High-efficiency Free-electron Laser Research Articles

The detuning effect of crystal monochromator in self-seeding and oscillator free electron laser.

Self-amplified spontaneous emission (SASE) free electron laser (FEL) is capable of generating ultra-short, high power and high brightness X-ray pulses, but its temporal coherence is poor. Self-seeding scheme is an approach to improve the temporal coherence by employing a crystal monochromator. The crystal detuning effect is the phenomenon that the Bragg angle deviates from the middle of the reflection domain due to the refraction effect, and can affect the seed power of hard X-ray self-seeding (HXRSS) FEL. In this paper, we introduce a novel idea to maximize the seed power by tuning the incident angle off the Bragg condition where the Bragg photon energy is corresponding to the central photon energy of the input X-ray pulse. We present the numerical analysis of the detuning effect in different reflecting atomic planes and different asymmetry angles of diamond crystal. Moreover, we analyze how the detuning affects the seed efficiency of HXRSS FEL, and discuss the application to X-ray FEL oscillator (XFELO). We find when the detuning is much smaller than the bandwidth of input X-ray pulse, we can neglect the detuning effect. However, if the detuning is much larger than or comparable with the bandwidth of input X-ray pulse, the detuning effect can not be ignored. This work can give a guidance to HXRSS FEL and XFELO commissioning for high efficiency FEL output.

Tapering enhanced stimulated superradiant oscillator

In this paper, we present a new kind of high power and high efficiency free-electron laser oscillator based on the application of the tapering enhanced stimulated superradiant amplification (TESSA) scheme. The main characteristic of the TESSA scheme is a high intensity seed pulse which provides high gradient beam deceleration and efficient energy extraction. In the oscillator configuration, the TESSA undulator is driven by a high repetition rate electron beam and embedded in an optical cavity. A beam-splitter is used for outcoupling a fraction of the amplified power and recirculate the remainder as the intense seed for the next electron beam pulse. The mirrors in the oscillator cavity refocus the seed at the undulator entrance and monochromatize the radiation. In this paper we discuss the optimization of the system for a technologically relevant example at 1 $\mu$m using a 1~MHz repetition rate electron linac starting with an externally injected igniter pulse.

Generation of subterawatt-attosecond pulses in a soft x-ray free-electron laser

We propose a novel scheme to generate attosecond soft x rays in a self-seeded free-electron laser (FEL) suitable for enabling attosecond spectroscopic investigations. A time-energy chirped electron bunch with additional sinusoidal energy modulation is adopted to produce a short seed pulse through a self-seeding monochromator. This short seed pulse, together with high electron current spikes and a cascaded delay setup, enables a high-efficiency FEL with a fresh bunch scheme. Simulations show that using the Linac Coherent Light Source (LCLS) parameters, soft x-ray pulses with a FWHM of 260 attoseconds and a peak power of 0.5 TW can be obtained. This scheme also has the feature of providing a stable central wavelength determined by the self-seeding monochromator.

Optimization of a high efficiency free electron laser amplifier

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 tapering.

Design and performance of the JAERI superconducting linac for high-power free-electron laser

A superconducting rf linac has been required to realize high-duty operation and low-electricity consumption for a high-average-power and high-efficiency free-electron laser (FEL). The JAERI FEL superconducting linac consists of a 250 kV electron gun, a subharmonic normal-conducting buncher (SHB) of 83.3 MHz, and two single-cell and two five-cell superconducting cavities of 499.8 MHz. The gun was typically operated around 200 kV to reduce space-charge effects. A combination of the SHB and the two single-cell cavities enabled a high-current beam of more than 10 A by utilizing a thermionic cathode and a grid pulser. The full width at half maximum bunch length and energy resolution were measured to be 22 ps and 1% or less, respectively. The measured results agreed with calculations utilizing the modified PARMELA code. This showed the validity of the linac design for the FEL. With this beam quality, high-average FEL power of 0.1 kW at a 24–28 μm wavelength was obtained in a quasi-cw mode.

Theoretical and experimental comparison of FEL-oscillators with conventional and reversed guide field

Results of theoretical and experimental studies of free-electron laser (FEL)-oscillators with guide magnetic field of both conventional and reversed orientation are presented. It is shown that in regimes far from the cyclotron resonance (including reversed guide field regimes) the quality of the electron beam formed in slowly up-tapered wiggler entrance is better than in regimes close to the cyclotron resonance. As a result, a FEL operating in the reversed guide field regime possesses a rather small sensitivity to the initial spread of beam parameters. A capability of high-efficiency FEL operation in reversed guide field regime is corroborated experimentally, and a record for millimeter-wavelength FEL-oscillators efficiency of 26% is obtained. In accordance with the simulations in conventional guide field regimes, under a magnetic field smaller than the resonance value, the process of RF-generation was accompanied with beam current losses caused by electron fall-out on the wall of the transporting channel.

Stochastic electronic motion and high-efficiency free-electron lasers.

We consider the asymptotic behavior of high-power Compton free-electron laser oscillators. We observe that the electronic motion along the wiggler presents intrinsic stochasticity. We show that the high efficiency in the broad spectrum regime is due to chaotic diffusion of the electrons toward lower energies, rather than to standard synchrotron motion. We obtain simple estimates of the properties of the system at saturation, which agree with complete numerical simulations. Namely, both efficiency and spectral width are predicted to behave like the square roots of the electronic density, of the cavity quality factor, and of the wiggler length, in such a way that their ratio is a universal brightness of the order of \ensuremath{\surd}3 /2.

High efficiency free-electron laser operation in a symmetrical confocal resonator

A tapered wiggler is used in a free-electron laser (FEL) oscillator to improve the saturation efficiency. During signal buildup the tapered wiggler does not provide optimum phase synchronism between the electron beam and the electromagnetic wave, resulting in an appreciable loss in small-signal gain. This problem can be ameliorated by using a multicomponent wiggler, which is a combination of a uniform wiggler and a tapered section. During buildup, gain is primarily contributed by the linear element, and at high power levels the gain and efficiency are enhanced by the taper. Ideally, one would like to have the optical waist location near the linear section at small-signal levels and near the tapered section at high power levels. Placing the FEL in a symmetrical confocal resonator approaches this desired effect automatically since it has the unique characteristic that a stable mode exists for all locations of the waist of a Gaussian beam along the axis of the interferometer.

Performance of the APEX free-electron laser at Los Alamos National Laboratory

The APEX (APLE Prototype Experiment) FEL (free-electron laser) is an advanced technology test-bed for the Average Power Laser Experiment (APLE). APEX is a photoinjector-driven FEL that is currently operated as an oscillator with untapered wigglers. Photocathodes are rapidly becoming the source of choice for high-brightness electron beams to drive high-efficiency FELs. The electron accelerator is a 40-MeV L-band rf linac with a 6-MeV photoinjector. Since the commissioning of the FEL in 1991, we have been engaged in accelerator and laser experiments. We have demonstrated that the accelerator is capable of providing a high-brightness electron beam at high current. Initial lasing has been at wavelengths near 3 μm, and our electron-beam brightness is such that harmonic lasing at much shorter wavelengths is possible. We have recently lased on the third harmonic at 0.83 μm. In this paper, we report the results of the accelerator and FEL performance in the oscillator mode. The results show that a photoinjector can be integrated into an FEL system to produce stable and reliable operation.

Topics in a high-efficiency FEL design

Abstract Various topics concerning a high-efficiency free-electron laser (FEL) design are discussed. The concept of applying an accelerating microwave field to the electrons as they pass through the wiggler to maintain constant electron energy is reviewed. Efficiencies of up to 48%, for practical values of FEL parameters, are calculated, and advantages of the scheme in comparison with tapered wiggler FELS are discussed. A unique structure for use in the high-efficiency scheme, combining the functions of a linear accelerator and a wiggler, and including a pre-bunching section, is reviewed. The effect of optical field strength variation along the resonator axis as well as the effect of operation at different wavelengths on the attainable efficiency are investigated.

Compact high efficiency free-electron lasers using multipass optical cavities

Abstract Compact and very effecient free-electron lasers may be realized by using novel, multipass optical cavities. These cavities may also be used externally to form very compact photon storage devices. We describe the essential characteristics of such cavities and establish critical design requirements for high efficiency FEL operation.

A novel wiggler design for use in a high-efficiency free-electron laser

A wiggler, for use in a high-efficiency FEL system utilizing microwave acceleration to maintain synchronism, is proposed. The wiggler not only provides a periodic transverse magnetic field, but also acts as a loaded waveguide capable of supporting microwaves. Measurements on the wiggler confirm that it can be utilized in a high-efficiency design to attain conversion efficiencies of approximately 42% in a 2 m length at 10 μm wavelength.

Novel approaches to FEL operation: The gas-loaded FEL and a high efficiency FEL design

Two novel methods for improving free-electron laser (FEL) oscillator performance are discussed: (a) The gas-loaded FEL (GFEL) allows operation at snorter wavelengths for a given accelerator energy and wiggler. Experimental results of laser operation with a gas retention foil in the electron beam line and with the introduction of gas to the wiggler cavity are presented, (b) An FEL design utilizing a time-ramped microwave field to accelerate electrons as they lose energy to radiation allows for high conversion efficiencies. Parameter constraints for such an FEL are discussed, leading to a structure that integrates a wiggler with a linac. It is shown that conversion efficiencies of 50% at λ = 10 μm with a 2m wiggler length can be achieved for typical FEL parameter values without sacrificing small-signal gain

Field certification of a high strength tapered hybrid undulator

Short wavelength, high efficiency free electron laser (FEL) operation imposes stringent specifications on undulator performance in order to maintain interaction strength. As part of the Boeing Aerospace Company, Spectra Technology Visible Oscillator Experiment, the Tapered Hybrid Undulator (THUNDER) meets the requirements imposed by high-efficiency visible rf-linac operation. The magnetic measurements used to validate the performance of THUNDER are presented. The specific design criteria and the tolerances they establish are discussed as well as the magnetic measurements that validate them. The electron beam diagnostic stations and their performance is also presented. The performance of THUNDER serves as a bench mark for undulator/wiggler designs central to long-wiggler FELs and advanced synchrotron radiation sources.

Subharmonic Triple Buncher for a High-Efficiency Free Electron Laser

A high-efficiency free electron laser oscillator experiment is being constructed at Los Alamos National Laboratory. A buncher system has been designed to deliver 30-ps, 5-nC electron bunches to a 20-MeV standing-wave linac at the 60th subharmonic of the 1300-MHz accelerator frequency. The first 108.3-MHz buncher cavity accepts a 5-ns, 5-A peak current pulse from a triode gun. Following a 120-cm drift space, a second 108.3-MHz cavity is used, primarily to enhance the bunching of the trailing half of the bunch. A 1300-MHz cavity with 20-cm drift spaces at each end completes the beamline components. The bunching process continues into the linac's first three accelerating cells. Two thin iron-shielded lenses and seven large-diameter solenoids provide axial magnetic fields for radial focusing.

High-Efficiency Free-Electron Laser Results

Results obtained with a tapered-wiggler free-electron laser demonstrate the concepts proposed by Morton for enhanced efficiency. They show deceleration of electrons by as much as 7%, and extraction of more than 3% of the total electron-beam energy as laser energy when the laser is operated as an amplifier. The experiment is presently being reconfigured to examine its performance as a laser oscillator.

Nonlinear Saturations of Parasitic Instabilities in High-Efficiency Free-Electron Lasers

A broad spectrum of unstable parasitic waves can arise from the interaction between the trapped electrons and the radiation generated by a free-electron laser. Imposing a dc electric field with appropriate strength at the onset of trapping can substantially narrow the unstable spectrum and allows considerable enhancement in the radiation intensity. The nonlinear mechanisms which limit the enhancement process are observed to be due to nonlinear frequency shift and detrapping.