Free-electron Laser Oscillator Research Articles

A Study of the Spectral Properties of the Radiation in a Free-electron Laser Oscillator

A Study of the Spectral Properties of the Radiation in a Free-electron Laser Oscillator

Transverse effects in a free-electron laser oscillator with two electron beams

We have analyzed the transverse effects in a free-electron laser (FEL) oscillator using two electron beams of different energies referring to a proposed FEL facility operated in the far-infrared and infrared regions. The transverse effects in a free-electron laser oscillator were studied taking into account the electron beam’s emittance, offset misalignment errors, energy spreads, and higher-order modes using an extended two-dimensional (2D) FEL code with two electron beams that we developed. The variation in the radiation amplitude depending on the electron beam emittance, offset misalignment errors, and energy spreads was also calculated in the case of betatron oscillations for multiparticles and multipass numbers using a new 2D code. The mode construction was studied for higher-order modes of the wiggler with and without offset misalignment errors for FEL operation with a high-quality electron beam.

Effects of a tapered undulator in a free-electron laser oscillator with two electron beams

We have analyzed the effects for a tapered undulator in a free-electron laser (FEL) oscillator by using two electron beams of different energies based on the proposed FEL facility that is to be operated in the far-infrared and infrared regions. The effects of the tapered undulator in a free-electron laser oscillator with two electron beams on the electron beam’s emittance, the offset misalignment errors, the energy spreads, and the higher-order modes were studied using a twodimensional (2D) FEL code with two electron beams that we had developed. The effects of the variations in the radiation amplitude on the electron beam’s emittance, the offset misalignment errors, and the energy spreads with a tapered undulator and with an untapered undulator were also calculated for the multi-particle and the multi-pass number by using a 2D code. The results for the gains of a higher energy beam with and without a tapered undulator in the two-beam system were compared to the results obtained using a single electron beam with and without tapering. The mode construction was studied for higher-order modes with a tapered undulator, and the results for the emittance and the offset misalignment errors were compared to those of an untapered undulator.

X-ray comb generation from nuclear-resonance-stabilized x-ray free-electron laser oscillator for fundamental physics and precision metrology

An x-ray free-electron laser oscillator (XFELO) is a next-generation x-ray source, similar to free-electron laser oscillators at VUV and longer wavelengths but using crystals as high-reflectivity x-ray mirrors. Each output pulse from an XFELO is fully coherent with high spectral purity. The temporal coherence length can further be increased drastically, from picoseconds to microseconds or even longer, by phase-locking successive XFELO output pulses, using the narrow nuclear resonance lines of nuclei such as $^{57}\mathrm{Fe}$ as a reference. We show that the phase fluctuation due to the seismic activities is controllable and that due to spontaneous emission is small. The fluctuation of electron-bunch spacing contributes mainly to the envelope fluctuation but not to the phase fluctuation. By counting the number of standing-wave maxima formed by the output of the nuclear-resonance-stabilized (NRS) XFELO over an optically known length, the wavelength of the nuclear resonance can be accurately measured, possibly leading to a new length or frequency standard at x-ray wavelengths. A NRS-XFELO will be an ideal source for experimental x-ray quantum optics as well as other fundamental physics. The technique can be refined for other, narrower resonances such as $^{181}\mathrm{Ta}$ or $^{45}\mathrm{Sc}$.

Resonator stability and higher-order modes in free-electron laser oscillators

Three-dimensional simulation codes genesis and opc are used to investigate the dependence of the resonator stability of free-electron laser (FEL) oscillators on the stability parameter, laser wavelength, outcoupling hole size and mirror tilt. We find that to have stable lasing over a wide range of wavelengths, the FEL cavity configuration should be carefully chosen. Broadly, the concentric configuration gives near-Gaussian modes and the best performance. At intermediate configurations the dominant mode often switches to a higher-order mode, which kills lasing. For the same reason, the outcoupled power can also be less. We have constructed a simple analytic model to study resonator stability which gives results that are in excellent agreement with the simulations. This suggests that modes in FEL oscillators are determined more by the cavity configuration and radiation propagation than by the details of the FEL interaction. We find (as in experiments at the CLIO FEL) that tilting the mirror can, for some configurations, lead to more outcoupled power than a perfectly aligned mirror because the mode is now a more compact higher-order mode, which may have implications for the mode quality for user experiments. Finally, we show that the higher-order mode obtained is usually a single Gauss-Laguerre mode, and therefore it should be possible to filter out the mode using suitable intracavity elements, leading to better FEL performance.

The effects of two electron beams on a free-electron laser oscillator

We have analyzed the characteristics of a free-electron laser (FEL) oscillator by using two electron beams of different energies. The evolution of the harmonic output intensity of the two-beam oscillator, the harmonic radiation field intensity relative to the beam’s current ratio, the harmonic field obtained by using both the lower and the higher electron energy beams, and the harmonic field for the higher energy electron beam alone were studied by using an extended 2D simulation code that described the effects of two electron beams with different energies. The 2D effects of the energy spread, higher-order modes, and the transverse mode construction of the optical field were also studied in the two electron beam oscillator model for the low-energy region.

Optimization of the lattice function in the planar undulator applied for terahertz FEL oscillators

Since the beta function of the electron beam within the undulator has a great influence on the power gain of the free electron laser (FEL), optimization of the undulator lattice becomes important. In this paper, the transfer matrix of the planar undulator is obtained from differential equations of the electron motion. Based on this, the lattice function of the planar undulator in a terahertz FEL oscillator proposed by Huazhong University of Science and Technology (HUST-FEL) is optimized and the expressions of the average beta function are derived. The accuracy of the optimization result was confirmed well by the numerical method. The application range of this analytical method is given as well. At last, the emittance growth in the horizontal direction due to the attenuation of the magnetic field is discussed.

Narrow-Band Emission in Thomson Sources Operating in the High-Field Regime

We present a novel and quite general analysis of the interaction of a high-field chirped laser pulse and a relativistic electron, in which exquisite control of the spectral brilliance of the up-shifted Thomson-scattered photon is shown to be possible. Normally, when Thomson scattering occurs at high field strengths, there is ponderomotive line broadening in the scattered radiation. This effect makes the bandwidth too large for some applications and reduces the spectral brilliance. We show that such broadening can be corrected and eliminated by suitable frequency modulation of the incident laser pulse. Furthermore, we suggest a practical realization of this compensation idea in terms of a chirped-beam-driven free electron laser oscillator configuration and show that significant compensation can occur, even with the imperfect matching to be expected in these conditions.

Spectral properties of a far-infrared free-electron laser oscillator based on a two-frequency wiggler

The spectral properties and the nonlinear interactions between an electron beam and the emitted radiation field were studied in a far-infrared free-electron laser oscillator. A two-frequency wiggler was used to obtain a high efficiency and a narrow spectrum, and a multi-particle simulation code was developed using a modified free-electron laser model based on the two-frequency wiggler. Numerical simulations were performed to obtain the optimal condition for the two-frequency wiggler, and the emitted field amplitude and the spectral properties were found to be improved when the two-frequency ratio parameter of the wiggler was optimized.

Characteristics of a free-electron laser oscillator with optical hole-coupling

We have analyzed the characteristics of a free-electron laser (FEL) oscillator with optical holecoupling. The effects of the hole-coupling on the amplitude, the acceptable misalignment error, the normalized FEL amplitude with offset misalignment, and the tilt tolerance of the misalignment were studied using a 3D FEL code that we developed. The variation in the radiation amplitude with the mirror tilting of the hole-coupling mirror and the variation in the radiation amplitudes on the spot sizes with the pass number were also calculated using a 3D code. The mode construction was simulated and studied for higher-order modes with and without errors.

Double free-electron laser oscillator for photon–photon collisions

In view of a quantum electrodynamics test, a “double” free-electron laser (FEL) oscillator is proposed as a possible device for head-on photon–photon collisions in vacuum. The oscillator is conceived in order to produce two laser beams in the same cavity by two counterpropagating electron beams. The latter are in turn exploited to produce gamma photons by backward Compton scattering of the intracavity FEL radiation itself. In view of an effective device design, specific ranges of values for the various parameters, that characterize the system, are individualized for operation at the maximum of the γ–γ scattering cross section. An estimate of the collision rate in definite device configurations is provided.

Harmonic lasing of X-ray free electron laser: on the way to smaller and cheaper

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 their cost and size. Harmonic lasing of FELs has 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 worldwide, this paper reviews the progress on harmonic lasing of X-ray FELs, mainly concentrating on the recently proposed harmonic lasing of X-ray FEL oscillators and further ideas on harmonic lasing of single pass X-ray FEL amplifiers.

Nonlinear theory of a free electron laser with a helical wiggler and an axial guide magnetic field

A 1D nonlinear theory of a free electron laser (FEL) with a helical wiggler and an axial guide magnetic field is developed based on averaged equations of the electron motion. By averaging we separated two different cases of the e-beam/rf-wave interaction. The first one corresponds to the traditional wiggler synchronism (resonance) of rf wave with the electrons moving along stationary helical trajectories. The second one corresponds to combination resonances distinguishing by excitation of oscillation of the electrons near the stationary helical trajectory. Comparative analysis of the FEL operation in different regimes has been studied under the traditional wiggler synchronism condition. It was shown that FELs operated far from cyclotron resonance (including a reversed guide field orientation) possess low sensitivity to the initial velocity spread in the driving beam resulting in high electron efficiency. In contrast, under the weak guide field (the gyrofrequency is less than the bounce frequency) of a conventional orientation, the FEL efficiency is restricted by a significant increase in the transverse velocity of the electrons during the interaction with the rf wave that results in violation of the synchronism conditions and is accompanied by electron current losses. An additional mechanism of FEL efficiency enhancement under the conventional guide field orientation in the conditions when the gyrofrequency is higher than the bounce frequency, based on the dependence of the effective mass of the oscillating electrons on their energy, was demonstrated. Results of the theoretical analysis are compared with the results of experimental studies of FEL oscillators. The specific features of energy extraction from the electron beam under condition of an abnormal Doppler effect in the case of the combination resonance are described. This regime is beneficial to increase radiation frequency keeping wiggler period and electron energies.

Design considerations of a planar undulator applied in a terahertz FEL oscillator

To fulfill the physical requirement of a 50–100μm Free Electron Laser (FEL) oscillator, design considerations of a planar undulator are described. The main undulator parameters are optimized for a trade off between the gain and the FEL's natural extraction efficiency, and the technical aspects including the structure of the permanent magnet (PM), the choice of PM materials and field tolerances are discussed. The lattice of the designed undulator is studied and the beam matching at the entrance of the undulator corresponding to various working points of electron beam energy is performed.

A double FEL oscillator: A possible scheme for a photon–photon collider

Exploration of the mutual scattering of photons in vacuum is considered as a fundamental test of the quantum electrodynamics theory. In this connection, we propose a “double” free-electron laser oscillator as a possible device for head-on photon–photon collisions. The device is conceived to comprise two undulator sections within the same cavity, where then two laser beams are produced by two counterpropagating electron beams. The latter are in turn exploited to produce gamma photons by backward Compton scattering of the intracavity FEL radiation itself. A preliminary analysis of the collision rate of the backscattered photons is presented specifically at the maximum of the relevant cross section.

Photon Source Capabilities of the Jefferson Lab FEL

Jefferson Lab operates a superconducting energy recovered linac which is operated with CW RF and which powers oscillator-based IR and UV Free-Electron Lasers (FELs) with diffraction limited sub-picosecond pulses with >1013 photons per pulse (1.0%BW) at pulse repetition frequencies up to 75 MHz. Useful harmonics extend into the vacuum ultraviolet (VUV). Based on FEL model calculations validated using this facility, we have designed both an oscillator-based VUV-FEL that would produce 6 × 1012 coherent (0.5% BW) 100 eV photons per pulse at multi-MHz repetition rates in the fundamental, and a dual FEL configuration that would allow simultaneous lasing lasing at THz and UV wavelengths. The VUV-FEL would utilize a novel high gain, low Q cavity, while the THz source would be an FEL oscillator with a short wiggler providing diffraction limited pulses with pulse energy exceeding 50 microJoules. The THz source would use the exhaust beam from a UV FEL. Such multiphoton capabilities would provide unique opportunities for out of equilibrium dynamical studies at time-scales down to 50 fs. The fully coherent nature of all these sources results in peak and average brightness values that are many orders of magnitude higher than storage rings.

A nanoradian x-ray beam deviation-correction device for x-ray free electron laser oscillator

An energy-recovery linac (ERL)-based x-ray free electron laser (X-FEL) combined with a low-loss x-ray optical cavity can produce a high-intensity and fully coherent x-ray beam. The idea is to let the x-ray emission beam in the cavity become the seed beam of the FEL to realize x-ray stimulated emission conditions. Because the electron bunch size in the ERL is sub-micrometer and the x-ray cavity crystals are a few tens of meters away from the electron bunch, a deviation of a few nanoradians in the beam direction can cause the seed beam miss the electron bunches. Therefore, it is necessary to develop a high-precision beam-direction correction device that is independent of the x-ray cavity tuning to ensure collision of the narrow x-ray seed beam and electron bunches. To this end, we have assembled a pair of x-ray prisms on two rotary stages. We operated the device using 1.086 Å wavelength x-rays and demonstrate that the angular resolution can reach 1 nrad within a deviation range of ±3.1 mrad.

Oscillator seeding of a high gain harmonic generation free electron laser in a radiator-first configuration

A longitudinally and transversely coherent, high repetition rate x-ray source with widely tunable wavelength is desired for a variety of experimental applications. A free electron laser (FEL) powered by an electron beam from a superconducting linac can reach the desired peak and average x-ray power levels with transverse coherence. However, generating longitudinally coherent x-ray pulses is a significant challenge, especially at high repetition rate. This paper presents a one-dimensional theoretical and numerical investigation of a method to achieve longitudinal coherence and high repetition rate simultaneously. We propose a ``radiator-first'' configuration, wherein an FEL oscillator follows a high gain harmonic generation (HGHG) FEL. The oscillator generates seed power that is directed upstream to initiate the HGHG process in a following electron bunch. This configuration allows for the generation of radiation at short wavelength, which is highly sensitive to energy spread, to occur before the longer wavelength oscillator, whose performance is not seriously degraded by the beam heating in the upstream radiator. The dynamics and stability of this radiator-first scheme is explored analytically and numerically. A single-pass, 1D map is derived using a semianalytic model for FEL gain and saturation. Iteration of the map is shown to be in good agreement with simulations. A numerical example is presented for a soft x-ray FEL.

Effect of wiggler harmonics in a free-electron laser oscillator

We have analyzed multi-wiggler harmonics in a free-electron laser oscillator with a single laser harmonic. The wiggler magnetic field is expanded into first and higher harmonics, and the radiation field is decomposed into higher order modes. Comparative studies of the radiation output, and of the phase and energy of the first and higher wiggler harmonics that were done using a 3D FEL code that we have developed are presented.

Threshold conditions for lasing of a free electron laser oscillator with longitudinal electrostatic wiggler

The system of the nonlinear non-stationary equations describing spatial-temporal dynamics of the amplitudes of an ondulator radiation and a space-charge wave of a relativistic electron beam in the resonator is obtained. A free electron laser resonator with longitudinal electrostatic wiggler is considered. In the linear approximation, the threshold conditions of lasing for Raman and Compton regimes under excitation of forward and backward electromagnetic wave are achieved. In the various physical situations, the variation of the minimum length of the resonator with the amplitude of wiggler, density of electron beam, and with the reflection coefficients of resonator's mirrors is investigated.