Conventional FEL Research Articles

Čerenkov free-electron laser with side walls

In this paper, we have proposed a Čerenkov free-electron laser (CFEL) with metallic side walls, which are used to confine an electromagnetic surface mode supported by a thin dielectric slab placed on top of a conducting surface. This leads to an enhancement in coupling between the optical mode and the co-propagating electron beam, and consequently, performance of the CFEL is improved. We set up coupled Maxwell–Lorentz equations for the system, in analogy with an undulator based conventional FEL, and obtain formulas for the small-signal gain and growth rate. It is shown that small signal gain and growth rate in this configuration are larger compared to the configuration without the side walls. In the nonlinear regime, we solve the coupled Maxwell–Lorentz equations numerically and study the saturation behaviour of the system. It is found that the Čerenkov FEL with side walls saturates quickly, and produces powerful coherent terahertz radiation.

A non-conventional ERL configuration for high-power EUV FELs

We show that a standard Linac configuration (consisting of accelerating sections, linearizing section, and magnetic chicane compressor) currently used in drivers for single-pass EUV/x-ray FELs is compatible with energy recovery, provided that certain timing constraints are met. By circulating the spent, rather than the fresh beam as in a conventional high-power ERL FEL design, the beam brightness can be more easily preserved thus facilitating lasing at short wavelength. As in a conventional ERL, the proposed design allows for energy-spread compression, enabling low-energy beam dumping and high energy-recovery efficiency. Results from numerical simulations presented in this paper show that this configuration could, in principle, support the generation of multi-kW average radiation power required for high-volume production EUV lithography.

Effect of two-stream instability on the saturation mechanism of a two-stream free-electron laser with a helical wiggler pump

In this paper we will investigate the effect of two-stream instability on saturation mechanism in Two-Stream Free Electron Laser (TSFEL) with a helical wiggler pump. A two-velocity relativistic electron beam propagates through the helical wiggler field. The relativistic electron stream is assumed to be cold. The slippage of the electromagnetic wave with respect to the electron beam is ignored. Self-consistent evolution of an electromagnetic wave in the presence of two-velocity electron beam is described by a set of coupled nonlinear differential equations in 1D approximation. Slowly varying envelope approximation is used and by the Runge–Kutta method algorithm is solved numerically. The power versus the axial distance has been plotted. It has been found that the TSFEL reaches the saturation regime in a shorter axial distance in comparison with the conventional FEL.

Development of a femtosecond soft X-ray SASE FEL at DESY

In this paper we describe the extension of the soft X-ray SASE FEL at the TESLA Test Facility (TTF) at DESY for generation of femtosecond pulses. The proposed scheme operates as follows. The first stage is a conventional FEL amplifier seeded by 523 nm external laser. A zero area optical pulse (i.e. the pulse with zero value of optical field in the central area of the pulse) is timed to overlap with the electron bunch. Radiation power is amplified up to the saturation level. Following the first stage the electron beam enters the main 6 nm SASE undulator. Large energy spread is induced in the significant fraction of the electron beam due to the FEL interaction process, and only a small part of the electron bunch (near the center of zero area light pulse) is able to produce radiation in the 6 nm SASE FEL. The SASE FEL described in this paper will provide soft X-ray pulses with 30 fs (FWHM) duration. On the basis of the TTF parameters it should be possible to achieve an average brilliance of 10 22 photons/ s 1/ mrad 2/ mm 2/ (0.1% BW). The average number of photons can exceed 10 12 photon/pulse.

X-ray FEL with a meV bandwidth

A new design for a single pass X-ray Self-Amplified Spontaneous Emission (SASE) FEL was proposed by Feldhaus et al. (Opt. Commun. 140 (1997) 341) and named “two-stage SASE FEL”. The scheme consists of two undulators and an X-ray monochromator located between them. For the Angström wavelength range the monochromator can be realized using Bragg reflections from crystals. We propose a scheme of monochromator with a bandwidth of 20meV for the 14.4keV X-ray SASE FEL being developed in the framework of the TESLA linear collider project. The spectral bandwidth of the radiation from the two-stage SASE FEL (20meV) is determined by the finite duration of the electron pulse. The shot-to-shot fluctuations of energy spectral density are dramatically reduced in comparison with the 100% fluctuations in a SASE FEL. The peak and average brilliance are three orders of magnitude higher than the values which could be reached by a conventional X-ray SASE FEL.

Possible application of X-ray optical elements for reducing the spectral bandwidth of an X-ray SASE FEL

A new design for a single pass X-ray Self-Amplified Spontaneous Emission (SASE) FEL is proposed. The scheme consists of two undulators and an X-ray monochromator located between them. The first stage of the FEL amplifier operates in the SASE linear regime. After the exit of the first undulator the electron bunch is guided through a non-isochronous bypass and the X-ray beam enters the monochromator. The main function of the bypass is to suppress the modulation of the electron beam induced in the first undulator. This is possible because of the finite value of the natural energy spread in the beam. At the entrance to the second undulator the radiation power from the monochromator dominates significantly over the shot noise and the residual electron bunching. As a result the second stage of the FEL amplifier operates in the steady-state regime when the input signal bandwidth is small with respect to that of the FEL amplifier. Integral losses of the radiation power in the monochromator are relatively small because grazing incidence optics can be used. The proposed scheme is illustrated for the example of the 6 nm option SASE FEL at the TESLA Test Facility under construction at DESY. As shown in this paper the spectral bandwidth of such a two-stage SASE FEL (Δλ/λ⋍ 5 × 10−5) is close to the limit defined by the finite duration of the radiation pulse. The average brilliance is equal to 7 × 1024photons/(s × mrad2 × mm2 × 0.1% bandw.) which is by two orders of magnitude higher than the value which could be reached by the conventional SASE FEL. The monochromatization of the radiation is performed at a low level of radiation power (about 500 times less than the saturation level) which allows one to use conventional X-ray optical elements (grazing incidence grating and mirrors) for the monochromator design.

Possible application of X-ray optical elements for reducing the spectral bandwidth of an X-ray SASE FEL

A new design for a single pass X-ray SASE FEL is proposed. The scheme consists of two undulators and an X-ray monochromator located between them. The first stage of the FEL amplifier operates in the SASE linear regime. After the exit of the first undulator the electron bunch is guided through a non-isochronous bypass and the X-ray beam enters the monochromator. The main function of the bypass is to suppress the modulation of the electron beam induced in the first undulator. This is possible because of the finite value of the natural energy spread in the beam. At the entrance to the second undulator the radiation power from the monochromator dominates significantly over the shot noise and the residual electron bunching. As a result, the second stage of the FEL amplifier operates in the steady-state regime. The proposed scheme is illustrated for the example of the 6 nm option SASE FEL at the TESLA Test Facility under construction at DESY. The spectral bandwidth of such a two-stage SASE FEL ( Δλ λ ⋍ 5 × 10 −5 ) is close to the limit defined by the finite duration of the radiation pulse. The average spectral brilliance is equal to 2 × 10 24 photons/(sec×mrad 2×mm 2×0.1% bandwidth) which is by two orders of magnitude higher than the value which could be reached by the conventional SASE FEL.

Axial free-electron laser interaction between an annular electron beam and an axisymmetric TM mode

Free-electron lasers (FEL's) operating with helical wiggler fields and axial guide fields are subject to orbit instability which causes additional particle loss as saturation is reached, A new type of FEL interaction possible between an annular electron beam and a TM/sub 0m/ mode is described here. This interaction is between the axial electron motion and an RF mode that is radially wiggled a in periodically rippled waveguide. The net transverse force integrated over a wiggle period on each of the particles in this type of device can be made to vanish by choosing the proper waveguide radius. The resulting particle orbits are more stable than those in conventional FEL's with helical wiggler fields. >

The tandem FEL dynamic behavior

A tandem FEL consists of two conventional FEL oscillators separated by a drift section. The same e-beam drives both oscillators, and since the first oscillator, where conventional evolution occurs, pilots the behavior of the second, an interesting dynamic behavior arises. We discuss the dynamic aspects of the problem from low signal to saturation, analyze the efficiency of the device, and present preliminary design considerations.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Chaotic electron trajectories in electromagnetic wiggler free-electron laser with a guide magnetic field

The Hamiltonian for an electron travelling through a large-amplitude backward electromagnetic wave, an axial guide magnetic field and radiation field is formulated. Poincare surface-of-section plots show that this Hamiltonian is non-integrable, and leads to chaotic trajectories. Equilibrium conditions are derived in the limit where the radiation field approaches zero. Compared to conventional FEL, the total energy of the system at pondermotive resonanceE c is large, while the electron's critical energyγ c is low for electromagnetic wiggler FEL. Moreover, the threshold wave amplitude (A r=A c) of beam chaoticity is found at lower values of the radiation field amplitude compared to magnetostatic wiggler FEL. Previous features confirmed that electromagnetic wiggler FEL can operate more coherently and more efficiently at moderated particle's energy compared to magnetostatic wiggler FEL.

The high gain optical klystron

We analyze the high-gain optical klystron regime, discussing the conditions under which deviations occur from the linear theory. We show that scaling relations, of the same type exploited for the conventional FEL gain, hold for optical klystron devices. We also explore problems related to the inhomogeneous broadening gain reduction and find that, in analogy to conventional FEL, the high gain corrections to the optical klystron operation are more severely reduced than the linear contributions.

A free electron laser with ion-focussed double undulator for VUV and X-ray generation

The use of an ion-focussing channel together with a double-period undulator enables one to enhance the FEL gain greatly in the VUV and soft X-ray region. The mechanism of the gain enhancement is an additional bunching of the electron beam due to driven betatron oscillations. The amplitude of the driven oscillations is sensitive to the electron beam energy. As a result the variation of the longitudinal electron velocity in response to a variation of the beam energy is increased as compared with a conventional FEL.

Experimental study of millimeter waves and microwaves from a circular free electron laser

Coherent microwave and millimeter wave emission from a rotating electron beam in a circular wiggler magnetic field (circular free electron laser) has been studied. The circular wiggler, which consists of a coaxial waveguide and an azimuthally periodic magnetic field, is compact as compared with a conventional linear FEL. Tunable radiation frequency (TM coaxial waveguide mode) was observed to be typically 11–38 GHz using a mildly relativistic electron beam of energy less than 500 kV, about 150 A current and about 8 μs pulse width. The total radiation power was about 840 kW at 11.5 GHz of TM 6,1.

Behavior of the TE pj1 mode in the Photon Storage Ring

Behavior of the TE pj1 mode in the Photon Storage Ring (PhSR) is studied analytically as well as by computer simulations. Simulations were carried out for an exact circular electron storage ring with 511 MeV electron energy. The growth rate of the TE mode obtained with the simulation is in good agreement with the analytical one. The observed microbunching in the simulation manifests the lasing mechanism similar to a conventional FEL. The nature of the PhSR is discussed.

The electromagnetic-wiggler harmonic amplifier FEL

Abstract In this paper, we present a variation of the two-stage FEL based on the HARmonic amPlifier FEL (HARP/FEL) [1,2] called the electromagnetic-wiggler HARP/FEL (EHARP). In the EHARP, the first stage is a conventional magnetic-wiggler FEL while the second stage's wiggler is an electromagnetic standing wave that is formed within the first stage's optical resonator by pumping with a microwave source. This two-stage FEL approach differs from that proposed by Elias [3] in that the first stage is not used to pump the second stage's optical wiggler; rather, its funtion is to enhance the second stage's gain by appropriately modulating the electron beam as it passes through the device. Such modulation occurs when the pump frequency is chosen such that the ratio of the bunching wavenumbers for the stages is a small integer. The advantage of the EHARP scheme is that the second stage does not need to reach laser threshold in order to radiate and its power is linear in the pump power. We compute the requirements of emission for an EHARP design based on the UCSB-FEL system where the first stage lases at wavelengths from 100 to 160 μm and the secons stage emits at a multiple of the first stage frequency. We predict that the two-stage mechanism may be demonstrated with an existing microwave device (such as the Hughes Mark-II FEL) for the pump source.

High gain distributed optical klystron

The optical klystron configuration comprising two undulators and a buncher between them, proposed and theoretically investigated by Vinokurov and Skrinsky in 1977 [Preprint of INP 77-59, Novosibirsk (1977)] gives a possibility of substantial gain enhancement, compared to the low gain conventional FEL. A logical development of this idea for high gain FELs is the distributed optical klystron (DOK). It consists of several periodical cells, each comprising one undulator and one buncher. A simple comparison for the conventional high gain FEL and the DOK is presented here.

Optical-klystron configuration for a high-gain X-ray free electron laser

We present the theory and numerical simulations of the performance of an X-ray free electron laser in the amplified spontaneous-emission mode with an optical-klystron undulator configuration. This device can produce picosecond pulses of high brightness with a significantly shorter undulator than a conventional FEL.

Self-consistent calculation of spontaneous emission and small-signal gain in high-voltage free-electron lasers

Using the Weisskopf–Wigner technique, we develop a self-consistent quantum-electrodynamic (SCQED) theory of spontaneous emission of radiation and single-photon small-signal gain for high-voltage free-electron lasers (FEL’s). For the first time to our knowledge, excellent agreement is obtained between the predictions and the experimental observations simultaneously for line shift, linewidth, and gain. The SCQED theory predicts line shift and broadening caused by quantum-mechanical effects, which are not predicted by the classical or conventional FEL theories but have been observed. Excellent agreement is obtained between the spontaneous-emission spectra predicted by the SCQED theory and the 1980–1981 ACO [ J. Phys. Lett.41, L-549 ( 1980); Free Electron Generators of Coherent Radiation ( Addison–Wesley, Reading, Mass., 1981, 1982), Vols. 7, 8; the Stanford 10.6-μm [ Phys. Rev. Lett.36, 717 ( 1976)]; and the Stanford 3.4-μm [ Phys. Rev. Lett.38, 892 ( 1977)] FEL experimental spectra. This agreement is much better than that of the prediction from the classical or conventional FEL theory, which gives highly noticeable errors. We show that the spontaneous-emission spectrum that is obtained from classical or conventional FEL theories is valid only in the limit of a short undulator that contains a small number of periods. The small-signal gain derived from the SCQED theory is shown to reduce to Colson’s gain formula [ Phys. Lett.64A, 190 ( 1977)] in the classical limit. However, the SCQED theory predicts significant reductions in the small-signal gain that agree well with the ACO gain data [ Opt. Commun.40, 373 ( 1982)] and are not predicted well by Colson’s formula. Because of the nonnegligible finite electron state lifetime, it is discovered that a fundamental physical gain limit exists that is universal to all types of FEL within the limits of the single-photon transition scheme considered (i.e., if multiphoton effects are ignored). Finally, the implications of the theoretically obtained results are discussed for practical conditions of experimental interest. It is shown that under practical experimental conditions quantum effects can be quite important in FEL’s.

Optical klystron configuration for a high gain x-ray free-electron laser

We present theory and numerical simulations of the performance of an x-ray free-electron laser in the amplified spontaneous emission mode with an optical klystron undulator configuration. This devicecan produce picosecond pulses of high brightness with a significantly shorter undulator than a conventional FEL.

An FEL driven by transverse gradients in the wiggler field

The inherent periodicity of the strong magnetic focusing in a planar wiggler can lead to an FEL-like instability. This instability differs from the conventional FEL in a number of ways: (1) The EM electric field component responsible for the interaction is perpendicular to the wiggler plane. (2) The EM waves couples to the betatron oscillations instead of the plasma oscillations. As such, it relies on transverse gradients in the wiggler field. (3) The “fundamental” mode is at the second harmonic: Δk = 2kw rather than Δk = kw. (4) Coupling goes as the fourth power of the wiggler parameter.The dispersion relation for this instability is derived for two cases: a cold sheet beam and a cold string beam, both in a planar wiggler configuration. An absolute instability is found and its growth rate is given in terms of the beam plasma density and the wiggler parameter.