• D. Dowell, E. N. Jongewaard, J. R. Lewandowski, Z. Li, C. Limborg-Deprey, J. F. Schmerge, A. E. Vlieks, J. W. Wang, L. Xiao
  SLAC, Menlo Park, California

The beam quality and operational requirements for the Linac Coherent Light Source (LCLS) currently being constructed at SLAC are exceptional, requiring the design of a new RF photocathode gun for the electron source. Based on operational experience at GTF at SLAC, SDL and ATF at BNL and other laboratories, the 1.6cell s-band (2856MHz) gun was chosen to be the best electron source for the LCLS injector, however a significant re-design was necessary to achieve the challenging parameters. Detailed 3-D analysis and design was used to produce nearly-perfect rotationally symmetric rf fields to achieve the emittance requirement. In addition, the thermo-mechanical design allows the gun to operate at 120Hz and a 140MV/m cathode field, or to an average power dissipation of 4kW. Both average and pulsed heating issues are addressed in the LCLS gun design. The first LCLS gun is now fabricated and has been operated with high-power RF. The results and analysis of these high-power tests will be presented.

• D. Dowell, E. N. Jongewaard, C. Limborg-Deprey, J. F. Schmerge, A. E. Vlieks
  SLAC, Menlo Park, California

The field emission was measured during the high-power testing of the LCLS photocathode RF gun. A careful study and analysis of the field emission electrons, or dark current is important in assessing the gun's internal surface quality in actual operation, especially those surfaces with high fields. The charge per 2 microsecond long RF pulse (the dark charge) was measured as a function of the peak cathode field for the 1.6 cell, 2.856GHz LCLS RF gun. Faraday cup data was taken for cathode peak RF fields up to 120MV/m producing a maximum of 0.6nC/RF pulse for a diamond-turned polycrystalline copper cathode installed in the gun. The field dependence of the dark charge is analyzed using a temperature-dependent Fowler-Nordheim (FN) theory to obtain the field enhancement factor and other emitter parameters. Digitized images of the dark charge were taken using a 100 micron thick YAG crystal for a range of solenoid fields to determine the location and angular distribution of the field emitters. The FN plots and emitter image analysis will be described in this paper.

• P. Emma, R. Akre, J. Castro, Y. T. Ding, D. Dowell, J. C. Frisch, A. Gilevich, G. R. Hays, P. Hering, Z. Huang, R. H. Iverson, P. Krejcik, C. Limborg-Deprey, H. Loos, A. Miahnahri, C. H. Rivetta, M. E. Saleski, J. F. Schmerge, D. C. Schultz, J. L. Turner, J. J. Welch, W. E. White, J. Wu
  SLAC, Menlo Park, California
• L. Froehlich, T. Limberg, E. Prat
  DESY, Hamburg

The Linac Coherent Light Source (LCLS) is a SASE x-ray Free-Electron Laser (FEL) project presently under construction at SLAC. The injector section, from drive-laser and RF photocathode gun through the first bunch compressor chicane, was installed during the Fall of 2006. Initial system commissioning with an electron beam takes place in the Spring and Summer of 2007. The second phase of construction, including the second bunch compressor and the FEL undulator, will begin later, in the Fall of 2007. We report here on experience gained during the first phase of machine commissioning, including RF photocathode gun, linac booster section, energy spectrometers, S-band and X-band RF systems, the first bunch compressor stage, and the various beam diagnostics.

• B. Sayyar-Rodsari, E. Hartman, C. A. Schweiger
  Pavilion Technologies, Inc, Austin, Texas
• M. J. Lee, P. Lui, J. M. Paterson, J. F. Schmerge
  SLAC, Menlo Park, California

High brightness electron beam sources such as rf photo-injectors as proposed for SASE FELs must consistently produce the desired beam quality. We report the results of a study in which a combined neural network (NN) and first-principles (FP) model is used to model the transverse phase space of the beam as a function of quadrupole magnet current, while beam charge, solenoid field, accelerator gradient, and linac voltage and phase are kept constant. The parametric transport matrix between the exit of the linac section and the spectrometer screen constitutes the FP component of the combined model. The NN block provides the parameters of the transport matrix as functions of quad current. Using real data from SLAC Gun Test Facility, we will highlight the significance of the constrained training of the NN block and show that the phase space of the beam is accurately modelled by the combined NN and FP model, while variations of beam matrix parameters with the quad current are correctly captured. We plan to extend the combined model in the future to capture the effects of variations in beam charge, solenoid field, and accelerator voltage and phase.