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| MOPPH007 | Theory of Nonlinear Harmonic Generation in Free-Electron Lasers with Helical Wigglers | electron, undulator, radiation, resonance | 30 | |||||
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Coherent Harmonic Generation (CHG), and in particular Nonlinear Harmonic Generation (NHG), is of importance for both short wavelength Free-Electron Lasers (FELs), in relation with the achievement of shorter wavelengths with a fixed electron-beam energy, and high-average power FEL resonators, in relation with destructive effects of higher harmonics radiation on mirrors. In this paper we present a treatment of NHG from helical wigglers with particular emphasis on the second harmonic. Our study is based on an exact analytical solution of Maxwell's equations, derived with the help of a Green's function method. In particular, we demonstrate that nonlinear harmonic generation (NHG) from helical wigglers vanishes on axis. Our conclusion is in open contrast with results in literature, that include a kinematical mistake in the description of the electron motion. (See DESY 07-058)
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| MOPPH011 | Comparison Between Kinetic and Fluid Description of Plasma-Loladed Free-Electron Laser | plasma, radiation, electron, space-charge | 42 | |||||
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In the kinetic treatment of the plasma-loaded FEL in Ref.* single particle equation of motion, for both beam and plasma electrons in the radiation fields, are used. Therefore, interaction terms between the wiggler and the space-charge wave, in the transverse velocity of electrons, which are important elements in the fluid model, are neglected. A dispersion relation of a plasma-loaded FEL with kinetic theory is found in Ref.* that takes into account the velocity spread of both beam and plasma electrons. In the present analysis, a dispersion relation is obtained, by the fluid theory, with the interaction terms between the wiggler and the space-charge wave in the transverse velocity of electrons taken into account. Since these interaction terms are inherently missing in the kinetic theory the two dispersion relation are compared to find out about the importance of these terms. It was found that although the absence of these terms has considerable effects on the growth rate, the general kinetic dispersion relation may be used to study the temperature effects of a warm beam/plasma on the instability of a free-electron laser with a plasma background.
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* S. Babaei and B. Maraghechi, "Kinetic description of plasma-loaded free-electron laser", Phys. Plasmas, to be published. |
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| MOPPH069 | 3D Modelling of the ERLP IR-FEL | simulation, radiation, electron, alignment | 167 | |||||
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An Energy Recovery Linac Prototype (ERLP) facility is currently being commissioned at Daresbury Laboratory; it serves as a testbed for technologies to be used in the proposed 4th Generation Light Source (4GLS) facility. As part of the ERLP facility, an infra-red oscillator FEL is due to be commissioned early in 2008. In this paper we present full three dimensional, time-dependent modelling of the ERLP IR-FEL using Genesis 1.3 in combination with a paraxial optical propagation code (OPC). We also discuss how this work will be used to inform commissioning of the FEL.
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| WEPPH056 | Gain and Coherence Enhancement for SASE FEL using Laser pre-modulated Electrons | electron, bunching, acceleration, radiation | 484 | |||||
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SASE FEL built up from shot noises exhibits noisy temporal and spectral structures at the output. We propose to use a laser to modulate the electron density at low beam energy and improve the emission characteristics of the SASE FEL at high beam energy. In this scheme, a laser beat wave is incident on the photocathode of an electron gun to generate periodically bunched electrons at the beat-wave frequency. The density modulated electrons can generate superradiance at the harmonics of the beat frequency in all types of single-pass FEL, including Smith-Purcell FEL, Cherenkov FEL, and undulator FEL. The bunching frequency can be further increased by a factor of 10-100 by compressing a chirped, density-modulated, low-energy electron pulse in an alpha magnet, which is subsequently accelerated to high energy for SASE FEL. Computer simulation using particle-in-cell codes, including ASTRA, ELEGANT, and GINGER, shows significant improvements on the gain and coherence of SASE FEL. We will report our detailed study and experimental progress in the conference.
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| FRAAU04 | Re-Commissioning of the Far-Infrared Free Electron Laser for Stable and High Power Operation after the Renewal of the L-Band Linac at ISIR, Osaka University | linac, klystron, electron, controls | 521 | |||||
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We have been developing a far-infrared FEL since late 1980s based on the 40 MeV, L-band electron linac at the Institute of Scientific and Industrial Research (ISIR), Osaka University. The first lasing was obtained at 32~40 um in 1994 and since then we progressively modified the FEL system and continued experiment in between to expand the wavelength region toward the longer wavelength. We finally obtained lasing at 150 um in 1998. We could not obtain power saturation because the macropulse duration is 2 us, though the RF pulse is 4 us long, due to a long filling time of the acceleration tube of the L-band linac and the number of amplification times is limited to 50 only. The linac was constructed approximately 30 years ago and it was not suitable for stable and high power operation of FEL, so that we suspended the development of the FEL. In 2002, we had an opportunity to remodel the linac largely for higher stability and reproducibility of operation. We also added a new operation mode for FEL in which the macropulse duration can be extended to 8 us. I took time to remodel the linac and commission it, but finally the operation mode for FEL is being commissioned and we are resuming the FEL again after the long suspension. We will report the progress and the current status of the re-commissioning of the FEL.
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