Single-pass Free-electron Laser Research Articles

Optimization of a Seeded Free-Electron Laser with Helical Undulators

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.

A compact free-electron laser for generating coherent radiation in the extreme ultraviolet region

Single-pass free-electron lasers based on self-amplified spontaneous emission1,2,3,4 are enabling the generation of laser light at ever shorter wavelengths, including extreme ultraviolet5, soft X-rays and even hard X-rays6,7,8. A typical X-ray free-electron laser is a few kilometres in length and requires an electron-beam energy higher than 10 GeV (refs 6, 8). If such light sources are to become accessible to more researchers, a significant reduction in scale is desirable Here, we report observations of brilliant extreme-ultraviolet radiation from a 55-m-long compact self-amplified spontaneous-emission source, which combines short-period undulators with a high-quality electron source operating at a low acceleration energy of 250 MeV. The radiation power reaches saturation at wavelengths ranging from 51 to 61 nm with a maximum pulse energy of 30 µJ. The ultralow emittance (0.6π mm mrad) of the electron beam from a CeB6 thermionic cathode9 is barely degraded by a multiple-stage bunch compression system that dramatically enhances the beam current from 1 to 300 A. This achievement expands the potential for generating X-ray free-electron laser radiation with a compact 2-GeV machine. Free-electron lasers can produce powerful pulses of radiation at very short wavelengths, even in the hard-X-ray region. In general, however, they comprise facilities several kilometres in length. A 55-m-long laser could open up the technology to a broader range of researchers.

Experimental Characterization of Nonlinear Harmonic Generation in Planar and Helical Undulators

We present an experimental characterization of the process of coherent harmonic generation in single-pass free electron lasers. The harmonic radiation is obtained by seeding the electron beam stored in the Elettra storage ring with a Ti:sapphire laser. Different methods for generating harmonics are compared between them, and a detailed characterization of the emitted light is performed for different polarizations. Our results also contribute to the debate about the possible presence of a coherent on-axis signal in helical undulators. In this respect, we provide an experimental confirmation of recent theoretical studies that predict no coherent on-axis signal.

Effects of ion-channel guiding on the saturation mechanism of a single-pass free-electron laser

In this paper we study the nonlinear dynamics of a single-pass free-electron laser in the presence of an ion-channel guide. A set of coupled nonlinear differential equations in 1D approximation is employed, which governs the self-consistent evolution of an electromagnetic wave in the presence of a cold, cylindrically symmetric electron beam. The electron beam propagates at a relativistic velocity through the combination of a wiggler field and of a channel of immobile ions. The effects of the ion channel on the saturation of an amplifier free-electron laser are illustrated. Numerical simulations are performed using the parameters of the Fermi@Elettra single-pass free-electron laser as a reference. The effect of the ion channel is shown to have an impact on both the saturation length and the output power.

Direct Measurement of the Double Emittance Minimum in the Beam Dynamics of the Sparc High-Brightness Photoinjector

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.

Maximum entropy principle and coherent harmonic generation using a single-pass free-electron laser

Single-pass free-electron lasers constitute an example of systems with long-range interactions. The light-particle interplay leading to the power growth and successive relaxation towards a quasi-stationary state is governed by the Vlasov equation. A maximum entropy principle inspired to Lynden-Bell's theory of "violent relaxation" for the Vlasov equation can be invoked to analytically characterize the behaviour of the saturated system. In particular, we here concentrate on the case of coherent harmonic generation obtained from an externally seeded free-electron laser and provide a simple strategy to predict the laser intensity as well as the final electron-beam energy distribution.

High-power tunable, 05-3 THz radiation source based on nonlinear difference frequency mixing of CO_2 laser lines

Terahertz (THz) pulses with a peak power of ∼2 kW were generated in a noncollinear phase-matched GaAs crystal at room temperature. Two 200 ns pulses from a dual-beam TEA CO2 laser were used for difference frequency mixing in the crystal. A comb of narrow lines (Δν/ν∼10−4) was obtained in the 0.5-3 THz range with a step of 40 GHz. By comparing the effective nonlinearity of GaSe with that of GaAs for THz generation, the electro-optic nonlinear coefficient for GaSe was measured to be deo=24.3±10% pm/V. Using simulations we show that a 1 kW THz pulse could be amplified by a factor of 2×104 to a 10 MW level in a 2 m long single-pass free-electron laser.

Soft-X-Ray Coherent Radiation Using a Single-Cascade Free-Electron Laser

Coherent harmonic generation using single-pass free-electron lasers is a promising method for generating coherent radiation in the vacuum ultraviolet and x-ray spectral region. We propose a simple scheme allowing one to generate powerful coherent radiation in the soft x-ray region by making use of present available technology. The method relies on the possibility of creating substantial bunching in a relativistic electron beam, while limiting the growth of its energy spread. The validity of the scheme is demonstrated using a simple one-dimensional model. Results are confirmed by three-dimensional simulations.

SASE FELs: Interactions with Atoms and Ions

Attainment of tunable laser-like radiation in the VUV and X-ray spectral regions is becoming a reality with the construction of single-pass Free Electron Lasers (FELs) based on the principle of Self-Amplified Spontaneous Emission (SASE). The paper introduces the development of short wavelength sources from an atomic physics perspective, describes the SASE FEL process and discusses examples from first user experiments at FLASH (Free Electron Laser at DESY (Hamburg). The perspective for further progress to shorter wavelength devices is outlined.

STUDY OF COMPTON BACKSCATTERING BETWEEN RELATIVISTIC ELECTRON BEAM AND SASE LIGHT

Study of Compton backscattering with relativistic high-intense electron beam and single-pass free electron laser (FEL) is carried out to produce high-brightness short X-ray pulse. The single-pass FEL such as SASE is high power coherent light source and the wavelength of the FEL can be tuned changing magnetic field strength of wiggler or undulator continuously. In our study, the relativistic electron beam is generated using a linear accelerator, which is a driver for the FEL. The electron beam is used for both the Compton backscattering and the generation of SASE light. The preliminary experiment of X-ray generation based on Compton backscattering with high-intensity electron beam and infrared SASE light is planed using the L-band linear accelerator at the Institute of Scientific and Industrial Research (ISIR), Osaka University. We will describe the preliminary experiment and the result of numerical studies.

Seeded free-electron and inverse free-electron laser techniques for radiation amplification and electron microbunching in the terahertz range

A comprehensive analysis is presented that describes amplification of a seed THz pulse in a single-pass free-electron laser (FEL) driven by a photoinjector. The dynamics of the radiation pulse and the modulated electron beam are modeled using the time-dependent FEL code, GENESIS 1.3. A 10-ps (FWHM) electron beam with a peak current of 50 ‐100 A allows amplification of a � 1k Wseed pulse in the frequency range 0.5‐3 THz up to 10 ‐100 MW power in a relatively compact 2-m long planar undulator. The electron beam driving the FEL is strongly modulated, with some inhomogeneity due to the slippage effect. It is shown that THz microbunching of the electron beam is homogeneous over the entire electron pulse when saturated FEL amplification is utilized at the very entrance of an undulator. This requires seeding of a 30cm long undulator buncher with a 1‐3 MW of pump power with radiation at the resonant frequency. A narrow-band seed pulse in the THz range needed for these experiments can be generated by frequency mixing of CO2 laser lines in a GaAs nonlinear crystal. Two schemes for producing MW power pulses in seeded FELs are considered in some detail for the beam parameters achievable at the Neptune Laboratory at UCLA: the first uses a waveguide to transport radiation in the 0.5‐3 THz range through a 2-m long FEL amplifier and the second employs high-gain third harmonic generation using the FEL process at 3‐9 THz.

Enhancement of particle trapping in the wave-particle interaction

The saturated dynamics of a Single-Pass Free Electron Laser is considered within a simplified mean-field approach. A method is proposed to increase the size of the macro-particle, which is responsible for the oscillations of the intensity of the wave. This approach is based on the reconstruction of invariant tori of the dynamics of test particles. To this aim a dedicated control term is derived, the latter acting as a small apt perturbation of the system dynamics. Implications of these findings are discussed in relation to the optimization of the laser source.

Fully Coherent X-Ray Pulses from a Regenerative-Amplifier Free-Electron Laser

We propose and analyze a regenerative-amplifier free-electron laser (FEL) to produce fully coherent, hard x-ray pulses. The method makes use of narrow-bandwidth Bragg crystals to form an x-ray feedback loop around a relatively short undulator. Self-amplified spontaneous emission (SASE) from the leading electron bunch in a bunch train is spectrally filtered by the Bragg reflectors and is brought back to the beginning of the undulator to interact repeatedly with subsequent bunches in the bunch train. The FEL interaction with these short bunches regeneratively amplifies the radiation intensity and broadens its spectrum, allowing for effective transmission of the x rays outside the crystal bandwidth. The spectral brightness of these x-ray pulses is about 2 to 3 orders of magnitude higher than that from a single-pass SASE FEL.

Statistical mechanics and Vlasov equation allow for a simplified Hamiltonian description of Single-Pass Free Electron Laser saturated dynamics

A reduced Hamiltonian formulation to reproduce the saturated regime of a single pass free electron laser, around perfect tuning, is here discussed. Asymptotically, $N\_m$ particles are found to organize in a dense cluster, that evolves as an individual massive unit. The remaining particles fill the surrounding uniform sea, spanning a finite portion of phase space, approximately delimited by the average momenta $\omega\_+$ and $\omega\_-$. These quantities enter the model as external parameters, which can be self-consistently determined within the proposed theoretical framework. To this aim, we make use of a statistical mechanics treatment of the Vlasov equation, that governs the initial amplification process. Simulations of the reduced dynamics are shown to successfully capture the oscillating regime observed within the original $N$-body picture.

ENHANCEMENT OF PARTICLE TRAPPING IN THE FREE ELECTRON LASER

The saturated dynamics of a Single Pass Free Electron Laser is here considered within a simplified mean field approach. A method is proposed to increase the size of the macro-particle. This approach is based on the reconstruction of invariant tori of the dynamics of test particles. To this aim a dedicated control term is derived, the latter acting as a small apt perturbation of the system dynamics. Implications of these findings are discussed in relation to the optimization of the laser source.

Nonlinear pulse evolution in seeded free-electron laser amplifiers and in free-electron laser cascades

The advances in laser technology have made available very short and intense laser pulses which can be used to seed a high-gain single-pass free-electron laser (FEL) amplifier. With these seed pulses, a regime of the FEL interaction where the radiation evolution is simultaneously dominated by nonlinear effects (saturation) and time-dependent effects (slippage) can be explored. This regime is characterized by the propagation of a solitary wavelike pulse where the power of the optical wave grows quadratically with time, its pulse length decreases and the spectral bandwidth increases. We analyze the interplay between the field and particle dynamics of this propagation regime which was studied before and termed super-radiance. Furthermore we analyze the properties of the strong higher-order harmonic emission from this wave and its behavior when propagating in a cascade FEL. The super-radiant pulse is indeed capable of passing through the stages of a cascade FEL and to regenerate itself at the wavelength of the higher-order harmonic. The optical pulse obtained is shorter than a cooperation length and is strongly chirped in frequency, thus allowing further longitudinal compression down to the attosecond time scale.

High gain single pass free electron laser dynamics and pulse propagation effects

RiassuntoIn questo lavoro si dimostra che la dinamica di laser ad elettroni liberi operanti in regime di singolo passaggio ad alto guadagno può essere riprodotta accuratamente per via analitica utilizzando la funzione logistica, in modo da includere contributi che vanno sugli effetti di pre-modulazione a quelli di impulso corto.

X-ray free-electron lasers

In a free-electron laser (FEL) the lasing medium is a high-energy beam of electrons flying with relativistic speed through a periodic magnetic field. The interaction between the synchrotron radiation that is produced and the electrons in the beam induces a periodic bunching of the electrons, greatly increasing the intensity of radiation produced at a particular wavelength. Depending only on a phase match between the electron energy and the magnetic period, the wavelength of the FEL radiation can be continuously tuned within a wide spectral range. The FEL concept can be adapted to produce radiation wavelengths from millimetres to Ångstroms, and can in principle produce hard x-ray beams with unprecedented peak brightness, exceeding that of the brightest synchrotron source by ten orders of magnitude or more. This paper focuses on short-wavelength FELs. It reviews the physics and characteristic properties of single-pass FELs, as well as current technical developments aiming for fully coherent x-ray radiation pulses with pulse durations in the 100 fs to 100 as range. First experimental results at wavelengths around 100 nm and examples of scientific applications planned on the new, emerging x-ray FEL facilities are presented.

Wave-particle interaction: from plasma physics to the free-electron laser

We discuss the analogies between two classical models of the single-pass free electron laser dynamics and of the beam-wave plasma instability. Moreover, a formal bridge between the two areas of investigation is established. This connection is here exploited to derive a reduced Hamiltonian formulation for the saturated regime of the free electron laser.

The FEL Program at DESY: From First Results at 100 nm Wavelength to a True 1 Å X-ray Laser

Gigawatt level radiation pulses at wavelengths between 80 and 120 nm and a pulse duration below 100 fs have recently been produced by a single-pass free electron laser at DESY. The radiation has been characterized with respect to pulse energy, statistical properties, spectral distribution and coherence. It has also been used for first experiments on solids and gases demonstrating the potential of the new source. The facility is currently upgraded to higher energy and is planned to start user operation in 2004. Based on this work a large free electron laser laboratory with several experimental stations for the hard X-ray regime down to 1 Å will be built at DESY which could start user operation at the beginning of the next decade.