| Paper |
Title |
Page |
| MOPKF083 |
Inverse Free Electron Laser Heater for the LCLS
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512 |
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- R. Carr, L.D. Bentson, P. Bolton, D. Dowell, P. Emma, A. Gilevich, Z. Huang, J.J. Welch, J. Wu
SLAC, Menlo Park, California
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The LCLS Free Electron Laser employs an RF photocathode gun that yields a 1 nC charge bunch a few picoseconds long, which must be further compressed to yield the high current required for SASE gain. The very cold electron beam from the RF photocathode gun is quite sensitive to microbunching instabilities such as coherent synchrotron radiation (CSR) in the compressor chicanes and longitudinal space charge (LSC) in the linac. These effects can be Landau damped by adding energy spread to the electron bunch prior to compression. We propose to do this by interacting an infrared laser beam with the electron bunch in an undulator added to the LCLS gun-to-linac injector. The undulator is placed in a 4-bend chicane to allow the IR laser beam to propagate co-linearly with the e-beam while it oscillates in the undulator. The IR laser beam is derived from the photocathode gun laser. Simulations presented elsewhere in these proceedings show that the laser interaction damps the microbunching instabilities to a very great extent. This paper is a description of the implementation of the laser heater
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| TUPLT162 |
Computation of the Longitudinal Space Charge Effect in Photoinjectors
|
1506 |
| |
- C. Limborg-Deprey, P. Emma, Z. Huang, J.J. Welch, J. Wu
SLAC, Menlo Park, California
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The LCLS Photoinjector produces a 100A, 10 ps long electron bunch which is later compressed down to 100 fs to produce the peak current required for producing SASE radiation. SASE saturation will be reached in the LCLS only if the emittance and uncorrelated energy spread remain respectively below 1.2 mm.mrad and 5. 10-4. This high beam quality will not be met if the Longitudinal Space Charge (LSC) instability develops in the injector and gets amplified in the compressors. The Longitudinal Space Charge instability originates in the injector beamline, from an initial modulation of the current density. Numerical computations, performed with Multiparticle Space Charge tracking codes, showing the evolution of the longitudinal phase space along the LCLS Photoinjector beamline, are presented. Those results are compared with an analytical model for various regimes of energy and acceleration. This study justifies the necessity to insert a "laser heater" in the LCLS Photoinjector beamline to warm up the beam and thus prevent the amplification of the LSC instability in the compressors. Numerical calculations of the 'laser heater' performances are presented.
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| WEPLT155 |
Effect of Dark Currents on the Accelerated Beam in an X-band Linac
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2200 |
| |
- V.A. Dolgashev
SLAC/ARDA, Menlo Park, California
- K.L.F. Bane, G.V. Stupakov, J. Wu
SLAC, Menlo Park, California
- T.O. Raubenheimer
SLAC/NLC, Menlo Park, California
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X-band accelerating structures operate at surface gradients up to 120-180 MV/m. At these gradients, electron currents are emitted spontaneously from the structure walls ("dark currents") and generate additional electromagnetic fields inside the structure. We estimate the effect of these fields on the accelerated beam in a linac using two methods: a particle-in-cell simulation code MAGIC and a particle tracking code. We use the Fowler-Nordheim dependence of the emitted current on surface electric field with field enhancement factor beta. In simulations we consider geometries of traveling wave structures that have actually been built for the Next Linear Collider project.
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| WEPLT156 |
Suppression of Microbunching Instability in the Linac Coherent Light Source
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2203 |
| |
- Z. Huang, P. Emma, C. Limborg-Deprey, G.V. Stupakov, J.J. Welch, J. Wu
SLAC, Menlo Park, California
- M. Borland
ANL/APS, Argonne, Illinois
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A microbunching instability driven by longitudinal space charge, coherent synchrotron radiation and linac wakefields is studied for the linac coherent light source (LCLS) accelerator system. Since the uncorrelated (local) energy spread of electron beams generated from a photocathode rf gun is very small, the microbunching gain may be large enough to significantly amplify shot noise fluctuations of the electron beam. The uncorrelated energy spread can be increased by an order of magnitude without degrading the free-electron laser performance to provide strong Landau damping against the instability. We study different damping options in the LCLS and discuss an effective laser heater to minimize the impacts of the instability on the quality of the electron beam.
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