Direction des Sciences de la Matière
Département de Recherche sur l’Etat Condensé, les Atomes et les Molécules
Service des Photons, Atomes et les Molécules

To the attention of Professor Nobuhiro Kosugi
Director of UVSORInstitute for Molecular Science
MyodaijiOkazaki 444-8585

Saclay, 2005, the 15th of March

Dear Professor Kosugi,

UVSOR is a world-class research synchrotron radiation facility for VUV andsoft X ray radiation. Regular up-grades on the machine and on the beamlines make itvery competing, even now, in the new landscape of third generation synchrotron lightsources such as ALS (Berkeley), ELETTRA (Trieste), BESSY-II (Berlin), MAX II(Lund), SLS (Villengen), and the newly coming Diamond (Oxford) and SOLEIL(Saclay). Besides, UVSOR hosts a storage ring Free Electron Laser (SRFEL) source. I
had the chance to collaborate with FEL scientists during short time stays, and a longer
three months stay this summer in Okazaki, after the shut down of the Super-ACO Free
Electron Laser at the end of 2003, on which I have been working for 14 years.
Key advances for the FEL community were made on the UVSOR FEL since its
first lasing in 1992. Since 1994 in UVSOR, very early systematic sophisticatedexperimental analysis of the FEL micropulse temporal and spectral distributions versus
time with the use of a double sweep streak camera in the early days of storage ring
FELs could provide a deeper insight on FEL dynamics, for different operating
conditions especially versus detuning (i.e. synchronism between the electron stored in
the ring and the optical pulses bouncing in the optical resonator). Detailed studies on
FEL performances for different momentum compaction factors were carried out. Agreat progress was also made with the installation of the first helical to planar optical
klystron for FEL operation, in 1996. UVSOR contributed also to the progress in short
FEL wavelength operation, with FEL lasing at 238 nm in 1996. A longitudinal feedback
system was developed to maintain the FEL pulse at perfect synchronism, leading tosuccessful pump-probe two-colour experiments in gas phase using FEL light andsynchrotron radiation for the first time. The production of monochromatic gamma-raysby Compton Back-Scattering with SRFELs was first demonstrated on the UVSOR FELin 1996 and broadened the use of FELs into the field of nuclear physics. ProfessorHiroyuki Hama was awarded the FEL Prize in 2004 jointly with V. Litvinenko for thesepioneering works.
Commiss ariat à l’Energie Atomique -DSM/DRECAMSPAM
Centre de Saclay
91191 G if-Sur-Yvette Cedexiii

Recent upgrade of UVSOR II with significantly lower emittance resulting from achromatic optics provides a unique opportunity to investigate more deeply theinteraction between the FEL and the electron beam, and to compare it to the wellestablished saturation process in the usual achromatic mode. Indeed, in the chromaticcase, the saturation process in which the electron beam is heated by the light source(enhancement of energy spread) induces not only a lengthening of the electron bunch in
the longitudinal plane but also in the transverse domain, leading to a significant increaseof the Touschek lifetime induced by the FEL. Besides, a very high current can be stored,
allowing the appearance of various types of instabilities, with which the FEL iscompeting, and under given circumstances, to the level of suppressing them. Such anunderstanding of the interplay between the FEL and the electron beam will be fruitfulfor the new slicing project to be conducted by Prof. Masahiro Katoh, with a high intense
Titanium:Sapphire laser interacting with the electron beam in an undulator, for theproduction of femtosecond synchrotron radiation.

Apart from the increasing interest for Linac based FEL for short wavelength
radiation (i.e. VUV and X ray range) and from the FEL user facilities operating in theinfra-red, Storage Ring Based FELs appear to be quite valuable sources for the UV andVUV range. At present, after the closing of Super-ACO in France, four installations areoperating. The European ELETTRA FEL (Trieste, Italy), installed on a third generation
light source, detains the record of the shortest wavelength (172-182 nm) for FEL
oscillators, and a program of harmonic generation is under development. On the NIJI-IV
FEL (Tsukuba, Japan), different studies on the FEL sources are in progress. Besides, theDUKE FEL (DUKE University, USA), after various successful results (gamma-rays
produced by Compton Back Scattering exploited by users, short wavelength operationin the oscillator and harmonic generation configuration…) is planning the installation ofan multiple optical klystron (OK5) on a long straight section.
The FEL source quality of UVSOR II should be pointed out, and it offers a highstability level, very unique nowadays. This is of crucial importance for the quality of thedata of the user experiments, and especially requested for pump-probe two-colourexperiments. Pump-probe two-color experiments have been performed only on theSRFELs installed on UVSOR and Super-ACO so far, because of the very limited
intensity fluctuations of these particular sources. Besides, the possibility of changing thepolarisation of the UVSOR FEL, will be exploited soon at 250 nm for chirality studies
in biological molecules.
The high FEL stability also permits to progress further in the FEL dynamics,
which is understood in this case with the point of view of the conventional laser
systems. Indeed, the study of pass to pass equations of the FEL electric field exhibits
some phase singularities, which signature can be given by the presence of holes in thespectral distribution and temporal drifts in the temporal domain. These correlated
phenomena were clearly observed on the UVSOR FEL. The nonlinear dynamics
approach also allowed the pulsed regimes at the millisecond time scale to be controlled,thanks to the existence of an unstable cw state coexisting with the limit cycles regimes.
The region of the detuning curve in which the FEL is cw can then be further extended,and offers a wider zone of operation for users applications.

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Finally, it is quite relevant to stress the advantage of developing an FEL in theenvironment provided by the synchrotron radiation community of UVSOR, and thelaser oriented laboratories of IMS. Such a double culture is key issue for theestablishment and fruitful exchanges between sources builders and user scientists, forconventional lasers, high harmonics produced in gases and accelerator based lightsources such as synchrotron radiation centres and fourth generation light sources basedon Free Electron Laser devices. IMS can be considered as combined facility providing
short and ultra-short light sources for a multi-disciplinary community. Gathering
accelerator light sources and conventional lasers facilities at the same location provides
an enhancement expertise and opens the way for new opportunities of scientificchallenges. Besides, the interaction between scientists takes place in a very niceatmosphere, which makes the exchanges even more fruitful.

Yours sincerely,

Marie-Emmanuelle Couprie
Commissariat à l’Énergie Atomique,Service de Photons, Atomes et Molécules, ATTO groupv