Search Results (16 total)

The new generation of linac injectors driving free electron lasers in the self-amplified stimulated emission (SASE-FEL) regime requires high brightness electron beams to generate radiation in the wavelength range from UV to x rays. The choice of the injector working point and its matching to the linac structure are the key factors to meet this requirement. An emittance compensation scheme presently applied in several photoinjectors worldwide is known as the “Ferrario” working point. In spite of its great importance there was, so far, no direct measurement of the beam parameters, such as emittance, transverse envelope, and energy spread, in the region downstream the rf gun and the solenoid of a photoinjector to validate the effectiveness of this approach. In order to fully characterize the beam dynamics with this scheme, an innovative beam diagnostic device, the emittance meter, consisting of a movable emittance measurement system, has been designed and built. With the emittance meter, measurements of the main beam parameters in both transverse phase spaces can be performed in a wide range of positions downstream the photoinjector. These measurements help in tuning the injector to optimize the working point and provide an important benchmark for the validation of simulation codes. We report the results of these measurements in the SPARC photoinjector and, in particular, the first experimental evidence of the double minimum in the emittance oscillation, which provides the optimized matching to the SPARC linac.

In this Letter we report the first experimental observation of the double emittance minimum effect in the beam dynamics of high-brightness electron beam generation by photoinjectors; this effect, as predicted by the theory, is crucial in achieving minimum emittance in photoinjectors aiming at producing electron beams for short wavelength single-pass free electron lasers. The experiment described in this Letter was performed at the SPARC photoinjector site, during the first stage of commissioning of the SPARC project. The experiment was made possible by a newly conceived device, called an emittance meter, which allows a detailed and unprecedented study of the emittance compensation process as the beam propagates along the beam pipe.

A Reply to the Comment by R. Bonifacio et al..

In this Letter we report the first experimental characterization of superradiance in a single-pass high-gain free-electron laser (FEL) seeded by a 150 femtosecond (FWHM) Ti:sapphire laser. The nonlinear energy gain after an exponential gain regime was observed. We also measured the evolution of the longitudinal phase space in both the exponential and superradiant regimes. The output FEL pulse duration was measured to be as short as 81 fs, a roughly 50% reduction compared to the input seed laser. The temporal distribution of the FEL radiation as predicted by a numerical simulation was experimentally verified for the first time.

The ultimate performance of a storage-ring free-electron laser in terms of light stability and extracted power depends on the possibility of simultaneously controlling the electron-beam and laser dynamics. As a preliminary requirement, the level of longitudinal and transverse electron-beam stability must be high enough to guarantee the laser start-up and growth. This is usually obtained by means of dedicated feedback systems. Once such a requirement is satisfied, the possibility of establishing and maintaining a continuous-wave operation mode finally resides in a deep understanding of the strongly coupled laser-electrons dynamics. For this purpose, we have developed a simple theoretical model which has been proved to be able to provide insight into the evolution of the laser intensity. In this framework, we have also proposed the possibility of utilizing a derivative closed-loop feedback to create or enlarge the region of stable signal. A feedback of this type has been implemented on the Elettra storage-ring free-electron laser. The obtained results, which fully confirm our predictions, are discussed in this paper.

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.

The TREDI Monte Carlo program is briefly described, devoting some emphasis to the Lienard-Wiechert potentials approach followed to account for self-field effects and the covariant technique devised to achieve regularization of electromagnetic fields. Some guidelines to the choice of the correct parameters to be used in the simulation are also sketched. The predictions obtained for the reference work point of the space-charge compensated SPARC photoinjector and a benchmark chicane designed to study coherent synchrotron radiation effects in a magnetic compressor are compared to those of other well-established simulation codes.

The high brightness electron beam required by a short wavelength self-amplified spontaneous emission free-electron laser (FEL) may be reached only with an accurate design of the beam dynamics from the generation in the rf injector up to the undulator. The beam dynamics is affected by strong self-consistent effects at injection, in the compression stage, and during the FEL process. The support of numerical simulations is extensively used in the predictions of the beam behavior in these nonlinear dynamical conditions. I present a review of available simulation techniques, currently exploited in the design of short wavelength free-electron lasers.

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 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.

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)].

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.