• J.N. Corlett, K.M. Baptiste, J.M. Byrd, A.E. Charman, P. Denes, R.W. Falcone, J. Feng, D. Filippetto, C.M.R. Greaves, J. Kirz, D. Li, H.A. Padmore, C. F. Papadopoulos, G.C. Pappas, G. Penn, J. Qiang, M.W. Reinsch, R.D. Ryne, F. Sannibale, R.W. Schoenlein, J.W. Staples, C. Steier, T. Vecchione, M. Venturini, W. Wan, R.P. Wells, R.B. Wilcox, J.S. Wurtele
  LBNL, Berkeley, California
• E. Kur
  UCB, Berkeley, California
• A. Zholents
  ANL, Argonne

The Next Generation Light Source (NGLS) is a design concept, under development at LBNL, for a 10‐beamline soft x‐ray FEL array powered by a 2.4 GeV superconducting linear accelerator, operating with a 1 MHz bunch repetition rate. The CW superconducting linear accelerator is supplied by a high-brightness, high-repetition-rate photocathode electron gun. Beam is distributed from the linac to the array of independently configurable FEL beamlines with nominal bunch rates up to 100 kHz, with even pulse spacing. Individual FELs may be configured for EEHG, HGHG, SASE, or oscillator mode of operation, and will produce high peak and average brightness x-rays with a flexible pulse format ranging from sub-femtoseconds to hundreds of femtoseconds.

• J. Qiang
  LBNL, Berkeley, California
• J. Wu
  SLAC, Menlo Park, California

In this paper, we propose a scheme to generate atto-second to femto-second tunable water window (~2-4 nm) coherent X-ray radiation for future light source applications. This scheme improves the previously proposed modulation compression method [1] by using a 10 pC, 100 μm electron beam at 2 GeV energy, a 200 nm seeding laser, an X-band linac, two opposite sign bunch compressors, and a long wavelength laser to generate a prebunched, kilo-Amper current beam with a modulation wavelength within the water window. Such a beam will be sent into an undulator to generate a short pulse transverse and temporal coherent soft X-ray radiation. The requirement of initial seeding laser power is small. The electron beam at the entrance of undulator can have sub micron normalized emittance.

[1] J. Qiang, "Short wavelength seeding through compression for free electron lasers," NIM-A,10.{10}16/j.nima.2010.04.053, 2010.

• C. F. Papadopoulos, J.N. Corlett, D. Filippetto, J. Qiang, F. Sannibale, J.W. Staples, M. Venturini, M.S. Zolotorev
  LBNL, Berkeley, California

The Advanced Photoinjector Experiment (APEX) is a part of the Next Generation Light Source (NGLS), a proposed soft x-ray FEL concept being studied at LBNL. The requirements for the beam delivered to the FELs pose restrictions on the beam parameters at the injector. In addition, different modes of operation of the machine may pose different requirements on the beam. In order to optimize the performance of the injector, a genetic multiobjective algorithm has been used. A genetic algorithm is used because of the inherent complexity of the beam dynamics at the energy range in question (0-30 MeV) and the large number of parameters available for optimization. On the other hand, the multiplicity of requirements on the beam, which include beam emittance, beam pulse length, energy chirp, as well as pulse shape and peak current, leads to a mutliobjective approach for the optimization technique. In this paper, we present the status of the optimization simulations, using the ASTRA particle-in-cell code. Different injector setups are presented and the resulting transport solutions are compared to each other and the requirements of the downstream sections of the accelerator.

• F. Sannibale, B.J. Bailey, K.M. Baptiste, A.L. Catalano, D. Colomb, J.N. Corlett, S. De Santis, L.R. Doolittle, J. Feng, D. Filippetto, G. Huang, R. Kraft, D. Li, H.A. Padmore, C. F. Papadopoulos, G.J. Portmann, S. Prestemon, J. Qiang, J.W. Staples, M.E. Stuart, T. Vecchione, R.P. Wells, M.S. Zolotorev
  LBNL, Berkeley, California
• M. J. Messerly, M.A. Prantil
  LLNL, Livermore, California
• M. Yoon
  POSTECH, Pohang, Kyungbuk

The fabrication and installation at the Lawrence Berkeley National Laboratory of a high-brightness high-repetition rate photo-gun, based on a normal conducting 187 MHz (VHF) RF cavity operating in CW mode, is in its final phase. The cavity will generate an electric field at the cathode plane of ~20 MV/m to accelerate the electron bunches up to ~750 keV, with peak current, energy spread and transverse emittance suitable for FEL and ERL applications. The gun vacuum system has been designed for pressures compatible with high quantum efficiency but "delicate" semiconductor cathodes to generate up to a nC bunches at MHz repetition rate with present laser technology. Several photo-cathode/laser systems are under consideration, and in particular photo-cathodes based on K2CsSb are being developed and have already achieved a QE of 8% at 532 nm wavelength, or close to 20% including the Schottky barrier lowering. The cathode will be operated by a microjoule fiber laser in conjunction with refractive optics to create a flat top transverse profile, as well as a birefringent pulse stacker to create a flat top temporal profile. The present status and the plan for future activities are presented.