Search Results (41 total)

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 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.

Nonlinear harmonic generation can be a very useful and important phenomenon for single-pass free-electron lasers (FELs) operating in the high-gain regime. Strong bunching at the fundamental wavelength and its associated higher harmonic content allow significant radiation at shorter wavelengths to be emitted without serious effects upon the output power at the fundamental. Here, we analyze the relative sensitivities to beam quality variations of the output fundamental and harmonic powers for a visible-wavelength FEL operating in the high-gain regime.

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.

We derive the equation governing the evolution of the optical field amplitude of a free-electron-laser oscillator operating in the optical-klystron configuration. The equation includes short pulse effects, due to the finite length of the electron bunches, and are valid under the assumption of small signal regime and low gain. Stationary solutions of the supermode type, analogous to those of conventional free-electron-laser configurations, are shown to exist for this case, too. Analytical expressions for the optical pulse shape are derived, without any assumption of dominance of the dispersive section, in the case of the long bunch regime, namely when the bunch length is larger than the slippage length. We present useful gain formulas, including the combined effect of dispersive section and pulse effects, and discuss the limits of validity of our treatment.

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.

We exploit the Fokker-Planck equation to investigate the longitudinal phase-space dynamics of a FEL amplifier operating with a storage ring. We study both standard and optical-klystron configurations and prove that in both cases the damping times of the electron longitudinal modes are modified by the system operating conditions. In particular, they decrease with increasing laser power when the input laser is tuned at the resonant frequency.

We discuss the storage-ring longitudinal dynamics by including the contributions from a free-electron-laser amplifier and from the microwave instability. We show that the two effects exhibit a mutual interplay that leads, under certain conditions, to the inhibition of the start up of the microwave instability and/or to a reduction of the electron-beam heating due to the free-electron laser. The agreement with the experimental results is also discussed.

The combined effects of electron-beam betatron motion and non-negligible emittance provide a rich phenomenology of the radiation emitted in linearly polarized undulators. Indeed, even harmonics may be radiated on axis and the induced inhomogeneous broadenings depend on the electron-beam matching condition. We discuss the dependence of undulator brightness on electron-beam betatron motion and also analyze how the harmonic spectrum depends on the electron-beam emittance and Twiss parameters. We present independent analytical and numerical analyses, make a comparison between the two methods, and comment on the limits of the analytical treatment. (c) 1995 The American Physical Society

We use the split-operator technique (SOT) to solve evolution equations of the Liouville type. The method we propose is based on an iterative application of the evolution operator, associated with the equation under study, on the initial distribution. The SOT approximation of the evolution operator leads to analytical expressions that can be easily programmed. We discuss the validity of the method, solving the Liouville equation governing the longitudinal phase-space dynamics of an e beam undergoing free-electron-laser interaction and the phase-space evolution of a quartic anharmonic oscillator.

A prebunched electron beam can be exploited to reduce the rise time of a free-electron laser (FEL) oscillator, or to provide the start-up signal for an amplifier. We discuss the dynamical behavior of FEL’s operating with prebunched electron beams, and analyze different regimes of operation, from low signal up to saturation. We also include short pulses effects, discussing the modifications occurring in the FEL pulse propagation equation and analyzing the relevant small signal dynamics.

In this paper we study the spectroscopic details of the radiation emitted by electrons propagating at relativistic velocities in undulators, whose on-axis field consists of the usual periodic term plus a contribution at a given harmonic. We present a fully analytic study and show that the undulator brightness can be written in a closed form using a new class of Bessel functions, which allows a clear understanding of the harmonic selection mechanisms pattern. We compare the analytic results with a fully numerical procedure and prove the equivalence of the two methods. We also discuss the relevance of the obtained results within the context of free-electron laser (FEL) physics.

In this paper we reconsider the problem of the optimum cavity losses for the free-electron-laser (FEL) operation. We use the results of a recently proposed one-dimensional saturation model and show that for large intracavity intensities the FEL behaves like conventional laser systems.

In this paper we discuss the spectroscopic details of the two-frequency undulator, a recently proposed device to suppress the sideband instability in free-electron lasers. We present an analytic and numerical approach to the problem and discuss the intrinsic differences between the brightnesses of one- and two-frequency undulators. We study the dependence of the emission on various physical parameters including the electron-beam qualities.

The Wigner phase-space formalism has been shown to provide the natural tools to treat charged-beam transport. In this article we show how simple experiments, exploiting electron beams propagating through quadrupole magnets and drift-tube sections, can be designed to measure the Wigner rotation.

We propose a model of gain saturation in free-electron lasers operating with a bunched electron beam. A simple method to evaluate the steady-state intensity as a function of the optical cavity length is given and the result is in close agreement with the intensity measured in the Los Alamos free-electron-laser experiment [B. E. Newnam et al., Nucl. Instrum. Methods A 237, 187 (1985)].

We reconsider the theory of pulse propagation in free-electron lasers operating with arbitrary gain. We show that the dynamics can be accounted for by collective longitudinal excitations of supermode type and discuss their properties. We study the interplay between the optical-packet longitudinal dimension and the electron-beam energy spread, along with the induced gain depression. Semianalytical formulas for the gain, pulse length, etc., are presented.

In this paper we discuss the theory of emission by relativistic electrons in linearly polarized undulators. We show that the analytical computations are greatly simplified by the introduction of the so-called generalized Bessel functions, originally introduced to treat problems for which the dipolar approximation does not hold. We also discuss the modification induced in the brightness by the inclusion of the sextupolar terms, which arise when electrons are injected off the undulator axis. We show that the phenomonology inherent to this problem is very rich, as additional harmonics come into play and may cause sizable effects. The analysis we develop may be useful to obtain further insight in the physics underlying the so-called inhomogeneously broadened regime. Finally, we also discuss the inclusion of energy corrections up to 1/γ³ and the relevant physical consequences.

In this paper we analyze the dynamics of a free-electron laser in the strong-signal regime, drawing a parallel with that of a conventional laser. A simple model of gain saturation is developed and a scaling relation of the gain versus the intracavity laser intensity is derived.

In this Brief Report we propose a simple phenomenological model of gain saturation for free-electron lasers operating with a bunched electron beam. The model is based on an analogy with that of conventional lasers, and in spite of its simplicity, the obtained results are in close agreement with the numerical simulations performed within the context of a more complete theory.

In this paper we discuss in detail the dynamics of a two-state system ruled by a non-Hermitian Hamiltonian. The relevance of the results to multiphoton ionization is also discussed and a comparison to earlier works is presented.

In this Brief Report we derive analytically the spectral structure of supermodes in the long electron bunch limit.

In this paper we use a recently suggested analytical solution of the pulse propagation equation in free-electron lasers (FEL) to get further information on the lethargy and on the velocity of propagation of the optical packet. We also introduce a refractive index for the FEL interaction. Finally, the supermodes relevant to the regime of long electron bunch are found analytically and a comparison with earlier numerical results is carried out.

We propose an analytical solution to the propagation equation of the optical pulse both in free-electron lasers and in optical klystrons. Our results include in a natural way the lethargy and wave-packet spreading.

We obtain the time-ordered form of the evolution operator for Hamiltonian linear combinations of the generators of the SU(1,1), SU(2), and h(4) groups. The relevance of the obtained results to physics problems such as the generation of non-Poissonian statistics in laser-plasma scattering and the pulse propagation in free-electron lasers is also briefly discussed.