X-ray SASE FEL Research Articles

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.

Development of ultra-short pulse, single coherent spike for SASE X-ray FELs

There is a large interest in the production of high power, ultra-short, 1 fs or less, coherent X-ray pulses, for atomic physics and other applications. However, the present design of X-ray SASE FELs leads to an X-ray pulse about 100 times longer. The time structure of the X-ray pulse is determined by several factors, mainly the electron bunch length and the FEL cooperation length. Until now all FELs have been working in the regime where the electron bunch length is much larger than the cooperation length. In the case of an X-ray SASE FEL this means that the X-ray pulse consists of a series of spikes, a few hundred in the case of the LCLS, and the bunch length is of the order of 200 fs. Several methods to reduce the bunch length to the 10 - 1 fs region have been proposed. These methods are based on electron bunch manipulation to cut the lasing part of the bunch to a fraction of the total length, thus reducing the X-ray pulse length. We are considering here a different method, using ultra-short, very low charge electron bunches, with a length of the order or shorter than the FEL cooperation length. In this case the X-ray pulse length after amplification in the undulator is a few times the electron bunch length. Our simulations show that in the case of LCLS and SPARX, operating with a 1 pC electron beam, we obtain X-ray pulses as short as 300 as, transform limited in the SPARX case, and almost transform limited in the LCLS case. Compared to other modes of producing short X-ray bunches this one offers the advantage of very small background signal, since all electrons are used to lase, and the charge is very small.

Two-color FEL amplifier for femtosecond-resolution pump-probe experiments with GW-scale X-ray and optical pulses

The paper describes a scheme for pump-probe experiments that could be performed at the soft X-ray SASE FEL at the TESLA Test Facility (TTF) at DESY and determines what additional hardware developments will be required to bring these experiments to fruition. Pump-probe experiments combining pulses from a XFEL and optical femtosecond laser are very attractive for sub-picosecond time-resolved studies. Since the synchronization between the two light sources to an accuracy of 100 fs is not yet solved, it is proposed to derive both femtosecond radiation pulses from the same electron bunch but from two insertion devices. This eliminates the need for synchronization and developing tunable, high power femtosecond quantum laser. In the proposed scheme for pump-probe experiments, GW-level soft X-ray pulse is naturally synchronized with his GW-level optical pulse and cancel jitter. The concept is based on generation of the optical radiation in the master oscillator-power FEL amplifier configuration. An attractive feature of the FEL amplifier scheme is the absence of limitation which would prevent operation in the femtosecond regime in a wide (200– 900 nm ) wavelength range. The problem of tunable quantum seed laser can be solved with commercially available long pulse dye laser. An important feature of the proposed scheme is that optical radiator uses the spent electron beam. As a result, saturation mode of operation of the optical FEL does not interfere with the main mode of the soft X-ray SASE FEL operation.

Terawatt-scale sub-10-fs laser technology – key to generation of GW-level attosecond pulses in X-ray free electron laser

We propose a technique for the production of attosecond X-ray pulses which is based on the use of X-ray SASE FEL combined with a femtosecond laser system. A few-cycle optical pulse from a Ti:sapphire laser interacts with the electron beam in a two-period undulator resonant to 800 nm wavelength and produces energy modulation within a slice of the electron bunch. Following the energy modulator the electron beam enters the X-ray undulator and produces SASE radiation. Due to energy modulation the frequency is correlated to the longitudinal position within the few-cycle-driven slice of SASE radiation pulse. The largest frequency offset corresponds to a single-spike pulse in the time domain which is confined to one half-oscillation period near the central peak electron energy. The selection of single-spike pulses is achieved by using a crystal monochromator after the X-ray undulator. Our studies show that the proposed technique is capable to produce 300-as long single pulses with GW-level output power in the 0.1 nm wavelength range, and is applicable to the European X-Ray Laser Project XFEL and the Linac Coherent Light Source at SLAC.

Photon ring multi-user distribution system for soft X-ray SASE FEL laboratory

It is shown that although the soft X-ray SASE FEL photon beam from undulator is in principle a single user tool, just like an optical laser, the optical distribution system based on multifacet reflectors provide efficient ways to generate a multi-user facility—very similar to present-day synchrotron radiation facilities. Multifacet reflectors involve multiple reflections from a series of plane mirrors. These make use of the principle of total external reflectance. The candidate mirror materials include C, Mo, and Si which should yield a reflectance of 40–80% for an arc angle of 90° in the wavelength range 6–100nm. Multifacet reflector arcs can be repeated a number of times to form a complete ring. In order to make efficient use of the new SASE FEL source, it is proposed to install this photon ring distribution system in the experimental hall of soft X-ray FEL laboratory. A few tens beam lines with different experiments can be served by a single FEL source. Using movable mirrors in each photon ring cell, it is possible to quickly switch the FEL photon beam from one experiment to the other, thus providing simultaneous multi-user capability.

Conceptual design of a high-brightness linac for soft X-ray SASE-FEL source

FELs based on SASE are believed to be powerful tools to explore the frontiers of basic sciences, from physics to chemistry to biology. Intense R&D programs have started in the USA and Europe in order to understand the SASE physics and to prove the feasibility of these sources. The allocation of considerable resources in the Italian National Research Plan (PNR) brought about the formation of a CNR–ENEA–INFN–University of Roma “Tor Vergata” study group. A conceptual design study has been developed and possible schemes for linac sources have been investigated, leading to the SPARX proposal. We report in this paper the results of a preliminary start to end simulation concerning one option we are considering based on an S-band normal conducting linac with high-brightness photoinjector integrated in an RF compressor.

Scheme for time-resolved experiments based on the generation of femtosecond pulses by a sideband-seeded soft X-Ray SASE FEL

In this paper, we describe the extention of the soft X-ray SASE FEL facility for two-color time-resolved experiments. A new type of pump–probe technique is proposed which is based on the use of soft X-ray SASE FEL combined with an fs quantum laser. An ultrashort laser pulse is used for modulation of the energy and density of the electrons within a slice of the electron bunch at a frequency ω opt. The density modulation exiting the modulator (energy-modulation undulator and dispersion section) is about 10%. Following the modulator the beam enters an X-ray SASE FEL undulator, and is bunched at a frequency ω 0. This leads to an amplitude modulation of the beam density at the sidebands ω 0± ω opt. The sideband density modulation takes place at the part of the electron pulse defined by the duration of the seed laser pulse that is much shorter than the electron pulse. Following the SASE FEL undulator the beam and SASE radiation enter undulator section (radiator) which is resonant at the frequency ω 0− ω opt. Because the beam has a large component of bunching at the sideband, coherent emission is copiously produced within fs slice of the electron bunch. Separation of the sideband frequency from the central frequency by a time-compensated monochromator is used to distinguish the fs pulses from the sub-ps intense SASE pulses. Finally, the fs soft X-ray pulses are combined with optical pulses generated in the seed laser system. In the proposed scheme for pump–probe experiments, fs soft X-ray pulse is naturally synchronized with his fs optical pulse and cancel jitter.

Generation of high power femtosecond pulses by a sideband-seeded X-ray FEL

New proposal of a fs X-ray facility, which is described in this paper, is based on the use of X-ray SASE FEL combined with a fs quantum laser. An ultrashort laser pulse is used for modulation of the energy and density of the electrons within a slice of the electron bunch at a frequency ω opt. The density modulation exiting the modulator (energy-modulation undulator and dispersion section) is about 10%. Following the modulator the beam enters an X-ray SASE FEL undulator, and is bunched at a frequency ω 0. This leads to an amplitude modulation of the beam density at the sidebands ω 0± ω opt. The sideband density modulation takes place at the part of the electron pulse defined by the duration of the seed laser pulse that is much shorter than the electron pulse. Following the SASE FEL undulator the beam and SASE radiation enter undulator section (radiator) which is resonant at the frequency ω 0− ω opt. Because the beam has a large component of bunching at the sideband, coherent emission is copiously produced within fs slice of the electron bunch. Separation of the sideband frequency from the central frequency by a monochromator is used to distinguish the fs pulses from the sub-ps intense SASE pulses.

Laser driven attosecond SASE X-ray FEL

Modern lasers with emission wavelengths ∼1 μm can be used as electromagnetic wigglers and have sufficient output power to obtain wiggler parameters K (normalized vector potential) close to unity. With this kind of wiggler, in order to make a 10-keV X-ray SASE FEL, electron beam energies of ∼25 MeV are sufficient. In this presentation, scaling laws for the operation of FELs will be discussed. Using these scaling laws, requirements for electron sources will be obtained. The required electron beams must be about six orders of magnitude brighter than those available with existing sources. We propose a novel source based on vacuum laser pondermotive acceleration that will produce an electron beam meeting these requirements. This opens up a realistic possibility of realizing the dream of a “table top” X-ray SASE FEL.

Development of a femtosecond soft X-ray SASE FEL at DESY

In this paper we describe the extension of the soft X-ray SASE FEL at the TESLA Test Facility (TTF) at DESY for generation of femtosecond pulses. The proposed scheme operates as follows. The first stage is a conventional FEL amplifier seeded by 523 nm external laser. A zero area optical pulse (i.e. the pulse with zero value of optical field in the central area of the pulse) is timed to overlap with the electron bunch. Radiation power is amplified up to the saturation level. Following the first stage the electron beam enters the main 6 nm SASE undulator. Large energy spread is induced in the significant fraction of the electron beam due to the FEL interaction process, and only a small part of the electron bunch (near the center of zero area light pulse) is able to produce radiation in the 6 nm SASE FEL. The SASE FEL described in this paper will provide soft X-ray pulses with 30 fs (FWHM) duration. On the basis of the TTF parameters it should be possible to achieve an average brilliance of 10 22 photons/ s 1/ mrad 2/ mm 2/ (0.1% BW). The average number of photons can exceed 10 12 photon/pulse.

Design considerations for a SASE X-ray FEL

The well developed theory of short wavelength SASE-FELs is now being used to design two X-ray lasers, LCLS and Tesla-FEL. However, the physics and technology of these projects present some unique challenges, related to the very high peak current of the electron beam, the very long undulator needed to reach saturation, and the importance of preserving the beam phase-space density even in the presence of large wake-field effects. In the first part of this paper, we review the basic elements of the theory, the scaling laws for an X-ray SASE-FEL, and the status of the experimental verification of the theory. We then discuss some of the most important issues for the design of these systems, including wake-field effects in the undulator, and the choice of undulator type and beam parameters.

Optimization of an X-ray SASE-FEL

The most important characteristics of an X-ray SASE-FEL are determined by the electron beam energy, transverse and longitudinal emittance, and by choice of the undulator period, field, and gap. Among them are the gain and saturation length, the amount and spectral characteristics of the spontaneous radiation, the wake fields due to the vacuum pipe. The spontaneous radiation intensity is very large in all X-ray SASE-FELs now being designed, and it contributes to the final electron beam energy spread, thus affecting the gain. It also produces a large background for the beam and radiation diagnostics instrumentation. The wake fields due to the resistivity and roughness of the beam pipe through the undulator, also affects the beam 6-dimensional phase space volume, and thus the gain and the line width. In this paper, we discuss ways to optimize the FEL when considering all these effects. In particular we consider and discuss the use of a hybrid iron-permanent magnet helical undulator to minimize some of these effects, and thus optimize the FEL design.

Development of a pump-probe facility with sub-picosecond time resolution combining a high-power ultraviolet regenerative FEL amplifier and a soft X-ray SASE FEL

This paper presents the conceptual design of a high power radiation source with laser-like characteristics in the ultraviolet spectral range at the TESLA Test Facility (TTF). The concept is based on the generation of radiation in a regenerative FEL amplifier (RAFEL). The RAFEL described in this paper covers a wavelength range of 200–400 nm and provides 200 fs pulses with 2 mJ of optical energy per pulse. The linac operates at 1% duty factor and the average output radiation power exceeds 100 W. The RAFEL will be driven by the spent electron beam leaving the soft X-ray FEL, thus providing minimal interference between these two devices. The RAFEL output radiation has the same time structure as the X-ray FEL and the UV pulses are naturally synchronized with the soft X-ray pulses from the TTF FEL. Therefore, it should be possible to achieve synchronization close to the duration of the radiation pulses (200 fs) for pump-probe techniques using either an UV pulse as a pump and soft X-ray pulse as a probe, or vice versa.

High power femtosecond pulses from an X-ray SASE-FEL

We discuss how to use the large-gain bandwidth of an X-ray SASE-FEL to produce femtosecond long pulses by chirping and compressing the output FEL radiation. We consider the power level, spectral width, and intensity fluctuations of the compressed X-ray pulses, compared to the case with no compression.

Study of electron beam with modulated energy distribution for optical klystron FELs

The development of an intense and coherent light source is expected in the X-ray region, because of its many applications in this region. But it is difficult to obtain such a short wavelength with ordinary FEL using the resonator and conventional laser. SASE FEL without the resonator is a promising scheme due to the improvement of accelerator technology. The high power FEL in the 2—4 nm range has already been proposed at SLAC [1,2]. The quasi-stable optical klystron [3,4] can reduce the wiggler length significantly for a soft x-ray SASE FEL. According to the numerical study the gain curve of the quasi-stable optical klystron has multiple gain peaks with narrow width and high gain. Fig. 1a shows the gain curve of the quasi-stable operation of the optical klystron and modulated energy distribution of the electron beam coupling to the positive gain peaks. Fig. 1b represents the gain curve of the uniform wiggler. When the energy spread of an electron beam is larger than a gain peak, the FEL gain becomes very low due to the negative gain of the gain function. But if the distribution of electron energy is modulated and its modulation peaks couple to the positive peaks of the optical klystron gain function, we can obtain the high FEL gain although the total energy spread of the electron beam is large. With the latest technology of laser pulse shaping, it is possible to produce femtosecond pulses [5,6]. This pulse shaping apparatus called liquid crystal spatial light modulator (LC SLM). Two LC SLMs are combined between a pair of polarizes. When the LC modulator is operated by applying a voltage, the liquid crystals rotate towards the direction of propagation of light and produce the modified pulses. With this apparatus, the user can specify the waveform with arbitrary profiles. When the modulated laser that is obtained in this way irradiates the photocathode, the modulated electrons are extracted with the same temporal profile, because the response time of metal photocathode is of femtosecond order. This is a very important property of our concept. This is the advanced operation of quasi-stable optical klystron with energymodulated electron beam. The electrons extracted with temporal modulation are accelerated with an RF gun. Then these electrons are accelerated in different RF phases.

Possible application of X-ray optical elements for reducing the spectral bandwidth of an X-ray SASE FEL

A new design for a single pass X-ray SASE FEL is proposed. The scheme consists of two undulators and an X-ray monochromator located between them. The first stage of the FEL amplifier operates in the SASE linear regime. After the exit of the first undulator the electron bunch is guided through a non-isochronous bypass and the X-ray beam enters the monochromator. The main function of the bypass is to suppress the modulation of the electron beam induced in the first undulator. This is possible because of the finite value of the natural energy spread in the beam. At the entrance to the second undulator the radiation power from the monochromator dominates significantly over the shot noise and the residual electron bunching. As a result, the second stage of the FEL amplifier operates in the steady-state regime. The proposed scheme is illustrated for the example of the 6 nm option SASE FEL at the TESLA Test Facility under construction at DESY. The spectral bandwidth of such a two-stage SASE FEL ( Δλ λ ⋍ 5 × 10 −5 ) is close to the limit defined by the finite duration of the radiation pulse. The average spectral brilliance is equal to 2 × 10 24 photons/(sec×mrad 2×mm 2×0.1% bandwidth) which is by two orders of magnitude higher than the value which could be reached by the conventional SASE FEL.