P. Elleaume

Special multipole wiggler design producing circularly polarized synchrotron radiation

Abstract A planar multipole wiggler device which can produce both linearly and circularly polarized synchrotron radiation is proposed. Standard wiggler designs have a quasi sinusoidal magnetic field distribution resulting in a - more or less - perfect cancellation of right/left handed circularly polarized photons emitted from successive poles. We show here that one can easily generate a periodic but asymmetric magnetic field distribution having a field integral still equal to zero in order to maintain a straight overall direction of the particle trajectory. Due to the asymmetry of the field distribution in our device, the synchrotron radiation along the wiggler axis can keep a high degree of circular polarization above or below the particle trajectory plane. As compared to standard bending magnets, the proposed device can afford a significant increase of the available flux of circularly polarized light which is typically proportional to the number of magnetic periods. As compared now to the two crossed field undulator system, our wiggler device definitively offers a much broader spectral range of emission and far more flexibility regarding energy scanned experiments which might prove to be very difficult with a two crossed field undulator device. The wiggler described here should also accomodate any future or already existing machine with common lattices while a two crossed field undulator system is only to work on very low emittance storage rings.

Theory of the optical klystron

Abstract The optical klystron is a modification of an undulator which can be used to improve the gain of an FEL oscillator or to enhance the coherent spontaneous emission originating from electron bunching. It is most useful when long enough straight sections are not available or when the energy spread is extremely small.

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.

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.

Storage ring FEL theory

Abstract The theory of storage ring free electron lasers is reviewed. The main theoretical aspects which are developed concern the undulator optimization, the saturation mechanisms, the macrotemporal, transverse, microtemporal and spectral structures.

Energy spread measurements on the ACO storage ring

Abstract Absolute and relative energy spread measurements have been made on the ACO storage ring using two different methods. The first one consists of a spectral analysis of the electron bunch length based on the proportionality of the longitudinal bunch length versus energy spread. The second one constitutes a direct measurement using the on axis synchrotron radiation emitted by an optical klystron. The two measurement techniques present a very high signal/noise ratio allowing time resolved records. Examples are presented of the energy spread time evolution in the FEL oscillator and of harmonic generation experiments.

Modification of the electron bunch shape induced by the Orsay storage ring FEL

Abstract Experiments have been performed on ACO to study the change of shape of the electron bunch induced by the free electron laser. The method of measurement used here exploits the Fourier transform properties by analysing the frequency spectrum of the signal from a pick-up electrode. When the laser is on, the electron bunch is no longer Gaussian, and the anomalous electron bunch oscillations are stabilized.

The Orsay storage ring free electron laser: New results

A modification of the operating point of the machine is reported. Smaller transverse sizes of the electron beam have been achieved producing a higher optical gain, and a laser tunability ranging from 5700 to 6400 A. The dependence of the temporal and spectral structure of the laser beam versus detuning and Q-switch frequency is discussed.

Optical frequency multiplication by an optical klystron on the ACO storage ring

Abstract The optical klystron previously successfully optimized for the realization of an FEL on the ACO storage ring has been used as a non-linear medium aimed at producing harmonics of an external laser. For the first time we have demonstrated experimentally this effect by looking at the third harmonic of a 1.06 μm Nd:YAG laser focused into it. An enhancement of 102 to 103 of the spontaneous emission at 355 nm has been recorded.

Status of the Orsay FEL experiment

Abstract The FEL installed on the storage ring ACO at Orsay has been operated down to a wavelength of 463 nm, at a ring energy of 200 MeV. This is probably the lowest wavelength attainable on ACO with TiO 2 SiO 2 dielectric mirrors because of the near-UV TiO 2 broad absorption band. A careful analysis of the optical gain, taking into account the measured beam characteristics, shows a spectral dependence scaling as λ 1,1 , with a maximum at λ = 650 nm close to 0.4%. Therefore the ACO FEL may be tuned over almost the whole visible spectrum at the expense of time consuming mirror changes. Due to the poor transmission of these very high reflectivity mirrors the average extracted power is only a few mW throughout the visible region. However the maximum intracavity peak power (at low frequency Q -switching) reaches about 0.5 MW. The next step will be to test new Al 2 O 3 SiO 2 coated mirrors in the near-UV spectral range.