• G. De Ninno
  ELETTRA, Basovizza, Trieste
• W.M. Fawley, G. Penn
  LBNL, Berkeley, California
• W. Graves
  MIT, Middleton, Massachusetts

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

Presently there is significant interest by multiple groups (e.g. BNL, ELETTRA, LBNL, BESSY, MIT) to reach short output wavelengths via a harmonic cascade FEL using an external seed laser. In a multistage device, there are a number of "free" parameters such as the nominal power of the input seed, the lengths of the individual modulator and radiator undulators, the strengths (i.e. the R56's) of the dispersive sections, the choice of the actual harmonic numbers to reach a given wavelength, etc., whose optimization is a non-trivial exercise. In particular, one can choose whether to operate predominantly in the "high gain" regime such as was proposed by Yu [1] in which case each radiator undulator is many gain lengths long or, alternatively, in the "low gain" regime in which case all undulators (except possibly the last radiator) are a couple gain lengths or less long and the output from each radiator essentially corresponds to coherent spontaneous emission from a pre-bunched beam. With particular emphasis upon the proposed two-stage FEL device for FERMI@Elettra, we discuss strategies for determining optimal cascade layouts based upon both analysis and numerical simulation results.

[1] L.H. Yu, Phys. Rev. A, 44, 5178 (1991).

• A.E. Charman, J.S. Wurtele
  UCB, Berkeley, California
• G. Penn
  LBNL, Berkeley, California

Funding: Division of High energy Phyiscs, DOE; and DARPA, DOD

Within the framework of a Hilbert space formalism, we derive a maximum-power variational principle (MPVP) applicable to classical spontaneous radiation from prescribed current sources. The principe appears similar to, but actually is distinct from, other well-known variational principles associated with Hamilton's principle of stationary action or Rumsey's methods involving "reaction." The techniques have been developed for and applied to the case of undulator radiation from relativistic electron beams, specifically to X-ray generation using an harmonic cascade. Such processes are currently evaluated using extensive calculations or simulation codes which can be slow to evaluate and difficult to set up. The variational principle emerged as a natural step in a simple analytic algorithm to predict the output of a harmonic generation beamline in the low-gain regime based on trial functions for the output radiation. Full three-dimensional effects are included, and it may be generalized to include further effects such as asymmetric beams and misalignments. This method has been implemented and compared with simulation results using the FEL code GENESIS, both for single stages of harmonic generation and for the LUX project.

• 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.