High-gain FEL Research Articles

A Short Note on Application of Unconventional Modes in X-ray Free-Electron Lasers with Spatially Varying Gain and Loss in Production of Renewable Energy

Recently, the field of unconventional lasers has been drawing the attention of various researchers mainly from the point of view of exploring the possibilities of newer applications. One Emerging area in their use is the field of Renewable energy. In this paper, we suggest the use of X-ray free-electron laser (XFEL) in production of Renewable energy. This is because of the fact that it is comparatively easier to assemble and capable of providing the required power for Renewable energy and even other forms of conventional energy. For the Optimization of their application, this paper presents the Theoretical Analysis of X-ray free-electron lasers in detail. The higher- order effects and the high-gain FEL interaction have also been discussed. In addition, the Unconventional modes in lasers with spatially varying gain and loss have been technically analyzed. The paper should be useful for the researchers and the designers in this fascinating field.

First operation of a harmonic lasing self-seeded free electron laser

Harmonic lasing is a perspective mode of operation of X-ray FEL user facilities that allows to provide brilliant beams of higher energy photons for user experiments. Another useful application of harmonic lasing is so called Harmonic Lasing Self-Seeded Free Electron Laser (HLSS FEL) that allows to improve spectral brightness of these facilities. In the past, harmonic lasing has been demonstrated in the FEL oscillators in infrared and visible wavelength ranges, but not in high-gain FELs and not at short wavelengths. In this paper we report on the first evidence of the harmonic lasing and the first operation of the HLSS FEL at the soft X-ray FEL user facility FLASH in the wavelength range between 4.5 nm and 15 nm. Spectral brightness was improved in comparison with Self-Amplified Spontaneous emission (SASE) FEL by a factor of six in the exponential gain regime. A better performance of HLSS FEL with respect to SASE FEL in the post-saturation regime with a tapered undulator was observed as well. The first demonstration of harmonic lasing in a high-gain FEL and at short wavelengths paves the way for a variety of applications of this new operation mode in X-ray FELs.

High-gain, short wavelength FEL in the Raman regime

While FEL technology has reached the EUV and X-ray regime at existing machines such as LCLS and SACLA, the scale of these projects is often impractical for university research and industrial applications. Sub-millimeter period undulators can reduce the size of a high-gain EUV FEL, but will impose stringent conditions on the electron beam. In particular, a high-gain EUV FEL based on undulators with a sub-mm period [1] requires electron beam currents upwards of 1kA at energies below 100MeV. Coupled with the small gap of such undulators and their low undulator strengths, K<0.1, such beam parameters bring longitudinal space-charge effects to the foreground of the FEL process. When the characteristic length associated with electron beam plasma behaviour becomes comparable to the gain-length, the relevant theoretical FEL model transitions from the Compton to the Raman limit [2], where collective field effects dominate. In this work, we investigate the behaviour of the FEL's gain-length and efficiency in these two limits. The starting point for the analysis is the one-dimensional FEL theory including space-charge forces. The derived results is compared to numerical results of Genesis 1.3 simulations. This theoretical model predicts that in the Raman limit, the gain-length scales as the beam current to the one-fourth power while the efficiency grows as the square root of the beam current. This advantage in efficiency may prove critical in the development of very compact short wavelength FELs, such as the Keck Foundation-funded SAMURAI FEL at UCLA.

Crossed undulator polarization control for X-ray FELs in the saturation regime

The crossed-planar undulator is a promising scheme for full and rapid control of X-ray polarization in a high-gain FEL. In this paper, we study the effectiveness of a crossed undulator for SASE FELs in the saturation regime in order to achieve stable operation. Statistical analysis is performed to show that the degree of polarization remains high in the saturation regime, and controllable polarization at the third harmonic of the FEL wavelength is obtained by adjusting the strength of the vertical undulator. We also propose a modified scheme for polarization control using a short pair of crossed undulators tuned to the second harmonic of the main undulator. The scheme is straightforward to implement and can reach high degree of polarization with high stability.

Steady-state analysis of short-wavelength, high-gain FELs in a large storage ring

Storage ring FELs have operated successfully in the low-gain regime using optical cavities. Discussions of a high-gain FEL in a storage ring typically involve a special bypass to decouple the FEL interaction from the storage ring dynamics. In this paper, we investigate the coupled dynamics of a high-gain FEL in a large storage ring such as PEP and analyze the equilibrium solution. We show that an FEL in the EUV and soft X-ray regimes can be integrated into a very bright storage ring and potentially provides three orders of magnitude improvement in the average brightness at these radiation wavelengths. We also discuss possibilities of seeding with HHG sources to obtain ultra-short, high-peak power EUV and soft X-ray pulses.

A virtual dielectric eigenmode expansion of high-gain FEL radiation for study of paraxial wave mode coupling

An expansion of guided free-electron laser radiation is presented in terms of eigenmodes of a transversely varying dielectric waveguide. For a particular quadratic distribution of the refractive index, the constituent eigenmode expansion basis is equivalent to a free-space paraxial basis evaluated at the mode waist, which facilitates investigations of guided coupling to naturally diffracting Gaussian modes over many Rayleigh lengths. A set of coupled differential equations is derived for the slowly varying field mode amplitude coefficients and the longitudinal electron density bunching in the cold-beam fluid approximation. Preliminary results calculated in this formulation show that selective amplification of dominant higher-order modes, particularly azimuthal modes, can be achieved by seeding with higher-order spatial modes and by selective density pre-bunching of the electron beam.

STARS: A Cascaded High-Gain Harmonic-Generation FEL

Femtosecond pulses from linac-based free-electron lasers are unique tools for future time-resolved experiments. In March 2004, BESSY published the Technical Design Report (TDR) for a free-electron-laser user facility that covers the VUV to soft X-ray range (BESSY FEL) [1]. This second-generation FEL facility is seeded and uses the High-Gain Harmonic-Generation (HGHG) [2] scheme to produce coherent radiation down to the 1 nm range.

Quantum self-amplified spontaneous emission regime of the x-ray Compton laser

In this work we consider in general the high-gain SASE x-ray Compton FEL in the quantum regime of amplification arising from the self-consistent set of the Maxwell and relativistic quantum kinetic equations. The analytical solutions are obtained which are valid before the saturation. The quantum seed-shot noise and subsequent SASE power are calculated. A considerable increase in start-up and narrowing of the spectrum of the SASE power for amplifying x-ray frequencies is shown compared with the classical SASE regime.

The SPARC/X SASE-FEL Projects

SPARC and SPARX are two different initiatives toward an ItalianFreeElectronLaser (FEL) source operating in theSelfAmplifiedSpontaneousEmission (SASE) mode, in which several national research institutions are involved. SPARC is a high gain FEL project devoted to provide a source of visible and VUV radiation while exploiting the SASE mechanism. An advanced Photo-Injector system, emittance compensating RF-gun plus a 150 MeV Linac, will inject a high quality e-beam into the undulator to generate high brilliance FEL radiation in the visible region at the fundamental wavelength, (∼500 nm). The production of flat top drive laser beams, high peak current bunches, and emittance compensation scheme will be investigated together with the generation of higher harmonic radiation in the VUV region. SPARX is the direct evolution of such a high gain SASE FEL toward the 13.5 and 1.5 nm operating wavelengths, at 2.5 GeV. To get the required value for the bunch peak current, Ipeak≈ 2.5 kA, the “hybrid” scheme, RF-compression stage plus magnetic chicane, is analyzed and compared with the more standard double stage of magnetic compression. The two options are reviewed considering the tolerance to the drive laser pulse phase jitter.

Using VUV high-order harmonics generated in gas as a seed for single pass FEL

In this paper we illustrate the main features of a new scheme which aims at obtaining coherent radiation at very short wavelengths (down to 1.5 nm) by using the Harmonics of an intense laser, generated in a gas, as a seed to inject in a long undulator, either in the pure SASE or in the HGHG configuration. The unique spatial and temporal coherence properties of the High-Order Harmonics are then conserved, and the High Gain FEL amplifier system saturates up to the GW level. An experiment is here proposed, to be performed on the new compact LINAC-based source SCSS, currently under development in SPring-8.

Free-electron lasers as pumps for high-energy solid-state lasers

High average-power free-electron lasers may be useful for pumping high peak-power solid-state laser-amplifiers. At very high peak-powers, the pump source for solid-state lasers is non-trivial: flash lamps produce thermal problems and are unsuitable for materials with short florescence-times, while diodes can be expensive and are only available at select wavelengths. FELs can provide pulse trains of light tuned to a laser material's absorption peak, and florescence lifetime. An FEL pump can thus minimize thermal effects and potentially allow for new laser materials to be used. This paper examines the design of a high average-power, efficient high-gain FEL for use as pump source. Specifically, the cases of a 100 J class pump, and a 100 TW-class laser at a planned fourth-generation light-source are considered.

Saturation of a high-gain single-pass FEL

We study a perturbation expansion for the solution of the nonlinear one-dimensional FEL equations. We show that in the case of a monochromatic wave, the radiated intensity satisfies a scaling relation that implies, for large distance z traveled along the undulator, a change in initial value of the radiation field corresponds to a translation in z (lethargy). Analytic continuation using Padé approximates yields accurate results for the radiation field early in saturation.

Saturation of a high-gain single-pass FEL

We study a perturbation expansion for the solution of the nonlinear one-dimensional FEL equations. We show that in the case of a monochromatic wave, the radiated intensity satisfies a scaling relation that implies, for large distance z traveled along the undulator , a change in initial value of the radiation field corresponds to a translation in z (lethargy). Analytic continuation using Padé approximates yields accurate results for the radiation field early in saturation.

Free-electron lasers as pumps for high-energy solid-state lasers

High average-power free-electron lasers may be useful for pumping high peak-power solid-state laser-amplifiers. At very high peak-powers, the pump source for solid-state lasers is non-trivial: flash lamps produce thermal problems and are unsuitable for materials with short florescence-times, while diodes can be expensive and are only available at select wavelengths. FELs can provide pulse trains of light tuned to a laser material's absorption peak, and florescence lifetime. An FEL pump can thus minimize thermal effects and potentially allow for new laser materials to be used. This paper examines the design of a high average-power, efficient high-gain FEL for use as pump source. Specifically, the cases of a 100 J class pump, and a 100 TW-class laser at a planned fourth-generation light-source are considered.

Possibility of a high-power, high-gain FEL amplifier

High-gain FEL amplifiers offer many unique advantages such as robust operation without a high-Q optical cavity and potentially high extraction efficiencies with the use of tapered wigglers. Although a high average power, cw FEL amplifier has not been demonstrated, many key physics issues such as electron beam brightness requirements, single-pass gain, saturation, etc. have been resolved. In this paper, we study the feasibility of a high-power FEL based on the high-gain, tapered-wiggler amplifier. We show that with suitable electron beam parameters, a peak output power of 1GW and a micropulse energy of 8mJ can be achieved. We also outline a possible configuration of a high-power, high-gain FEL amplifier with energy recovery.

High-gain FEL on the coherent Bremsstrahlung of a relativistic electron beam in a crystal

In this paper a possible way to achieve lasing in the X-ray domain due to coherent Bremsstrahlung of an electron beam in the crystal is considered. The high-gain regime is investigated when charged particles move close to the crystal lattice plane or axis. The consideration is based on the self-consistent set of the Maxwell–Vlasov equations. We illustrate the resulting formulae with some speculative examples.

Results of the VISA SASE FEL experiment at 840 nm

VISA (Visible to Infrared SASE Amplifier) is a high-gain self-amplified spontaneous emission FEL, which achieved saturation at 840nm within a single-pass 4-m undulator. A gain length shorter than 18cm has been obtained, yielding the gain of 2×108 at saturation. The FEL performance, including spectral, angular, and statistical properties of SASE radiation, has been characterized for different electron beam conditions. The results are compared to 3-D SASE FEL theory and start-to-end numerical simulations of the entire injector, transport, and FEL system. Detailed agreement between simulations and experimental results is obtained over the wide range of the electron beam parameters.

SPARC project and SPARX proposal

SPARC and SPARX are two distinct phases of an Italian FEL SASE project involving different national institutions. SPARC is a high gain FEL project aimed at developing a source of visible and VUV radiation exploiting SASE mechanisms. A 150MeV Linac will provide a high quality e-beam to generate high brilliance FEL radiation at visible region at the fundamental wavelength and at VUV wavelengths with the harmonics. SPARC will allow to investigate velocity bunching mechanisms in the bunch compressor and coherent harmonic generation in the undulators. SPARX is a project aimed at the realization of an FEL SASE source operating at 13.5 and 1.5nm in two separate undulators channels with a 2.5GeV electron beam. SPARX development is expected to take profit of the SPARC activities and part of the equipment. We describe here the main features of these two projects including accelerators system, FEL sections and applications.

FEL options for the proposed UK fourth generation light source (4GLS)

4GLS is a novel low-energy light source proposed as a complementary facility to the DIAMOND X-ray project for the UK, and could probably replace the present SRS at Daresbury in about 6 years time. This facility will use a combination of three separate FELs, undulators and bending magnets to provide a unique source of high-brightness continuous and pulsed radiation from the IR to XUV (∼100eV). Here, we give a brief description of the proposed FELs in the IR and VUV followed by a pre-design parameter study of the more technically challenging XUV high-gain FEL. The electron beam source for this FEL is a 600MeV superconducting energy recovery linac with peak currents of a few kA, normalised rms emittance ∼3πmmmrad and rms energy spread ∼5×10−4. Computer simulations using the 3-D FEL code GENESIS 1.3 are used to investigate a feasible undulator and beam focussing scheme, allowing estimates for achievable radiation power and saturation length to be made.

A comparison between high gain Cerenkov and undulator based FELs

In this paper we show that Cerenkov and undulator high gain FEL dynamics can be studied by means of a fully equivalent formalism. We prove that the system evolution can be treated analytically by identifying a few crucial parameters. Finally we investigate the reasons of the inherent compactness of Cerenkov devices.