Free-electron Laser Oscillator Research Articles

Demonstration and consideration for UV free-electron laser oscillation around 270 nm

Free-electron laser oscillation around 270 nm has been demonstrated on the UVSOR storage ring. In order to overcome an increase of cavity losses due to a shorter wavelength, the FEL gain was enhanced by reducing beam energy from an ordinal value of 500 to 430 MeV. We have investigated the reflectance of dielectric multilayer mirrors in the UV, fabricated by deposition techniques of electron-beam evaporation and ion-beam sputtering. A possibility of high-reflectance multilayer at shorter wavelength is discussed.

What limits the wavelength of an FEL? Challenge at the shortest wavelength lasing limit of linac-based FELs using the FELI linac with a thermionic gun

We have demonstrated that a medium-emittance (26π mm mrad), medium-current (60 A), 24 μs pulse 155 MeV electron beam from the FELI linac with a thermionic gun allows free electron laser (FEL) oscillations at 0.28–0.36 μm. The peak and average FEL powers are 1.8 MW and 4 mW at 0.35 μm. Gain estimation and effects of electron beam quality and filling factor on gain are also discussed. Experimental verification of gain estimates and the criteria on the electron beam quality and wavelength limit due to photon diffraction are tested with our data from visible- and ultraviolet-FEL oscillations.

Short wavelength free electron lasers in 1996

Feedback control of cavity detuning length in a short-pulse free-electron laser oscillator system is suggested and numerical simulation results are presented. By changing the detuning length with reference to the position of the optical pulse at the end of every pass, the system operation is very stable, and very high power and comparably good spectral characteristics have been achieved. Feasibility of this scheme is also discussed.

Numerical analysis of high-gain free-electron laser oscillators

Lasing dynamics in a short-pulse FEL oscillator driven by a high-peak current RF linac is numerically studied. Chaotic lasing due to the nonlinear coupling between electron and laser pulses is observed. We suggest a chaos suppression method using a Michelson resonator having path difference shorter than electron bunch length.

Superradiance in a Short-Pulse Free-Electron-Laser Oscillator

We demonstrate that superradiance (SR) is a general feature of free-electron-laser (FEL) devices, incuding high gain amplifiers as well as low gain oscillators, and that SR pulses and spiking, due to synchrotron instabilities, have the same origin. In particular, we present a direct observation of SR from an FEL oscillator. The ultrashort SR pulses are characterized by a peak intensity and pulse duration which depend, respectively, on the square and the inverse square root of the electron current. The spectral brightness, $B$, defined as the ratio between the efficiency and the relative rms spectral width, is constant and equal to $0.86$.

Gain and saturation in free-electron laser oscillators

We present a calculation of the free-electron laser (FEL) gain, in the small gain, large signal regime that is of relevance to FEL oscillators. We derive an analytic expression for the gain as a function of both the detuning parameter and the optical intensity. We use this analysis to analytically determine the parameters of a three-parameter generalized gain function that is valid to larger intensities and into the saturation regime. We find that with increasing optical intensity the peak of the detuning curve shifts towards higher values of the detuning parameter, and that at higher detuning the gain can actually increase with intensity before falling. We use this parametrization to predict the dependence of the saturated power in an FEL oscillator on the detuning parameter. Our results are in good agreement with one-dimensional numerical simulations that do not make the small gain approximation and also with experimental data from the Institute of High Energy Physics, Beijing and Stanford University FIREFLY FELs. Our analysis should therefore be useful in the design of FEL oscillators.

Theory and simulation of transverse supermode evolution in a free-electron laser oscillator

We derive the conditions for establishment of a transverse supermode in a free-electron laser (FEL) oscillator, and demonstrate the evolution of a supermode by means of a three-dimensional nonlinear code. Both the analytical formulation and the numerical code are based on coupled-mode theory. The oscillator supermode is a combination of transverse modes that keeps its field profile at any point along the oscillator intact after each round trip, and therefore it is the steady-state result of the oscillation buildup process. In the FEL, as in any laser, the oscillator supermode is identical with the amplifier supermode only if the feedback process is entirely nondispersive. If this is not the case, the steady-state supermode field profile varies along the oscillator axis. The simulations demonstrate that the transverse supermode evolution process is primarily a linear regime process and can be proceeded or even completed before saturation.

First lasing at 0.287-0.37 μm free electron lasers using an S-band electron linear accelerator with a thermionic gun

We report on first operation of the shortest wavelength free electron lasers driven by a linear accelerator with a thermionic gun. We have proved that a medium-emittance (26 πmm mrad), medium-current (60A) but 24-μs pulse 155-MeV electron beam allows free electron laser (FEL) oscillations at 0.287-0.37 μm. The peak and average FEL powers are 1.8 MW and 4 mW at 0.35 μm. Tunable wavelength range depends on characteristic optical loss of used multilayer mirrors and agrees with gain simulations. Our results provide the confidence to proceed with shorter wavelength FEL projects using a low-emittance (πmm mrad), high-current (> 100 A) beam from a high-energy linac with a photocathode.

Optical power evolution in the Boeing 1 kW FEL oscillator

A four-dimensional simulation in x, y, z, and t, including betatron motion of the electrons, is used to study optical power evolution and the trapped-particle instability in the Boeing I kW FEL oscillator. Boeing is designing a visible, high power free electron laser (FEL) oscillator, to produce I kW average power at a wavelength A = 0.63 Rm [1]. The large electron beam current of 1-500 A will pass through an L = 5 m long, N = 220 period undulator, creating strong optical fields inside a resonator cavity. Simulations show this may lead to sideband growth and the onset of the trapped-particle instability. Increasing the outcoupling and tapering the undulator would lessen these effects. Fig. I shows the results from a multi-mode four-dimensional simulation in (x, y, , t) of the Boeing oscillator, for n = 50 passes. The longitudinal coordinate z is scaled to the slippage length NA, and the transverse coordinates (x, y) are scaled to the characteristic optical mode radius (LA/ITr) 2 . The model includes the betatron motion of the

Transverse mode coupling and supermode establishment in a free-electron laser oscillator

A three-dimensional study of transverse mode evolution in a free-electron laser (FEL) oscillator is presented. The total electromagnetic field circulating in the resonator is represented as a superposition of transverse modes of the cavity. Coupled-mode theory is employed to derive a generalized 3-D steady-state oscillation criterion. from which the oscillator superrnode can be found analytically in the linear gain approximation. The oscillator supermode. which is the eigenmode solution of the oscillator at steady-state, keeps its transverse profile and polarization after each round-trip. Relations between the oscillator super-mode and the umplijier supermode are discussed. It is shown that they are identical only when the feedback process is entirely non-dispersive and non-discriminating. We employ a 3-D nonlinear simulation code based on the same transverse mode expansion to demonstrate the evolvement of transverse modes in the oscillator towards formation of a supermode in a simple example. The simulation shows that the steady-state result of the oscillation buildup simulation is identical to the supermode predicted by the analytical approach.

Coherent harmonic production using a two-section undulator FEL

Abstract We present measurements of a new method of generating harmonic radiation in a free-electron laser oscillator using a two-section undulator. To produce coherent harmonic radiation the undulator is arranged so that the resonance frequency of the downstream undulator section matches an harmonic of the upstream undulator. Both the fundamental and the harmonic optical fields evolve in the same optical cavity and are coupled out with different extraction efficiency, through a hole in one of the cavity mirrors. We present measurements that show that the optical power at the second and third harmonic can be enhanced by more than an order of magnitude. We examine the dependence of the harmonic power on the optical cavity desynchronism and the cavity roundtrip number to evaluate the influence of the fundamental power on the evolution of the harmonic. We compare our measurements with predictions of a multi-electron numerical model to evaluate the bunching at the harmonic frequency. This fundamental/harmonic mode of operation of the FEL may have useful applications in the production of coherent X-ray and VUV radiation, a spectral range where high reflectivity optical cavity mirrors are difficult or impossible to manufacture.

Theory of short pulse FEL oscillators

Abstract A simple model for the nonlinear low gain evolution of a short pulse Compton free electron laser (FEL) oscillator is derived. An analysis of the small signal regime allows the calculation of the shape of the produced optical pulse. For small excess over threshold, a Landau-Ginzburg equation is obtained, which allows us to show that a strong superradiant efficiency enhancement occurs at small detuning. Furthermore, the knowledge of the eigenfrequencies in the small signal regime allows one to infer a scale law for the period of limit cycles occurring at larger excess over threshold. Theoretical results are compared to experimental results from FELIX.

Plans for a far-infrared free-electron laser in India

The Centre for Advanced Technology is building the INDUS complex of synchrotron radiation sources. As part of this programme it is also proposed to build a far-infrared free-electron laser oscillator. This will use a microtron injector and a 50-period, Halbach-configuration, permanent-magnet undulator. We discuss details of the FEL design and the present status of activities on this project.

Account of the feedback frequency dispersion in spatio-temporal equations of a free-electron laser

The nonstationary dynamics of the free-electron laser oscillator with a low- Q cavity and a broad feedback frequency bandwidth is studied. A method for accounting for the feedback frequency dispersion within the framework of the spatio-temporal approach is proposed. It is shown that broadband dispersion changes the dynamics of the laser excitation, even when its bandwidth is large compared with the amplification bandwidth. Because of the reduction of the number of competing longitudinal modes, the transient time is significantly shortened. Moreover, the current threshold of stability of the single-frequency generation is considerably enlarged.

Free electron laser dynamics, evolution of the bunching coefficients, and coherent harmonic generation

Free electron laser dynamics is characterized by the interplay between bunching and emission processes. The first mechanism can be quantitatively specified in terms of the so-called bunching coefficients, which determine the degree of the bunching at the fundamental and higher harmonics. We numerically solve the Liouville equation, which rules the longitudinal phase-space dynamics, and evaluate the bunching coefficients and associated harmonic generation. We develop a semianalytical method to study the harmonic generation and evolution in free electron laser oscillators.

A model for the saturation of a storage ring free electron laser

The dynamical properties of a storage ring free electron laser oscillator are reproduced using a model combining three different saturation mechanisms: laser induced energy spread of the e-beam, associated bunch lengthening and longitudinal synchronism between laser and electron pulses. The stationary output power is shown to be always linked to the power emitted via synchrotron radiation in one machine turn by the circulating e-beam.

Analytical theory of short-pulse free-electron laser oscillators.

A simple model for the nonlinear evolution of a short-pulse free-electron laser oscillator in the small gain regime is derived. An analysis of the linearized system allows the definition and calculation of the eigenmodes characterizing the small signal regime. An arbitrary solution of the nonlinear system can then be expanded in terms of these supermodes. In the single-supermode approximation, the system reduces to a Landau-Ginzburg equation, which allows the efficiency and saturated power to be obtained as functions of cavity detuning and cavity losses. In the limit of small cavity detuning, electrons emit superradiantly, with an efficiency inversely proportional to the number of radiation wavelengths within the optical pulse, and power proportional to the square of the bunch charge. In the multisupermode regime, limit cycles and period doubling behavior are observed and interpreted as a competition between supermodes. Finally, the analytical and numerical results are compared with the experimental observations from the Free-Electron Laser for Infrared eXperiments experiment.

Experimental indications for non-Gaussian transverse modes in the hole-coupled, short-pulse free-electron lasers of FELIX

The radial structure of intra-cavity light pulses is considered in the hole-coupled free-electron laser (FEL) oscillators of the Dutch FEL facility, FELIX. In a previous numerical study, it was shown that the radial profile can vary significantly over the light pulse, in particular when the electron pulses are short with respect to the slippage length. In this paper experimental indications are presented for the predicted transverse profile of the radiation. The decay of the power coupled out through the hole has been measured, after the electron beam was switched off, for the two different resonators employed in FELIX. These resonators differ in cavity length, mirror curvature and aperture radius, and in the width of the vacuum tube inside the undulator. The decay rate of the extracted radiation is interpreted according to the calculated intra-cavity mode structure from ELIXER. This simulation code describes the time-dependent radial profile in terms of a finite number of Gauss-Laguerre functions. Two extreme cases are discussed in further detail, one in which edge losses are large and one in which they are small.

Tunability of free-electron lasers.

In a free-electron laser oscillator, a variation in the electron beam energy leads to a change in the resonant optical frequency. Simulations are used to study the optical response to an electron beam energy change. A step change in the electron beam energy is used to define a characteristic response time for changing the optical frequency.

Free-electron laser oscillation with a multibunch electron beam of the 38 MeV L-band linear accelerator at ISIR

Free-electron laser oscillation has been achieved with a multibunch electron beam generated with the 38 MeV L-band linear accelerator at The Institute of Scientific and Industrial Research in Osaka University. The wavelengths of the observed FEL are 32 to 40 μm for energies of the electron beams of 17 to 19 MeV. The macropulse length of the beam is 1.8 μs. The net FEL gain measured at a wavelength of 40 μm is 25%. The tuning range of the length of the optical cavity and the FEL spectra have also been measured. The total output FEL energy of 12 mJ has been obtained in the recent experiments at a wavelength of 40μm. The intracavity peak power of the FEL is estimated to be about 150 MW.