Super-ACO Free Electron Laser Research Articles

Suppression of limit cycles in a free electron laser

To take complete benefit of the Free Electron Laser (FEL) features, we have developed a feedback control scheme designed to suppress the dynamical instabilities affecting the FEL output intensity. The principle is to stabilize an unstable stationary state that coexists with the unwanted limit cycle. This paper gives a theoretical and experimental description of the stabilized stationary regime of a storage ring FEL in presence of the feedback control. We present a numerical model of the FEL with control, that describes the longitudinal dynamics of the laser pulse in condition of detuning between the FEL and the electron beam. These theoretical results are compared with experiments performed on the Super-ACO FEL. The established feedback control provides stable operation of the FEL.

Advection-Induced Spectrotemporal Defects in a Free-Electron Laser

We evidence numerically and experimentally that advection can induce spectrotemporal defects in a system presenting a localized structure. Those defects in the spectrum are associated with the breakings induced by the drift of the localized solution. The results are based on simulations and experiments performed on the super-ACO free-electron laser. However, we show that this instability can be generalized using a real Ginzburg-Landau equation with (i) advection and (ii) a finite-size supercritical region.

Chaotic nature of the super-ACO FEL

The response of the Free Electron Laser (FEL) to an external perturbation can be studied by applying a laser gain modulation. When its frequency is close to the laser resonance one, a non-linear gain variation of the Super-ACO storage ring FEL can lead to a chaotic dynamic behaviour of the laser macro-temporal structure. At a fixed frequency of the modulation, and increasing its amplitude, the systematic analysis of the macro-temporal structure of the laser, shows that chaos appears between two periodic regimes. The attractors, i.e. the phase-space representation of the macro-temporal structure, characterize the regime as periodic, quasi-periodic, or chaotic. The bifurcation scheme evolves with the frequency of the modulation. The experimental macro-temporal sequences can be reproduced by LAS the numerical empirical code developed in the Super-ACO FEL group. Here the comparison between the experimental and numerical results obtained on the non-linear dynamics of the Super-ACO FEL is presented.

Super-ACO FEL oscillation with longitudinal to transverse coupled beam dynamics

The Super-ACO storage ring Free Electron Laser (FEL) has been operated using an alternative operating point characterized by a non-zero dispersion function in the undulator straight section, linking the transverse motion of the electrons to the energy spread, and by a low momentum compaction factor. The longitudinal to transverse coupled electron beam motion induces a modified dynamics of the Super-ACO FEL. The FEL characteristics, such as the FEL power, pulse duration and spectral width are presented and analyzed in terms of storage ring FEL dynamics.

Chaos studies on the super-ACO free electron laser

A laser with parameters modulated at a frequency f may respond not only at f and its harmonics, but also at is sub-harmonic frequencies f/n. The laser behaviour can become irregular though remaining deterministic. A gain modulation has been applied on the Super-ACO Free Electron Laser close to its natural frequency, via a change of the synchronization between the electron bunch and the optical pulse (detuning). Different macrotemporal structures can be observed such as 1T regime in which the laser is pulsed at the modulation period, 2T, chaos and so on. Such transitions can also be measured on the Free Electron Laser pulse temporal position. Bifurcation diagrams and attractors have also been recorded. Comparisons with simulations will be also given.

Steady state of a fully detuned storage-ring free-electron laser

This paper gives an analytical description of the stationary regime of a storage-ring free-electron laser in the presence of the maximum detuning (compatible with the laser onset) between the laser pulse and the electron beam when they pass and interact in the optical cavity. In this condition, the conservation of the first moments of the laser intensity distribution allows one to express the peak gain of the light amplification process and the maximum detuning as a function of system parameters that are directly measurable. These theoretical results are compared with experiments performed on the Super-ACO free-electron laser.

Chaotic dynamics in a storage-ring free electron laser

The temporal dynamics of a storage-ring Free Electron Laser is here investigated with particular attention to the case in which an external modulation is applied to the laser-electron beam detuning. The system is shown to produce bifurcations, multi-furcations as well as chaotic regimes. The peculiarities of this phenomenon with respect to the analogous behavior displayed by conventional laser sources are pointed out. Theoretical results, obtained by means of a phenomenological model reproducing the evolution of the main statistical parameters of the system, are shown to be in a good agreement with experiments carried out on the Super-ACO Free Electron Laser.

Internal micropulse structure of a storage-ring free-electron laser.

The longitudinal distribution of a free-electron laser (FEL) may present a complex internal structure. This phenomenon has been already observed in the case of LINAC based oscillators and self-amplified spontaneous emission devices (for which the presence of "spikes" in the temporal distribution is systematically observed). We investigate here the physical process responsible for the growth of complex substructures inside the micropulse of a storage-ring free-electron laser. This "hole-burning-like" process results from the localized character of the interaction between the ultrarelativistic electron beam circulating in the storage ring and the laser pulse. Experimental results concerning the case of the super-ACO FEL are presented and interpreted by means of a pass-to-pass tracking code containing all the relevant features of the system dynamics.

Super-ACO free-electron laser (FEL) operation with a reduced momentum compaction factor

A new optics providing an emittance close to the minimum theoretical value has been proposed for the Super-ACO storage ring with a reduced value of the momentum compaction factor and non-zero dispersive function in the Free-Electron Laser (FEL) section. It has been adapted to the FEL operation mode, with two RF cavities at 100 and 500 MHz. The obtained results concerning this new optics and the FEL oscillation are presented.

Detuned dynamics of a storage ring free electron laser

This paper is devoted to a theoretical analysis of the dynamical properties of a Storage Ring Free Electron Laser in the presence of an imperfect synchronism between the electron bunch and the laser pulse at each pass inside the optical cavity. The resulting “cw” and pulsed regimes are investigated in detail for the case of the Super-ACO Free Electron Laser. Numerical results are shown to be in good agreement with experimental data.

Local energy exchange in a storage-ring free-electron laser

In this paper a theoretical model is presented, which is based on a pass to pass analysis of the localized interaction between a short laser pulse with a wider electron distribution. It can be applied to a large class of physical phenomena and, in particular, to the case of a storage-ring free-electron laser (FEL). Numerical results are confirmed by experimental measurements done on the ACO and Super-ACO FELs.

Gamma rays produced by inverse Compton scattering in the Super-ACO storage ring free electron laser

The inverse Compton scattering between positron bunches and the free electron laser (FEL) of the Super-ACO storage ring generates a collimated and tunable gamma-ray beam. The use of the FEL instead of an external conventional laser, automatically provides the transverse alignment and synchronisation of the electron and optical beams. The Super-ACO FEL is operated in the 300– 430 nm range at 800 MeV with a high repetition rate (8 MHz) and an average transmitted power of 300 mW at 350 nm . High energy gamma-rays of 35 MeV are produced with a rate of 1.5×10 7 photons/ s and good monochromaticity (about 10%).

The super-ACO free electron laser source in the UV and its applications

Free electron lasers (FELs) are new sources of tuneable coherent radiation, based on the interaction of a relativistic electron beam with a permanent magnetic field. The Super-ACO FEL operates in the UV (down to 300 nm ) at 800 MeV , the nominal energy of the electron beam, with a high repetition rate (8 MHz) . It presents a high average extracted power (up to 300 mW ), short pulses (15–50 ps FWHM) and small bandwidth (3×10 −2 nm) . Taking advantage of these characteristics, we demonstrated for the first time the possibility of using a storage ring FEL as a coherent source of radiation for scientific applications. In particular, the tuneable Super-ACO FEL source, combined with synchrotron radiation covering the X-ray to infra-red range, is a unique tool for the time-dependent studies of excited states. Such analysis benefits from the natural synchronisation of both sources at a high repetition rate, their mutual tunability, high intensity and coherence. Several experimental set-ups are now under operation.

Free-electron lasers sources for scientific applications

The free-electron laser (FEL) has proven to be an invaluable source of radiation in the UV to far infrared spectral ranges. It is based on the amplification of an optical wave by a high energy electron beam oscillating transversely to its prop a- gation. In the last decade, several FELs have been designed and built as user dedicated facilities. Presently, seven infrared FEL s, such as CLIO at Orsay (France), are running more than 1000 hours/year for users performing experiments in various scientific fields. In the UV spectral range, the Super-ACO (Orsay, France) FEL is promoting applications in various fields. The main appli- cations are described and prospects are given.

Surface photovoltage in semiconductors under pulsed optical excitation, and its relevance to synchrotron radiation spectroscopy

Abstract We used the SuperACO Free Electron Laser (FEL) to excite photocarriers in semiconductors at room temperature, and synchrotron radiation photoemission spectroscopy to measure the consequently induced surface photovoltage (SPV). The natural synchronization between the two sources allowed us to study with sub-nanosecond resolution the temporal dependence of the non-equilibrium charge carrier distribution for two prototype systems, Ag/GaAs(110) and Si(111)2×1. We found that to correctly interpret the effects of the SPV on the band position, the interplay between instrumental factors (pulse duration and repetition rate) and semiconductor parameters (such as surface and bulk recombination rates) must be taken into account. Since the FEL has the typical temporal structure of a synchrotron radiation source and a flux comparable to the one obtainable at third generation storage rings, these results are of relevance to the most advanced spectroscopic techniques used to study the electronic structure of semiconductors; therefore, in this paper we discuss the possible effects of intense synchrotron radiation beams on position and lineshape of photoemission features at semiconductor surfaces.

The Super-ACO FEL operation with shorter positron bunches

The recent installation of a 500 MHz RF cavity on the storage ring Super-ACO allows the positron bunch to be reduced by a factor of 3. The first tests on the positron beam started at the end of March this year: besides the bunch length reduction observed on a double sweep streak camera, a vertical unstability usually appears and could be damped under given circumstances (current, voltage on the harmonic cavity, value of the chromaticity…). The Free Electron Laser (FEL) was nevertheless successfully operated with a higher gain at 350 nm: the laser threshold at low current is 7 mA instead of 25 mA with the same set of mirrors, the laser tunability is enhanced by a factor of 2, and the pulse duration is shortened by a factor of 2. Besides, the laser establishment stabilizes the vertical beam instability, in a complex dynamical process involving transverse to longitudinal coupling. Stability analysis will be carried out in more details.

The Super-ACO FEL dynamics measured with a streak camera

The dynamics of the Super-ACO Free Electron Laser (FEL) is followed with a Hamamatsu double-sweep streak camera (C5680). In its natural regime, the FEL micropulse distribution can sometimes exhibit a few internal short pulses which evolve differently; and generally, the FEL pulse position drifts with respect to the positron bunch. The stability issues have been more carefully analysed in relation to coherent synchrotron oscillations, high-current operation and longitudinal feedback systems on the beam and the FEL. Finally, the influence of the FEL on the positron bunch distribution has been measured.

Source challenges resulting from the first applications of a UV storage ring FEL on Super-ACO

Abstract On the Super-ACO free electron laser were developed the first applications of a storage ring based FEL and of a UV FEL. The feasibility of use was demonstrated in 1993 with a time resolved polarised fluorescence experiment in biology. After these experiments, the stability of the laser source was significantly improved, with the installation of a longitudinal feedback. A particular advantage of a storage ring FEL source is to provide as well naturally synchronized synchrotron radiation. A first pump-probe time-resolved experiment coupling such two tunable, synchronized light sources was then performed in surface physics.

First use of the UV Super-ACO free-electron laser: Fluorescence decays and rotational dynamics of the NADH coenzyme

Significant improvements in the performances of the Super-ACO storage ring free-electron laser (FEL) at 800 MeV have been obtained recently: enhancement of the output power in the ultraviolet, laser duration of 10 h for the same injection of positrons, long-term stability with a micropulse of 60 ps FWHM. A first series of experiments using this FEL has then been successfully performed. Taking advantage of the time structure, the polarization and the high power of the source at 350 nm, the polarized fluorescence decays of the reduced nicotinamide adenine dinucleotide coenzyme (NADH) were studied in aqueous solution, using the single-photon counting (SPC) technique. The experimental setup is described, including the Super-ACO FEL characteristics and diagnostics. The FEL working point has been first optimized by monitoring the SPC apparatus function. A complete fluorescence experiment required about 30 min of data acquisition, during which the best integrated instrumental response had a FWHM of 110 ps. Measurements performed in such a way lead to the unambiguous separation of two close lifetime components of 0.28 and 0.62 ns in the fluorescence decays of NADH at 20 °C, in good agreement with previous works. The thermodynamic parameters obtained from temperature studies show that the NADH fluorescence heterogeneity is consistent with the ground-state folding equilibrium of the coenzyme, as characterized by many other spectroscopic techniques. From the fluorescence anisotropy decays, an apparent hydrodynamic radius of about 6 Å is determined, while on the other hand, a large initial depolarization of the fluorescence indicates a fast independent motion of the nicotinamide ring. The quality of the collected data fully meets the requirements for the study of more complex systems such as fluorescent compounds bound to proteins or membranes. Thus, the feasibility of use of a storage ring UV FEL for this type of time-resolved experiments on the subnanosecond time scale has been demonstrated.

Temporal behaviour and longitudinal instabilities on the Super-ACO free electron laser

In a Free Electron Laser (FEL) the amplification of a stored light pulse varies according to different conditions of gain. This implies, for a Storage Ring FEL (SRFEL), a particular behaviour of the temporal structure. Measurements of the temporal width of the laser pulse in the Super-ACO FEL versus stored ring current and versus the so called “detuning” have been performed with a dissector. The results presented here show how the temporal width is influenced by the evolution of the gain. The position of the laser pulse with respect to the center of the positron bunch in equilibrium condition has been also measured. These results seem to confirm, along with the existence of longitudinal instabilities of the laser pulse for a perfect synchronisation, the important role played by detuning in SRFEL saturation process.