M. Veronese

Highly coherent and stable pulses from the FERMI seeded free-electron laser in the extreme ultraviolet

Researchers demonstrate the FERMI free-electron laser operating in the high-gain harmonic generation regime, allowing high stability, transverse and longitudinal coherence and polarization control.

The FERMI free-electron lasers

FERMI is a seeded free-electron laser (FEL) facility located at the Elettra laboratory in Trieste, Italy, and is now in user operation with its first FEL line, FEL-1, covering the wavelength range between 100 and 20 nm. The second FEL line, FEL-2, a high-gain harmonic generation double-stage cascade covering the wavelength range 20-4 nm, has also completed commissioning and the first user call has been recently opened. An overview of the typical operating modes of the facility is presented.

Multicolor High-Gain Free-Electron Laser Driven by Seeded Microbunching Instability.

Laser-heater systems are essential tools to control and optimize high-gain free-electron lasers (FELs) working in the x-ray wavelength range. Indeed, these systems induce a controllable increase of the energy spread of the electron bunch. The heating suppresses longitudinal microbunching instability which otherwise would limit the FEL performance. Here, we demonstrate that, through the action of the microbunching instability, a long-wavelength modulation of the electron beam induced by the laser heater at low energy can persist until the beam entrance into the undulators. This coherent longitudinal modulation is exploited to control the FEL spectral properties, in particular, multicolor extreme-ultraviolet FEL pulses can be generated through a frequency mixing of the modulations produced by the laser heater and the seed laser in the electron beam. We present an experimental demonstration of this novel configuration carried out at the FERMI FEL.

TeraFERMI: A Superradiant Beamline for THz Nonlinear Studies at the FERMI Free Electron Laser Facility

During the last few decades, we have witnessed an impressive advancement in the progress of THz technologies. THz spectroscopy can now be considered a mature technique, providing a formidable tool ...

Status and achievements at FERMI@Elettra: the first double cascade seeded EUV-SXR FEL facility open to users

FERMI@Elettra is the first seeded VUV/soft X-ray FEL source. It is composed of two undulatory chains: the low energy branch (FELl) covering the wavelength range from 20 nm up to 100 nm, and the high energy branch (FEL2, employing a double stage cascade), covering the wavelength range from 4 nm up to 20 nm. At the end of 2012 FELl has been opened to external users while FEL2 has been turned on for the first time having demonstrated that a double cascade scheme is suitable for generating high intensity coherent FEL radiation. In this paper we will share our experience and will show our most recent results for both FERMI FELl and FEL2 sources. We will also present a brand new machine scheme that allows to perform two-colour pump and probe experiments as well as the first experimental results.

FERMI@Elettra, a seeded free electron laser source for a broad scientific user program

After less than two years of commissioning the FERMI@Elettra free electron laser is now entering into the operation phase and is providing light to the first user experiments. To reach the final ambitious goals of providing high power coherent pulses with fundamental wavelengths down to 4 nm, the system will need further studies and additional commissioning time in 2011 when fine tuning of the major systems such as the electron gun and the main accelerator will take place. Nevertheless, FERMI is already able to provide light with unique characteristics allowing Users to perform experiments not possible with other facilities. Based on a 1.5 GeV electron linear accelerator, FERMI@Elettra has two seeded FEL lines that cover the whole spectral range from 100 nm down to 4 nm with fully coherent pulses. The use of the high gain harmonic generation scheme initiated by a tunable laser in the UV allows FERMI to produce light characterized by both transverse and full temporal coherence. The use of specially designed undulators allows full control of the FEL polarization and can be continuously varied from linear to circular in any orientation or ellipticity. Here we will report about the first results and the future plans for FERMI@Elettra.

The FERMI seeded-FEL facility: Status and perspectives

The FERMI Free Electron Laser (FEL) in Trieste, Italy operates in the extreme ultraviolet and soft x rays (EUV-SXR) wavelength range delivering high-fluence, stable, ultra-short pulses. Its unique design based on a seeded scheme and on tunable undulators allows unprecedented control of pulse parameters such as wavelength, phase, polarization, synchronization, pulse duration and implementation of multi-color FEL schemes. Both FEL-1 and FEL-2 lines with nominal wavelength range 100–20 nm and 20–4 nm, respectively, are open to users. We report on the unique features of FERMI, in particular those that have evolved beyond the original design, and on their application to pioneering experiments. We also present the upgrades that are planned to further expand the capabilities of this unique light source.

Coherent THz Emission Enhanced by Coherent Synchrotron Radiation Wakefield

We demonstrate that emission of coherent transition radiation by a ∼1 GeV energy-electron beam passing through an Al foil is enhanced in intensity and extended in frequency spectral range, by the energy correlation established along the beam by coherent synchrotron radiation wakefield, in the presence of a proper electron optics in the beam delivery system. Analytical and numerical models, based on experimental electron beam parameters collected at the FERMI free electron laser (FEL), predict transition radiation with two intensity peaks at ∼0.3 THz and ∼1.5 THz, and extending up to 8.5 THz with intensity above 20 dB w.r.t. the main peak. Up to 80-µJ pulse energy integrated over the full bandwidth is expected at the source, and in agreement with experimental pulse energy measurements. By virtue of its implementation in an FEL beam dump line, this work promises dissemination of user-oriented multi-THz beamlines parasitic and self-synchronized to EUV and x-ray FELs.

FERMI longitudinal diagnostics: results and future challenges

The seeded FEL FERMI has completed the commissioning of both the FEL lines, and it is now providing the user community with a coherent and tunable UV radiation (from 100 nm to 4 nm) in a number of different configurations. These also include original FEL-pump - FEL-probe schemes with twin-seeded FEL pulses. Among the key systems for the operation of FERMI, there is the femtosecond optical timing system and dedicated longitudinal diagnostics, specifically developed for FERMI. In this paper, after a short review of the FERMI optical timing system and of its routinely achieved performances, we focus on the results obtained from the suite of longitudinal diagnostics (Bunch Arrival Monitor, Electro Optical sampling station and RF deflectors) all operating in single shot and with 10s fs resolution which demonstrate the FERMI achieved performances. The longitudinal diagnostics measurements are compared between these device and other device on shot-to-shot basis, looking for correlations between machine parameters. Finally future challenges in terms of improvement of existing diagnostics, planned installations and possible upgrades are discussed.

Experimental characterization of the FERMI laser heater and its impact on the FEL operations

FERMI is the first user facility based upon an externally seeded free-electron laser (FEL) that delivers a coherent and tunable UV radiation (down to 4 nm at the fundamental) in a number of different configurations. A microbunching instability (MBI) developing in the bunch compressors and in the rest of the linac can degrade the quality of the high brightness electron beam sufficiently to reduce the FEL output intensity and spectral brightness. A laser heater installed in the low energy (100 MeV) part of the FERMI accelerator increases the local energy spread to provide Landau damping against this instability. In this paper we summarize the main results obtained with the FERMI laser heater since it commissioning in 2012. We present the measurement of the reduction of the incoherent energy spread at the linac exit induced by the heating of the electron beam at the beginning of the linac. We also discuss the positive effects of such heating upon the emission of coherent optical transition radiation and the FEL performances both in terms of intensity and spectrum. Moreover, we report about results that have been used to experimentally demonstrate that for transversely uniform heating the local energy spread augmentation is characterized by a non-Gaussian distribution that can be preserved up to the FEL undulator entrance with a significant impact on the performance of high-gain harmonic generation (HGHG) FELs, especially at soft x-ray wavelengths.