Orsay Storage Ring Research Articles

Spontaneous emission of the Super-ACO fel optical klystron domino

New free electron laser experiments are planned on the Orsay storage ring Super-ACO, which has been commisioned in 1987 at LURE (Laboratoire d'Utilisation du Rayonnement Electromagnétique). For this purpose, a new insertion device, DOMINO, was set up on Super-ACO in January 1988; it consists of a permanent magnet optical klystron composed of two undulators separated by a dispersive section; the magnetic gaps can be changed independently. Here, the parameters of the optical klystron are specified, and the main steps of the optimization are described. Then, the effect of the insertion device on the stored beam is discussed. The spontaneous emission of the optical klystron has been observed and analysed. Consequently, with the ring parameters measurements, the free electron laser gain versus the current can be evaluated.

Measurement of the reaction e +e −→ ηπ+π− in the center of mass energy interval 1350–2400 MeV

The e +e −→ ηπ + π − reaction has been measured in the center of mass energy interval 1350–2400 MeV by the magnetic detector DM2 at the Orsay storage ring DCI. Under the hypothesis of only one large resonance the cross section is not fit in a satisfactory way. The branching ratio τ −→ η − π 0 ν τ =(0.13 ± 0.02)% is deduced via CVC from the above measurement.

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

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 Å. The dependence of the temporal and spectral structure of the laser beam versus detuning and Q-switch frequency is discussed.

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

Due to its low gain, the Orsay storage ring free-electron laser necessitates the use of high reflectivity mirrors. Three techniques for measuring the mirror losses are presented, based on cavity decay time measurements using either an external laser, the synchrotron radiation stored in the cavity, or the free-electron laser itself. The high signal-to-noise ratio allowed the detection of loss variations as low as 10(-7)/sec(1/2). From these diagnostics three distinct processes of UV-induced degradation of TiO2/SiO2 dielectric mirrors were identified. One was a surface absorption of the upper SiO2-air interface; it was not affected by annealing. The other two corresponded to a volume absorption of the layers which completely recovered after annealing.

Results of the Orsay storage ring free electron laser oscillation

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 Å) 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.

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

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.

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.

Photoabsorption and electron yields of La and Gd in the vicinity of the 3d thresholds

A comparative study of the 3d photoabsorption and electron-yield spectra of La and Gd using synchrotron radiation emitted by the Orsay storage ring (ACO) is presented. The effects of sample thickness on each kind of spectrum are discussed. The mean sampling depths for La and Gd are determined from the variations in intensity of the main excitation lines as a function of film thickness in the yield spectra. The advantages and potential of the yield technique for the study of lanthanide spectra are pointed out.

Study of e +e − → K S0K ±π∓ in the 1.4–2.18 GeV energy range: A new observation of an isoscalar vector meson: φ′ (1.65 GeV)

First measurements of the e +e − → K S 0 K ± π ∓ reaction in the 1.4–2.18 GeV energy range have been performed with the magnetic detector DM1 at the Orsay storage rings DCI. The cross section is rather large. The production is mainly K ∗0K 0 which reveals an interference between isovector and isoscalar amplitudes. These results show again the existence of an isoscalar vector at 1.65 GeV shown to be a φ′ meson.

Gain measurements versus theory for the ACO storage ring laser

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.

Study of the reaction e+e− → 3 π+3 π− in the total energy range 1400–2180 MeV

The e + e − → 3 π +3 π − cross section has been measured between 1400 and 2180 MeV with the magnetic detector DM1 at the Orsay storage rings DCI. The cross section increases continuously above 1600 MeV and reaches 2 nb at the maximum explored energy, much larger than VDM previous estimates.

DM2, A Magnetic Detector for the Orsay Storage Ring DCI

DM2 is the magnetic detector built to study e+e- interactions in the c.m. energy range 1.2-3.7 GeV with the Orsay Storage Ring DCI. The magnet is a solenoid of 2 m diameter and 3 m length with a conventional aluminium coil producing a 0.5 T field.Inside the coil, the charged particles detector is made of two proportional chambers with anode and cathode read-outs, 13 layers of drift chambers, one layer of water Cerenkov counters and one layer of scintillation counters.Photon detection is provided outside the 1 rad length coil by stacks of 14 planes of wire tubes with longitudinal read-outs, separated by lead sheets, and by five layers of scintillation counters. Inside the coil, the end caps are covered by shower detectors made of lead sheets and MWPC planes.Muon separation uses the flux return iron or concrete absorbers, and planes of wire tubes with longitudinal read-out.The charged particle detector covers 87% or 4π and the photon detector 82% of 4π. Charged particle identification by time of flight and Cerenkov radiation can discriminate pions from kaons up to 600 MeV/c and kaons from protons up to 1100 MeV/c. The photon detection efficiency is 50% at 50 MeV and near 100% at 100 MeV. It allows a neutral trigger to be operated. The detector is starting operation on the storage ring DCI at Orsay.

Study of the reaction e +e − → K S0K L0 in the total energy range 1.4–2.18 GeV and interpretation of the K + and K 0 form factors

The e +e − → K S 0K L 0 cross section has been measured between 1400 and 2180 MeV. About 58 K S 0K L 0 events were in the magnetic detector DM1 at the Orsay storage ring DCI. The charged and neutral kaon form factor behaviour suggests the existence of a new isoscalar vector meson at 1.65 GeV.

Study of the reaction e +e − → K +K − in the total energy range 1400–2060 MeV

The e +e − → K +K − cross section has been measured between 1400 and 2060 MeV. About 500 K +K − events were detected in the magnetic detector DM1 at the Orsay storage rings DCI. The charged kaon form factor is appreciably higher than predicted by only the ϱ, ω, ø tails, suggesting contributions from higher vector mesons.

Measurement of lepton and pion pair production in photon-photon collisions at DCI

We report a photon-photon experiment performed at the Orsay storage rings. 300e +e −, μ +μ − and π +π − pairs produced with low invariant masses have been observed. For each event, one or both protons have been tagged at a very small angle. The γγ→μ +μ − and γγ→π +π − cross-sections have been measured near threshold, the γγ→e +e − process being used as a normalization. The observed invariant mass distribution is compared to theoretical calculations for each of the three processes. The μ +μ − data are in good agreement with QED predictions. The π +π − cross-section, in our experimental acceptance, is somewhat larger than the one expected from the Born terms only.

The development of toroidal grating monochromators at L.U.R.E.

In the last two years grazing incidence monochromators based on holographic toroidal gratings have been developed at L.U.R.E. The model described in this paper is a 3 m one (entrance slits-grating distance). Due to the large grazing angle (15°) it can be used between 10 and 150 eV ∗. With the Orsay storage ring (A.C.O.) very high flux ( ∼4 × 10 12 phot/ A ̊ · s at 100 eV and good resolution (0.1 Å for 100 μm slits) have been obtained. Due to the large size of the grating (50 × 210 mm 2) no decrease of the intensity has been observed after more than one year of use (>1200 h). Examples of angular resolved photoemission and fluorescence spectra will be shown.

Cross section of the reaction e +e −→ π+π−π0 for C.M. energies from 750 to 1100 MeV

We present the results obtained with the magnetic detector DM1 at the Orsay storage ring ACO for the reaction e +e −→ π + π − π 0 from 483 to 1100 MeV in the center of mass. Our data show without ambiguity an interference effect between the ω and φ mesons, which corresponds to a negative coupling constant product ratio Re( g γω g ω→3 π/ g γφ g φ→3 π ) ; however our measurements above the φ, performed using kinematical analysis, can only be explained by a higher energy contribution. In addition, the parameters of the ω have been obtained with an improved accuracy compared to other experiments, and particularly the branching ratio B ω→e +e − = (6.75±0.69) × 10 −5 . We confirm that the reaction e +e −→ π + π − π 0 proceeds essentially via a quasi-two-body state ϱπ, at the energy of the φ.

Measurement of lepton pair production in γγ processes from e +e + collisions at DCI

82 ± 2 lepton pairs produced in gamma;γ collisions have been observed withthe Orsay storage rings operated with positron beams. The e +e + → ℓ +ℓ − (e +e +) cross section for lepton pairs with low invariant masses has been measured. Several relevant distributions are compared, for the first time, to predictions based on the theoretical differential cross section.

Cross section of the reaction e +e − → π+π−π+π− for center of mass energies from 890 to 1100 MeV

We present the results of a measurement of the cross section oof the reaction e +e − → π + π − π + π − from 890 to 1100 MeV in the center of mass, obtained with a magnetic detector at the Orsay Storage Ring ACO. With respect to previous experiments, the present one offers the possibility of reconstructing events with at least one constraint and his improved statistics. We find that our measurement of the cross section for e +e − → π + π − π + π − is compatible with quasi two-body production of πA 1( m A1 = 1.1 GeV, Γ A1 ∼ 0.2−0.3 GeV), through the ϱ and ϱ′(1600) intermediate states. We were able to states. We were able to determine the cross section of this reaction at the energy of the φ meson and consequently an upper limit on the branching ratio of φ → π + π − π + π −.