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LCLS

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MOPCH048 Linac Coherent Light Source Electron Beam Collimation linac, undulator, collimation, cathode 148
 
  • J. Wu, D. Dowell, P. Emma, C. Limborg-Deprey, J.F. Schmerge
    SLAC, Menlo Park, California
  This paper describes the design and preliminary simulations of the electron beam collimation system in the Linac Coherent Light Source (LCLS) linac. Dark current is expected from the gun and some of the accelerating cavities. Particle tracking of the expected dark current through the entire LCLS linac, from L0-linac exit to FEL undulator entrance, is used to estimate final particle extent in the undulator as well as expected beam loss at each collimator or aperture restriction. A table of collimators and aperture restrictions is listed along with halo particle loss results, which includes an estimate of average continuous beam power lost on each individual collimator. In addition, the transverse wakefield alignment tolerances are calculated for each collimator.  
 
MOPCH049 Trajectory Stability Modeling and Tolerances in the LCLS quadrupole, linac, undulator, CSR 151
 
  • J. Wu, P. Emma
    SLAC, Menlo Park, California
  To maintain stable performance of the Linac Coherent Light Source X-ray Free-electron laser, one has to control undulator trajectory stability to a small fraction of the rms beam size. BPM based feedback loops running at 120 Hz will be effective in controlling jitter at low frequencies less than a few Hz. On the other hand, linac and injector stability tolerances must control jitter at higher frequencies. In this paper, we study the possible sources of such high frequency jitter, including: 1) steering coil current regulation; 2) quadrupole (and solenoid) transverse vibrations; 3) quadrupole (and solenoid) current regulation in presence of typical 200-micron misalignments; 4) charge jitter coupling to RF cavity transverse wakefield due to alignment errors; and 5) bunch length jitter coupling to Coherent Synchrotron Radiation in Chicane. Based on this study, we then set tolerances on each item.  
 
THPCH072 Wakefields in the LCLS Undulator Transitions undulator, radiation, SLAC, linac 2952
 
  • K.L.F. Bane
    SLAC, Menlo Park, California
  • I. Zagorodnov
    DESY, Hamburg
  We have studied longitudinal wakefields of very short bunches in non-cylindrically symmetric (3D) vacuum chamber transitions using analytical models and the computer program ECHO. The wake (for pairs of well-separated, non-smooth transitions) invariably is resistive, with its shape proportional to the bunch distribution. For the example of an elliptical collimator in a round beam pipe we have demonstrated that—as in the cylindrically symmetric (2D) case—the wake can be obtained from the static primary field of the beam alone. We have obtained the wakes of the LCLS rectangular-to-round transitions using indirect (numerical) field integration combined with a primary beam field calculation. For the LCLS 1 nC bunch charge configuration we find that the total variation in wake-induced energy change is small (0.03% in the core of the beam, 0.15% in the horns of the distribution) compared to that due to the resistive wall wakes of the undulator beam pipe (0.6%).  
 
THPCH073 Reflectivity Measurements for Copper and Aluminum in the Far Infrared and the Resistive Wall Impedance in the LCLS Undulator undulator, impedance, SLAC, FEL 2955
 
  • K.L.F. Bane, G.V. Stupakov
    SLAC, Menlo Park, California
  • J. Tu
    City College of The City University of New York, New York
  Reflectivity measurements in the far infrared, performed on aluminum and copper samples, are presented and analyzed. Over a frequency range of interest for the LCLS bunch, the data is fit to the free-electron model, and to one including the anomalous skin effect. The models fit well, yielding parameters dc conductivity and relaxation times that are within 30-40\% of expected values. We show that the induced energy in the LCLS undulator region is relative insensitive to variations on this order, and thus we can have confidence that the wake effect will be close to what is expected.  
 
THPCH193 Comparison between H-ion and Heat Cleaning of Cu-metal Cathodes cathode, gun, DIAMOND, GTF 3245
 
  • D. Dowell, F. King, R.E. Kirby, J.F. Schmerge
    SLAC, Menlo Park, California
  Understanding the quantum efficiency (qe) of a metal photocathode in an s-band RF gun is important to limit the drive laser energy requirement and provide the best quality electron beam. Systematic measurements of the qe vs. wavelength for varying surface contamination have been performed on copper samples using x-ray photoelectron spectroscopy (XPS). The sample is first cleaned to the theoretical limit of qe using a 1 keV hydrogen ion beam. The H-ion beam cleans an area approximately 1cm in diameter and has no effect on the surface roughness while removing essentially all contaminants and lowering the work function to 4.3eV. The sample is then exposed to atmospheric contaminants (nitrogen and oxygen) and measured again with XPS to determine the degree of contamination and the effect on the qe. The goal is to determine the best procedure for transferring and installing cathodes in an s-band gun. These results and comparison with a heat cleaned cathode are presented.