| Paper | Title | Page |
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| MOPPH044 | Optical Beam Quality in Free-Electron Lasers | 134 |
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| The quality of the FEL optical beam is an important consideration for many applications. The quantity M-squared is a single parameter that is used to quantify the higher-order transverse mode content of the beam. For steady state propagation in the paraxial limit, equations for the axial variation of the laser spot size and M-squared are derived. The quantity M-squared for the output of an FEL can also be determined by making measurements of the spot size at three locations and making use of the parabolic propagation law. We consider the optical beam quality for a MW-class amplifier. In this configuration the radiation is optically guided, maintaining a constant spot size through the wiggler, and is pinched at the wiggler exit. This leads to a relatively good optical beam quality, short growth length, short wiggler length, and good efficiency. Diffractive spreading of the FEL output beam can be sufficiently large to allow the first relay mirror to be close to the exit of the wiggler without exceeding the mirror damage intensity threshold, particularly in a grazing incidence configuration. The minimum distance to the relay mirror is shown to be inversely proportional to M-squared. | ||
| MOPPH055 | Coherent Harmonic Emission of the Elettra Storage-Ring Free-Electron Laser in Single-Pass Configuration: a Numerical Study for Different Undulator Polarizations | 170 |
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| The optical klystron installed on the Elettra storage-ring is normally used as interaction region for an oscillator free-electron laser, but, removing the optical cavity and using an external seed laser, one obtains an effective scheme for the single-pass harmonic generation. In this configuration, which is presently under development, the high-power external laser is synchronized with the electron beam entering the first undulator of the optical klystron. The laser-electron beam interaction produces a spatial partition of electrons in micro-bunches separated by the seed wavelength. The micro-bunching is then exploited in the second undulator (radiator) to produce coherent light at the harmonics of the seed wavelength. The Elettra radiator is an APPLE type undulator and this allows to explore different configurations of polarization. We present here numerical results obtained using the code Medusa for both planar and helical configurations. We also draw a comparison with predictions of the numerical code Genesis. | ||
| MOPPH065 | The First Experimental Observation of FEL Amplifier Efficiency Improvement using Electron Beam Energy Detuning at the NSLS SDL | 190 |
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| We report on the first observation of efficiency enhancement in a single-pass laser seeded FEL amplifier by detuning electron beam energy away from resonance. The dependence of the output FEL energy on the electron energy was measured; a maximum and average enhancement of 100 % and 70 % were observed. The spectral output of the seeded FEL both with and without an energy detuning was also measured. It was verified that the peak wavelength was dominated by the seed laser. The experimental results are compared with the analytical theory and the numerical simulation code, GENESIS 1.3. | ||
| TUPPH071 | Simulation of Mirror Distortion in Free-Electron LASER Oscillators | 477 |
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| Thermal distortion in cavity mirrors in high-power FELs can alter mode quality and degrade performance. Hence, it is important to be able to predict the character of the distortions to model their effect on FEL performance. To this end, we address these key issues by developing modeling and simulation tools that can accomplish these goals, and then benchmarking the simulation against observations on the 10 kW-Upgrade experiment at the Thomas Jefferson National Accelerator Facility. The modeling and simulation will rely on the MEDUSA code, which is a 3-D FEL simulation code 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 resonantor modes and then analytically propagating these modes through the resonator back to the entrance of the wiggler in synchronism with the next electron bunch. Knowledge of the power loading on the mirrors allows us to model the mode distortions using Zernicke polynomials and this technique has been incorporated into MEDUSA. | ||
| THAAU03 | A Scalloped Electron Beam Free-Electron Laser | 509 |
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| Typical high-gain FEL amplifiers employ an electron beam that is matched to the wiggler so that the envelope remains constant throughout the wiggler. This paper describes a novel approach in which the electrons undergo natural betatron scalloping motion along the wiggler because the beams are deliberately mismatched at the wiggler entrance. We present an analysis of the electron scalloping motion and the FEL interaction with a scalloped electron beam. For a representative set of beam and wiggler parameters, we discuss the effect of the pinching the electron beams on the interaction in the FEL and on the focusing and propagation of the FEL radiation. | ||
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| THBAU04 | Stair-Step Tapered Wiggler for High-Efficiency FEL | 545 |
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A new concept of a high-efficiency wiggler called the stair-step tapered wiggler is presented. The stair-step tapered wiggler differs from the traditional continuously tapered wigglers in that there are several uniform wiggler segments with decreasing wiggler periods (or decreasing Krms). Thanks to the relatively large ponderomotive potential in each segment, a substantial fraction of the electrons is captured while the electrons execute synchrotron motion down the energy scale. This leads to high FEL extraction efficiencies and partial optical guiding in the tapered wiggler sections. The stair-step tapered wiggler provides other advantages, such as ease of fabrication and flexibility in the taper rate. Numerical simulations using the code MEDUDA* will be presented to show the high-efficiency performance of a representative FEL with a stair-step tapered wiggler.
* H. P. Freund, S. G. Biedron, and S. V. Milton, IEEE J. Quantum Electron. 36, 275 (2000) |
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