Pump-probe experiments combining pulses from a X-ray FEL and an optical femtosecond laser are very attractive for sub-picosecond time-resolved studies. Since the synchronization between the two independent 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 a tunable high power femtosecond quantum laser. In the proposed scheme a GW-level soft X-ray pulse is naturally synchronized with a GW-level optical pulse, independent of any jitter in the arrival time of the electron bunches. The concept is based on the generation of optical radiation in a master oscillator-power FEL amplifier (MOPA) configuration. X-ray radiation is generated in an X-ray undulator inserted between the modulator and radiator sections of the optical MOPA scheme. An attractive feature of the FEL amplifier scheme is the absence of any apparent limitations which could prevent operation in the femtosecond regime in a wide (200-900 nm) wavelength range. A commercially available long (nanosecond) pulse dye laser can be used as seed laser.
A model is presented which demonstrates that the attosecond pulse structure of a High Harmonic Generation (HHG) seed may be retained through to saturation in an FEL amplifier. At wavelengths of ~12nm a train of attosecond pulses of widths ~300 attoseconds with peak powers in excess of 1 GW are predicted from full 3D simulation. Methods for improving these results are discussed.
The problem of an efficiency increase of an FEL amplifier is now of great practical importance. Technique of undulator tapering in the post-saturation regime is used at the existing x-ray FELs LCLS and SACLA, and is planned for use at the European XFEL, Swiss FEL, and PAL XFEL. There are also discussions on the future of high peak and average power FELs for scientific and industrial applications. In this paper we perform detailed analysis of the tapering strategies for high power seeded FEL amplifiers. Application of similarity techniques allows us to derive universal law of the undulator tape…
The simplest high gain free electron laser (FEL) amplifier concept is proposed. A klystron amplifier has the useful property that the various electronic processes take place in separate portions of the amplifier, rather than overlapping as in FEL amplifier with an uniform undulator. The klystron consists of two fundamental parts: succession of 2-3 cascades (modulator), and an output undulator (radiator) in which the modulated electron beam coherently radiates. Each cascade consists of uniform undulator and dispersion section. Unlike distributed optical klystrons, we have a high gain per cascad…
Since spring, 2005 vacuum ultraviolet free electron laser (VUV FEL) at DESY operates as user facility in the wavelength range around 30 nm. Electron beam formation system at the VUV FEL is essentially nonlinear and naturally results in a formation of a short high-current leading peak in the density distribution that produces FEL radiation. The main feature of the femtosecond mode of operation of the VUV FEL is the production of short, down to 20 fs radiation pulses with GW-level peak power. This paper is devoted to detailed studies of the short-pulse effects in the VUV FEL at DESY operating in…
We propose and analyze a novel regenerative amplifier free electron laser (FEL) to produce fully coherent 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 not only amplifies the radiation intensity but also…
A method is proposed to generate trains of few-cycle x-ray pulses from a Free-Electron Laser (FEL) amplifier via a compact 'afterburner' extension consisting of several few-period undulator sections separated by electron chicane delays. Simulations show that in the hard x-ray (wavelength ~0.1 nm; photon energy ~10 keV) and with peak powers approaching normal FEL saturation (GW) levels, root mean square pulse durations of 700 zeptoseconds may be obtained. This is approximately two orders of magnitude shorter than that possible for normal FEL amplifier operation. The spectrum is discretely multi…
Arbitrary manipulation of the temporal and spectral properties of X-ray pulses at free-electron lasers (FELs) would revolutionize many experimental applications. At the Linac Coherent Light Source at Stanford National Accelerator Laboratory, the momentum phase-space of the FEL driving electron bunch can be tuned to emit a pair of X-ray pulses with independently variable photon energy and femtosecond delay. However, while accelerator parameters can easily be adjusted to tune the electron bunch phase-space, the final impact of these actuators on the X-ray pulse cannot be predicted with sufficien…
X-ray free-electron laser oscillator (XFELO) is expected to be a cutting edge tool for fully coherent X-ray laser generation, and undulator taper technique is well-known for considerably increasing the efficiency of free-electron lasers (FELs). In order to combine the advantages of these two schemes, FEL amplifier seeded by XFELO is proposed by simply using a chirped electron beam. With the right choice of the beam parameters, the bunch tail is within the gain bandwidth of XFELO, and lase to saturation, which will be served as a seeding for further amplification. Meanwhile, the bunch head whic…
By utilizing higher harmonics of undulator radiation, harmonic lasing is helpful in the development of compact x-ray free electron lasers (FELs), i.e. reducing its costs and sizes. Harmonic lasing of FELs have been experimentally demonstrated in the low-gain FEL oscillators from terahertz, infrared to ultraviolet spectral range. Based on the current status and future directions of short-wavelength FELs in the worldwide, this paper reviews the progresses on harmonic lasing of x-ray FELs, mainly concentrating on the recently proposed harmonic lasing of x-ray FEL oscillators and further ideas on …
Initial studies of a 2-colour FEL amplifier using one monoenergetic electron beam are presented. The interaction is modelled using the unaveraged, broadband FEL code Puffin. A series of undulator modules are tuned to generate two resonant frequencies along the FEL interaction and a self-consistent 2-colour FEL interaction at widely spaced non-harmonic wavelengths at 1nm and 2.4nm is demonstrated.
A technique is proposed to generate attosecond pulse trains of radiation from a Free-Electron Laser amplifier. The optics-free technique synthesises a comb of longitudinal modes by applying a series of spatio-temporal shifts between the co-propagating radiation and electron bunch in the FEL. The modes may be phase-locked by modulating the electron beam energy at the mode spacing frequency. Three-dimensional simulations demonstrate the generation of a train of 400as pulses at giga-watt power levels evenly spaced by 2.5fs at a wavelength of 124 Angstrom. In the X-ray at wavelength 1.5 Angstrom, …
Free-electron lasers (FELs) have been built ranging in wavelength from long-wavelength oscillators using partial wave guiding through ultraviolet through hard x-ray FELs that are either seeded or start from noise (SASE). Operation in the x-ray spectrum has relied on single-pass SASE due either to the lack of seed lasers or difficulties in the design of x-ray mirrors. However, recent developments in the production of diamond crystal Bragg reflectors point the way to the design of regenerative amplifiers (RAFELs) which are, essentially, low-Q x-ray free-electron laser oscillators (XFELOs) that o…
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 w…
As a new-generation light source, free-electron lasers (FELs) provide high-brightness X-ray pulses at the angstrom-femtosecond space and time scales. The fundamental physics behind the FEL is the interaction between an electromagnetic wave and a relativistic electron beam in an undulator, which consists of hundreds or thousands of dipole magnets with an alternating magnetic field. Here, we report the first observation of the laser-beam interaction in a pure dipole magnet, in which the electron beam energy modulation with 40-keV amplitude and 266-nm period is measured. We demonstrate that such …
We propose a scheme for generation of single 100 GW 300-as pulse in the X-ray free electron laser with the use of a few cycles optical pulse from Ti:sapphire laser system. Femtosecond optical pulse interacts with the electron beam in the 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 first part of the baseline gap-adjustable X-ray undulator and produces SASE radiation with 100 MW-level power. Due to energy modulation the frequency is correlated to the longitudina…
Recent initiatives in ultra-short, GeV electron beam generation have focused on achieving sub-fs pulses for driving X-ray free-electron lasers (FELs) in single-spike mode. This scheme employs very low charge beams, which may allow existing FEL injectors to produce few-100 as pulses, with high brightness. Towards this end, recent experiments at SLAC have produced ~2 fs rms, low transverse emittance, 20 pC electron pulses. Here we examine use of such pulses to excite plasma wakefields exceeding 1 TV/m. We present a focusing scheme capable of producing ~200 nm beam sizes, where the surface Coulom…
The high peak current required in free-electron lasers (FELs) is realized by longitudinal compression of the electron bunches to sub-picosecond length. In this paper, a frequency-domain diagnostic method is described that is capable of resolving structures in the femtosecond regime. A novel in-vacuum spectrometer has been developed for spectroscopy of coherent radiation in the THz and infrared range. The spectrometer is equipped with five consecutive dispersion gratings and 120 parallel readout channels; it can be operated either in short wavelength mode (5 - 44 um) or in long wavelength mode …
Studies of a broad bandwidth, two-colour FEL amplifier using one monoenergetic electron beam are presented. The two-colour FEL interaction is achieved using a series of undulator modules alternately tuned to two well-separated resonant frequencies. Using the broad bandwidth FEL simulation code Puffin, the electron beam is shown to bunch strongly and simultaneously at the two resonant frequencies. Electron bunching components are also generated at the sum and difference of the resonant frequencies.
This paper discusses the potential for enhancing the LCLS hard X-ray FEL capabilities. In the hard X-ray regime, a high longitudinal coherence will be the key to such performance upgrade. The method considered here to obtain high longitudinal coherence is based on a novel single-bunch self-seeding scheme exploiting a single crystal monochromator, which is extremely compact and can be straightforwardly installed in the LCLS baseline undulator. We present simulation results dealing with the LCLS hard X-ray FEL, and show that this method can produce fully-coherent X-ray pulses at 100 GW power lev…
We present the response time measurements of a Cs2Te photocathode illuminated with two 100 fs duration, variable time separation laser pulses at 266 nm wavelength. The response time was confirmed in dispersive region downstream of a 12-cell standing wave S-band acceleration structure using a well-known RF zero-crossing technique. At the same time it was also measured by changing mechanical path-length difference between two micro-bunches. Both methods agree that Cs2Te photocathode time response is of the order of 250 fs and thereby it is possible to generate and control a THz sequence of relat…
Longitudinal space charge (LSC) driven microbunching instability in electron beam formation systems of X-ray FELs is a recently discovered effect hampering beam instrumentation and FEL operation. The instability was observed in different facilities in infrared and visible wavelength ranges. In this paper we propose to use such an instability for generation of VUV and X-ray radiation. A typical longitudinal space charge amplifier (LSCA) consists of few amplification cascades (drift space plus chicane) with a short undulator behind the last cascade. If the amplifier starts up from the shot noise…
Free-electron lasers (FELs) have been designed to operate over virtually the entire electromagnetic spectrum from microwaves through x-rays and in a variety of configurations including amplifiers and oscillators. Oscillators can operate in both the low and high gain regime and are typically used to improve the spatial and temporal coherence of the light generated. We will discuss various FEL oscillators ranging from systems with high-quality resonators combined with low-gain undulators to systems with a low-quality resonator combined with a high-gain undulator line. The FEL gain code MINERVA a…
Self-seeding options for the LCLS baseline were recently investigated using a scheme which relies on a single-crystal monochromator in Bragg-transmission geometry. The LCLS low-charge (0.02 nC) mode of operation was considered in order to demonstrate the feasibility of the proposed scheme. The wakefield effects from the linac and from the undulator vacuum chamber are much reduced at such low charge, and can be ignored. In this paper we extend our previous investigations to the case of the LCLS mode of operation with nominal charge. Based on the LCLS start-to-end simulation for an electron beam…
Electron beam quality is paramount for X-ray pulse production in free-electron-lasers (FELs). State-of-the-art linear accelerators (linacs) can deliver multi-GeV electron beams with sufficient quality for hard X-ray-FELs, albeit requiring km-scale setups, whereas plasma-based accelerators can produce multi-GeV electron beams on metre-scale distances, and begin to reach beam qualities sufficient for EUV FELs. We show, that electron beams from plasma photocathodes many orders of magnitude brighter than state-of-the-art can be generated in plasma wakefield accelerators (PWFA), and then extracted,…
Modern x-ray free-electron lasers (XFELs) produce x-ray pulses of exceptional transverse coherence. This is due largely to the process of optical guiding by which the radiation is both refractively guided by the bunched electron beam and gain guided by the preferential amplification of on-axis radiation. These effects may be summarized by an effective index of refraction, which has been used in the past to study the transverse dynamics of the FEL process with significant simplifications and approximations, but never fully self-consistently. We present here a self-consistent method for studying…
The spectroscopic techniques for time-resolved fine analysis of matter require coherent X-ray radiation with femtosecond duration and high average brightness. Seeded free-electron lasers (FELs), which use the frequency up-conversion of an external seed laser to improve temporal coherence, are ideal for providing fully coherent soft X-ray pulses. However, it is difficult to operate seeded FELs at a high repetition rate due to the limitations of present state-of-the-art laser systems. Here, we report the novel self-modulation method for enhancing laser-induced energy modulation, thereby signific…
The laser invention more than fifty years ago was a major scientific revolution. Among the different possible gain media, the Free Electron Lasers (FEL) uses free electrons in the periodic permanent magnetic field of an undulator, covering wavelengths from far infrared to X-ray, with easy tuneability and high peak power. Nowadays, the advent of tuneable intense (mJ level) short pulse FELs with record peak power (GW level) in the X-ray domain sets a major step in laser development, and enables to explore new scientific areas, such as deciphering molecular reactions in real time, understanding f…
Self-seeding schemes, consisting of two undulators with a monochromator in between, aim to reduce the bandwidth of SASE X-ray FELs. We recently proposed a new method of monochromatization exploiting a single crystal in Bragg-transmission geometry for self-seeding in the hard X-ray range. A straightforward extension is to use such kind of monochromator setup with two -or more- crystals arranged in a series to spectrally filter the SASE radiation at two closely-spaced wavelengths within the FEL gain band. This allows for the production of doublet -or multiplet- spectral lines. Applications invol…
The growing requirements of pump-probe techniques and nonlinear optics experiments greatly promote the studies of two-color free-electron lasers (FELs). We propose a new method to generate coherent two-color pulses in a high-gain harmonic generation (HGHG) FEL. In this scheme, an initial tilted electron beam is sent though the modulator and dispersive section of an HGHG FEL to generate the bunching at harmonics of the seed laser. Then a transverse gradient undulator (TGU) is adopted as the radiator and in such radiator, only two separated fractions of the tilted beam will resonate at two adjac…
Free-electron lasers (FELs) have been built ranging in wavelength from long-wavelength oscillators using partial wave guiding through ultraviolet through hard x-ray that are either seeded or start from noise (SASE). In addition, FELs that produce different polarizations of the output radiation ranging from linear through elliptic to circular polarization are currently under study. In this paper, we develop a three-dimensional, time-dependent formulation that is capable of modeling this large variety of FEL configurations including different polarizations. We employ a modal expansion for the op…
X-ray free-electron lasers (FEL) deliver ultrabright X-ray pulses, but not the sequences of phase-coherent pulses required for time-domain interferometry and control of quantum states. For conventional split-and-delay schemes to produce such sequences the challenge stems from extreme stability requirements when splitting Angstrom wavelength beams where tiniest path length differences introduce phase jitter. We describe an FEL mode based on selective electron bunch degradation and transverse beam shaping in the accelerator, combined with a self-seeded photon emission scheme. Instead of splittin…
The SASE3 soft X-ray beamline at the European XFEL has been designed and built to provide experiments with pink or monochromatic beam in the photon energy range 250 eV - 3000 eV. Here, we focus on the monochromatic operation of the SASE3 beamline and report on design and performance of the SASE3 grating monochromator. The unique capability of an FEL source to produce short femtosecond pulses of high degree of coherence challenges the monochromator design by a demand to control both photon energy and temporal resolution. The aim to transport close to transform-limited pulses poses very high dem…
Accelerator-based X-ray free-electron lasers (XFELs) are the latest addition to the revolutionary tools of discovery for the 21st century. The two major components of an XFEL are an accelerator-produced electron beam and a magnetic undulator which tend to be kilometer-scale long and expensive. Here, we present an ultra-compact scheme to produce 10s of attosecond X-ray pulses with several GW peak power utilizing a novel aspect of the FEL instability using a highly chirped, pre-bunched and ultra-bright electron beam from a plasma-based accelerator interacting with an optical undulator. The self-…
In this paper we describe a measurement technique capable of resolving femtosecond X-ray pulses from XFEL facilities. Since these ultrashort pulses are themselves the shortest event available, our measurement strategy is to let the X-ray pulse sample itself. Our method relies on the application of a "fresh" bunch technique, which allows for the production of a seeded X-ray pulse with a variable delay between seed and electron bunch. The shot-to-shot averaged energy per pulse is recorded. It turns out that one actually measures the autocorrelation function of the X-ray pulse, which is related i…
In this paper we propose a new method for measurements of the longitudinal profile of 100 femtosecond electron bunches for X-ray Free Electron Lasers (XFELs). The method is simply the combination of two well-known techniques, which where not previously combined to our knowledge. We use seed 10-ps 1047 nm quantum laser to produce exact optical replica of ultrafast electron bunches. The replica is generated in apparatus which consists of an input undulator (energy modulator), and the short output undulator (radiator) separated by a dispersion section. The radiation in the output undulator is exc…
Mutually phase-stable tunable attosecond soft X-ray attosecond pulses from a free-electron laser
SLAC National Accelerator Laboratory（SLAC国家加速器实验室）；Stanford PULSE Institute, SLAC National Accelerator Laboratory（斯坦福PULSE研究所，SLAC国家加速器实验室）；Department of Applied Physics, Stanford University（斯坦福大学应用物理系）；J.R. Macdonald Laboratory, Kansas State University（堪萨斯州立大学J.R.麦克唐纳实验室）
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 radiati…
