D. Nutarelli

Electron beam properties and impedance characterization for storage rings used for free electron lasers

Abstract Good electron beam qualities and stability are the crucial features of Storage Rings dedicated to synchrotron radiation sources or to Free Electron Laser. Most of these characteristics depend on the coupling of the e-beam with the machine environment, which can be in turn modelled in terms of a characteristic impedance, whose absolute value and structure can be used to specify both the stability (longitudinal and transverse) of the beam and its qualities (energy spread, bunch length, peak current, etc.). In this paper we consider two specific examples of Storage Rings used for FEL operation and analyze their performances by means of semianalytical and numerical methods. The analysis is aimed at clarifying the dependence of beam energy spread and bunch length on beam current and at providing a set of parameters useful for the optimization of Free Electron Laser or synchrotron radiation sources.

Two color experiments combining Free Electron Laser and Synchrotron Radiation

Abstract We present the results of the first two color experiment coupling FEL and Synchrotron Radiation, where the SuperACO FEL was used to photoexcite carriers at semiconductor surfaces and the consequently induced surface photovoltage was studied with soft X-ray photoemission spectroscopy. Thanks to the subnanosecond temporal resolution and high surface sensitivity of our experimental setup, we were able to measure for the first time the effect of surface states on band bending dynamics. Based on our direct experience during these experiments, and on the future developments related to other two color experiments proposed on SuperACO, we will discuss the advantages and the technical aspects of this mode of operating the storage ring and the FEL.

Towards the Fourier limit on the super-ACO Storage Ring FEL

Abstract Systematic studies on the Free Electron Laser (FEL) line and micropulse have been performed on the Super-ACO storage ring FEL with a monochromator and a double-sweep streak camera under various conditions of operation (detuning, “CW” and Q-switched mode). From these data, it appears that the FEL is usually operated very close to the Fourier limit.

Applications of UV-storage ring free electron lasers: the case of super-ACO

Abstract The potential of UV-storage ring free electron lasers (SRFELs) for the performance of original application experiments is shown with a special emphasis concerning their combination with the naturally synchronized synchrotron radiation (SR). The first two-color FEL+SR experiment, performed in surface science at Super-ACO is reported. The experimental parameters found to be the most important as gathered from the acquired experience, are underlined and discussed. Finally, future prospects for the scientific program of the Super-ACO FEL are presented with two-color experiments combining the FEL with SR undulator-based XUV and VUV beamlines as well as with a SR white light bending magnet beamline emiting in the IR-UV ( 20 μm – 0.25 μm ) .

Inter-dependence of the electron beam excitations with the free electron laser stability on the super-ACO storage ring

Abstract Storage ring free electron lasers have a complex dynamics as compared to the LINAC driven FEL sources since both the laser and the recirculating electron beam behaviours are involved. Electron beam perturbations can strongly affect the FEL operation (start-up, stability) whereas the FEL can stabilize beam instabilities. Experimental analysis together with simulations are reported here. Improvements of the Super-ACO FEL for users is discussed, and consequences are given in terms of electron beam tolerances for a source development for users.

Two-color experiments combining the UV storage ring free-electron laser and the SA5 IR beamline at Super-ACO

The UV-storage ring Free Electron Laser (FEL) operating at Super-ACO is a tunable, coherent and intense (up to 300 mW) photon source in the near-UV range (300 - 350 nm). Besides, it has the unique feature to be synchronized in a one-to-one shot ratio with the Synchrotron Radiation (SR) at the high repetition rate of 8.32 MHz. This FEL + SR combination appears to be very powerful for the performance of pump- probe time-resolved and/or frequency-resolved experiments on the sub-ns and ns time-scales. In particular, there is a strong scientific case for the combination of the recently- commissioned SA5 Infra-Red Synchrotron Radiation beamline with the UV-FEL, for the performance of transient IR- absorption spectroscopy on FEL-excited samples with a Fourier-transform spectrometer coupled with a microscope allowing high spectral and spatial resolution. The principle and interest of the two-color combination altogether with the description of both the FEL and the SA5 IR beamline are presented. The first synchronization signal between the IR and the UV beams is shown. The correct spatial overlap between the UV (FEL) and the IR (SR) photon beams is demonstrated by monitoring via IR-spectro-microscopy the time evolution of a single mineral particulate (kaolinite) under UV-FEL irradiation.

Dynamic filling factor in the Super-ACO free electron laser

Abstract Here, the Free Electron Laser evolution before the start-up from the spontaneous radiation of an undulator, is considered. This analysis is carried on through the dynamical behaviour of the Filling-factor, representing the transverse overlap between the light pulse and the electron bunch. In such a framework, the gain optimization before the laser start-up is presented and more generally applied to low-gain storage-ring free electron lasers, such as on Super-ACO.

Achromatic damage investigations on mirrors for UV-free electron lasers

Storage Ring Free Electron Laser (FEL) are attractive, full of promise, tuneable and powerful laser sources for the UV range. High reflectivity dielectric mirrors should be produced in order to allow lasing at very short wavelength, with a long stability in a strongly harsh environment and to optimize the extracted FEL power required for most of the newest applications. The front mirror of the laser cavity receives all the synchrotron radiation (SR) emitted by the wiggler, which is responsible for the mirror degradation, combined with the contamination by the vacuum residuals. We are tackling the problem of tests and manufactures of reliable robust mirrors and explore themes such as resistance analysis of UV mirrors to FEL multiscale power, broadband (X-UV) mirror robustness. Under drastic SR conditions, multiscale wavelength damages could be observed. Specific measurement techniques, able to investigate localized spatial modification induced by the non-uniform synchrotron radiation are presented. A local crystalline structure modification of the high index material appears together with a severe increase of the roughness.

Transient absorption spectroscopy in biology using the Super-ACO storage ring FEL and the synchrotron radiation combination

The Super-ACO storage ring FEL, covering the UV range down to 300 nm with a high average power (300 mW at 350 nm) together with a high stability and long lifetime, is a unique tool for the performance of users applications. We present here the first pump-probe two color experiments on biological species using a storage ring FEL coupled to the synchrotron radiation. The intense UV pulse of the Super-ACO FEL is used to prepare a high initial concentration of chromophores in their first singlet electronic excited state. The nearby bending magnet synchrotron radiation provides, on the other hand a pulsed, white light continuum (UV–IR), naturally synchronized with the FEL pulses and used to probe the photochemical subsequent events and the associated transient species. We have demonstrated the feasibility with a dye molecule (POPOP) observing a two-color effect, signature of excited state absorption and a temporal signature with Acridine. Applications on various chromophores of biological interest are carried out, such as the time-resolved absorption study of the first excited state of Acridine.

Toward resistant UV mirrors at 200 nm for free electron lasers: manufacture, characterizations, and degradation tests

Storage Ring Free Electron Laser are attractive and full of promise tunable and powerful laser sources for the UV range. Concerning the optical cavity, the relatively small gain obtained in the UV calls for the necessity to use high reflectivity multilayer mirrors with reliable longevity in synchrotron environment. It is also crucial to limit their absorption in order to optimize the extracted power required for most of the applications. Indeed, the front mirror of the laser cavity receives not only the first harmonic where the lasers operates but all the synchrotron radiation emitted by the undulator: a wide spectrum extending towards X rays. These short wavelengths are responsible for the mirror degradation which results from changes in the coating materials (high induced absorption, color centers, heating...) as well as from carbon contamination due to cracked hydrocarbons originating from the residual vacuum atmosphere. Deposition technologies which allow the manufacture of very dense oxide coatings with low absorption and high reflectivity in UV spectral region were optimized and characterized for this purpose. We report here degradation studies performed on UV mirrors for Storage Ring Free Electron Lasers down to wavelengths as short as 200 nm.