Short-pulse Free-electron Laser Research Articles

Few-Cycle Infrared Pulse Evolving in FEL Oscillators and Its Application to High-Harmonic Generation for Attosecond Ultraviolet and X-ray Pulses

Generation of few-cycle optical pulses in free-electron laser (FEL) oscillators has been experimentally demonstrated in FEL facilities based on normal-conducting and superconducting linear accelerators. Analytical and numerical studies have revealed that the few-cycle FEL lasing can be explained in the frame of superradiance, cooperative emission from self-bunched systems. In the present paper, we review historical remarks of superradiance FEL experiments in short-pulse FEL oscillators with emphasis on the few-cycle pulse generation and discuss the application of the few-cycle FEL pulses to the scheme of FEL-HHG, utilization of infrared FEL pulses to drive high-harmonic generation (HHG) from gas and solid targets. The FEL-HHG enables one to explore ultrafast science with attosecond ultraviolet and X-ray pulses with a MHz repetition rate, which is difficult with HHG driven by solid-state lasers. A research program has been launched to develop technologies for the FEL-HHG and to conduct a proof-of-concept experiment of FEL-HHG.

Influence of waveguide dispersion on short-pulse free electron laser detuning curves

In this paper we present results of numerical studies on the cavity desynchronization in the short-pulse waveguided free electron laser (FEL) oscillator FLARE, which is a new THz FEL at the Radboud University Nijmegen, with the emphasis to investigate the influence of the waveguide dispersion. In particular, the peaks of the detuning curves are predicted to be essentially broader. We also predict that the extent of the peak broadening increases as the waveguide dispersion becomes more significant.

Multi-mode competition in an FEL oscillator at perfect synchronism of an optical cavity

The sustained saturation in a short pulse free electron laser (FEL) oscillator at perfect synchronism of an optical cavity has been observed recently by Japan Atomic Energy Research Institute (JAERI) FEL group by using their super-conducting linac (Phys. Rev. Lett., in preparation). The experiments have clearly shown that FEL efficiency becomes maximum at perfect synchronism, although it has been considered that only a transient state exists at perfect synchronism due to the lethargy effect. Through careful analyses of the experimental condition of JAERI FEL, we found that, in spite of the short length of the electron micro-bunch, the saturation appears due to the following features, which were different from other FEL experiments: (1) very large ratio of the small signal gain to losses, (2) very long electron macro-bunch which can tolerate a slow start up. The saturation and high efficiency at perfect synchronism were benefited from the contribution of the weak sideband instability. In order to analyse these new experimental phenomena, we expanded our three-dimensional time-dependent FEL code applicable to longitudinal mode competition, and obtained satisfactory numerical results, which were qualitatively in agreement with the experimental results.

Resonantlike phenomenon in short-pulse free-electron-laser oscillators with modulated desynchronism.

Recently a new method of controlling the pulse length of a short-pulse free-electron laser (FEL) has been developed. By modulating the synchronism between the optical and electron pulses in the FEL cavity, it was found that the output power and the micropulse length of the FEL beam oscillates at the modulation frequency. In this paper, we study theoretically the behavior of the micropulse length, both in the high loss (steady state) regime and the low loss (limit cycle) regime, when a modulated desynchronism is applied. In order to do this, we analyze the dynamics of a short-pulse FEL oscillator. The modulation frequency value plays an important role in the dynamics. We find that there is a resonantlike phenomenon between the externally applied desynchronism modulation and the limit cycle oscillation without modulation of a free-electron laser.

Modulated desynchronism in a free-electron laser oscillator

We study experimentally and theoretically, the effects of desynchronism modulation on short pulse free-electron laser (FEL) oscillators. We find that the output power and the micropulse length of the FEL beam oscillate periodically at the modulation frequency and the minimum micropulse length can be significantly shorter than that obtained without modulation. The FEL can operate during part of the modulation cycle in the normally inaccessible portion of the output power curve where the FEL gain is less than the cavity loss.

Modulated desynchronism in short pulse free-electron laser oscillators

We present an experimental and theoretical study of the effect of desynchronism modulation on short pulse free-electron laser (FEL) oscillators. We find that the output power and the micropulse length of the FEL beam oscillate periodically at the modulation frequency and that the minimum micropulse length during the cycle can be significantly shorter than that which can be obtained without modulation. For example, when the desynchronism of our FEL is modulated at 40 kHz, the minimum measured micropulse length is 300 fs. Without modulation the minimum is about 700 fs. We show that when the desynchronism is modulated, the FEL can operate for part of the cycle in the normally inaccessible portion of the output power curve where the FEL gain is less than the cavity losses. It is even possible for the FEL to operate periodically in the region of negative desynchronism where gain, as normally defined, does not exist.

Continuously tunable, high-power, single-mode radiation from a short-pulse free-electron laser.

This paper gives the first demonstration of high-power, continuously tunable, narrowband radiation that is produced by means of a free-electron laser (FEL) in the far-infrared region of the electromagnetic spectrum. A Fox-Smith intracavity étalon was used to induce phase coherence between the 40 optical micropulses that were circulating in the laser cavity. The corresponding phase-locked spectrum consisted of a comb of discrete frequencies separated by 1 GHz. A pair of external Fabry-Pérot étalons was used to filter out a single line from this spectrum. The power in the selected narrow line at 69 microm wavelength was equal to 250 mW during the macropulse of the laser. The spectral width of the selected line is as small as that of a single cavity mode, i.e., a fraction of 25 MHz, in single macropulses of the laser. The average bandwidth of 25 MHz is determined by mode hopping of the phase-locked FEL. The selected frequency hops over 25 MHz between the extrema of this band. The influence of partially coherent spontaneous emission and mode hopping on the final linewidth was studied. The narrow-linewidth radiation was scanned in frequency over 1 GHz. We show that the possibilities to scan over smaller or larger frequency intervals are unlimited.

Feedback control of cavity detuning in short-pulse free-electron lasers

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.

Superradiant, single-supermode and nonlinear regimes of short pulse free electron laser oscillators

Superradiant, single-supermode and nonlinear regimes of short pulse free electron laser oscillators

Intracavity filtering of the spectrum of a short-pulse FEL by inducing 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.

The response of the short pulse free-electron laser oscillator to periodic perturbation of the cavity synchronism

The detailed optical pulse structure of the short pulse free-electron laser depends on the synchronism between optical and electron pulses in the cavity. For certain cavity detuning a regular train of optical pulses are produced and the macroscopic energy of the optical pulses exhibits a limit cycle. We show that the instabilities associated with the limit cycle and chaotic behaviour can be examined by periodically adjusting the cavity desynchronism. Resonances have been observed at the limit cycle, the period doubled, etc. frequencies. Moreover, the optical power close to these resonances is enhanced and the frequency of the optical pulse microstructure in the cavity is locked to the perturbing frequency leading to possible new methods of synchronising external lasers to the free-electron laser.

Observation of superradiance in a short-pulse FEL oscillator

Superradiance has been experimentally studied, in a short-pulse free-electron laser (FEL) oscillator. Superradiance is the optimal way of extracting optical radiation from an FEL and can be characterised by the following scale laws: peak optical power P , scales as the square of electron charge, Q, ( P , ∝ Q 2); the optical pulse duration, σ z , scales as the inverse of the square root of the charge, Q( P,∝ Q 2); the efficiency, η, scales as the inverse of optical pulse length (n ∝ 1 σ z ∝ Q ); , which also implies that the relative spectral brightness defined by η ( σ λ λ ) remains constant and close to 0.86. To characterise the properties of the superradiant emission, we have measured the efficiency, optical pulse energy, pulse duration and spectral width as functions of electron beam current and cavity loss for the optimum cavity length detuning. The efficiency has been deduced from measurements of electron beam energy spectra. The optical pulse duration has been determined from second-order autocorrelation measurements and the optical spectra determined using a grating spectrometer. We show that the superradiance in the oscillator has properties similar to that in a high-gain amplifier and discuss the links with spikes created by synchrotron instabilities.

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.

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.

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.

Free-electron laser efficiency enhancement, gain enhancement, and spectral control using a step-tapered undulator.

We present a new method of enhancing the efficiency of a Compton free-electron laser. Measurements and a theoretical analysis show that the efficiency of a short pulse free-electron laser can be almost doubled using a two-section step-tapered undulator with the downstream undulator having the higher magnetic field, i.e., reversed with respect to conventional tapering. We also show that the stability of the laser and the spectral purity are improved at the highest output powers because sidebands in the spectrum due to synchrotron oscillations are suppressed. Smooth subpicosecond optical pulses with peak powers of 100 MW have been produced in the midinfrared.

Experimental observation of limit-cycle oscillations in a short-pulse free-electron laser.

The first experimental observation of limit-cycle power oscillations in a short-pulse free-electron laser is presented. These are due to a nonlinear modulation of the optical micropulse shape and phase by the electrons, which leads to the formation of a train of subpulses. Experimentally, the oscillations have been found to depend on the slippage distance and on the desynchronism between optical pulses and electron bunches, comparing well with theoretical predictions.

Selection of single-mode radiation from a short-pulse free-electron laser.

We demonstrate the selection of narrow-band radiation from the output of an infrared free-electron laser operating with electron pulses of a few picoseconds duration. A continuous output with a linewidth of less than ${10}^{\mathrm{\ensuremath{-}}3}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ and a powder in the order of 10 W in macropulses with a duration around 10 \ensuremath{\mu}s can be obtained. The number of active cavity modes is reduced by a factor of 40 by inducing coherence between separate optical pulses in the cavity. An external Fabry-P\'erot interferometer is used to select one of the remaining modes.

Bifurcations in a short-pulse free-electron laser oscillator

With one-dimensional Maxwell-Lorentz equations, we have investigated the bifurcations in a short-pulse free-electron laser oscillator. A generic Hopf bifurcations and chaotic transition via period-doubling cascade have been described in the parametric space of three branching parameters: the cavity detuning parameter D, the slippage parameter S, and the superradiant parameter K. The present theory of the period of limit-cycle oscillation in the radiation power is well consistent with recent experimental results.

Phase locking in an infrared short-pulse free-electron laser

The feasibility of the phase-locking procedure for the external selection of a single mode with reasonable power has been tested by simulation of the optical pulse evolution in a short-pulse free-electron laser using a model based on the self-consistent solution of the equations of motion for the electrons and the wave equation driven by single-particle currents. The simulations show that a high degree of coherence between successive pulses can be induced by a low-finesse etalon. Saturated operation in a greatly reduced number of modes, but with the same total power, is attained with a slight delay in the growth of the power as compared to the case without phase locking.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>