Wiggler FEL Research Articles

Coherent emission from a bunched electron beam: superradiance and stimulated-superradiance in a uniform and tapered wiggler FEL

We outline fundamental coherent radiation processes from a charge particles beam: Spontaneous Superradiance (SR), Stimulated Superradiance (ST-SR), and in the context of undulator radiation: Tapering-Enhanced Superradiance (TES) and Tapering-Enhanced Stimulated Superradiance Amplification (TESSA). Both single bunch and periodic bunching (in phasor and spectral Fourier frequency formulations) are considered in a model of radiation mode expansion.

Controlling chaotic behavior of the equilibrium electrons by simultaneous using of two guiding fields in a free-electron laser with an electromagnetic-wave wiggler

In the present paper the effects of the combination of the axial-guide magnetic field and the ion-channel guiding on the chaotic trajectories in a free-electron laser with electromagnetic-wave wiggler have been considered. It is shown that the simultaneous using of the two guiding fields in the certain conditions causes chaotic behavior in the electron motion. It is also illustrated that the chaotic trajectories decrease as the ion-channel density or the strength of the axial magnetic field increases. The transition from the chaotic trajectories to regular trajectories, occurs at a special ion-channel density, ω−it, or a particular amount of the strength of the axial magnetic field, at. Furthermore numerically calculation shows that the normalized ion-channel frequency of the transition, ω−it, reduces by increasing the axial magnetic field. Also ω−i increase causes the trajectories to be regular at the weaker at. The electron motion has been altered significantly by the self-fields effects. It is demonstrated that, the self-fields cause a decrement in the chaotic trajectories. This is in contrast to the idealized helical wiggler FEL with the axial magnetic field guiding.

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.

3D simulations of a superradiant harmonic cascade in the XUV region

Short wavelength generation via superradiant emission on higher harmonics in a high gain multiple wiggler FEL configuration has been originally proposed some years ago on the basis of 3D numerical simulation [1] and of analytical calculations [2] . The main advantages, in principle, with respect to the SASE configuration are: 1) spectral purity and linewidth ; 2) shorter wiggler; and 3) relaxation of requirements on the electron beam quality . More recently an optimization scheme which uses dispersive sections between the different wigglers has been proposed [3,4] on the basis of simple 1D calculations, and was tested by 3D simulations . The 3D simulations presented in ref . [1] for a case of resonant tripling of a 240 nm radiation input were assuming a very poor beam quality so that it has not been possible to demonstrate the exponential growth and saturation on the harmonic field in the second wiggler, which is important for the subsequent tapering and efficiency enhancement. In Table 1 we present the results of a series of GINGER [5] simulations which show the exponential gain starting from a signal at 240 nm and

Long-pulse, low-voltage infrared free-electron laser

Free-electron lasers (FEL's) have been developed as powerful IR sources at several sites around the world. This paper considers the design of a relatively compact, low-cost, tunable IR FEL based on a 3-MeV electrostatic accelerator and a submillimeter-wavelength electromagnetic wiggler. The system should be sufficiently portable for use in the field, and applications such as remote sensing and industrial processing are envisioned. The wiggler is powered by a second-harmonic quasioptical gyrotron. A prescription for optimizing the gain of an electromagnetic wiggler FEL is derived and the nonlinear regime is treated by relating the electromagnetic wiggler FEL to the universal normalized FEL equations. A point design is given showing accelerator, gyrotron, and output parameters for a system capable of operating over the entire 3-13-/spl mu/m wavelength atmospheric windows. A wide range of output characteristics is available. >

Gain analysis of a strong-pump FEL operating at the fundamental and high harmonics

An analytical study of a planar wiggler FEL in the cold electron-beam strong-pump regime is presented. Such an FEL can lase at high odd harmonics due to the periodic variation in the longitudinal electron velocity, and can also emit superradiant emission at harmonic frequencies. Based on a parametric interaction approach, we develop an extended gain dispersion equation for a laser operating at the fundamental or high harmonics taking collective effects (Raman regime) into account. The analysis applies also to the case a w (βγ) ∼ 1 , where the customary use of the Taylor expansion of V 2 does not apply. This case is of interest mostly for mildly relativistic FELs, and allows excitation of very high harmonics.

INEX simulations of the Los Alamos HIBAF free-electron laser MOPA experiment

We present results of Integrated Numerical Experiment (INEX) simulations of the performance of a 1 m untapered wiggler FEL oscillator driving a 2 m wiggler FEL amplifier for the new HIBAF (high-brightness accelerator free-electron laser) facility at Los Alamos. INEX simulations utilize a numerically generated electron micropulse, from ISIS/PARMELA calculations of the photoinjector/ linac/beam-transport system, in the 3D FEL simulation code FELEX.

Computation of emittance growth in a focusing wiggler FEL

Computed electron trajectories in a free electron laser show significant emittance growth in the wiggle plane when there are wiggler field gradients. For a low gain free electron laser using a focusing planar wiggler, phase space plots show the development of this 3-dimensional effect as well as rotation of the phase space region occupied by the beam due to betatron oscillations in the focusing wiggler field. Matching the input electron beam to the wiggler eliminates emittance growth.

Corrections to “skewed” FEL expressions

Previous derivations of pendulum/small signal gain expressions have been shown to be in error for a “nonskewed” (wiggler, electron beam and radiation propagation axes are colinear), planar wiggler FEL [1]. The incorrectly derived expressions are, however, correct due to a fortuitous cancelation of non-negligible forces acting on the electrons. It was suggested [B.W.J. McNeil and W.J. Firth, Nucl. Instr. and Meth. A259 (1987) 240] that this cancelation of forces may not occur in a skewed FEL (e.g. where the electron beam is injected off-axis to the wiggler), so that the form of previously derived skewed pendulum/gain expressions will be in error. We report here that this cancelation does not occur and give the corrected pendulum/gain expressions for both planar and helical wiggler FELs, and at harmonics of the radiation field.

Experiment and theory on CO2 laser powered wiggler and induction linac FEL

As an electromagnetic wiggler experiment, CO2 laser light (100 J/100 ns) is injected into a high current electron beam (1 kA, 0.5 MeV). Scattered visible light (λ = 8000-5000 Å) is observed. From the spontaneous emission power the gain for the CO2 laser powered FEL is estimated.The induction linac for the FEL which is developed at our institute and the FEL driven by the induction linac are also reported.

The axial-velocity distribution function and the longitudinal susceptibility of an e-beam in a planar wiggler FEL

This paper introduces a development of a normalized axial velocity distribution function and a susceptibility term for an electron beam in a planar wiggler FEL. The model includes the independent contributions of the energy spread and the angular spread, the emittance and the betatron motion, to the axial velocity distribution function.In the case of an emittance dominated spread, the resulting distribution function is a skewed, asymmetrical function. The e-beam susceptibility is described in this case by the first derivative of the complex error function (the plasma dispersion function), convolved with a decaying exponent.The “exact” distribution function and the susceptibility integral that are represented in this paper may be used in any linear, kinetic FEL model to improve its accuracy in cases where the angular spread, the emittance or the betatron motion, are dominant spread sources.

The free electron laser operating with millimetre wave undulator

The paper reports small-signal analysis and large-signal simulation for a new type of FEL operating with millimetre wave (MMW) undulator. This device is characterized by greatly lowered beam accelerating voltage for a defining laser output wavelength comparable with a static magnet wiggler FEL. Some probable configurations of MMW undulator are suggested.

Analysis and nonlinear simulation of a quadrupole wiggler FEL at millimeter wavelengths

The use of a helical quadrupole wiggler field in a circular guide for amplification of millimeter waves is discussed. The effects of space charge and axial field are considered for beam stability for near-axis orbits. When space charge is neglected, the orbit equations and properties and characteristic betatron oscillation of frequencies are examined by a linearization of the orbit equations. A nonlinear simulation code for dipole wigglers is modified to examine optimum gain TE/sub 21/ waveguide modes for a quadrupole wiggler in the 30-300 GHz range. >

Free-electron lasers with electromagnetic standing wave wigglers

A detailed analysis of the electromagnetic standing wave wiggler for free-electron lasers (FEL's) is conducted for both circular and linear wiggler polarizations, following a single-particle approach. After determination of the unperturbed electron orbits in the wiggler field, the single-particle spontaneous emission spectrum and subsequently the gain in the low gain Compton regime (using the Einstein coefficient method) are explicitly calculated. This analysis results in a clear understanding of the resonance conditions and the coupling strength associated with each resonance of this type of FEL. In particular, a striking feature obtained from this investigation is that the electromagnetic standing wave wiggler FEL, under certain circumstances, exhibits a rich harmonic content. This harmonic content is caused by the presence of both the forward and backward wave components of the standing wave wiggler field. In addition, the nonlinear self-consistent equations for this type of FEL are also presented, permitting further investigation of it by the theoretical techniques and numerical codes developed for conventional FEL's.

Gain in planar wiggler free electron lasers

We present a detailed derivation of the Bessel function factor in the gain/pendulum expressions for a planar wiggler FEL. We show three individually nonnegligible corrections to the standard derivation of Colson (IEEE J. Quantum Electron. QE-17 (8) (1981) 1417). All three corrections however cancel and give the “standard” Bessel function factor of Colson. The gain of a sectional planar wiggler, is investigated for a variation in the deflection parameter, K, in one of its sections. Results produced are compared with and are in good agreement with experiment.

The gain calculation of media and electrostatic free-electron lasers by the madey theorem

The Madey theorem has been used to calculate the small-signal gain of a planar magnetic wiggler FEL and optical klystron. The analysis in this paper shows that the Madey theorem can also be applied to media FEL's, i.e., Cerenkov and transition radiation FEL's, as well as electrostatic FEL's. Here we use the Madey theorem to calculate the small-signal gain of these two kinds of FEL's.

Radiation force corrections to planar wiggler FEL gain

The longitudinal electron dynamics in an FEL are governed by the coupling of the electrons transverse velocity, V ⊥, to the combined magnetic fields of the wiggler and radiation in the Lorentz force equation. The derivation of the “standard” gain expression neglects the radiation contribution to V ⊥ and assumes V ⊥ results from the wiggler field alone [1]. We show, however, that the radiation contribution to V ⊥ couples with the wiggler field to produce a force of the same order as that from the wiggler field coupling with the radiation field [2]. When this former force, the “radiation force”, is included in the electron dymanics, the “difference of Bessel function factor” in the pendulum equation differs significantly from that of the standard expression, being given by: 1 2 1+ 1 f J (f−1) 2 (fξ)− 1− 1 f J (f+1) 2 (fξ) (−1) (f−1) 2 instead of {J (f−1) 2 (fξ)−J (f+1) 2 (fξ)}(−1) (f−1) 2 The gain is then proportional to the product of these two factors, rather than the square of the latter. The corrections to the pendulum/gain expressions result in approximately a 20% increase in gain at the fundamental but a 20% decrease in gain at the third harmonic for deflection parameter K = 2. The standard expressions for helical wiggler gain and spontaneous emission cross-sections are unaffected by inclusion of the radiation force term.

A kinetic small signal gain analysis of a planar wiggler FEL, operating in the high harmonic (strong wiggler) regime

A kinetic linear analysis of a strong planar wiggler FEL operating at the fundamental or odd harmonic frequencies is presented in this paper. The gain dispersion equation in the high harmonic regime is shown to be an extension of the known general FEL equation, and converges to it in the weak pump limit. In the low gain, cold, tenuous beam regime we get the known gain expression with the Bessel function correction term. A similar term also appears in the gain expression of the high gain strong pump limit. When space charge effects are significant (Raman regime), the strong pump gain dispersion relation is more complicated than the corresponding weak pump relation. In the cold beam limit it leads to a quintic polynomial dispersion relation instead of the known cubic equation. This brings about gain curve dependences on the detuning parameter which are double humped and with a relatively high bandwidth. The higher the order of the harmonic frequency, the stronger is the effect of the axial velocity spread on the gain and the tighter are the beam acceptance parameters. The analytical model permits an arbitrary axial velocity distribution. The gain damping effect is computed, both with a simple model of a Gaussian electron velocity distribution and with a more accurate model, taking into account an analytical expression for the asymmetrical skewed velocity distribution produced by the combined finite emittance and energy spread effects.

An upper limit to the gain-energy acceptance product of the free electron laser

We show that the gain-energy acceptance product of the uniform wiggler FEL is a maximum with respect to variations in phase shift or dispersion along the length, and any additional phase shift or dispersion cannot increase this product. For example, in the case of an optical klystron, the gain is increased by introducing a dispersive section, but the gain-energy acceptance product falls to less than that of the uniform wiggler FEL.

Visible and ultraviolet radiation generation using a gas-loaded free-electron laser

Gas loading of a free-electron laser modifies the phase-matching condition while the small-signal gain expression remains the same when written in appropriate form. This permits a wider parameter space than the vacuum FEL, which is particularly advantageous at shorter wavelength operation. Scattering of the electrons by the gas limits the interaction length, but available gains are still high enough to allow oscillation build-up. For example, a 0.5-μm wavelength helical wiggler FEL utilizing a 41 MeV electron beam is restricted to a length of 14 cm, has a small signal gain of 21 percent, and builds up to saturation in less than 1μs. Tunability of this device over several microns is easily obtained by changing the gas pressure.