (optimized for pilot tone) to compare the long-term stability results. Findings show the pilot tone front end enables great features like selfdiagnostics and cable-fault compensation as well as small improvement in the long-term stability. Measurement resolution is in range of 10 nanometers RMS in 5 Hz bandwidth.

The operational status of the Italian 2.4/2.0 GeV third generation light source Elettra is presented together with upgrades especially concerning the next low emittance light source Elettra 2.0.

In this contribution, we describe the advantages of the pilot tone compensation technique that we implemented in a new BPM prototype for Elettra 2.0. Injecting a fixed reference tone upstream of cables allows for a continuous calibration of the system, compensating the different behaviour of every channel due to thermal drifts, variations of cable properties, mismatches and tolerances of components. The system ran successfully as a drop-in substitute for a Libera Electron not only during various machine shifts, but also during a user dedicated beamtime shift for more than 10 hours, behaving in a transparent way for all the control systems and users. The equivalent RMS noise (at 10 kHz data rate) for the pilot tone position was less than 200 nm on a 19 mm vacuum chamber radius, with a long-term stability better than 1 μm in a 12-hour window. Two main steps led to this important result: firstly, the development of a novel RF front end that adds the pilot tone to the signals originated by the beam, secondly, the realisation of an FPGA-based double digital receiver that demodulates both beam and pilot amplitudes, calculating the compensated X and Y positions.

In this paper we present a novel and original four-channel front end developed for a beam position monitor (BPM) system. In this work, we demonstrate for the first time the continuous calibration of the system by using a pilot tone for both beam current dependency and thermal drift compensation, completely eliminating the need for thermoregulation. By using this approach, we were also able to investigate several odd and well-known behaviours of BPM systems; the influence of important issues, like the non-linearity of ADCs and the gain compression of amplifiers, which do affect the reliability of the measurement, have been fully understood. To achieve these results, we developed a new radio-frequency front end that combines the four pick-up signals originated by the beam with a stable and programmable tone, generated within the readout system. The signals from a button BPM of Elettra storage ring have been acquired with a 16-bit, 160 MS/s digitizer controlled by a CPU that evaluates the acquired data and applies the correction factor of the pilot tone. A final resolution equal to 1 μm, for a vacuum chamber with an average radius of 19 mm, has been measured with a long-term stability better than 1 μm.

FERMI is the seeded Free Electron Laser (FEL) user facility at the Elettra laboratory in Trieste, operating in the VUV to EUV and soft X-rays spectral range; the radiation produced by the seeded FEL is characterized by wavelength stability, low temporal jitter and longitudinal coherence in the range 100-4 nm. During 2018 a dedicated experiment has shown the potential of the Echo Enabled Harmonic Generation (EEHG) scheme [1] to cover most of this spectral range with a single stage cascade [2]. Such a scheme, combined to an increment of the beam energy and of the accelerator performances, could extend the FERMI operating range toward the oxygen k-edge. With this perspective, we present the development plans under consideration for the next 3 to 5 years. These include an upgrade of the linac and of the existing FEL lines, consisting in the conversion of FEL-1 first, and FEL-2 successively, into EEHG seeded FELs.

FERMI has reached its nominal performance on both FEL lines, FEL-1 (12 eV to 62 eV) and FEL-2 (62 eV to 310 eV). After a brief overview of the activities with users, we will describe plans for LINAC, FEL and beamline upgrades for 2016-2018 and beyond. This includes EEHG schemes for FEL-2. INTRODUCTION FERMI is the seeded Free Electron Laser (FEL), operating in the VUV to soft X-rays range, located at the Elettra laboratory in Trieste, Italy [1]. FERMI is the only FEL user facility designed to operate in the seeded HGHG mode [2] and has successfully demonstrated operation from 100 nm down to 4 nm [3, 4]. The VUV to EUV FEL line, FEL-1, started operation with external users, i.e. selected by the FERMI Review Panel, in December 2012. Since then a period of actual commissioning of the source with users has been conducted, in a strong interaction between machine and laser team and the users. This resulted in proposing and implementing innovative solutions and schemes for pumpprobe experiments. A portion of the seed laser is delivered as optical laser pulse to the experimental stations for pump-probe experiments with extremely low jitter to the FEL pulse, less than 7 fs rms [5]. Several schemes to produce two colour, FEL-pump and FEL-probe, pulses have been implemented [6-9]; the most recent FEL scheme makes use of two seed laser beams of different wavelengths and of a split radiator section to generate two extreme ultraviolet pulses from distinct portions of the same electron bunch [10]. A parameter under observation during user’s beamtimes is of course the actual availability of the FEL compared to the scheduled time. The uptime has been always around 85% of the scheduled time and, thanks to some improvements on the machine systems, has lately attained 90%. FEL-2 is the second FEL line of FERMI. It uses a double stage cascade scheme and the fresh bunch technique. First lasing was observed in 2012 [4] and the performance was gradually improved [11], in parallel to FEL-1 operation, until it reached nominal performances in 2014 [1]. The FEL emission of FEL-2 is characterized by excellent spectral line shapes and the transverse profile of the FEL pulses is close to the TEM00 Gaussian mode. At wavelengths below 5 nm the FEL tuning turned out to be critical, with large shot-to-shot energy fluctuations and with the necessity of a seed energy in excess of 40 J. Based on the experience of the operation for users on FEL-1 and of the commissioning of FEL-2, and with the aim of increasing the uptime of the facility beyond the 90% mark, a number of upgrade actions have been activated in 2015 and are being planned for 2016-2018. These upgrades shall further improve the performance of FEL-1 in the various multicolour schemes and should allow on FEL-2 similar tuning possibilities as on FEL-1, along with improved stability and flexibility of operation. OPERATION FOR USERS FERMI has been in operation for 6528 hours during 2015; 3520 hours thereof, that is 54% of the total operation time, were dedicated to users with a clear increase compared to the 43% reached in 2014. The remaining time has been divided between machine commissioning (1312 hours equal to 20% of the total operation time; was 40% in 2014) and machine and beamlines tuning (1696 hours, equal to 26% of the total operation time; was 17% in 2014). The average FEL uptime for users in 2015 was equal to 88.0% of the scheduled FEL time, confirming the effectiveness of the constant effort in increasing the reliability of the machine. In fact, in 2014 the uptime was equal to 86.1% and in 2013 to 84.7%. In the last run of 2015, that lasted three months under continuous operation, a value of 91% was registered. A total of 27 Users’ beamtimes, 20 peer-reviewed and 7 in-house, were allocated on the three beamlines operated in 2015, namely Diffraction and Projection Imaging (DiProI, 12 experiments), Low Density Matter (LDM, 7 experiments) and Elastic Inelastic Scattering TIMEX ____________________________________________ † email address: alessandro.fabris@elettra.eu MOPOW015 Proceedings of IPAC2016, Busan, Korea ISBN 978-3-95450-147-2 744 C op yr ig ht © 20 16 C C -B Y3. 0 an d by th e re sp ec tiv e au th or s 02 Photon Sources and Electron Accelerators A06 Free Electron Lasers (EIS-TIMEX, 6 experiments). In-house beamtimes in 2015 also included commissioning of the EIS-TIMER experimental station, which started in July [12] and continued in November, when the first evidence of a transient-grating signal was observed from a BaF2 sample. Also the commissioning of the TeraFERMI beamline, that collects the THz radiation (ultrashort pulses in the 0.1-15 THz range) naturally emitted by the electron beam already spent by the FEL undulators, has started in December 2015 and the first THz photons have been observed. The sixth beamline at FERMI is MagneDyn, dedicated to the exploration of the ultimate limits of magneto-dynamic processes in conventional and advanced magnetic materials. This beamline is still under construction, with the commissioning expected to begin at the end of 2016. After five calls the number of the submitted proposals by users requiring beamtime for experiments at FERMI is still steadily increasing. In the last call for experiments, at the beginning of February, 72 proposals were submitted, 21 of them are experiments using FEL-2. The estimated oversubscription rate is about 3.4. UPGRADE RESULTS 2016 The first upgrade program started between 2014 and 2015, focused mainly on the LINAC and on FEL-2. The results collected in a dedicated commissioning period scheduled at the beginning of 2016 are discussed in the following sections. LINAC FERMI is driven by a 200 meter long S-band LINAC. In the injector region, two 3-meter long forward traveling wave accelerating structures, coming from the old Elettra injector, were installed. In order to improve the electron beam quality, it was decided to replace the first two existing structures with two dual-feed accelerating structures [13]. The structures were manufactured by Research Instruments GmbH and delivered to Elettra in July 2015. Figure 1: The new accelerating structures in the injector. After the high power testing in the FERMI cavity test facility, in January 2016 the structures were installed, on schedule, in the injector region, as shown in Fig. 1. The old structures were consequently moved into the high energy region of the LINAC to further increase the final energy by approximately 90 MeV. In February 2016 normalized emittances of 0.7 and 0.9 mm mrad in the horizontal and vertical plane, respectively, were measured in the 100 MeV diagnostic section. These values at 700 pC are 10-15% smaller than the previous ones. Peak LINAC energies as high as 1629 MeV were measured. The maximum operating energy, with compressed and linearized electron beam phase space, at 700 A of nominal current, is about 1550 MeV (see Fig. 2). Figure 2: Maximum LINAC operating energy. FEL-2 In 2014 the design parameters of FEL-2 were demonstrated down to the lower end of the wavelength range, 4 nm. However, it appeared evident that upgrades were needed to improve the stability and flexibility of FEL-2, and to achieve a frequency tunability similar to the one available on FEL-1 (OPA operation). The first of these upgrades addressed the LINAC energy, as described in the previous section. The second upgrade, specific to FEL-2, consists in the installation of a second regenerative amplifier. It produces a shorter seed pulse (infrared pulse duration less than 40 fs, 60-70 fs in the UV) that allows for better filling of the longitudinal phase space in fresh bunch mode. The laser also permits a wavelength tunability in the UV (+/-2%) allowing a limited FEL wavelength tuning with the third harmonic generation setup. As well a second OPA laser system was installed, so that double pulse seeding is possible on FEL-1 with two fully tunable sources. Another important upgrade of FEL-2 consists in the installation of a third Elliptically Polarized Undulator (EPU) in the radiator of the first HGHG stage of FEL-2, completing the design layout of this FEL line, shown in Fig. 3. Figure 3: FEL-2 with the new installed EPU. Proceedings of IPAC2016, Busan, Korea MOPOW015 02 Photon Sources and Electron Accelerators A06 Free Electron Lasers ISBN 978-3-95450-147-2 745 C op yr ig ht © 20 16 C C -B Y3. 0 an d by th e re sp ec tiv e au th or s The additional EPU reduces the seed energy required for operation in the shorter wavelength range, providing the possibility to operate with OPA laser system in a longer wavelength range. The EPU was built by Kyma srl with the same parameters of the two already existing ones: 55.2 mm period length, 42 periods, 10 mm gap. In March 2016, after the upgrades, FEL-2 reached unprecedented shot to shot stability, below 6% rms at 4.2 nm, as shown in Fig. 4, and average intensity greater than 15 J/pulse. The energy requirement from the seed laser could be reduced to less than 20 J. The new undulator configuration also offers the flexibility to operate efficiently with different harmonic conversion factors, as 16x4 which is almost equivalent to 13x5 used in the past. Figure 4: FEL-2 pulse by pulse intensity at 4.2 nm. Reasonable energy per pulse was measured down to 3.8 nm. Radiation spectra could be measured in single shot even at harmonic 3.1 nm, as shown in Fig. 5. Figure 5: Single shot spectrum at harmonic 17x5=85. The plots in the inset of Fig. 5 represent spectra from multiple shots during the same sequence. The green line is the projection of the single shot CCD acquisition displayed in the figure. The vertical axis represents the unfocused vertical distribution of the FEL pulse. PLANS FOR THE NEAR FUTURE The upgrade plans for 2017-2018 at FERMI foresee exploring the possibility of replacing the aging S-band Backward Traveling Wave structures (BTWs) of the LINAC and to implement the Echo Enabled Harmonic Generation scheme (EEHG) on FE

At Elettra, the Italian synchrotron light source, an internal project has been started to develop an electron beam position monitor capable of achieving sub-micron resolution with a self-compensation feature. In order to fulfil these requirements, a novel RF front end has been designed. A high isolation coupler combines the input signals with a known pilot tone which is generated by the readout system. This allows the parameters of the four channels to be continuously calibrated, by compensating the different responses of each channel. A similar technique is already known, but for the first time experimental results have shown the improvement in resolution due to this method. The RF chain was coupled with a 4-channel digitizer based on 160 MHz, 16 bits ADCs and an Altera Stratix FPGA. At first, no additional processing was done in the FPGA, collecting only the raw data from the ADCs; the position was calculated through the FFT of each signal. A simulation was also performed to verify the analytic relation between spatial resolution and signal-to-noise ratio; this was very useful to better understand the behaviour of the system with different sources of noise (aperture jitter, thermal noise, etc.). The experimental data were compared with the simulation, showing indeed a perfect agreement with the latter and confirming the capability of the system to reach sub-micrometric accuracy. Therefore, the use of the pilot tone greatly improves the quality of the system, correcting the drifts and increasing the spatial resolution by a factor of 4 in a time window of 24 hours.

FERMI is a seeded free-electron laser (FEL) delivering extreme ultraviolet (EUV) light in the wavelength range between 100 and 4 nm. An overview of generation and control of fully coherent stable EUV pulses is presented.

FERMI is the seeded FEL user facility in Trieste, Italy, producing photons from the VUV to the soft X-rays with a high degree of coherence and spectral stability. Both FEL lines, FEL-1 and FEL-2, are now available for users, down to the shortest wavelength of 4 nm. We will report on the completion of the commissioning of the high energy FEL line, FEL-2, and on the operational experience for users, in particular those requiring specific FEL configurations, like two-colour experiments. We will also give a perspective on the improvements and upgrades which have been triggered by our experience and are aiming to maintain as well as to constantly improve the performance of the facility for our user community.