Search Results (34 total)

We report the first experimental characterization of efficiency and spectrum enhancement in a laser-seeded free-electron laser using a tapered undulator. Output and spectra in the fundamental and third harmonic were measured versus distance for uniform and tapered undulators. With a 4% field taper over 3 m, a 300% (50%) increase in the fundamental (third harmonic) output was observed. A significant improvement in the spectra with the elimination of sidebands was observed using a tapered undulator. The experiment is in good agreement with predictions using the MEDUSA simulation code.

We describe a procedure for the simulation of free-electron-laser (FEL) oscillators. The simulation uses a combination of the MEDUSA simulation code for the FEL interaction and the OPC code to model the resonator. The simulations are compared with recent observations of the oscillator at the Thomas Jefferson National Accelerator Facility and are in substantial agreement with the experiment.

In this paper, we compare the 10 kW-Upgrade experiment at the Thomas Jefferson National Accelerator Facility in Newport News, VA, with numerical simulations using the MEDUSA code. MEDUSA is a three-dimensional FEL simulation code that is capable of treating both amplifiers and oscillators in both the steady-state and time-dependent regimes. MEDUSA employs a Gaussian modal expansion, and treats oscillators by decomposing the modal representation at the exit of the wiggler into the vacuum Gaussian modes of the resonator and then analytically determining the propagation of these vacuum resonator modes through the resonator back to the entrance of the wiggler in synchronism with the next electron bunch. The bunch length in the experiment is of the order of 380–420 fsec FWHM. The experiment operates at a wavelength of about 1.6 microns and the wiggler is 30 periods in length; hence, the slippage time is about 160 fsec. Because of this, slippage is important, and must be included in the simulation. The observed single pass gain is 65%–75% and, given the experimental uncertainties, this is in good agreement with the simulation. Multipass simulations including the cavity detuning yield an output power of 12.4 kW, which is also in good agreement with the experiment.

High-power free-electron laser (FEL) amplifiers present many practical design and construction problems. One such problem is possible damage to any optical beam control elements beyond the wiggler. The ability to increase the optical beam’s divergence angle after the wiggler, thereby reducing the intensity on the first optical element, is important to minimize such damage. One proposal to accomplish this optical beam spreading is to pinch the electron beam thereby focusing the radiation as well. In this paper, we analyze an approach that relies on the natural betatron motion to pinch the electron beam near the end of the wiggler. We also consider a step-tapered, two-stage wiggler to enhance the efficiency. The combination of a pinched electron beam and step-taper wiggler leads to additional optical guiding of the optical beam. This novel configuration is studied in simulation using the MEDUSA code. For a representative set of beam and wiggler parameters, we discuss (i) the effect of the scalloped beam on the interaction in the FEL and on the focusing and propagation of the radiation, and (ii) the efficiency enhancement in the two-stage wiggler.

Time-dependent free-electron laser simulations use a variety of techniques. Particle-in-cell codes have been used to simulate free-electron masers; however, this is not feasible at short wavelengths. Most simulations use a slowly varying envelope approximation in both in z and t, where the particles and fields are advanced in z using the same process as in steady-state simulations and then the time derivative describing slippage is applied. We describe the inclusion of this technique in the non–wiggler-averaged code MEDUSA, which is then applied to study temporal behavior in amplifiers and oscillators.

It is widely believed that harmonics are suppressed in helical wigglers. However, linear harmonic generation (LHG) occurs by an azimuthal resonance that excites circularly polarized, off-axis waves, where the hth harmonic varies as exp⁡(ihθ). Nonlinear harmonic generation (NHG) is driven by bunching at the fundamental and has different properites from LHG. While NHG has been studied in planar wigglers, there has been no analysis of NHG in helical wigglers. The 3D simulation code medusa has been modified for this purpose, and it is shown that NHG is substantial in helical wigglers and that the even and odd harmonics have comparable intensities.

Since many free-electron lasers use a segmented undulator, it is important to understand the requirements on phase matching between the undulators. A simulation that self-consistently determines the phase slippage between the light and the electrons is used to study these effects, The simulation is found to be in agreement with an analytic formulation of the phase slippage. A seeded x-ray free-electron laser is studied which makes use of a segmented undulator with quadrupoles in the gaps to provide strong focusing. Optimal performance is found for gap lengths corresponding to a phase slippage within about 20% of a wavelength through a unit cell consisting of an undulator and gap.

Efficiency enhancement in free-electron lasers (FELs) using rf beam acceleration in the wiggler is described. Since the beam tube is a waveguide, there are low and high frequency resonances. Injection of low frequency power can act as an inverse-FEL accelerator concurrently with high frequency power extraction. Simulation of a FEL using this technique shows that substantial efficiency enhancements are possible without significant increases in the beam energy spread, which facilitates the use of energy recovery schemes. The technique is applicable to amplifier and oscillator configurations.

Nonlinear harmonic generation can be a very useful and important phenomenon for single-pass free-electron lasers (FELs) operating in the high-gain regime. Strong bunching at the fundamental wavelength and its associated higher harmonic content allow significant radiation at shorter wavelengths to be emitted without serious effects upon the output power at the fundamental. Here, we analyze the relative sensitivities to beam quality variations of the output fundamental and harmonic powers for a visible-wavelength FEL operating in the high-gain regime.

Two-color operation in free-electron laser (FEL) amplifiers is studied using a 3D nonlinear polychromatic simulation. We assume the FEL is seeded at two closely spaced wavelengths within the gain band, and study the growth of the seeds and a discrete spectrum of beat waves that are outside the gain band. The beat waves grow parasitically due to electron bunching in the seeded waves with growth rates higher than the seeded waves. Injection of narrow-band seeds ensures a discrete spectrum. An example is discussed corresponding to an x-ray FEL; however, the physics is applicable to all spectral ranges.

It is commonly assumed that a matched electron beam optimizes free-electron-laser performance; however, this assumption has not been proven for operation in the high-gain regime. We test this hypothesis for a self-amplified spontaneous emission configuration using a 3D multimode simulation. The gain length predicted for a matched beam is in good agreement with analytic theory. Further, the simulation indicates that while the gain length is optimized for a matched beam the saturated power is not necessarily optimized.

The nonlinear evolution of free-electron laser (FEL) amplifiers is studied for infrared and shorter wavelengths. The configuration of interest consists in the propagation of an energetic electron beam through a drift tube in the presence of a periodic wiggler magnetic field with planar symmetry. A three-dimensional formulation is derived in which the electromagnetic field is represented as an expansion of Gaussian optical modes. Since the wiggler model is characterized by planar symmetry, the Gauss-Hermite modes are used for this purpose. A set of nonlinear differential equations is derived for the evolution of the amplitude and phase of each mode, and they are solved simultaneously in conjunction with the three-dimensional Lorentz force equations for an ensemble of electrons in the presence of the magneto-static wiggler, self-electric and self-magnetic fields due to the charge and current distributions of the beam, and the electromagnetic fields. It is important to note that no wiggler average is used in the integration of the electron trajectories. This permits the self-consistent modeling of effects associated with (1) the injection of the beam into the wiggler, (2) emittance growth due to inhomogeneities in the wiggler and radiation fields as well as due to the self-fields, (3) the effect of wiggler imperfections, and (4) betatron oscillations. The optical guiding of the radiation field is implicitly included in the formulation. This approach has important practical advantages in analyzing FELs, since it is necessary only to characterize the beam upon injection into the wiggler, and the subsequent evolution is treated self-consistently.

Numerical simulations are performed for two examples corresponding to an infrared FEL at wavelengths near 3.5 μm, and an x-ray FEL operating in the neighborhood of 1.4 Å wavelengths corresponding to the proposed linear coherent light source (LCLS) at the Stanford Linear Accelerator Center. Results for both cases indicate that the more severe limiting factor on the performance of the FEL is the beam emittance. For the infrared example, the transition to the thermal regime occurs for an axial energy spread of Δγ_z/γ₀≊0.19%, and optimal performance is obtained for Δγ_z/γ₀<0.1% and γ is the relativistic factor. This restriction is more severe for the LCLS parameters, for which the thermal transition is found for Δγ_z/γ₀≊0.05% and optimal performance requires Δγ_z/γ₀≤0.01%. Wiggler imperfections are found to be a much less important constraint on FEL design. Simulations indicate that there is no coherent ‘‘walkoff’’ of the beam from the symmetry axis due to wiggler imperfections, and that the radiation field is sufficiently guided by the interaction that no severe degradation is found in the extraction efficiency or growth rate for moderate levels of wiggler fluctuations.

A self-consistent analysis of wiggler-field errors in free-electron lasers is described using the three-dimensional simulation code wigglin. A random variation is chosen for the pole-to-pole wiggler amplitude, and a continuous map is used between the pole faces. On average, increases in the root-mean-square value of the field causes a decrease in the interaction efficiency; however, this is relatively benign for the specific case studied, and particular error distributions can result in efficiency enhancements.

The first experimental demonstration of a harmonic free-electron-laser amplifier utilizing a periodic position instability is described for a planar wiggler configuration. The interaction occurs at the even harmonics of the fundamental. A maximum gain of 7 dB was observed over a frequency band ranging from 14 to 15 GHz. The experimental results are compared with predictions from the three-dimensional simulation code wigglin with excellent agreement. Improvements due to a tapered wiggler for this interaction are discussed.

A nonlinear simulation of the Cherenkov maser amplifier is presented for a configuration in which an electron beam propagates through a dielectric-lined cylindrical waveguide. The parameters used correspond to an experiment at General Dynamics which measured a total efficiency of 11.5% at 8.6 GHz. The simulation is in agreement with this but indicates that the system was too short to reach saturation and that an efficiency of 30% would have been possible for a longer system, and the performance is not significantly degraded by thermal spreads up to 20%.

Optical guiding in the free-electron laser is studied for a configuration in which a relativistic electron beam propagates through a rectangular waveguide in the presence of a planar wiggler. The wiggler model includes the effect of parabolically tapered pole faces for enhanced focusing of the electron beam. A set of coupled nonlinear differential equations is solved in three dimensions, which governs the evolution of the fields in a loss-free rectangular waveguide as well as the trajectories of an ensemble of electrons that are integrated using the complete Lorentz force equations. The interaction in a free-electron laser arises from the beating of the wiggler and radiation fields, which produces both upper and lower beat waves. The upper beat wave results in a slowly varying ponderomotive wave, while the lower beat wave introduces an oscillation at half the wiggler period. The lower beat wave cancels out entirely for a helical wiggler, but is present in planar configurations and causes the power and phase of the wave to oscillate at half the wiggler period. This effect also results in an oscillation in the guiding of the signal. The three-dimensional, multimode waveguide code WIGGLIN has been adapted to treat the short-wavelength regime and is capable of examining the effect of the lower beat wave because the orbit equations are not averaged over a wiggler period. The analysis assumes the injection of a Gaussian mode from a master oscillator into the wiggler in synchronism with the electron beam. The Gaussian mode is assumed to be focused down to the minimum spot size upon entry to the wiggler and is then decomposed into the modes of the rectangular waveguide, which satisfy the boundary conditions. Results indicate that the optical guiding of the signal (i) is oscillatory at one half of the wiggler period, and (ii) is more marked in the direction of the wiggle motion rather than transverse to it. In addition, side lobes about the principal radiation signal are observed to grow, which may adversely affect the bulk guiding of the wave and could result in increased wall loading. Implications of these results upon future free-electron laser designs are discussed.

The nonlinear evolution of a free-electron laser (FEL) amplifier is investigated for a configuration in which an electron beam propagates through an overmoded rectangular waveguide in the presence of a planar wiggler with parabolically tapered pole pieces. The analysis is fully three dimensional and describes the evolution of an arbitrary number of resonant TE and/or TM modes of the rectangular guide as well as the trajectories of an ensemble of electrons. Numerical simulations are conducted for parameters consistent with the 35-GHz amplifier experiment performed by Orzechowski and co-workers [Phys. Rev. Lett. 54, 889 (1985); 57, 2172 (1986)], in which the TE₀₁, TE₂₁, and TM₂₁ modes were observed. The theory is found to be in good agreement with the experiment. Surprisingly, comparison with a single-mode analysis shows that the enhancement of the efficiency of the TE₀₁ mode obtained by means of a tapered wiggler is significantly greater (as well as being in substantial agreement with the experiment) when the TE₂₁ and TM₂₁ modes are included in the simulation.

A nonlinear formulation of the free-electron-laser amplifier with a linearly polarized wiggler magnetic field is used to study harmonic generation. Substantial emission is found to occur at the harmonics for a cold beam; however, the harmonics are far more sensitive to beam thermal effects than is the fundamental.

The nonlinear analysis of the orbitron maser is studied numerically for an amplifier configuration in which an electron beam propagates through a coaxial wave guide with a voltage applied between the inner and outer conductors. A set of coupled nonlinear differential equations is derived in three dimensions which governs the self-consistent evolution of the TE, TM, or TEM modes in a loss-free coaxial waveguide as well as the trajectories of an ensemble of electrons. The saturation efficiency as well as the linear growth rate are calculated. The linear growth rate (∼1 dB/cm) is largest for the TEM modes but the efficiency (∼6.0%) is highest for the TE modes. Severe limitations arise in the high-frequency operation of the orbitron amplifier in the fundamental mode due to the breakdown electric field and the extremely small radius of the inner conductor.

The nonlinear evolution of the free-electron-laser (FEL) amplifier is investigated numerically for a configuration consisting of a planar wiggler with parabolically tapered pole pieces. A set of coupled nonlinear differential equations is derived in three dimensions which governs the self-consistent evolution of the TE and TM modes in a loss-free rectangular waveguide as well as the trajectories of an ensemble of electrons. The initial conditions are chosen to model the injection of a cylindrically symmetric electron beam into the wiggler by means of a region with an adiabatically tapered wiggler amplitude, and the effect of an initial beam momentum spread is included in the formulation. Both self-field and space-charge effects have been neglected, and the analysis is valid for the high-gain Compton regime. In addition, the phase stability of the FEL amplifier against fluctuations in the beam voltage, the enhancement of the efficiency by means of a tapered wiggler amplitude, and harmonic generation are also studied. Numerical simulations are conducted to model a 35-GHz amplifier with an electron beam energy of 3.5 MeV, and good agreement is found between the simulation and an experiment conducted by Orzechowski and co-workers. Significantly, the results indicate that a tapered wiggler configuration is somewhat less sensitive to the beam thermal spread than a uniform wiggler system.

The small-signal gain for an electromagnetically pumped free-electron laser is calculated for an amplifier configuration which includes an axial-guide magnetic field. The large-amplitude electromagnetic wave acts like the magnetostatic wiggler in a conventional free-electron laser, and the expression for the gain is shown to reduce to the well-known result in the limit of a magnetostatic wiggler. Substantial enhancements in the gain are found when Ω₀≃γ₀(ω_w+k_wv_?), where Ω₀ is the axial gyrofrequency, γ₀ is the relativistic factor for the electron beam, v_? is the axial velocity of the electron beam, and (ω_w,k_w) are the frequency and wave vector of the electromagnetic wiggler.

The effect of finite momentum spread on the free-electron-laser amplifier is investigated for a configuration of a helical wiggler and an axial guide magnetic field. A set of coupled nonlinear differential equations in three dimensions describing the evolution of the electron trajectories and the radiation field is derived and solved numerically for several sets of parameters. The initial beam distribution is defined external to the wiggler field and the adiabatic injection of the beam into the wiggler is modeled by allowing the wiggler amplitude to increase slowly from zero to a constant amplitude over several wiggler periods. While the efficiency is found to decrease rapidly with increasing momentum spread, resonant effects between the wiggler and guide magnetic fields are found to substantially decrease the sensitivity of the saturation efficiency to the beam emittance. In addition, a relatively sharp change in the slope of the efficiency versus beam momentum spread is identified with the transition between the ‘‘cold’’ and ‘‘thermal’’ beam regimes.

A technique for efficiency enhancement in free-electron-laser and ‘‘ubitron’’ amplifiers (where the ubitron is essentially a free-electron laser operated at electron beam energies less than 500 keV) is analyzed which makes use of both tapered wiggler and axial guide magnetic fields. A set of model equations is derived which describes the coupling between an ensemble of electrons and the radiation field. The analysis is fully three dimensional, and treats the propagation of an electron beam of fi- nite cross-sectional area through a loss-free cylindrical waveguide in the presence of a helically symmetric wiggler field and an axial guide magnetic field. The model equations are solved numerically, and substantial enhancements in the interaction efficiency are found for a variety of choices of the model parameters. The efficiency enhancement is observed to be a sensitive function of both the degree of taper in the wiggler and axial magnetic fields as well as the point at which the taper is begun. In order to illustrate the physical mechanism underlying the efficiency enhancement, a modified pendulum equation which describes the interaction is derived from the orbit equations under a set of idealized assumptions, and used to construct a small-signal theory of the efficiency enhancement.

The nonlinear evolution of the free-electron-laser amplifier is investigated numerically for a configuration consisting of a helical wiggler and axial guide magnetic fields. A set of coupled nonlinear differential equations is derived in three dimensions which governs the self-consistent evolution of either the TE or TM modes in a loss-free cylindrical waveguide and the trajectories of an ensemble of electrons. The initial conditions are chosen to model the adiabatic injection of a cold, cylindrically symmetric electron beam into an interaction region in which the wiggler amplitude rises slowly from zero to a constant level in ten wiggler periods. Both self-field and space-charge effects have been neglected in the formulation, and the analysis is valid for the high-gain Compton regime of operation. Numerical simulations are conducted to model an amplifier operating in the neighborhood of 35 GHz, and for electron-beam energies of 250 keV and 1 MeV. (The free-electron-laser operating at electron-beam energies less than 500 keV is called the ubitron.) The growth rate in the linear regime prior to saturation is found to be in substantial agreement with the predictions based on a linear theory of the instability, and the saturation efficiency is consistent with that expected on the basis of simple, heuristic phase-trapping arguments. Substantial enhancements in the efficiency are found to occur due to the presence of the axial guide field.

A scheme for efficiency enhancement in free-electron lasers is described which employs a tapered axial guide field. An analytical description of this process is given in the strong-pump regime. The efficiency enhancement occurs in such a way that the axial velocity remains constant as kinetic energy is extracted from the beam. Results indicate that efficiency enhancements of as much as 100% are possible with only modest gradients in the axial field.