Storage Ring Free Electron Laser Research Articles

Generation of superposed orbital angular momentum beams using a free-electron laser oscillator.

With wavelength tunability, free-electron lasers (FELs) are well-suited for generating orbital angular momentum (OAM) beams in a wide photon energy range. We report here the first experimental demonstration of OAM beam generation using an oscillator FEL with the tens of picosecond pulse duration. Lasing around 458 nm, we have produced the four lowest orders of superposed Laguerre-Gaussian beams using a very long FEL resonator of 53.73 m. The produced beams have good beam quality, excellent stability, and substantial average power. We have also developed a pulsed operation mode for these beams with a highly reproducible temporal structure for a range of repetition rate of 1-30 Hz. This development can be extended to short wavelengths, for example to x-rays using a future x-ray FEL oscillator. The OAM operation of such a storage-ring FEL also paves the way for the generation of OAM gamma-ray beams via inverse Compton scattering.

Effective DC acceleration of charged particles in a circular ring and its potential application

A novel scheme is proposed for realizing effective DC acceleration of charged particles in a circular ring. The key is to use an induction acceleration cell with multi-insulating gaps that allow forced short circuit using solid-state switching devices such as SiC-MOSFET or unforced short circuit using diodes. A baseline design of the induction acceleration cell is shown. A coasting beam is accelerated in the circular ring without bunching. As its possible application, a continuous-wave terahertz free-electron laser (FEL) is proposed with numerical parameters, where this effective DC acceleration compensates for energy loss of the electron beam occupying the entire isochronous storage ring due to synchrotron radiation and FEL interaction. As a result, FEL microbunching of a coasting beam can be created, resulting in a high average output power that has not been possible with existing linac-driven FELs or RF storage-ring FELs.

Ultra-Short Pulses Generation of Free Electron Laser

The major problem facing the development of civil and military laser applications lies in the attempts to obtain short pulses close to the length of the bond that connects the atoms of certain materials. In this paper, an executable program has been constructed to simulate and analyze the generation of ultra-short free electron laser pulses; it contains several parameters directing the creation of short pulses within a time duration of femtoseconds (fs). On analyzing the simulation results, it can be concluded that it is possible to generate ultra-short pulses with a duration of about 7.4 – 87.4 fs with the storage ring free-electron laser Fabry–Perot resonator with noticeably short wavelengths (11.4–190.2) for the output laser beam. KEYWORDS Homogenous broadening; SR-FEL; undulator; energy spread gain; Q- switching

Lasing below 170 nm using an oscillator FEL

The short wavelength operation of free-electron laser (FEL) oscillators is limited by the availability of high-reflectivity, thermally stable, and radiation-resistant FEL mirrors in the vacuum UV (VUV) wavelength. We report our recent work to extend the shortest lasing wavelength of the oscillator FEL to 168.6 nm using a storage ring FEL. This progress has been made possible by developing a new FEL configuration with substantially reduced undulator harmonic radiation on the FEL mirror, a thermally stable FEL optical cavity, and a new type of high-reflectivity fluoride-based multilayer coating with a protective capping layer. Using these fluoride-based mirrors, we have demonstrated storage ring FEL lasing from 168.6 to 179.7 nm with excellent beam stability. Employing this VUV FEL in Compton scattering, we have produced the first 120 MeV gamma rays at the High Intensity Gamma-ray Source (HIGS). Operating the HIGS in this new high-energy region will create many new opportunities for photonuclear physics research, in particular, the low-energy quantum chromodynamics research.

Electron-bunch lengthening on higher-harmonic oscillations in storage-ring free-electron lasers.

The influence of higher-harmonic free-electron laser (FEL) oscillations on an electron beam have been studied by measuring its bunch length at the NIJI-IV storage ring. The bunch length and the lifetime of the electron beam were measured, and were observed to have become longer owing to harmonic lasing, which is in accord with the increase of the FEL gain. It was demonstrated that the saturated FEL power could be described by the theory of bunch heating, even for the harmonic lasing. Cavity-length detuning curves were measured for the harmonic lasing, and it was found that the width of the detuning curve was proportional to a parameter that depended on the bunch length. These experimental results will be useful for developing compact resonator-type FELs by using higher harmonics in the extreme-ultraviolet and the X-ray regions.

Widely Tunable Two-Color Free-Electron Laser on a Storage Ring.

With a wide wavelength tuning range, free-electron lasers (FELs) are well suited for producing simultaneous lasing at multiple wavelengths. We present the first experimental results of a novel two-color storage ring FEL. With three undulators and a pair of dual-band mirrors, the two-color FEL can lase simultaneously in infrared (IR) around 720 nm and in ultraviolet (UV) around 360 nm. We have demonstrated independent wavelength tuning in a wide range (60 nm in IR and 24 nm in UV). We have also realized two-color harmonic operation with the UV lasing tuned to the second harmonic of the IR lasing. Furthermore, we have demonstrated good power stability with two-color lasing, and good control of the power sharing between the two colors.

Wide-Range Third Harmonic Oscillations in Free-Electron Lasers Using the Storage Ring NIJI-IV

We developed third-harmonic free-electron laser (FEL) oscillations that operated from the visible to near-infrared region. Our infrared FEL system uses the storage ring NIJI-IV, which produced the first third-harmonic storage ring FEL oscillations. By measuring the characteristics of the third-harmonic FEL oscillations, we found that behavior of the third-harmonic FEL could be explained by a one-dimensional FEL theory. We demonstrated that the higher harmonic was effective in shortening the FEL wavelength, so it will save electron-beam energy in X-ray FEL oscillators based on a cw energy-recovery linac.

Suppression of self-pulsing instabilities in free-electron lasers using delayed optical feedback

We show that the macropulse instability affecting storage ring free-electron laser (FEL) oscillators can be suppressed using a delayed optical feedback. The principle, known as coherent photon seeding, consists in reinjecting a very small part of the laser output in the laser cavity in order to create a new deterministic solution. The feedback is shown to be efficient over a large range of the detuning parameter of the FEL cavity, even with very small fractions of reinjected power ($<{10}^{\ensuremath{-}8}$ here from inside to inside the cavity). The experiments have been performed on the UVSOR-II storage ring free-electron laser.

Possibility for mode-locked operation of a femtosecond UV storage ring free-electron laser using a low-loss Fabry–Perot resonator

A storage ring free-electron laser (SR-FEL) is inherently a self-mode-locked optical system. The gain broadening due to electron energy spread affects the small signal gain in order to determine the output coupling. Here, the dependence of the small signal gain, the optimum output coupling, and pulse duration on both electron energy spread and loss of a Fabry–Perot resonator in UV SR-FEL were investigated. It was shown that the output coupling strongly affects the mode-locked pulse duration and the present picosecond pulses can be shortened to femtosecond ones using a proper low-loss resonator.

Equivalence between free-electron-laser oscillators and actively-mode-locked lasers: Detailed studies of temporal, spatiotemporal, and spectrotemporal dynamics

We show experimentally and numerically that free-electron-laser (FEL) oscillators behave in a very similar way to conventional actively-mode-locked lasers. This stems from the similar structures of their underlying Haus equations. A comparative study of the temporal evolutions of the pulse train shapes and spatiotemporal regimes is performed on a Nd:YVO${}_{4}$ laser and a storage-ring free-electron laser. Furthermore, since direct observations of time-resolved pulse shapes and spectra are more accessible on free-electron lasers, the analogy also potentially enables one to investigate mode-locked laser dynamics using existing FEL facilities.

Lasing of middle-infrared free-election lasers using the storage ring NIJI-IV

We report for the first time to our knowledge the experimental realization of a storage ring free-electron laser (FEL) in the middle-infrared (MIR) region. A technique to adjust the optical cavity using higher harmonic FELs was developed for a fundamental FEL in the MIR region. The MIR FELs were oscillated in the wavelength region of 2475 to 2673 nm, and the relative linewidth was 5×10⁻⁴. A quasi-monochromatic x-ray beam with an energy of 700 keV was generated using FEL Compton backscattering. We were able to realize a quasi-monochromatic x-ray beam, whose energy is difficult to generate even in advanced synchrotron radiation facilities.

Systematic Study on Ce:LuLiF4 as a Fast Scintillator Using Storage Ring Free-Electron Lasers

Ce:LuLiF4 is evaluated as a fast scintillator using a storage ring free-electron laser operating in the deep-ultraviolet region. The relatively flat spectral response across its absorption bands makes this material an attractive scintillator for various excitation sources. Its decay time is acceptable for ignition timing in fast-ignition, inertial confinement nuclear fusion using laser. This report is the first systematic study on Ce:LuLiF4 as a scintillator, wherein the storage ring free-electron laser is also shown as a powerful tool for material survey.

Third Harmonic Lasing in a Storage Ring Free-Electron Laser

We report the first experimental achievement of a storage ring free-electron laser (FEL) oscillation on the third harmonic in the near-infrared region. The FEL gain was evaluated as 0.029% per mA. The measured linewidth of the third-harmonic FEL was less than that of the fundamental FEL, and the measured pulse width of the third-harmonic FEL was wider than that of the fundamental FEL. A higher-harmonic FEL should be developed not only to enhance the wavelength region of the FEL but also to vary the characteristics of an FEL micropulse.

Photon Science at Accelerator-Based Light Sources

Accelerator-based light sources stimulated progress in photon science in a truly extraordinary manner. The spectral brightness of storage-ring-based facilities increased by three orders of magnitude every 10 years since the 1960s. The extreme peak brightness at single-pass free electron X-ray lasers with pulse durations variable between about 1 and 300 femtoseconds will allow transformative experiments in many areas of science. This article is an attempt to show how progress in accelerator science and technology stimulated advancement in photon science, by discussing a limited number of examples of work at third generation storage ring facilities and free electron lasers. Hopes for further improvements in specific beam properties are expressed.

Operational radiation protection in synchrotron light and free electron laser facilities

The third-generation synchrotron radiation (SR) facilities are storage ring-based facilities with many insertion devices and photon beamlines, and have low injection beam power, but extremely high stored beam power. The fourth-generation X-ray free electron laser (FEL) facilities are based on an electron Linac with a long undulator and have high injection beam power. Due to its electron and photon beam characteristics and modes of operation, storage ring and photon beamlines have unique safety aspects, which are the main subjects of this paper. The shielding design limits, operational modes and beam losses are reviewed. Shielding analysis (source terms and methodologies) and interlocked safety systems for storage ring and photon beamlines (including SR and gas bremsstrahlung) are described. Specific safety issues for storage ring top-off injection and FEL facilities are discussed. Operational safety programme elements, e.g. operation authorisation, commissioning, training and radiation measurements, for SR facilities are also presented.

Chaos in free electron laser oscillators

The chaotic nature of a storage-ring free electron laser (FEL) is investigated. The derivation of a low embedding dimension for the dynamics allows the low-dimensionality of this complex system to be observed, whereas its unpredictability is demonstrated, in some ranges of parameters, by a positive Lyapounov exponent. The route to chaos is then explored by tuning a single control parameter, and a period-doubling cascade is evidenced, as well as intermittence.

Lasing of infrared free-election lasers using the storage ring NIJI-IV

We report for the first time to our knowledge the experimental achievement of a storage ring free-electron laser (FEL) device dedicated to applications in the IR region. IR FELs were oscillated in the wavelength region of 1392-1502 nm, and the relative linewidth was 3 x 10(-4). The maximum power of the IR FEL transmitted through a vacuum window was approximately 0.3 mW, and the intracavity power was approximately 2 W. We have already observed an intensive quasi-monochromatic x ray by using FEL Compton backscattering. The results of our study are expected to lead to applications in which an IR FEL and a quasi-monochromatic x ray will be used simultaneously.

Sub-picosecond coherent VUV source on the Elettra storage ring

Taking advantage of the storage ring free electron laser beamline at Elettra, we have implemented an experimental setup for the generation of sub-picosecond (ps) coherent optical pulses in the VUV range. The setup is based on the frequency up-conversion of a high-power external signal (provided by a Ti:Sapphire laser) and makes use of a relativistic electron bunch as resonating medium. The produced VUV pulses have peak power in MW range, variable polarization, high shot to shot stability and control of the timing parameters at the ps level. In this paper, we present the first characterization of the temporal and spectral features of the emitted light. The radiation can be exploited for new experiments in the fields of dynamical phenomena, non-linear physics, magnetism and biology.

Evaluation of Synchrotron Damping Time with Storage Ring Free Electron Lasers

We measured the bunch length of an electron beam in the macropulse mode of a storage ring free electron laser (SRFEL) with the compact storage ring NIJI-IV. The energy spread was evaluated from measured data of the bunch length. The synchrotron damping time of the electron bunch was evaluated by applying the evolution of the energy spread to an expression based on the one-dimensional theory of bunch heating. The synchrotron damping time evaluated at five electron-beam currents was in accord with that calculated by computer simulation. This finding suggests that the synchrotron damping time can be measured by the new evaluation method in the SRFEL macropulse mode. It was clarified that the synchrotron damping time measured in a Q-switching operation is shorter than that measured in the macropulse mode. This difference would suggest that the peak intensity of the SRFEL micropulse affects the synchrotron damping time.

Study of mirror degradation in short-wavelength FELs and recent FEL application experiments on the storage ring NIJI-IV

Cavity-mirror degradation is one of the serious problems especially in short-wavelength storage ring free-electron lasers (FELs). To reduce the degradation problem, we tried to treat the Al 2O 3/SiO 2 mirror, degraded through irradiation by undulator radiation, with both oxygen plasma and thermal annealing. As a result, the optical loss of the degraded mirrors optimized around 200 nm was successfully restored. With such efforts, FEL application experiments are being performed. The recent progress on surface observation using a photoelectron emission microscopy (PEEM) at NIJI-IV is also reported.