Search Results (18 total)

In spatiotemporal systems with advection, suppression of noise-sustained structures involves questions that are outside of the framework of deterministic dynamical systems control (such as Ott-Grebogi-Yorke-type methods). Here we propose and test an alternate strategy where a nonlocal additive feedback is applied, with the objective to create a new deterministic solution that becomes robust to noise. As a remarkable fact—though the needed parameter perturbations required have essentially a finite size—they turn out to be extraordinarily small in principle: 10^-8 in the free-electron laser experiment presented here.

Seeded free-electron lasers (FELs) are among the future fourth-generation light sources in the vacuum ultraviolet and x-ray spectral regions. We analyze the seed temporal coherence preservation in the case of coherent harmonic generation FELs, including spectral narrowing and structure degradation. Indeed, the electron synchrotron motion driven by the seeding laser can cause sideband growth in the FEL spectrum.

Seeded single pass free-electron lasers are promising coherent, short-duration, and intense light sources, from the visible to x rays. Operated with adjustable undulators, they are also a unique device for providing fully variable polarized radiation. We report here the first seeding of helical undulators with a variable polarized source. We demonstrate that the adjustment of the seed polarization and focusing allows the free-electron laser radiation to be optimized in terms of intensity and quality.

The new trend is to operate storage ring based light sources in a “chromatic mode” with a distributed dispersive function in the straight sections for low emittance. The electron bunch heating induced by a storage ring free electron laser (FEL) has been investigated for such optics, and exhibits a more complex saturation process as compared to a usual achromatic mode of operation without dispersion in the straight sections. The correlated measured FEL power is then interpreted in terms of the electron bunch heating and compared to theoretical expectations. Experiments performed at UVSOR-II are here reported. The theoretical interpretation of the new saturation phenomenon is then discussed.

We evidence numerically and experimentally that advection can induce spectrotemporal defects in a system presenting a localized structure. Those defects in the spectrum are associated with the breakings induced by the drift of the localized solution. The results are based on simulations and experiments performed on the super-ACO free-electron laser. However, we show that this instability can be generalized using a real Ginzburg-Landau equation with (i) advection and (ii) a finite-size supercritical region.

Collective effects, such as the microwave instability, influence the longitudinal dynamics of an electron beam in a storage ring. In a storage ring free electron laser (FEL) they can compete with the induced beam heating and thus be treated as a further concomitant perturbing source of the beam dynamics. Bunch length and energy spread measurements, carried out at the Super-ACO storage ring, can be correctly interpreted according to a broad-band impedance model. Quantitative estimations of the relative role that is played by the microwave instability and the laser heating in shaping the beam longitudinal dynamics have been obtained by the analysis of the equilibrium laser power. It has been performed in terms of either a theoretical limit, implemented with the measured beam longitudinal characteristics, or the numerical results obtained by a macroparticle tracking code, which includes the laser pulse propagation. Such an analysis, carried out for different operating points of the Super-ACO storage ring FEL, indicates that the laser heating counteracts the microwave instability.

We show that the pulsed regimes observed in free-electron lasers (FELs) can be suppressed using feedback control. By applying tiny parameter perturbations, the feedback allows to keep the systems onto a stationary state that is naturally existing in phase space, but is usually inaccessible because of its unstable nature. We test this method numerically on a master equation derived from the classical iterative model. Then we present the experimental results obtained on the super-ACO FEL. This method is in principle directly applicable to the other free-electron lasers, whose instabilities have a dynamical (deterministic) origin.

We present a phenomenological treatment of free-electron laser storage ring saturation dynamics. The model includes longitudinal instabilities, Touschek intrabeam scattering, and nonzero off-energy-function contributions. The model predictions are compared with Super ACO experimental results and the agreement is shown to be satisfactory.

This paper gives an analytical description of the stationary regime of a storage-ring free-electron laser in the presence of the maximum detuning (compatible with the laser onset) between the laser pulse and the electron beam when they pass and interact in the optical cavity. In this condition, the conservation of the first moments of the laser intensity distribution allows one to express the peak gain of the light amplification process and the maximum detuning as a function of system parameters that are directly measurable. These theoretical results are compared with experiments performed on the Super-ACO free-electron laser.

The longitudinal distribution of a free-electron laser (FEL) may present a complex internal structure. This phenomenon has been already observed in the case of LINAC based oscillators and self-amplified spontaneous emission devices (for which the presence of “spikes” in the temporal distribution is systematically observed). We investigate here the physical process responsible for the growth of complex substructures inside the micropulse of a storage-ring free-electron laser. This “hole-burning-like” process results from the localized character of the interaction between the ultrarelativistic electron beam circulating in the storage ring and the laser pulse. Experimental results concerning the case of the super-ACO FEL are presented and interpreted by means of a pass-to-pass tracking code containing all the relevant features of the system dynamics.

The stabilization of nonlinear excitations by noise is a topic of fundamental importance in many physical problems. We discuss a genuine example within the context of storage ring-free electron laser physics, by presenting a model which allows the characterization of the system evolution and the determination of the conditions leading to the suppression of instabilities of sawtooth type. The conclusions of the model are confirmed by a comparison with experimental results on the Super Aco Storage Ring-Free Electron Laser.

In this paper a theoretical model is presented, which is based on a pass to pass analysis of the localized interaction between a short laser pulse with a wider electron distribution. It can be applied to a large class of physical phenomena and, in particular, to the case of a storage-ring free-electron laser (FEL). Numerical results are confirmed by experimental measurements done on the ACO and Super-ACO FELs.

We used time-resolved pump-probe core level photoemission spectroscopy to study the transient regime of the charge distribution at SiO₂/Si interfaces after photoexcitation with an UV free electron laser. We found that electrons generated in the Si substrate can accumulate at the surface of the oxide layer, strongly affecting the electric field at the interface. For n-type silicon, this effect can lead to an enhancement of the curvature of the bands, rather than to the expected flattening due to surface photovoltage. The characteristic decay time of this vacuum transient charging at the surface of the oxide layer depends markedly on its thickness; our results indicate that for about 12-Å oxide thickness, it is comparable to the typical excess carrier recombination time in silicon space charge layers.

The operation of storage-ring free-electron lasers (SRFEL) at high current still represents a challenge because of the growth of longitudinal beam instabilities. One of these, the quadrupolar coherent synchrotron oscillation, is very harmful for free-electron-laser (FEL) operation. On the Super-ACO storage ring, they either prevent the FEL start-up, or result in a very poor stability of the FEL source. A new feedback system to damp the quadrupolar coherent synchrotron oscillation has been installed on the ring and the stabilized beam parameters have been systematically measured. As a result, the FEL gain is higher and the FEL operates more easily and with a higher average power. Its stability, which is very critical for user applications, has been significantly improved as it has been observed via systematic measurements of FEL dynamics performed with a double sweep streak camera.

We develop a heuristic dynamical model to analyze the interplay between free-electron-laser (FEL) storage-ring dynamics and instabilities of the head-tail type. We show that, under given conditions, the FEL may inhibit the onset of the instability and may provide a reduction of the electron-beam transverse dimensions. Some experimental results that can be used in support of the model are also reported.

The growth and saturation of a storage ring free electron laser (SRFEL) is driven by the beam behavior, including bunch lengthening or coherent modes of longitudinal motion (the bunch length being related to the energy spread), detuning effects, and a modification of the bunch distribution by the FEL interaction; all of these phenomena are accumulated for various passes, leading to complex dynamical processes. The knowledge and understanding of the dynamics, together with the stability over time are essential for efficient use of SRFEL sources. This is illustrated with the Super-ACO FEL experiment, analyzed from growth from the positron bunch to laser saturation and induced positron beam modification. Stability analysis (jitter, shape, intensity) is then performed carefully. A longitudinal feedback system can significantly improve it. Information provided with a streak camera reveals the distribution of a single FEL micropulse or synchrotron radiation pulse without any averaging or sampling. © 1996 The American Physical Society.

Equilibrium and dynamical properties of a storage-ring free-electron laser are shown as resulting from the combined effects of two different saturation mechanisms. The first one is the laser-induced energy spread of the electron beam. The second mechanism is due to an imperfect longitudinal synchronism between the electron and laser pulses. Consequences are briefly described.

A storage-ring free-electron laser at Orsay has been operating since 1989 in the visible wavelength range. In contrast with previous experiments, it operates with positrons and at higher energies (600–800 MeV), with the storage ring Super-ACO (ACO denotes Anneau de Collisions d’Orsay). The optical gain, the laser power, the transverse profile, and the macrotemporal structure of the laser are analyzed. In particular, we show that the gain matrix possesses many off-diagonal elements, which results in lasing on a combination of noncylindrical Gaussian modes. The eigenmode of the laser oscillation is a combination of one or two main Gaussian modes and several higher-order modes, which results in most of the power being extracted in these modes.