• F. Banfi, G. Ferrini, P.G. Galimberti, C. Giannetti, S. Pagliara, F. Parmigiani, E. Pedersoli
  Universita Cattolica-Brescia, Brescia
• J.N. Corlett, S.M. Lidia
  LBNL, Berkeley, California
• B. Ressel
  ELETTRA, Basovizza, Trieste

Funding: U.S. DOE, Office of Science, under Contract No. DE-AC03-76SF00098

A Monte Carlo study of electron transport in Cs2Te films is performed to investigate the transverse emittance epsilon at the cathode surface. We find the photoemitted electron angular distribution and explain the physical mechanism involved in the process, a mechanism hindered by the statistical nature of the Monte Carlo method. The effects of electron-phonon scattering are discussed. The transverse emittance is calculated for different radiation wavelengths and a laser spot size of 1.5*10(-3) m. For a laser radiation at 265 nm we find epsilon = 0.56 mm-mrad. The dependence of epsilon and the quantum yield on the electron affinity Ea is also investigated. The data shows the importance of aging/contamination on the material.

• F. Banfi, G. Ferrini, P.G. Galimberti, C. Giannetti, S. Pagliara, F. Parmigiani, E. Pedersoli
  Universita Cattolica-Brescia, Brescia
• J.N. Corlett, S.M. Lidia
  LBNL, Berkeley, California
• B. Ressel
  ELETTRA, Basovizza, Trieste

Funding: U.S Department of Energy, Office of Science under Contract No. DE-AC03-76SF00098.

Quantum Efficiency (QE) measurements of single photon photoemission from a Cu(111) single crystal and a Cu polycrystal photocathodes, irradiated by 150~fs-6.28~eV laser pulses, are reported over a broad range of incidence angle in both s and p polarizations. The maximum value of QE for the Cu polycrystal sample is Y~4*10(-4), obtained with p polarization at an angle of incidence theta=65°. Our data confirm the vectorial photoemission model. Issues concerning surface roughness and symmetry considerations are addressed. An explanation in terms of non local conductivity tensor is proposed. Advantages of a 6.28~eV photon as compared to the standard 4.71~eV photon in use with Cu photocathodes are discussed.

• M. Trovo, M.B. Danailov, G. Penco
  ELETTRA, Basovizza, Trieste
• W. Graves
  MIT, Middleton, Massachusetts
• S.M. Lidia
  LBNL, Berkeley, California

In the framework of the FERMI@ELETTRA project, aimed to build an X-ray FEL source, a crucial role is played by the electron source, which has to produce a very high quality bunch, in terms of low emittance and uncorrelated energy spread. We have investigated the effects of low- (100-300 5m) and high- ( 10^-50 5m) frequency modulation of the beam charge deriving from intensity modulation of the laser pulse incident on the photocathode on the downstream beam distribution. Following other proposals, we have investigated the use of a short laser 'heater' to increase the effective incoherent energy spread and reduce the gain in the longitudinal density modulation instability. We present results from simulation of the beam generation at the photocathode, and transport through the photoinjector, initial acceleration modules and the laser heater.

• G. Penco, M. Trovo
  ELETTRA, Basovizza, Trieste
• S.M. Lidia
  LBNL, Berkeley, California

In the framework of the FERMI@ELETTRA project, aimed to build an x-ray FEL source based on laser-seeded harmonic generation, a crucial role is played by the electron source, which has to produce a very high quality beam, in terms of low emittance and uncorrelated energy spread. A very attractive solution is the SLAC/BNL/UCLA 1.6 cell s-band gun III based upon the demonstrated high performance of this design and its descendants. This paper describes the results of the optimization studies based on the gun III design and carried out with two space charge tracking codes (GPT and ASTRA) for nominal operating parameters. In particular two different bunch charge regimes has been explored: low (few hundreds of pC) and high (~1nC) . In the first case, the limited charge extracted from the photo-cathode allows to propagate a bunch with an initial higher density and to compress it along the linac down to a few hundreds of fs, attaining a high peak current bunch with a very low slice emittance. The second case has been investigated in order to verify the possibility to produce a "1 ps plateau" bunch with acceptable peak current and a slice emittance lower than 2 mm mrad. We present simulation results for both cases.

• 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.

• 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.