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| MOPPH007 | Theory of Nonlinear Harmonic Generation in Free-Electron Lasers with Helical Wigglers | electron, undulator, wiggler, radiation | 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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| MOPPH009 | Undulator Radiation in a Waveguide | undulator, radiation, vacuum, diagnostics | 34 | ||
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We propose an analytical approach to characterize undulator radiation near resonance, when the presence of the vacuum-pipe considerably affects radiation properties. This is the case of the far-infrared undulator beamline at the Free-electron LASer (FEL) in Hamburg (FLASH), that is designed to deliver pulses in the TeraHertz (THz) range. This undulator can be used for pump-probe experiments where THz pulses are naturally synchronized to the VUV pulse from the FEL, as well as the development of novel electron-beam diagnostics techniques. Since the THz radiation diffraction-size exceeds the vacuum-chamber dimensions, characterization of infrared radiation must be performed accounting for the presence of a waveguide. We developed a theory of undulator radiation in a waveguide based on paraxial and resonance approximation. We solved the field equation with a tensor Green's function technique, and extracted figure of merits describing in a simple way the influence of the vacuum-pipe on the radiation pulse as a function of the problem parameters. Our theory, that makes consistent use of dimensionless analysis, allows treatment and physical understanding of many asymptotes of the parameter space, together with their region of applicability. (See DESY 07-031)
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| MOPPH028 | On NMR probing of the kinetics of a free-electron laser-induced chemical exchange | free-electron-laser, laser | 81 | ||
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Determination of the quantum yields of photochemical reactions (in particular, molecular photoisomerization reactions) is an efficient application of NMR spectroscopy*,**. We are proposed a combined approach for study the kinetics of photo-induced chemical exchange (PICE) reactions (see Scheme), which involves a free-electron laser (FEL) and NMR spectroscopy**. Scheme hv A↔B Continuous and pulse IR-irradiation of the chemical system in the presence of PICE can be achieved using FEL. Based on the analytical equations derived in the work we consider the time-dependent dynamics of variation of the instant NMR line shape for two-site chemical exchange. If one induce an abrupt increase in the rate constant of chemical exchange (for example, by quickly heating the system or PICE), then the NMR line shape will change with time, ultimately reaching another new steady state. Our calculations show that there are beats and oscillations (on the NMR line shape) damped with time. The information about the time-dependent instant line shapes provides possibility to predict the real kinetics of PICE. Prospective objects are molecular conformers and spin isomers. So, FEL can be used in the setup for PICE studies in combination with a NMR spectrometer.
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* S. P. Babailov, E. M. Glebov, et al., Concepts in Magn. Reson., 2006, 8, 571. ** S. P. Babailov, Russ. Chem. Bull., Int. Ed., 2006, 10, 1631. |
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| TUPPH002 | High Order Mode Analyses for the Rossendorf SRF Gun | gun, emittance, coupling, focusing | 228 | ||
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High Order Modes (HOM) excited by the beam in a superconducting RF gun (SRF gun) could destroy the quality of the electron beam. This problem is studied on the base of frequency domain description by considering of the equivalent RLC circuit contour for each HOM, periodical excited by a pulsed current source. Expression for the voltage, the field amplitude and the phase of the excited HOM has been obtained. The equations for the coupling impedances of monopole TM-HOM and TE-HOM in the RF gun cavity has been derived. In this calculation the change of the particle velocity due to acceleration is taken into account. Resonance frequencies, coupling impedances, unloaded and external quality factors, excitation voltages and field distributions for each HOM including trapped HOM are calculated for Rossendorf SRF gun up to the frequency of 7.5 GHz, using the complex field solver CLANS. The dependence of the calculated parameters from a cavity deformation has been studied. The influence of the seven most dangerous HOM on the beam quality has been estimated by particle tracking using the ASTRA code.
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| TUPPH020 | Quiet Start Method in small signal HGHG FEL Simulation | bunching, simulation, electron, laser | 264 | ||
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Quiet start scheme is broadly utilized in Self Amplified Spontaneous Radiation (SASE)FEL simulations, which is proven to be correct and efficient. Nevertheless, due to the energy modulation and dispersion section, the High Gain Harmonic Generation (HGHG) FEL simulation will not be improved by the traditional quiet start method. A new approach is presented to largely decrease the number of macro-partilces per slice that can be implemented in both time-independent and time-dependent simulation, accordingly expedites the high order harmonic cascade simulation or other small signal HGHG cases.
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