• C. Sung, C. Joshi, C. Pellegrini, J.E. Ralph, S. Reiche, J.B. Rosenzweig, S. Tochitsky
  UCLA, Los Angeles, California

The lack of a high-power, relatively low-cost and compact terahertz (THz) source in the range 0.3-3x10(12) Hz is the major obstacle in progressing on biomedical and material studies at these wavelengths. A high-gain, single pass seeded FEL technique allows to obtain high power THz pulses of a high spectral brightness. We describe an ongoing project at the Neptune laboratory where a ~ 1kW seed pulse generated by difference frequency mixing of CO2 laser lines in a GaAs nonlinear crystal is injected into a waveguide FEL amplifier. The FEL is driven by a 5 ps (r.m.s) long electron pulse with a peak current up to 100A provided by a regular S-band photoinjector. According to 3-D, time dependent simulations, up to ~ 10 MW THz power can be generated using a 2 meter long planar undulator. By mixing different pairs of CO2 laser lines and matching resonant energy of the electron beam, tunability in the 100-400 mm range is expected. A tunable Fabri-Perot interferometer will be used to select a high-power 5ps THz pulse. This pulse is synchronized both with 1mm (photoinjector driver) and 10 mm lasers allowing time resolved pump-probe measurements.

• G. Andonian, A.Y. Murokh, C. Pellegrini, S. Reiche, J.B. Rosenzweig, G. Travish
  UCLA, Los Angeles, California
• M. Babzien, I. Ben-Zvi, V. Litvinenko, V. Yakimenko
  BNL, Upton, Long Island, New York
• I. Boscolo, S. Cialdi, A.F. Flacco
  INFN-Milano, Milano
• M. Ferrario, L. Palumbo, C. Vicario
  INFN/LNF, Frascati (Roma)
• J.Y. Huang
  POSTECH, Pohang, Kyungbuk

The VISA II experiment entails use of a chirped beam to drive a high gain SASE FEL. The output radiation is diagnosed with a modified frequency resolved optical gating (FROG) technique. Sextupoles are implemented to correct the lonigtudinal aberrations affecting the high energy spread chirped beam during transport to the undulator. The double differential energy spectrum is measured with a pair of slits and a set of gratings. In this paper, we report on start-to-end simulations, radiation diagnostics, as well as intial experimental results; experimental methods are described.

• T. Watanabe, D.F.L. Liu, J.B. Murphy, I.P. Pinayev, J. Rose, T.V. Shaftan, J. Skaritka, T. Tanabe, T. Tsang, X.J. Wang, L.-H. Yu
  BNL, Upton, Long Island, New York
• S. Reiche
  UCLA, Los Angeles, California
• P. Sprangle
  NRL, Washington, DC

The design of a Short Rayleigh Length (SRL) FEL amplifier based on the strong focusing VISA undulator [1] is presented in this study. The SRL FEL amplifier will be operating in the IR (0.8 - 1 μm), and consists of a two-meter VISA undulator with a peak seed laser power of about 1 kW. The FEL power and transverse mode evolution along the undulator were investigated using the three-dimensional numerical code GENESIS1.3. The evolution of the FEL output from the undulator exit to the first downstream optics is also studied. The possibility of using the proposed amplifier for a two-stage cascaded HGHG FEL [2] at the BNL SDL is also explored. The design parameters and the numerical results will be presented.

[1] R. Carr et al., PRSTAB, Vol. 4, 122402 (2001). [2] J. Wuard and L.H. Yu, NIMA 475, 104 (2001).

• S. Reiche, C. Joshi, C. Pellegrini, J.B. Rosenzweig, S. Tochitsky
  UCLA, Los Angeles, California
• G. Shvets
  The University of Texas at Austin, Austin, Texas

Funding: The work was supported by the DOE Contract No. DE-FG03-92ER40727.

Free-Electron Laser in the THz range can be used to generate high output power radiation or to modulate the electron beam longitudinally on the radiation wavelength scale. Microbunching on the scale of 1-5 THz is of particular importance for potential phase-locking of a modulated electron beam to a laser-driven plasma accelerating structure. However the lack of a seeding source for the FEL at this spectral range limits operation to a SASE FEL only, which denies a subpicosecond synchronization of the current modulation or radiation with an external laser source. One possibility to overcome this problem is to seed the FEL with two external laser beams, which difference (beatwave) frequency is matched to the resonant FEL frequency in the THz range. In this presentation we study feasibility of an experiment on laser beat-wave injection in the THz FEL considered at the UCLA Neptune Laboratory, where both a high brightness photoinjector and a two-wavelength, TW-class CO₂ laser system exist. By incorporating the energy modulation of the electron beam by the ponderomotive force of the beat-wave in a modified version of the time-dependent FEL code Genesis 1.3, the performance of a FEL at Neptune is simulated and analyzed.

• 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).

• T. Watanabe, D.F.L. Liu, J.B. Murphy, J. Rose, T.V. Shaftan, Y. Shen, T. Tsang, X.J. Wang, L.-H. Yu
  BNL, Upton, Long Island, New York
• L. Giannessi, S. Spampinati
  ENEA C.R. Frascati, Frascati (Roma)
• P. Musumeci
  Universita di Roma I La Sapienza, Roma
• S. Reiche
  UCLA, Los Angeles, California

Funding: Work suppoted by the Brookhaven National Lab and Office of Naval Research

The SDL facility at BNL[1] is an excellent platform to explore some of the recent ideas related to superradiance in a seeded single pass FEL. At the SDL facility there is an operating FEL with a Ti:Sapphire seed laser and a high brightness e-beam with an energy up to 250 MeV. Seeding may be realized with pulses shorter than the e-beam bunch length to induce the superradiant regime. A status report concerning this experiment will be presented.

[1] A. Doyuran et al., PRSTAB, Vol. 7, 050701 (2004).

• S. Reiche
  UCLA, Los Angeles, California

The Linac Coherent Light Source operates as a Self-Amplified Spontaneous Emission Free-Electron Laser (SASE FEL), where transverse coherence is achieved by the domination of the FEL Eigenmode with the largest growth rate. However complete transverse coherence is not guaranteed because there are multiple eigenmodes with similar growth rates for a low-diffracting FEL, such as the LCLS. In addition the mode purity can be degraded by collective electron beam motion. In this presentation the transverse coherence for the LCLS pulse is investigated with respect to scattering on crystals. The degradation in the contrast and size of the Bragg peaks is analysed for a step wise improved modeling of the experiment (stead-state, time-dependent and start-end simulations). The impact on diffraction experiments, including the proposed experiment to measure the transverse coherence, is discussed.

• L. Palumbo
  Rome University La Sapienza, Roma
• D. Alesini, M. Bellaveglia, S. Bertolucci, M.E. Biagini, R. Boni, M. Boscolo, M. Castellano, A. Clozza, G. Di Pirro, A. Drago, A. Esposito, M. Ferrario, D. Filippetto, V. Fusco, A. Gallo, A. Ghigo, S. Guiducci, M.  Migliorati, A. Mostacci, L. Pellegrino, M.A. Preger, C. Sanelli, M. Serio, F. Sgamma, B. Spataro, A. Stella, F. Tazzioli, C. Vaccarezza, M. Vescovi, C. Vicario
  INFN/LNF, Frascati (Roma)
• F. Alessandria, A. Bacci
  INFN/LASA, Segrate (MI)
• F. Broggi, S. Cialdi, C. De Martinis, D. Giove, C. Maroli, M. Mauri, V. Petrillo, M. Rome, L. Serafini
  INFN-Milano, Milano
• L. Catani, E. Chiadroni, A. Cianchi, C. Schaerf
  INFN-Roma II, Roma
• F. Ciocci, G. Dattoli, A. Doria, F. Flora, G.P. Gallerano, L. Giannessi, E. Giovenale, G. Messina, P.L. Ottaviani, G. Parisi, L. Picardi, M. Quattromini, A. Renieri, C. Ronsivalle
  ENEA C.R. Frascati, Frascati (Roma)
• P. Emma
  SLAC, Menlo Park, California
• M. Mattioli
  Universita di Roma I La Sapienza, Roma
• P. Musumeci
  INFN-Roma, Roma
• S. Reiche, J.B. Rosenzweig
  UCLA, Los Angeles, California

The first phase of the SPARX project, now funded by MIUR (Research Department of Italian Government), is an R&D activity focused on developing techniques and critical components for future X-ray FEL facilities. This project is the natural extension of the activities under development within the ongoing SPARC collaboration. The aim is the generation of electron beams characterized by an ultra-high peak brightness with a linear accelerator based on the upgrade of the existing Frascati 800 MeV LINAC and to drive a single pass FEL experiment in the range of 3-5 nm, both in SASE and SEEDED FEL configurations, exploiting the use of superconducting and exotic undulator sections. In this paper we discuss the present status of the collaboration.