Chaos in free electron laser oscillators

The chaotic nature of a storage-ring Free Electron Laser (FEL) is investigated. The derivation of a low embedding dimension for the dynamics allows the low-dimensionality of this complex system to be observed, whereas its unpredictability is demonstrated, in some ranges of parameters, by a positive Lyapounov exponent. The route to chaos is then explored by tuning a single control parameter, and a period-doubling cascade is evidenced, as well as intermittence.Comment: Accepted in EPJ

The FEL SASE operation, bunch compression and the beam heater

We discuss the conditions required for an optimal SASE FEL operation when bunch compression techniques are exploited to enhance the bunch peak current. We discuss the case of velocity bunching and magnetic bunch compression. With the reference to the latter technique we provide a quantitative estimate of the amount of laser heater power necessary to suppress the micro-bunching instability without creating any problem to the SASE dynamic

SPONTANEOUS EMISSION OF UNDULATORS AND FREE ELECTRON LASERS

Les onduleurs montés sur des anneaux de stockage peuvent être utilisés comme sources de radiation synchrotron ou bien comme lasers à électrons libres. On montre que l'émission spontanée des onduleurs est une source de lumière puissante, quasi-monochromatique et accordable dans un large domaine spectral s'étendant des rayons X à l'infra-rouge. L'émission stimulée dans le domaine visible a également été obtenue et les premiers résultats acquis à Orsay sur l'anneau de stockage ACO sont présentés.In this paper, we report the main characteristics of undulators assembled on an electron storage ring, operating as a synchrotron radiation source and as a free electron laser. It is shown that the spontaneous emission is a powerful, quasi-monochromatic and tunable source of light in the infra-red, visible, ultra-violet and X-rays range. First results on the stimulated emission in the visible range are also reported

The super-ACO free electron laser in the UV, first application for time-resolved fluorescence in biology and prospects for soleil

Storage ring (SR) Free Electron Lasers (FEL) are very promising tunable laser sources in the UV. The Super-ACO UV FEL at 800 MeV was recently improved : enhanced power (roughly two orders of magnitude higher compared to other FELs), 10 hours duration per beam injection, long-term stability, 60 ps FWHM micropulses repeated at 8 MHz, generally CW at the ms range.This source was first successfully used at 350 nm : polarized fluorescence decays of the reduced nicotinamide adenine dinucleotide coenzyme in aqueous solution were recorded versus temperature, using the single photon counting technique, leading to accurate measurements and original results, thus demonstrating the quality of the Super-ACO FEL. Moreover, SRFELs appear as unique tools in various scientific fields for two-colours experiments, coupling both the UV FEL and synchrotron radiation, naturally synchronized. Projects are underway on Super-ACO. However, the small gain achieved on the Super-ACO FEL limits its performances such as tunability, short wavelength operation. A FEL implantation on SOLEIL will offer a powerful tunable source between 350 and 100 nm together with synchrotron radiation. Rather than a single by-pass, an efficient solution with two longer straight sections seems more attractive

Mesure du dichroïsme circulaire. Description de différents montages a très hautes performances

A number of devices are considered which perform circular dichroism measurements by a null-method. A theoretical study shows that they all possess similar characteristics. One particular actual achievement is fully described and its performance is illustrated with a number of magnetic circular dichroism curves. The spectral range 2 100-7 500 Å can be investigated. Our machine has both a good resolution (1-6 Å) and high accuracy. The smallest measurable dichroïc optical density is as small as 10-6 or 10-5 according to the resolution considered (6 Å or 1 Å respectively).On considère plusieurs dispositifs de mesure du dichroïsme circulaire par une méthode de zéro. Leur étude théorique montre qu'ils ont tous sensiblement les mêmes caractéristiques. L'une de nos réalisations expérimentales est ensuite décrite et ses performances sont illustrées par des courbes de dichroïsme circulaire magnétique. Notre appareil possède une excellente résolution (variable de 1 à 6 Å) et une grande sensibilité. Son domaine spectral s'étend de 2 100 à 7 500 Å. La densité optique dichroïque mesurable peut devenir inférieure à 10-6 lorsqu'on utilise une bande passante de 6 Å et reste de l'ordre de 10-5 pour une bande passante de 1 Å

Vérification expérimentale des relations de Kramers-Kronig entre indices de réfraction et d'extinction

The existence of a Kramers-Kronig type relationship between the refractive and extinction indices has been checked experimentally. The experiments on n and k have been conducted with a highly performing photoelectric refractometer and a commercially available spectrophotometer respectively. A large number of compounds have been studied — potassium, ferricyanide and bichromate, rare-earth trichlorides (RE = Nd, Er, Eu), ferrous orthophenanthroline, phenanthrene and anthracene — as solutions in the spectral range 2 200-6 000 Å. A single crystal of lead chloride was also run in the solid phase. In all cases, the Kramers-Kronig transform of the curve k(λ) fits correctly the experimentally measured curve n(λ). We are thus lead to the conclusion that the transform is valid, at least in all the above examples.Nous vérifions expérimentalement les relations de Kramers-Kronig, entre les indices de réfraction et d'extinction. Pour ceci nous mesurons ces deux indices sur le même échantillon à l'aide respectivement d'un réfractomètre photoélectrique de hautes performances et d'un spectrophotomètre commercial. La comparaison de la dispersion de l'indice de réfraction mesurée expérimentalement et de la transformée de Kramers-Kronig de l'indice d'extinction montre que les relations de Kramers-Kronig sont valables dans tous les cas étudiés. Ceux-ci sont relatifs au ferricyanure et bichromate de potassium, aux chlorures de néodyme, erbium et europium, à l'ortho-phénanthroline ferreuse, au phénanthrène et à l'anthracène, tous ces corps étant en solution, et à un monocristal de chlorure de plomb. Le domaine spectral étudié s'étendant de 2 200 à 6 000 Å. Dans tous les cas nous montrons que la transformée de Kramers-Kronig de l'indice d'extinction est identique, à la précision des mesures près, à l'indice de réfraction mesuré. Ceci confirme la validité des relations de Kramers-Kronig, au moins pour tous les exemples ci-dessus