Self-amplified Spontaneous Emission Free Electron Research Articles

Generation of Ultrashort Pulses in XUV and X-ray FELs via an Excessive Reverse Undulator Taper

The pulse duration in short-pulse schemes for Self-Amplified Spontaneous Emission Free Electron Lasers (SASE FELs) is limited by the FEL coherence time. A recently proposed concept allows to overcome the coherence time barrier and to obtain much shorter pulses. When the lasing part of an electron bunch is much shorter than the coherence time, one can suppress the radiation in the long main undulator while preserving microbunching within that short lasing slice. Then, a short radiation pulse is produced in a relatively short radiator. A possible suppression method, an excessive reverse undulator taper, is discussed and illustrated numerically in this paper. We also performed the first experimental tests of this method at the soft X-ray FEL user facility FLASH. The measured pulse duration approaches 1 fs (FWHM) at the wavelength of 5 nm.

Application of a modified chirp-taper scheme for generation of attosecond pulses in extreme ultraviolet and soft x-ray free electron lasers

Typically, in Self-Amplified Spontaneous Emission Free Electron Laser (SASE FEL) based short-pulse schemes, pulse duration is limited by FEL coherence time. For hard X-ray FELs, coherence time is in a few hundred attosecond range while for XUV and soft X-ray FELs it is in the femtosecond regime. In this paper the modification of so-called chirp-taper scheme is developed that allows to overcome the coherence time barrier. Numerical simulations for XUV and soft X-ray FEL user facility FLASH demonstrate that one can generate a few hundred attosecond long pulses in the wavelength range 2 - 10 nm with peak power reaching hundreds of megawatts. With several thousand pulses per second this can be a unique source for attosecond science.

Beam dynamic studies at accelerator system of gun for a self-amplified spontaneous emission free electron laser project

One of the important factors in high radiation generation facilities is the production of a high quality electron beam. To do that, the requirements of an optical electron source, such as current, radiation emission, etc. must be met. For this purpose, cavity beam dynamics studies of 1.4, 1.5, 1.6 and 1.8-cell SRF gun were performed. In addition, detailed studies have been made on the solenoid design and the location of the solenoid along the beam path. A design with the most optimized cavity geometry and RF fields for the gun cavity was posed. The injector system was designed for the Turkish SASE-FEL project with cavities of many different geometrical sizes. The most efficient high energy cavity system is determined as 1.6-cell cavity. The emittance value of the beam was achieved as 1.76±0.42π mm mrad and the beam energy reached at the end of cavity was obtained as 3.5MeV.

Revision of optical klystron enhancement effects in self-amplified spontaneous emission free electron lasers

We review the theory of optical klystrons, with its applications for self-amplified spontaneous emission free electron lasers in mind. We show that previous theories miss terms in the power gain factor that cannot be neglected, and we illustrate differences between the previously known analytical expressions, new ones found in this paper, and numerical calculations. We then consider the use of optical klystrons for electron energy-spread and radiation coherence-time diagnostics purposes.

Slice collective dynamics, projected emittance deterioration and free electron laser performances detrimental effects

Self-amplified spontaneous emission (SASE) free electron laser (FEL) devices have disclosed an unexpected interplay between the laser intensity growth and regions of the electron bunch of the order of the coherence length. They are usually identified with the bunch slice and contribute to the laser dynamics with their own characteristics. The dynamical effects inducing geometrical and phase space misalignment of bunch slice in X-ray operating FELs can be traced back to a plethora of phenomena, both in the Linac accelerating section or inside the beam transport optic magnet. They are responsible for spoiling of the beam projected qualities and, if not corrected properly, induce an increase of the saturation length and a decreasing of the output power. We discuss the inclusion of these effects in models employing scaling formulae.

The detuning effect of crystal monochromator in self-seeding and oscillator free electron laser.

Self-amplified spontaneous emission (SASE) free electron laser (FEL) is capable of generating ultra-short, high power and high brightness X-ray pulses, but its temporal coherence is poor. Self-seeding scheme is an approach to improve the temporal coherence by employing a crystal monochromator. The crystal detuning effect is the phenomenon that the Bragg angle deviates from the middle of the reflection domain due to the refraction effect, and can affect the seed power of hard X-ray self-seeding (HXRSS) FEL. In this paper, we introduce a novel idea to maximize the seed power by tuning the incident angle off the Bragg condition where the Bragg photon energy is corresponding to the central photon energy of the input X-ray pulse. We present the numerical analysis of the detuning effect in different reflecting atomic planes and different asymmetry angles of diamond crystal. Moreover, we analyze how the detuning affects the seed efficiency of HXRSS FEL, and discuss the application to X-ray FEL oscillator (XFELO). We find when the detuning is much smaller than the bandwidth of input X-ray pulse, we can neglect the detuning effect. However, if the detuning is much larger than or comparable with the bandwidth of input X-ray pulse, the detuning effect can not be ignored. This work can give a guidance to HXRSS FEL and XFELO commissioning for high efficiency FEL output.

Generating attosecond x-ray pulses through an angular dispersion enhanced self-amplified spontaneous emission free electron laser

We propose a scheme that can generate attosecond x-ray pulses through an angular dispersion enhanced self-amplified spontaneous emission (ESASE) free electron laser (FEL). In this scheme, an angular dispersion section is implemented into a conventional ESASE set-up, together with a few cycle laser pulse to manipulate the electron beam effectively. During the procedure, the transverse-longitudinal phase space coupling is introduced into the electron beam in addition to the energy modulation and momentum compaction processes. As a result, the peak current of the electron beam will be strengthened significantly within a narrow width of the electron bunch, which can be used to generate ultrashort FEL pulses. Comparing to a conventional ESASE scheme, the proposed scheme can reduce the required modulation laser power by at least one order of magnitude. The signal-to-noise ratio of the final FEL radiation pulses can also be improved. In addition, we discuss the possibility of using this scheme to generate two-pulse two-color x-ray FEL with tunable central wavelengths and time delays for a conceivable x-ray pump/x-ray probe experiment.

High-Power Femtosecond Soft X Rays from Fresh-Slice Multistage Free-Electron Lasers.

We demonstrate a novel multistage amplification scheme for self-amplified spontaneous-emission free electron lasers for the production of few femtosecond pulses with very high power in the soft x-ray regime. The scheme uses the fresh-slice technique to produce an x-ray pulse on the bunch tail, subsequently amplified in downstream undulator sections by fresh electrons. With three-stages amplification, x-ray pulses with an energy of hundreds of microjoules are produced in few femtoseconds. For single-spike spectra x-ray pulses the pulse power is increased more than an order of magnitude compared to other techniques in the same wavelength range.

Soft X-ray generation in cascade SASE FEL with two-frequency undulator

The generation of soft X-ray radiation in cascade self-amplified spontaneous emission free electron laser (SASE FEL) using harmonics of a two-frequency undulator is studied. An advanced phenomenological model of SASE FEL, accounting for the properties of the undulator and of the electron beam, including major losses, is employed. The account for the beam energy spread, divergence and diffraction is harmonic dependent, which is a step forward from the previous models and phenomenological approach, which consider all the losses independent of the harmonic number. The model is validated with the experimental measurements of the FEL harmonic generation and with relevant three-dimensional numerical simulations. The model is adopted for the description of cascading SASE FELs. With its help the dynamics of high-gain harmonic generation (HGHG) SASE FEL cascades is analyzed to obtain the maximum high harmonic power in the X-ray band with the lowest possible seed frequency and the shortest FEL length. The advantages of the two-frequency undulator for SASE FEL in this context are demonstrated and the possibility of its usage in cascading HGHG SASE FEL is shown. Several types of cascade FELs with two-frequency undulators are simulated. Frequency multiplication in two, three and four stage cascades is introduced to obtain soft X-ray radiation. The possibility of the generation of a powerful radiation, 75 MW at and a lower intensity 3rd harmonic, 100 kW at , is demonstrated in multi-cascade FELs at just 20–35 meters with the seed wavelengths and .

Two-frequency undulator usage in compact self-amplified spontaneous emission free electron laser in Roentgen range

AbstractThe generation of harmonics in two-frequency undulator in a self-amplified spontaneous emission free electron laser (SASE FEL) is studied in order to produce Roentgen radiation in a relatively compact sized installation. The dynamics of SASE FEL is analyzed with the help of the phenomenological model to obtain the maximum of the X-ray high-harmonic power. The model accounts for the properties of the undulator magnetic field and of the electron beam and includes the major sources of losses, such as the electron energy spread, etc. It is compared and calibrated with the existing data on a FEL experiment. The advantages of the two-frequency undulator for Roentgen SASE FEL are demonstrated and the possibility to generate powerful mild Roentgen radiation at already ~25 m length is shown. The evolution of the bunching coefficients for high harmonics is studied together with the evolution of the FEL-induced energy spread. The linear and non-linear regimes are explored for common and for two-frequency undulators The usage of the two-frequency undulator for cascade SASE FEL with high X-ray harmonic power and high-harmonic bunching coefficients with low-induced energy spread is proposed.

Optical Klystron Enhancement to Self Amplified Spontaneous Emission at FERMI

The optical klystron enhancement to a self-amplified spontaneous emission free electron laser has been studied in theory and in simulations and has been experimentally demonstrated on a single-pass high-gain free electron laser, the FERMI FEL-1, in 2014. The main concept consists of two undulators separated by a dispersive section that converts the energy modulation induced in the first undulator in density modulation, enhancing the coherent harmonic generation in the first part of the second undulator. This scheme could be replicated in a multi-stage: the bunching is enhanced after each dispersive section, consistently reducing the saturation length. We have applied the multi-stage optical klystron (OK) scheme on the FEL-2 line at FERMI, whose layout includes three dispersive sections. Optimizing the strength of the dispersions allowed a significant increase of the self-amplified spontaneous emission (SASE) intensity in comparison to a single-stage OK and extending to the soft-X rays the OK enhanced SASE previously demonstrated on FEL-1.

Emission and tuning of harmonics in a planar two-frequency undulator with account for broadening

AbstractWe analyze the undulator radiation from planar bi-harmonic undulators with account for all principle losses in real devices. The exact analytical expressions for the UR spectrum, intensity, and the line shape are obtained in terms of special functions. The interplay of various broadening contribution is elucidated and their role is explored. Suspension and boost of certain harmonics by fine tuning the undulator parameters is demonstrated. The constant non-periodic magnetic constituents are studied to compensate the divergency of the electronic beam. The examples of harmonic generation in various undulator schemes with single and double period magnetic field are explored. Influence of inhomogeneous and homogeneous broadening on these harmonics is demonstrated. The analysis is applied to evaluate the harmonics in self-amplified spontaneous emission free electron laser.

Self-amplified spontaneous emission free electron laser devices and nonideal electron beam transport

We have developed, at the SPARC test facility, a procedure for a real time self-amplified spontaneous emission free electron laser (FEL) device performance control. We describe an actual FEL, including electron and optical beam transport, through a set of analytical formulas, allowing a fast and reliable on-line ``simulation'' of the experiment. The system is designed in such a way that the characteristics of the transport elements and the laser intensity are measured and adjusted, via a real time computation, during the experimental run, to obtain an on-line feedback of the laser performances. The detail of the procedure and the relevant experimental results are discussed.

Transform-Limited X-Ray Pulse Generation from a High-Brightness Self-Amplified Spontaneous-Emission Free-Electron Laser

A method to achieve high-brightness self-amplified spontaneous emission (HB-SASE) in the free-electron laser (FEL) is described. The method uses repeated nonequal electron beam delays to delocalize the collective FEL interaction and break the radiation coherence length dependence on the FEL cooperation length. The method requires no external seeding or photon optics and so is applicable at any wavelength or repetition rate. It is demonstrated, using linear theory and numerical simulations, that the radiation coherence length can be increased by approximately 2 orders of magnitude over SASE with a corresponding increase in spectral brightness. Examples are shown of HB-SASE generating transform-limited FEL pulses in the soft x-ray and near transform-limited pulses in the hard x-ray. Such pulses may greatly benefit existing applications and may also open up new areas of scientific research.

Purified self-amplified spontaneous emission free-electron lasers with slippage-boosted filtering

We propose a simple method to significantly enhance the temporal coherence and spectral brightness of a self-amplified spontaneous emission (SASE) free-electron laser (FEL). In this purified SASE (pSASE) FEL, a few undulator sections (called slippage-boosted sections) resonant at a subharmonic of the FEL radiation are used in the middle stage of the exponential growth regime to amplify the radiation while simultaneously reducing the FEL bandwidth. In this slippage-boosted section, the average longitudinal velocity of electrons is reduced, which effectively increases the FEL slippage length that allows the radiation fields initially far apart to create a phase relation, leading to $n$ times increase in FEL cooperation length, where $n$ is the ratio of the resonant wavelength of the slippage-boosted section to that of the original FEL radiation. The purified radiation, as a seed with improved temporal coherence, is further amplified to saturation in the undulator sections tuned to the FEL wavelength. Using the linac coherent light source II (LCLS-II) parameters as an example, we show that with the proposed configuration the temporal coherence and spectral brightness of a SASE FEL can be significantly enhanced. This scheme may be applied to many SASE FEL light sources to enhance the FEL performance.

Attosecond X-ray free electron laser

For a real, meaningful pump-probe experiment with attosecond temporal resolution, an isolated attosecond pulse is in demand. In this vein we report the generation of an isolated ~ 148 attosecond pulse duration radiation pulse at 0.1 angstrom wavelength using current enhanced self-amplified spontaneous emission free electron laser. We consider the 10-GeV PAL-XFEL e-beam for enhanced self-amplified spontaneous emission (ESASE) scheme. In ESASE, X-ray SASE FEL is combined with a femtosecond laser system. An 800 nm wavelength, 5 fs FWHM carrier envelope phase stabilized laser was employed in ESASE scheme.

Femtosecond x-ray free electron laser pulse duration measurement from spectral correlation function

We present a novel method for measuring the duration of femtosecond x-ray pulses from self-amplified spontaneous emission free electron lasers by performing statistical analysis in the spectral domain. Analytical expressions of the spectral correlation function were derived in the linear regime to extract both the pulse duration and the spectrometer resolution. Numerical simulations confirmed that the method can be also used in the nonlinear regime. The method was demonstrated experimentally at the Linac Coherent Light Source by measuring pulse durations down to 13 fs FWHM.

A SASE free electron laser optimization based on new generation in-vacuum undulators

Optimization studies for an accelerator based light source, namely self-amplified spontaneous emission (SASE) free electron laser (FEL), based on new generation in-vacuum hybrid and superconducting undulator configurations, are compared and discussed. It is shown that the FEL wavelength should be down to soft X-rays ( ∼ 3 nm ) part of the spectrum while keeping the same linear accelerator (linac) energy about 1 GeV. On the other hand, numerical calculations and simulation results of the main performance parameters for SASE operation (1D gain length, saturation power and saturation length), are optimized. Finally, technological advantages and challenges for both cases, are briefly mentioned.

Wavefront Analysis of Nonlinear Self-Amplified Spontaneous-Emission Free-Electron Laser Harmonics in the Single-Shot Regime

The single-shot spatial characteristics of the vacuum ultraviolet self-amplified spontaneous emission of a free electron laser (FEL) is measured at different stages of amplification up to saturation with a Hartmann wavefront sensor. We show that the fundamental radiation at 61.5 nm tends towards a single-mode behavior as getting closer to saturation. The measurements are found in good agreement with simulations and theory. A near diffraction limited wavefront was measured. The analysis of Fresnel diffraction through the Hartmann wavefront sensor hole array also provides some further insight for the evaluation of the FEL transverse coherence, of high importance for various applications.

Self-Amplified Spontaneous Emission Free-Electron Laser with an Energy-Chirped Electron Beam and Undulator Tapering

We report the first experimental implementation of a method based on simultaneous use of an energy chirp in the electron beam and a tapered undulator, for the generation of ultrashort pulses in a self-amplified spontaneous emission mode free-electron laser (SASE FEL). The experiment, performed at the SPARC FEL test facility, demonstrates the possibility of compensating the nominally detrimental effect of the chirp by a proper taper of the undulator gaps. An increase of more than 1 order of magnitude in the pulse energy is observed in comparison to the untapered case, accompanied by FEL spectra where the typical SASE spiking is suppressed.