AM mode-locking of a free-electron laser oscillator

The authors present experimental and theoretical studies of a mode-locked free-electron laser (FEL) oscillator. In the experiment the FEL uses a continuous electron beam and operates in the microwave regime. AM mode-locking is performed by modulating the attenuation of the FEL ring cavity by a PIN diode modulator. The modulation period is tuned to match the RF roundtrip time in the ring cavity. The experimental results show the evolution of a single radiation macropulse, consisting of narrow micropulses in synchrony with the sinusoidal locking signal. The micropulse period (~37 ns) equals the roundtrip time and the modulation period. The micropulse width (~5 ns) is limited by the FEL slippage time and by the dispersion in the waveguide ring cavity. The effect of the mode locking consisting in suppressing asynchronous oscillations is clearly observed in the experiment. A theoretical model of the AM mode-locked FEL oscillator operating in the small signal regime is presented. This model includes the slow time variation of the e-beam energy and waveguide dispersion. The theoretical analysis agrees well with the experimental results     

FEL驱动激光器的研究

为了获得高亮度和高品质的电子束,在自由电子激光实验装置中采用光阴极注入器作为电子源,其驱动激光器系统采用主振功率放大结构,振荡器为二极管抽运连续锁模激光器,放大器为一级双程及两级单程二极管脉冲抽运放大,后接4倍频系统.经过工程研制及调试,获得的输出指标为:波长266nm,微脉冲能量4μJ,宏脉冲宽度2μs至6μs可调,微脉冲能量抖动优于3%.结果表明,采用Cs2Te阴极的光阴极注入器实验,该系统能够获得不低于40A的电子束流,达到了自由电子激光实验对电子源的束流要求.     

Generation of Intense Narrow-Band Tunable Terahertz Radiation from Highly Bunched Electron Pulse Train

We present the analysis and start-to-end simulation of an intense narrow-band terahertz (THz) source with a broad tuning range of radiation frequency, using a single-pass free electron laser (FEL) driven by a THz-pulse-train photoinjector. The fundamental radiation frequency, corresponding to the spacing between the electron microbunches, can be easily tuned by varying the spacing time between the laser micropulses. Since the prebunched electron beam is highly bunched at the first several harmonics, with the harmonic generation technique, the radiation frequency range can be further enlarged by several times. The start-to-end simulation results show that this FEL is capable of generating a few tens megawatts power, several tens micro-joules pulse energy, and a few percent bandwidth at the frequencies of 0.5–5 THz. In addition, several practical issues are considered.     

Cavity length adjustment and output FEL intensity optimization

The authors reconsider in more quantitative detail the possibility of enhancing the output intensity of a free electron laser (FEL) oscillator operating with bunched electron beam and finite macropulse duration, adjusting the cavity length while the signal grows. In a forthcoming publication G. Dattoli et al. point out that the output power can be enhanced by moving the cavity mirrors while the intercavity signal grows in such a way that the system is operating at the value of the cavity length providing the maximum gain. Practical solutions to realize the mirror movements are also discussed. The authors complete the analysis to be published by presenting simple formulas which allow a straightforward evaluation of the power enhancement     

A submillimeter free-electron laser experiment

A free-electron laser (FEL) Raman-backscattering oscillator, using afrac{1}{2}-1MeV, 20 kA electron beam pumped with a weak (∼ 250g) periodic (8 mm) transverse magnetic field, is operated as a source of high power millimeter and submillimeter radiation, with lasing developing during the 150 ns beam pulse. Measurements of lasing operation are described at 1.0 and 0.6 mm. The temporal behavior of cavity radiation is considered and effective gain is measured and compared with theory.     

太赫兹自由电子激光放大器研究

太赫兹自由电子激光(FEL)是获得高功率太赫兹辐射的重要途径,目前运行的太赫兹FEL装置基本上都采用振荡器结构,若采用FEL放大器模式,则可产生具有更高峰值功率的太赫兹辐射。本文以北京大学超导电子直线加速器的束流参数为基础,通过模拟分析确定了太赫兹FEL放大器对太赫兹种子源、电子束流及波荡器等的要求。模拟结果显示,太赫兹种子的峰值功率在10 W以上即可实现太赫兹FEL放大;在较易实现的参数条件下,可获得峰值功率数兆瓦的太赫兹辐射。     

太赫兹自由电子激光放大器研究

太赫兹自由电子激光(FEL)是获得高功率太赫兹辐射的重要途径,目前运行的太赫兹FEL装置基本上都采用振荡器结构,若采用FEL放大器模式,则可产生具有更高峰值功率的太赫兹辐射。本文以北京大学超导电子直线加速器的束流参数为基础,通过模拟分析确定了太赫兹FEL放大器对太赫兹种子源、电子束流及波荡器等的要求。模拟结果显示,太赫兹种子的峰值功率在10 W以上即可实现太赫兹FEL放大;在较易实现的参数条件下,可获得峰值功率数兆瓦的太赫兹辐射。     

Stimulated emission by E×B drifting electrons grazing adiffraction grating

It has been recently proposed that generation of relativistically upshifted radiation by an E×B drifting electron beam grazing a diffraction grating combines the Smith-Purcel free-electron laser (FEL) merits of short wavelengths at low beam energies, with efficiencies much higher than the 1/2 N_g conventional FEL limit. The small-signal theory is studied here, including the effect of drift velocity shear from the equilibrium (unbunched) space-charge fields. The dependence of the small-signal gain on the device parameters is determined and the startup current is evaluated. The effect of thermal beam spreads is analyzed and found that the oscillation startup current is much less sensitive to thermal spreads than a similar grating FEL     

A simple model of the free electron laser oscillator from low intohigh gain

An investigation of the Compton free electron laser (FEL) oscillator from the low into the high gain regime is presented. The one-dimensional Maxwell-Pendulum equations, describing the radiation-electron beam evolution in an FEL, are used to develop a simple, computationally efficient method of modeling the radiation evolution. The maximum efficiency for a range of values of the gain parameter G is calculated     

Coherent emission and gain from a bunched electron beam

The radiation from a modulated electron beam, such as that produced in a radio-frequency accelerator, passing through a magnetic undulator is analyzed. The authors show that in a waveguide free electron laser (FEL), this may lead to an emission of a significant amount of coherent radiation in the far infrared to millimeter wave range. A simple and powerful method of calculating the spectral distribution of the radiated power is presented along with an analysis of the gain and the saturated power. The experimental results of spontaneous emission measurements on an RF driven FEL, are presented and compared to theoretical predictions     

A prebunched free electron laser

The narrow width of an rf-accelerated electron micropulse can be exploited to yield gain significantly higher than in the conventional FEL. If the micropulses are much shorter than the wavelength of the generated wave and the electrons are resonant with the FEL beat wave, then a low order prebunched FEL interaction results. All electrons lose energy to the wave     

Intracavity filtering of the spectrum of a short-pulse FEL byinducing interpulse phase coherence

The number of active cavity modes in the short-pulse free-electron laser FELIX was reduced by a factor of 40 at a constant level of the saturated power. This was achieved by inducing phase-coherence between the 40 optical micropulses that are independently amplified at 1 GHz in the 25-MHz cavity. A 1-GHz Fox-Smith intracavity etalon was used to this aim. The resulting spectrum consists of a comb of frequencies that are spaced by 1 GHz. Based on a CW frequency analysis of the coupled cavities we predict that the individual frequencies have a linewidth of 170 kHz. The stability of the selected frequencies was analyzed. Mode hopping over 25 MHz was occasionally observed between macropulses of the laser, but not actually during the macropulse. Simulations by means of a simple pulse evolution model corroborated this behavior. We show that the comb of resonant frequencies can be scanned over a range of 1 GHz by scanning the length of the intracavity etalon. The work presented here gives the theoretical and experimental background of single-linewidth experiments that will be described in a separate paper. The latter experiments concern the selective transmission of a single cavity mode from the phase-locked signal by means of external etalons. This single line should be narrow, stable, and continuously tunable for high-power high-resolution experiments in the far-infrared region of the electromagnetic spectrum     

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     

A novel Smith-Purcell free electron laser

An experiment on a novel Smith–Purcell free electron laser (FEL) is described in this paper. The FEL is driven by a relativistic sheet electron beam of middle energy. The high frequency system of the device is a quasi-optical resonator composed of a diffraction grating and a three-mirror reflector. Coherent radiation with a peak power of tens of kW at the 3 mm waveband is sucessfully detected from an experimental facility. The main experimental parameters are: sheet beam energy from 400 kV to 500 kV; pulse length of voltage 70 ns; pulse beam current 0.2 kA; synchronous guide magnetic field up to 1.2 T with 10 ms pulse length; and grating period 2.2 mm.     

Free electron laser based on the Smith-Purcell radiation

The electromagnetic radiation is sti mulated when anelectron beampasses a metallic periodic structure like adiffraction grating at a fixed distance close to the sur-face.This phenomenon is known as the Smith-Purcelleffect .The radiation mechanism was pred…     

COMPARISON BETWEEN FEL AND CYCLOTRON RADIATION

In general,in free electron lasers there are two kinds of stimulated radiations:FELradiation and cyclotron radiation.This paper chows theoretically that if the initial transversevelocity of electron beam is large and the selected parameters for FEL are not suitable,thecyclotron radiation will be dominant,especially when the energy of electron beam is low.But thecyclotron radiation does not have double Doppler frequency upshift effect,its frequency is limitedby axial megnetic field,and when the energy of electron beam is high,the cyclotron radiationfrequency will be much lower than FEL radiation frequency.Therefore,in FEL experimentshow to distinguish these two kinds of radiations and to suppress the cyclotron radiation are veryimportant.     

自由电子激光辐射和迴旋辐射的比较研究

在自由电子激光器中一般存在两种受激辐射形式:自由电子激光辐射和迴旋辐射。本文从理论上阐明,如果电子束的初始横向速度较大,自由电子激光器的参数选用不当,则迴旋辐射将占优势,特别当电子束能量较低时,产生迴旋辐射的倾向更大。但是迴旋辐射并不具有双重多普勒频率上漂移的特性,由于受轴向磁场强度的限制,在电子束能量较高时,其辐射频率要比自由电子激光辐射频率低得多。因此在自由电子激光器的实验研究中,如何区别这两种辐射,并有效地抑制迴旋辐射,是一个十分重要的课题。     

High-gain free electron lasers using induction linear accelerators

High-power free electron lasers (FEL's) can be realized using induction linear accelerators as the source of the electron beam. These accelerators are currently capable of producing intense currents (10²-10⁴A) at moderately high energy (1-50 MeV). Experiments using a 500 A, 3.3 MeV beam have produced 80 MW of radiation at 34.6 GHz and are in good agreement with theoretical analysis. Future experiments include a high-gain, high-efficiency FEL operating at 10.6 μm using a 50 MeV beam.     

An X-ray free-electron laser loaded with a periodic dielectric

When a periodic medium is inserted along the axis of a free-electron laser (FEL), the synchronism condition between the electron beam and the electromagnetic wave is changed. This alteration permits a wider parameter selection than the conventional FEL, and provides for the possibility of X-ray operation. The device is a hybridization of the FEL and the stimulated-transition-radiation laser. Photon absorption by the periodic medium and scattering of the electrons limit the interaction length, but calculated gains are still high enough for oscillation. The analysis shows that ultrarelativistic electron beams and low-density foil materials give the most physically realizable parameters. Higher order resonant transition modes (r>1) should be used to maximize gain     

The Michelson resonator free-electron laser. I. Passive modestructure and mode decay

A general analysis of the evolution of phase locking in an RF linac-driven Michelson resonator free-electron laser (FEL) is presented. By providing a delay of one RF period in the secondary arm of the interferometer, successive optical pulses can be coupled at the beamsplitter, so that they build up from noise with a definite phase relationship. Phase locking is described in terms of longitudinal mode decay by extracting the mode losses directly from the passive frequency response of the resonator. The analyses predict significant mode structure simplification in microsecond macropulses, so that high-resolution spectroscopy is feasible on RF linac-driven FELs. Simulations of the perfectly tuned Michelson resonator FEL are described from spontaneous radiation to saturation, using a one-dimensional pulse propagation code. Excellent agreement with the analytical results in the small-signal regime is demonstrated