• K. Goldammer
  BESSY GmbH, Berlin
• P. Emma, Z. Huang
  SLAC, Menlo Park, California

The Linac coherent Light Source (LCLS) is an x-ray free-electron laser (FEL) project based on the SLAC linac. With its nominal set of electron beam, focusing and undulator parameters, it is designed to achieve SASE saturation at an undulator length of about 100m with an average power of 10GW. In order to keep the electron beam focused in the undulators, a FODO lattice is integrated along the entire length of the undulators. Nominally, the quadrupoles strengths are chosen to produce nearly constant beta function and beam size along the undulator, optimized for the FEL interaction in the exponential growth regime. Since these quadrupoles are electromagnetic, it is possible to adjust the individual quadrupole strength to vary the beta function and the beam size along the undulator, tailoring the FEL interaction in the startup and the saturation regimes. In this paper, we present simulation studies of the tapered beta function in the LCLS undulator and discuss the generated x-ray properties.

• S. Di Mitri, P. Craievich
  ELETTRA, Basovizza, Trieste
• M. Cornacchia, P. Emma, Z. Huang, J. Wu
  SLAC, Menlo Park, California
• D. Wang
  MIT, Middleton, Massachusetts
• A. Zholents
  LBNL, Berkeley, California

Funding: Sincrotrone Trieste and Director, Office of Science, of the U.S. Department of Energy, under contract No. DE-AC03-76SF00098.

Design studies are in progress to use the existing FERMI@Elettra linear accelerator for a seeded harmonic cascade FEL facility [1]. This accelerator will be upgraded to 1.2 GeV and equipped with a low-emittance RF photocathode gun, laser heater, two bunch compressors, and beam delivery system. We present an optimization study for all the components following the gun, with the aim of achieving high peak current, low energy spread and low emittance electron beam necessary for the FEL. Various operational scenarios are discussed. Results of accelerator simulations including effects of space charge, coherent synchrotron radiation, and wakefields are reported.

[1] C. Bocchetta, et al., FERMI@Elettra - A Seeded Harmonic Cascaded FEL for EUV and Soft X-rays, this conference.

• G.V. Stupakov, Z. Huang
  SLAC, Menlo Park, California

Funding: This work was supported by the Department of Energy, contract DE-AC02-76SF00515.

Several ideas have been proposed in the past to "condition" an electron beam prior to the undulator of a Free-Electron Laser (FEL) by increasing each particle's energy in proportion to the square of its transverse betatron amplitude. This conditioning enhances FEL gain by reducing the axial velocity spread within the electron bunch. Nevertheless, a practical solution for beam conditioning remains difficult. In this paper we consider a new approach to condition the beam using nonlinear effects in the RF field. We demonstrate that such effects can generate a radial variation of the particle's energy in the beam, and and calculate the induced energy spread in the limit of weak field. Methods to minimize the emittance growth in such a beam conditioner are also discussed.

• K.L.F. Bane, P. Emma, Z. Huang, H.-D. Nuhn, G.V. Stupakov
  SLAC, Menlo Park, California
• W.M. Fawley
  LBNL, Berkeley, California
• S. Reiche
  UCLA, Los Angeles, California

Funding: Work supported in part by the Office of Science,U.S. Dept. of Energy under Contracts DE-AC02-76F00515 and DE-AC03-76SF0098.

Energy loss due to wakefields within a long undulator, if not compensated by an appropriate tapering of the magnetic field strength, can degrade the FEL process by detuning the resonant FEL frequency. The wakefields arise from the vacuum chamber wall resistivity, its surface roughness, and abrupt changes in its aperture. For LCLS parameters, the resistive component is the most critical and depends upon the chamber wall material (e.g. Cu) and its radius. Of recent interest [1] is the so-called "AC" component of the resistive wake which can lead to strong variations on very short timescales (e.g. ~20 fs). To study the expected performance of the LCLS in the presence of these wakefields, we have made an extensive series of start-to-end SASE simulations with tracking codes PARMELA and ELEGANT, and time-dependent FEL simulation codes GENESIS1.3 and GINGER. We discuss the impact of the wakefield losses upon output energy, spectral bandwidth, and temporal envelope of the output FEL pulse, as well as the benefits of a partial compensation of the time-dependent wake losses obtained with an undulator field taper. We compare these results to those predicted analytically [2].

[1] K.Bane and G. Stupakov, SLAC PUB-10707 (2004). [2] Z. Huang and G. Stupakov, Phys. Rev. ST Accel. Beams 8, 040702 (2005).

• J. Wu, Z. Huang
  SLAC, Menlo Park, California

Funding: Work supported by the U.S. Department of Energy under contract DE-AC02-76SF00515

LINAC Coherent Light Source (LCLS) will be the world's first hard x-ray FEL based on self-amplified spontaneous emission (SASE). As one of the options to improve the temporal coherence of the SASE FELs, in this paper, we study the High-Gain Harmonic Generation (HGHG) scheme. With a High-order Harmonic-Generation (HHG) laser at 30 - 50 nm as the seed, and the LCLS nominal beam quality, our study shows that it is feasible to use a two-stage HGHG to reach a radiation wavelength of about 1 nm. We also discuss the possibilities of generating sub-nanometer coherent radiation using this method.

• J. Wu, P. Emma, Z. Huang
  SLAC, Menlo Park, California

Funding: Work supported by the U.S. Department of Energy under Contract No. DE-AC02-76SF00515.

Longitudinal space charge (LSC) force can be a main effect driving the microbunching instability in the linac for an x-ray free-electron laser (FEL). In this paper, the LSC-induced beam modulation is studied using an integral equation approach that takes into account the transverse (radial) variation of LSC field. Changes of beam energy and the transverse beam size can be also incorporated. We discuss the validity of this approach and compare it with other analyses as well as numerical simulations. We apply this approach to study the LSC effect in the LCLS accelerator

• W. Graves, F.O. Ilday, F.X. Kaertner, T. Zwart
  MIT, Middleton, Massachusetts
• M.B. Danailov, B. Diviacco, M. Ferianis, M. Marsi
  ELETTRA, Basovizza, Trieste
• Z. Huang
  SLAC, Menlo Park, California
• S.M. Lidia
  LBNL, Berkeley, California

The optical properties of an FEL amplifier are sensitively dependent on the electron beam current profile, energy spread, and transverse emittance. In this paper we consider using a short FEL amplifier operating on a low harmonic of a visible-IR input seed as a mildly destructive electron beam diagnostic able to measure these properties for sub-ps time slices. The optical methods are described as well as a planned implementation of the device for the FERMI@Elettra XUV FEL under construction at Sincrotrone Trieste, including its fiber-based seed laser closely coupled with the facility timing system, undulator parameters, and requirements on the electron and FEL pulses. This diagnostic is conveniently integrated with a "laser heater" designed to increase the very low electron beam energy spread produced by a photoinjector in order to avoid space charge and coherent synchrotron radiation instabilities.

• Z. Huang
  SLAC, Menlo Park, California

Funding: Work supported by US Department of Energy contract DE-AC02-76SF00515.

High-gain free electron lasers (FEL) are being developed as extremely bright x-ray sources of a next-generation radiation facility. In this paper, we review the basic theory and the recent progress in understanding the startup, the exponential growth and the saturation of the high-gain process, emphasizing the self-amplified spontaneous emission (SASE). We will also discuss how the FEL performance may be affected by various errors and wakefield effects in the undulator.

In memory of the late scientist Ming Xie.

• C.J. Bocchetta, D. Bulfone, P. Craievich, G. D'Auria, M.B. Danailov, G. De Ninno, S. Di Mitri, B. Diviacco, M. Ferianis, A. Gomezel, F. Iazzourene, E. Karantzoulis, G. Penco, M. Trovo
  ELETTRA, Basovizza, Trieste
• J.N. Corlett, W.M. Fawley, S.M. Lidia, G. Penn, A. Ratti, J.W.  Staples, R.B. Wilcox, A. Zholents
  LBNL, Berkeley, California
• M. Cornacchia, P. Emma, Z. Huang, J. Wu
  SLAC, Menlo Park, California
• W. Graves, F.O. Ilday, F.X. Kaertner, D. Wang, T. Zwart
  MIT, Middleton, Massachusetts
• F. Parmigiani
  Universita Cattolica-Brescia, Brescia

We describe the machine layout and major performance parameters for the FERMI FEL project funded for construction at Sincrotrone Trieste, Italy. The project will be the first user facility based on seeded harmonic cascade FELs, providing controlled, high peak-power pulses. With a high-brightness rf photocathode gun, and using the existing 1.2 GeV S-band linac, the facility will provide tunable output over a range from ~100 nm to ~10 nm, with pulse duration from 40 fs to ~ 1ps, and with fully variable output polarization. Initially, two FEL cascades are planned; a single-stage harmonic generation to operate > 40 nm, and a two-stage cascade operating from ~40 nm to ~10 nm or shorter wavelength. The output is spatially and temporally coherent, with peak power in the GW range. Lasers provide modulation to the electron beam, as well as driving the photocathode and other systems, and the facility will integrate laser systems with the accelerator infrastructure, including a state-of-the-art optical timing system providing synchronization of rf signals, lasers, and x-ray pulses. Major systems and overall facility layout are described, and key performance parameters summarized.