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

Abstract
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.