M. Velghe

Coherent harmonic generation in VUV with the optical klystron on the storage ring Super-ACO

Abstract The goal of this experiment is to explore the possibility of producing coherent light in the VUV spectral range using the nonlinear properties of the free electron laser gain that result when an electron beam is bunched by an external pulsed laser. The positive results, that have been obtained in 1987 on the old storage ring ACO, have lead us to install an experiment on Super-ACO, but with some modifications. This article presents the most recent results obtained on Super-ACO, where coherent photons have been produced at the 3rd (177 nm) and 5th (106 nm) harmonics of the incident laser. It presents also a brief summary of the practical interest and difficulties of this techniques.

Dynamical study of the Super-ACO free electron laser with the dissector

Abstract The Super-ACO free electron laser being “continuous” at the ms range under circumstances allows one to study the dynamical behaviour. The microtemporal laser line has been measured under several conditions: versus the tuning curve, the ring current, the dispersive section gap, and the laser temporal structure (“continuous”, pulsed, Q -switched). The effect of the lasing on the longitudinal bunch distribution is studied. For those experiments, the usual laser parameters have been followed, such as the output power, the laser linewidth, etc.

Gain Measurement on the ACO Storage Ring Laser

We report the results of the first gain measurements on the free electron laser being assembled on the storage ring ACO. The largest measured peak gain, averaged over the laser mode, is ¿ = 4.3 × 10-4 per pass. No laser induced bunch lengthening is observed within the experimental accuracy of 50 psec under a laser intensity of 1.6 kw/cm2 and under the conditions of strong anomalous bunch lengthening (¿¿/¿¿(I=0)) = 6.8 - 4.2.

Gain measurements versus theory for the ACO storage ring laser

Abstract We discuss the gain measurements made on the Orsay storage ring free electron laser, and show that the peak measured gain is identical to that predicted by a modified theory which takes into account the electron trajectory distortions. This is the first free electron laser experiment in which the quality of the data is sufficiently good to permit a detailed verification of the Madey theorem.

Optical klystron experiments for the ACO storage ring Free Electron Laser

To improve the gain in the Orsay storage ring Free Electron Laser (FEL) experiment, the 17 period permanent magnet undulator has been modified to form an optical klystron (OK). We report the measurement of spontaneous emission and the effects on it of energy spread and angular spread. Gain and laser induced bunch lengthening measurements with the OK are also reported and are in very good agreement with the FEL classical theory. The spontaneous emission spectrum which is easy to measure with good signal to noise ratio, turns out to be a very good diagnostic tool forenergy spread and angular spread measurements on storage rings. The factor of four increase in the small gain obtained by converting the undulator NOEL into an OK was the critical factor in the recent operation of the ACO storage ring laser above threshold.

Free-electron-laser oscillation on the Super-ACO storage ring at Orsay

Abstract In February 1989, free-electron-laser oscillation was obtained in the visible at Orsay. For such experiments, the Super-ACO storage ring is operated at 600 MeV with two opposite bunches, using the “low-emittance” optics. It provides a gain of 2%. The general features of the laser are described.

Coherent harmonic generation in the vacuum ultraviolet spectral range on the storage ring ACO

Abstract In this paper we summarize the various achievements made at Orsay on the “coherent harmonic generation”, using an external laser focused in an optical klystron. First we observed in 1984 the third harmonic (3547 A) of a NdYAG laser, on our storage ring ACO. In a later experiment, performed in 1987 on the same machine, VUV photons (1770 and 1064 A) have been generated by the same method, on the third and fifth harmonic of a doubled NdYAG laser. The perspectives offered for VUV production by the new storage ring Super-ACO are discussed, taking into account the parameters recently measured on this ring.

Optical properties of multilayer mirrors exposed to synchrotron radiation

Abstract One main limitation to the future development of free electron lasers towards lower wavelengths is the problem of optical degradation due to spontaneous emission of the undulator. In our case, a high reflectivity is also needed in order to maintain the gain over cavity losses. On the ACO storage ring at Orsay, some dielectric multilayer mirrors for the near UV spectral range have been exposed to synchrotron radiation, and the optical degradation has been studied. With highly degraded mirrors, for which the reflectivity is reduced, optical density curves are generally shifted towards lower wavelengths and sharpened. A theoretical model which explains some of these results has been developed by modifying the real or imaginary part of the optical indices of selected layers constituting the mirror.

Diagnostic techniques and VUV induced degradation of the mirrors used in the Orsay storage ring free electron laser

Abstract Due to its low gain, the Orsay storage ring free electron laser necessitated the use of high reflectivity mirrors. Three measurements techniques of the mirror losses are presented. They are all based on a cavity decay time measurement using either an external laser, or the synchrotron radiation stored in the cavity or the free electron laser itself. The high signal over noise ratio allowed measurement of loss variations as low as 10 −7 s − 1 2 . From these diagnostics we were able to identify three distinct processes of UV induced degradation of TiO 2 SiO 2 dielectric mirrors. One of them was a surface absorption of the upper SiO 2 -air interface; it was not affected by annealing. The two others corresponded to a volume absorption of the layers which completely recovered after annealing.

Short–Wave-Length Operation of a Storage Ring Free-Electron Laser

The Orsay storage ring free-electron laser has been operated in the short–wave-length side (blue) of the visible spectral range from 4630 A to 4860 A. General conclusions on storage ring free-electron laser are derived after our successive operations of a tunable laser over almost the entire visible spectral range.