C. Bazin

Realization of the permanent magnet undulator Noel

Abstract A permanent magnet undulator designed for the ACO storage ring has been built. The period and magnetic field have been optimized for FEL studies on the storage ring, working at 240 MeV. The peak magnetic field measured in the completed undulator agrees very well with the designed field. The undulator has been successfully placed on the beam since February 1982. Its central part is removable in order to replace it by a dispersive section leading to the so-called “Optical Klystron”. It will also constitute a suitable source of VUV synchrotron radiation with the ring working at 540 MeV.

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.

Optimization of a permanent magnet undulator for free electron laser studies on the ACO storage ring

We describe the magnetic design problems involved in the realization of the permanent magnet undulator ‘‘Nouvel Onduleur pour l’Etude du Laser (NOEL)’’currently being used for the free electron laser experiments on the storage ring ACO. The undulator has been compensated for the magnetic imperfections of the magnet elements used for its construction. Each pole piece was made from two elements so that we could cancel out the transverse field errors. Each undulator pole was selected from a two‐dimensional magnet distribution graph which was drawn with the results of a set of Hall‐probe measurements. Magnetic measurements of the assembled undulator as well as the first experiments on the ACO storage ring indicate that the method has been successful.

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.

Results of the Orsay storage ring free electron laser oscillation

Abstract Recent results obtained with the Orsay free electron laser are summarized in this paper. The laser oscillator was operated at two different wavelengths (6500 and 5800 A) and at energies ranging from 166 MeV to 224 MeV. The main results presented here concern the Q-switched operation, the macro-temporal structure and the saturation mechanism of the storage ring free electron laser.

First Operation Of A Storage Ring Free-Electron Laser

A storage ring free-electron laser oscillator has been operated at a visible wavelength A = 6500 A .