• H. Hama
  LNS, Sendai

More than 20 years ago, D.A.G. Deacon proposed an isochronous storage ring for FEL to avoid bunch heating and decreasing instantaneous gain [1]. Some of low momentum compaction (alpha) operations have been carried out, and recently coherent infrared radiation are observed on a 3rd generation light source. Because the 3rd generation rings are optimized to obtain very low emittance beam, the dispersion function in the arc sections are much reduced by introducing large bending radius, so that those are very big machines. Meanwhile N.A. Vinokurov et al. recently proposed a ring type SASE FEL based on a complete isochronous bending transport [2]. At least, experimental and theoretical study of the isochronous ring so far suggests nonlinear effects resulted from higher order dispersion and chromaticity declines the "complete" isochronous system. On the other hand, in a wavelength region of THz, tolerance of the path length along a turn of the ring seems to be within our reach. A concept to preserve of a form factor of microbunch or very short bunch by using no-dispersion bend for a ring source of THz coherent radiation and its extension toward short wavelength and SASE FEL will be discussed.

[1] D.A.G. Deacon, Phys. Rep. 76 (1981) 349. [2] N.A. Vinokurov, O.A. Shevchenko, Nucl. Instr. and Meth. A 528 (2004) 491.

• S. Schreiber
  DESY, Hamburg

The VUV-FEL is a free electron laser user facility being commissioned at DESY. It is based on the TTF-FEL, which was in operation until end of 2002 providing a photon beam for two pilot experiments in the wavelength range of 80 to 120 nm. In its final configuration, the new VUV-FEL is designed to produce SASE FEL radiation with a wavelength down to 6 nm with high brilliance. The commissioning started in fall 2004, and in January 2005 succeeded in first lasing in the SASE mode at a wavelength of 32 nm with a radiation power in the saturation range. This is a major milestone of the facility and of SASE FELs in general. This contribution reports on the present the electron linac driving the FEL, on properties of the electron beam and on the characterization of the FEL photon beam.

• M. Abo-Bakr, R. Follath, A. Meseck
  BESSY GmbH, Berlin

Funding: Funded by the Bundesministerium für Bildung und Forschung, the state of Berlin and the Zukunftsfonds Berlin

BESSY proposes a soft X-ray free electron laser (FEL) multi-user facility. It will consist of three undulator lines, each based on a cascaded High-Gain Harmonic-Generation (HGHG) scheme. With a seed laser, tunable between 230 nm and 460 nm, the desired output radiation wavelength range from 1.24 nm to 51 nm can be covered. Signal to noise ratio and coherence of the HGHG FEL radiation degrades quadratically with the harmonic number. For the short-wavelength BESSY-FEL line, operating on the 225th harmonic of the seed, a cure to this effect and maintaining the coherence is to improve the spectral purity of the output radiation by implementation of a "non-dispersive double-monochromator" system between two HGHG stages. Layout and parameters of such a monochromator section are described. To separate the electron beam path from the optical devices a bypass section is needed. Its design is presented and influences on the electron beam dynamics are discussed. Simulations of the full cascaded HGHG FEL, using the restored seed radiation and the bypassed electron beam, are presented.

• K. Goldammer, M. Abo-Bakr, R. Follath, A. Meseck
  BESSY GmbH, Berlin

Funding: Funded by the Bundesministrium für Bildung, und Forschung, the state Berlin and the Zukunftsfonds Berlin

BESSY proposes a linac-based High-Gain Harmonic-Generation (HGHG) free electron laser (FEL) facility with three independent FEL lines. In the BESSY High-Energy-FEL (HE-FEL), a seed laser wavelength of 280nm is downconverted to 1.24nm by a cascade of four HGHG-stages. This procedure requires a high brightness electron beam and a high power seed laser. With the nominal set of beam parameters, radiation power in the range of GWs can be achieved. However, the signal to noise ratio degrades in each HGHG stage. This motivated intensive studies on the possibilities to further optimize the performance of the BESSY HE-FEL. In this paper, we report on three methods aiming to control the signal to noise ratio. They include simulation studies of new seeding schemes with HHG-lasers at shorter wavelengths and seeding with higher seed powers. Also, a concept for the integration of monochromators between two HGHG-stages has been worked out, see also [1]. All methods were studied extensively with regard to their influence on FEL output power, pulse duration and spectral bandwidth.

[1] M. Abo-Bakr et al., these Proceedings

• K. Goldammer, A. Meseck
  BESSY GmbH, Berlin

In the FEL process, bunching and coherent radiation is produced at the fundamental frequency as well as its higher harmonics. BESSY proposes a linac-based cascaded High-Gain Harmonic-Generation (HGHG) free electron laser (FEL) multi-user facility. The BESSY soft X-ray FEL will be seeded by three lasers spanning the spectral range of 230nm to 460nm. Two to four HGHG stages downconvert the seed wavelength to the desired radiation range of 1.24nm to 51nm using higher harmonic bunching. As a surplus, higher harmonic radiation is intrinsically produced in each FEL stage. Radiation on a higher harmonic of the FEL frequency is of high interest because it yields the possibility to reduce the number of FEL stages. This paper details extensive studies of the higher harmonic content of the BESSY FEL radiation. Important aspects of FEL interaction on higher harmonics as resulting from theory and from numerical simulations are discussed. For the case of the BESSY FEL, methods for improving the harmonic content are presented. These methods are examined as to their influence on FEL output power, pulse duration and spectral bandwidth. Focus is laid on the application of higher harmonic radiation to seeding.

Funded by the Bundesministerium für Bildung und Forschung, the state of Berlin and the Zukunftsfonds Berlin

• 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.

• B.C. Kuske, M. Abo-Bakr, A. Meseck
  BESSY GmbH, Berlin

Funding: Bundesministerium für Bildung und Forschung, the state of Berlin and the Zukunftsfonds Berlin

In present FEL designs, undulators are usually optimized for an electron bunch with properties constant along the bunch length. The central energy, emittance and other parameters are assumed not to vary from slice to slice. Unavoidable timing jitter of the photo cathode laser, and phase and amplitude errors of the RF fields in the injector and the linac result in variations in emittance, energy spread and beam dimensions along the bunch, causing a jitter in the arrival time of the electron bunch. Due to the passage through bunch compressors, the bunch shows a considerable residual energy chirp. Even assuming a perfect and on-time seed laser pulse, the changing properties along the bunch in combination with the arrival time jitter cause varying conditions for the interaction of the electron bunch with the seed laser radiation. This paper talks about how far the BESSY-FEL radiation is affected by the expected time jitter and the realistic bunch profile, and investigates counter measures. The studies are confined to the low energy FEL line generating output at l = 10nm.

• B.C. Kuske, M. Abo-Bakr, K. Goldammer, A. Meseck
  BESSY GmbH, Berlin

Funding: Bundesministerium für Bildung und Forschung, the state of Berlin and the Zukunftsfonds Berlin

Contrary to storage rings, where the electron bunch properties are damped to equilibrium values due to the repeated passages through identical structures, every bunch in a single pass FEL will show individual imprints of it's passage through the linac. Based on ASTRA and ELEGANT tracking studies, realistic bunches were tracked through the BESSY-FEL undulators; the effect of timing errors of the photo cathode laser, and phase and amplitude errors of the RF fields in the injector and the linac on the FEL radiation were studied. The fluctuations of the bunch parameter due to these errors are of the order of magnitude of their variation over the bunch length, reflecting the initial electron distribution and the impact of the passed optics. The unavoidable residual energy chirp in connection with the timing jitter is of concern. The expected shot to shot variations in the FEL output are discussed.

• A. Meseck, M. Abo-Bakr, J. Bahrdt, B.C. Kuske
  BESSY GmbH, Berlin

Funding: Funded by the Bundesministrium für Bildung, und Forschung, the state Berlin and the Zukunftsfonds Berlin

BESSY proposes a soft X-ray free electron laser (FEL) multi-user facility. It will consist of three undulator lines, each based on cascaded High-Gain Harmonic-Generation (HGHG) scheme delivering photons in energy range of 24 eV to 1 keV. Start-to-end Simulations including error sources from the injector, and linac structure have been performed to provide realistic information about the expected radiation field (B. Kuske, FEL2005). However, the beamline designer needs to know the exact location and the size of the photon beam waist to maximize the brightness at the sample. This information can be derived from results of longitudinal propagation of the electric field distribution, which can be extracted from simulation results using the code GENESIS. The results of the Start-to-End simulation are used for realistic prediction of the photon beam properties of the BESSY Soft X-ray FEL.

• A. Meseck, J. Bahrdt
  BESSY GmbH, Berlin

Funding: Funded by the Bundesministrium für Bildung, und Forschung, the state Berlin and the Zukunftsfonds Berlin

Wide-range wavelength-tunability is one of the key aspects of proposed FEL facilities. Once the electron beam energy and undulator period length is given, the span of the available K-values determines the achievable wavelength range, according to the resonance condition. As the usable range of the K-values is limited by technical considerations like minimum acceptable gap or permanent magnet technology etc, alternatives to enlarge the output wavelength range are of high interest. Using revolver-undulator design, different magnetic structures can be incorporated in the same undulator segment. Thus it is possible to switch between different undulator periods, covering a wider wavelength range at a given FEL-line. Because of the transverse-positions dependency of the magnetic field, the alignment reproducibility of the revolver-undulators is of concern, in particular for the APPEL type devices. Simulation studies have been performed taking the BESSY FEL-lines as examples to investigate the alignment tolerances of these devices, to reveal their limits of applicability.

• E. Saldin, E. Schneidmiller, M.V. Yurkov
  DESY, Hamburg

Recent theoretical and experimental studies have shown that SASE FEL with a planar undulator holds a potential for generation of relatively strong coherent radiation at the third harmonic of the fundamental frequency. Here we present detailed study of the nonlinear harmonic generation in SASE FEL obtained with time-dependent FEL simulation code FAST. Using similarity techniques we present universal dependencies for temporal, spectral, and statistical properties of the third harmonic radiation from SASE FEL.

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

The FERMI at ELETTRA project at Sincotrone Trieste involves two FEL's, each based upon the principle of a seeded harmonic cascade and using the existing ELETTRA injection linac at 1.2 GeV beam energy. Scheduled to be completed in 2008, FEL-1 will operate in the 40-100 nm wavelength range and will involve one stage of harmonic up-conversion. The second phase, FEL-2, will begin operation two years later in the 10^-40 nm wavelength range and will involve two cascade stages. FEL design assumes wavelength tunability over the full wavelength range and polarization tunability of the output radiation including helical polarization. The design considers focusing properties and segmentation of realizable undulators and available input seed lasers. We discuss how the interplay between various limitations and self-consistent accelerator simulations [1,2] have led to our current design. We present results of simulations using GENESIS and GINGER simulation codes including studies of various shot-to-shot fluctuations and undulator errors. Findings for the expected output radiation in terms of the power, transverse and longitudinal coherence for the short pulse (50-200 fs) and long pulse (~1 ps) modes of operation are reported.

[1] S. Lidia et al. in these proceedings. [2] S. Di Mitri et al. in these proceedings.

• L. Giannessi, M.C. Carpanese, F. Ciocci, G. Dattoli, A. Dipace, A. Doria, G.P. Gallerano, E. Giovenale, G. Parisi, M. Quattromini, A. Renieri, C. Ronsivalle, E. Sabia, S. Spampinati, I.P. Spassovsky
  ENEA C.R. Frascati, Frascati (Roma)
• D. Alesini, M.E. Biagini, A. Drago, M. Ferrario, V. Fusco, A. Ghigo, B. Spataro, C. Vaccarezza, C. Vicario
  INFN/LNF, Frascati (Roma)
• M. Bougeard, B. Carre, M.-E. Couprie, D. Garzella, M. LABAT, G. Lambert, H. Merdji, P. Salieres
  CEA/Saclay, Gif-sur-Yvette
• M. Mattioli, P. Musumeci, M. Petrarca
  Universita di Roma I La Sapienza, Roma
• M.  Migliorati, L. Palumbo
  Rome University La Sapienza, Roma
• M. Nisoli, S. Stagira, S. de Silvestri
  Politecnico/Milano, Milano
• L. P. Poletto, G. T. Tondello
  Univ. degli Studi di Padova, Padova

Funding: Work supported by the EU Commission in the sixth framework programme, contract no. 011935 – EUROFEL.

Sources based on high order harmonics generated in gas with high power Ti:Sa lasers pulses represent promising candidates as seed for FEL amplifiers for several reasons, as spatial and temporal coherence, wavelength tunability and spectral range, which extends down to the 10(-9)m wavelength scale. This communication is devoted to the description of a research work plan that will be implemented at the SPARC FEL facility in the framework of the EUROFEL programme. The main goal of the collaboration is to study and test the amplification and the FEL harmonic generation process of an input seed signal obtained as higher order harmonics generated both in crystal (400nm and 266 nm) and in gas (266nm, 160nm, 114nm) from a high intensity Ti:Sa laser pulse.

• R. Bonifacio, R. Bonifacio
  Universidade Federal de Alagoas, Maceio
• M. Ferrario
  INFN/LNF, Frascati (Roma)
• N. Piovella
  Universita' degli Studi di Milano, MILANO
• G.R.M. Robb
  Strathclyde University, Glasgow
• A. Schiavi
  Rome University La Sapienza, Roma
• L. Serafini
  INFN-Milano, Milano

Funding: Istituto Nazionale di Fisica Nucleare (INFN), Italy

Quantum effects in high-gain FELs become relevant when ρ'=ρ(mcγ/ ћ k)<1. The quantum FEL parameter ρ' rules the maximum number of photons emitted per electrons. It has been shown that when ρ'<1 a "quantum purification" of the SASE regime occurs: in fact, the spectrum of the emitted radiation (randomly spiky in the usual classical SASE regime) shrinks to a very narrow single line, leading to a high degree of temporal coherence. From the definition of ρ it appears that in order to achieve the quantum regime, small values of ρ, beam energy and radiation wavelength are necessary. These requirements can be met only using a laser wiggler. In this work we state the scaling laws necessary to operate a SASE FEL in the Angstrom region. All physical quantities are expressed in terms of the normalized emittance and of two parameters: the ratio between laser and electron beam spot sizes and the ratio between Rayleigh range and electron β-function. The feasibility study of a Quantum SASE FEL experiment using parameters as those foreseen in the SPARC/PLASMONX projects in construction at the INFN Frascati is explicitly discussed.

• N. Thompson, M.W. Poole
  CCLRC/DL/ASTeC, Daresbury, Warrington, Cheshire
• B.W.J. McNeil
  Strathclyde University, Glasgow

Funding: We acknowledge the support of the European Framework Programme 6 EUROFEL Design Study, CCLRC, and the Scottish Universities Physics Alliance.

A Free-Electron Laser operating in the photon energy range 3-10eV is a component of the 4th Generation Light Source (4GLS) proposal at Daresbury Laboratory in the UK. In this paper we present a current design proposal which is based on the Regenerative Amplifier Free-Electron Laser (RAFEL) concept. We also present simulation results which illustrate the potential performance of the device.

• 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.

• 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.

• T. Watanabe, D.F.L. Liu, J.B. Murphy, J. Rose, T.V. Shaftan, T. Tsang, X.J. Wang, L.-H. Yu
  BNL, Upton, Long Island, New York
• P. Sprangle
  NRL, Washington, DC

Funding: Work supported by the Office of Naval Research.

A laser seeded near IR FEL amplifier experiment was initiated at the BNL SDL [1] to explore various schemes of FEL efficiency improvement and generation of short Rayleigh length (SRL) FEL output. The FEL achieved first SASE lasing at 0.8 μm on May 6, 2005. The experimental characterization of the laser seeded FEL output power, spectrum and transverse mode structure evolution will be presented.

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

• P. Michel, P. Evtushenko, U. Lehnert, Ch. Schneider, R. Schurig, W. Seidel, J. Teichert, D. Wohlfarth
  FZR, Dresden

A FEL in the mid infrared range is one of the applications of the ELBE cw-electron accelerator. The successful operation of the lasing process for the different wavelength is mainly determined by the alignment of the optical cavities, the bunch length and the energy spread of the electron beam so as the transversal adjustment of the beam through the FEL. The energy spread and the bunch length of the electron beam have their minima at different phase conditions of the accelerator. For various energy settings of the accelerator a special adjustment of both parameters has to be found for the lasing process. The presentation describes the diagnostic tools used at ELBE for the correct alignment of the optical cavity, the steering of the electron beam through the FEL and the adjustment of the electron beam parameters with respect to energy spread and bunch length.

• H. Schlarb, V. Ayvazyan, F. Ludwig, D. Noelle, B. Schmidt, S. Simrock
  DESY, Hamburg
• F.X. Kaertner
  MIT, Cambridge, Massachusetts
• A. Winter
  Uni HH, Hamburg

The control and stabilization of the longitudinal beam profile and the bunch arrival time in linac driven VUV or X-ray Free-Electron Lasers require special effort and new developments in the fields of low level RF controls, global synchronization systems and longitudinal beam feedbacks. In this paper we describe the required upgrades for the VUV-FEL at DESY to synchronize the FEL pulse and optical lasers to the level of hundred femtoseconds (FWHM).

• G. D'Auria, P. Craievich, M. Ferianis, M.M. Milloch
  ELETTRA, Basovizza, Trieste
• dc. Cheever, T. Zwart
  MIT, Middleton, Massachusetts
• L.R. Doolittle, A. Ratti
  LBNL, Berkeley, California

The VUV soft x-ray FEL user facility, FERMI@Elettra, will use the existing 1.2 GeV linac to produce, in two separate phases, 100-40 nm and 40-10 nm, intense photon beams with single stage and double stage harmonic generation schemes respectively. To fulfill the stringent requirements of the project the present RF systems will be completely revised and upgraded. The work presented here describes the present performances of the system and plans for the linac upgrades to meet the required system specifications for FEL operation.

• 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.

• M. Ferianis
  ELETTRA, Basovizza, Trieste
• J.M. Byrd, J.W.  Staples, R.B. Wilcox
  LBNL, Berkeley, California
• J. Chen, F.O. Ilday, F.X. Kaertner, J. Kim
  MIT, Cambridge, Massachusetts
• A. Winter
  Uni HH, Hamburg

Fermi is the fourth generation light source that is currently being designed at ELETTRA, in the frame of a collaboration that includes LBNL and MIT. The timing system will play a crucial role in achieving the expected performance of this and other Linac based FELs due to the sub-ps electron bunch length and the expanded use of fs-lasers as key components in future light sources. Furthermore, the requirements of the timing system are also tightly linked to the applications of the generated ultrafast x-ray pulses. In this paper we present the requirements for the FERMI timing system, which will be based on optical timing distribution concepts, currently seen to be the only technique to enable an RMS jitter at the 10fs level. The timing system, intended for a user facility that is operated on a 24-h, 7-d basis, must operate stable and reliable. The fundamental components of the system are analyzed, such as the optical reference oscillator, the fiber optic stabilized links and the local optical to electrical (O/E) converters, needed for the RF plant synchronization. Furthermore, solutions for the synchronization of the diagnostic tools for the Linac as well as user related synchronization issues are presented and discussed.

• G.M. Kazakevich
  Fermilab, Batavia, Illinois
• Y.U. Jeong, B.C. Lee, S.-H. Park
  KAERI, Daejon
• G.I. Kuznetsov
  BINP SB RAS, Novosibirsk
• V. M. Pavlov
  Strathclyde University, Glasgow

Funding: Budker Institute of Nuclear Physics RAS, Academician Lavrentyev 11, Novosibirsk, 630090, Russia; Laboratory for Quantum Optics, Korea Atomic Energy Research Institute, P. O. Box 105, Yusong, Taejon, 305-600, South Korea.

A reliable injection system of the widely tunable microtron-based terahertz Free Electron Laser (FEL) has been developed and during last few years provides stable operation of the FEL for users. The system is based on the long-life thermionic cathode assembly using 2.5 mm-in diameter monocrystalline LaB6 emitter, heated by the tungsten cylindrical filament with the power consumption less than 50 W. The cathode emits the macro-pulse current in the range of 1-1.4 A providing operation of the terahertz FEL during more than 1000 h. The cathode assembly is installed on the cover of the I-type microtron accelerating cavity in location providing an efficient injection for the acceleration with variable number of orbits. This variation widely changes the energy of the electron beam and allows on-the-fly retuning of the FEL in the range of 1-3 THz. Pulse-signal system stabilizing the emission current prevents randomized break-downs in the accelerating cavity and decreases macro-pulse power fluctuations of the FEL radiation. The fluctuations were measured to be less than 10% during long-time operation.

• 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.

• D. Hardy, S.V. Benson, M.D. Shinn, S. Zhang
  Jefferson Lab, Newport News, Virginia

Funding: This work supported by the Office of Naval Research, the Joint Technology Office, the Commonwealth of Virginia, the Air Force Research Laboratory, the Army Night Vision Lab, and by DOE Contract DE-AC05-84ER40150.

A real-time spectrograph with a 1Hz update rate was designed and installed at the JLab FEL facility using a Cal Sensors PbSe array and a Roper Scientific SpectraPro 300 monochrometer. This paper describes the implementation of EPICS channel access on a remote PC running LabView with modification of vendor supplied LabView VI's to allow display of FEL light spectra in real-time on a remote workstation. This allows PC based diagnostics to be used in EPICS

• G. Parisi, F. Ciocci, G. Dattoli, L. Giannessi, G. K. V. Voykov
  ENEA C.R. Frascati, Frascati (Roma)

In the framework of the SPARC FEL experiment, a 14-m long undulator divided into six sections is presently under construction (by ACCEL Instruments GmbH). In order to correct the phase difference between the electron beam and the radiation, a tunable device will be inserted at the end of each of the six sections of the undulator. In this paper a preliminary design of this device, performed with RADIA code, is proposed,. The phase shifter consists of two groups of a few permanent magnets, arranged as in the main undulator in a variable gap magnet assembly. Adjusting the gap allows to correct the electron-radiation phase difference. Results from beam dynamics simulation, performed with GENESIS code, show the impact of the phase shifter on the characteristics of the output laser field.

• T. Fukui, T. Kii, K. Kusukame, K. Masuda, Y. Nakai, H. Ohgaki, T. Yamazaki, K. Yoshikawa, H. Zen
  Kyoto IAE, Kyoto

We have constructed system for 3~20μmeter FEL oscillation for bio/chemical energy researches which consists of a thermionic RF gun and a 3-meter accelerating tube at the Institute of Advanced Energy, Kyoto University. [1] A 30 MeV electron beam has been successfully accelerated and studies on the beam characterization have also been carried out. [2] In parallel, evaluation of an FEL oscillation in KU-FEL has been performed. We will report our present expectations and future visions about undulators in KU-FEL. The former is the numerical studies on the expected FEL gain of the existing Halbach type undulator in KU-FEL based on experimental measurements of the undulator and beam parameters. The latter is the upgrade plan of the undulator and the design of a variable polarization undulator to obtain higher gain or circular polarized FEL. These undulator parameters are calculated by simulation code Radia and TDA3D.

[1] T. Kii, et al., Proceedings of the 2004 FEL conference, (2004) 447 [2] K. Masuda et al., Proceedings of the 2004 FEL conference, (2004) 450

• C. Pellegrini
  UCLA, Los Angeles, California

Funding: Work funded by the US Department of Energy, grant 4-444025-PG-57689

We study the use of Radio Frequency electromagnetic waves as undulators for short wavelength FELs and undulator radiation sources. Magnetostatic undulators have a gap much smaller than the period, limiting how short a period we can use. The relation between period and gap can be overcome using electromagnetic waves to produce the force wiggling the electrons. The wave frequency is chosen to optimize the system performance. In the case of centimeters or mm waves a waveguide is used to propagate the field over a long distance. We call an undulator based on a waveguide a TWU. In this paper we show that a TWU using X-band RF is a practical and convenient device for short wavelength FELs, and to produce sub-nanometer undulator radiation circularly or linearly polarized.The recent development of high power X-band microwave sources make it possible today to build TWUs of practical interest. In this paper we will discuss the characteristic of the TWU, how to control the effects of RF power losses in the waveguide walls, and how to optimize a TWU and the associated electron transport system for use in a synchrotron radiation source or FEL.

• L. Giannessi, S. Spampinati
  ENEA C.R. Frascati, Frascati (Roma)
• P. Musumeci
  Universita di Roma I La Sapienza, Roma

The advances in laser technology have made available very short and intense laser pulses which can be used to seed a high gain single pass Free-Electron Laser (FEL) amplifier. With these seed pulses, a regime of the FEL interaction where the radiation evolution is simultaneously dominated by non-linear effects (saturation) and time-dependent effects (slippage) can be explored. This regime is characterized by the propagation of a solitary wave-like pulse where the power of the optical wave grows quadratically with time, its pulse length decreases and the spectral bandwidth increases. We analyze the interplay between the field and particles dynamics of this propagation regime which was studied before and termed superradiance. Furthermore we analyze the properties of the strong higher order harmonics emission from this wave and its behaviour when propagating in a cascade FEL. The superradiant pulse is indeed capable of passing through the stages of a cascade FEL and to regenerate itself at the wavelength of the higher order harmonic. The optical pulse obtained is shorter than a cooperation length and is strongly chirped in frequency, thus allowing further longitudinal compression down to the attosecond time-scale.

• T.V. Shaftan, S. Krinsky, D.F.L. Liu, J.B. Murphy, J. Rose, X.J. Wang, T. Watanabe, L.-H. Yu
  BNL, Upton, Long Island, New York
• H. Loos
  SLAC, Menlo Park, California

Funding: The manuscript has been authored by Brookhaven Science Associates, LLC under Contract No. DE-AC02-98CH1-886 with the U.S. Department of Energy.

We discuss analysis of an experiment with High-Gain Harmonic Generation Free Electron Laser at BNL (DUV FEL). The tunability concept [1] of a seeded FEL with a fixed seed wavelength has been verified experimentally. During the experiment we recorded about 200 radiation spectra corresponding to different energy chirps in the electron beam. We have analyzed this set of spectral data to obtain properties of HGHG radiation. Correlations and trends in the radiation spectrum at 266 nm have been observed and studied.

[1] T. Shaftan and L.H. Yu, Phys. Rev. E 71, 046501 (2005)

• E. Saldin, E. Schneidmiller, M.V. Yurkov
  DESY, Hamburg

Present bunch compression scheme at the VUV FEL is essentially nonlinear and naturally results in a formation of a short high-current leading peak (spike) in the density distribution that produces FEL radiation. The main feature of the considered mode of operation is the production of short (15-50 fs FWHM) radiation pulses with GW-level peak power that are attractive for many users. In this paper we present main parameters of the SASE FEL radiation: temporal and spectral characteristics, intensity distributions, etc. We also analyze statistical properties of the radiation from a SASE FEL driven by short electron bunches. In the linear regime the radiation from a SASE FEL is a Gaussian random process. When approaching saturation point, statistical properties of the radiation change drastically on a scale of one field gain length. Particular attention is devoted to the analysis of fluctuations of total energy in the radiation pulse and after a narrow-band monochromator. It is shown that fluctuations at saturation are significantly suppressed when electron pulse length becomes comparable with cooperation length.

• I. Ben-Zvi, D. Kayran, V. Litvinenko
  BNL, Upton, Long Island, New York

Historically, the first demonstration of the FEL was in an amplifier configuration at Stanford University. There were other notable instances of amplifying a seed laser, such as the LLNL amplifier and the BNL ATF High-Gain Harmonic Generation FEL. However, for the most part FELs are operated as oscillators or self amplified spontaneous emission devices. Yet, in wavelength regimes where a conventional laser seed can be used, the FEL can be used as an amplifier. One promising application is for very high average power generation, for instance a 100 kW average power FEL. The high electron beam power, high brightness and high efficiency that can be achieved with photoinjectors and superconducting energy recovery linacs combine well with the high-gain FEL amplifier to produce unprecedented average power FELs with some advantages. In addition to the general features of the high average power FEL amplifier, we will look at a 100 kW class FEL amplifier is being designed to operate on the 0.5 ampere Energy Recovery Linac which is under construction at Brookhaven National Laboratory's Collider-Accelerator Department.

• Y.-C. Huang, Y.-Y. Lin
  NTHU, Hsinchu

Funding: Center for Advanced Information System and Electronics Research(CAISER)

A Smith-Purcell radiator produces transversely asymmetric radiation modes due to the arrangement of a grating on one side of the electron beam. This asymmetric output could limit the usefulness of such a device in the THz spectrum where diffraction of waves is severe. It is possible to produce symmetric radiation from a double-grating waveguide driven by an electron beam traversing the waveguide gap. We derive a theory that describes the modes and small signal gain of this novel grating-waveguide free-electron laser. Our theory shows that extremely high laser gain is obtained when the electron beam is phase matched to the middle or edge of the radiation bands where the radiation modes have zero group velocity. In our calculation we obtained 66dB/mm gain at 298 &micro;m for a 5 mA, 30keV driving beam in a grating waveguide with a 50-micron, 40% duty-cycle grating period, a 60-micron groove depth, and a 150 micron waveguide gap. This extremely high gain indicates that this novel device establishes resonance without resonator mirrors in a one-dimensional photonic-crystal lattice or from distributed feedbacks in the grating pairs. Experimental progress will be reported in the conference.

• W. Decking
  DESY, Hamburg

X-ray Free-Electron Lasers facilities are planned or already under construction around the world. This talk covers the X-Ray Free-Electron Lasers LCLS (SLAC), European XFEL (DESY) and SCSS (Spring8). All aim for self-amplified spontaneous emission (SASE) FEL radiation of approximately 0.1 nm wavelengths. The required excellent electron beam qualities pose challenges to the accelerator physicists. Space charge forces, coherent synchrotron radiation and wakefields can deteriorate the beam quality. The accelerator physics and technological challenges behind each of the projects will be reviewed, covering the critical components low-emittance electron gun, bunch-compressors, accelerating structures and undulator systems.

• G. Geloni, E. Saldin, E. Schneidmiller, M.V. Yurkov
  DESY, Hamburg

Nonlinear generation of coherent harmonic radiation is an important option in the operation of a X-ray FEL facility since it broadens the spectral range of the facility itself, thus allowing for a wider scope of experimental applications. We found that up-to-date theoretical understanding of second harmonic generation is incorrect. Derivation of correct radiation characteristics will follow our criticism.

• E. Saldin, E. Schneidmiller, M.V. Yurkov
  DESY, Hamburg

Influence of a linear energy chirp, imposed on the electron beam, on SASE process is studied analytically and numerically.

• V. Kumar, K.-J. Kim
  ANL, Argonne, Illinois

Funding: Work supported by U.S. Department of Energy, Office of Basic Energy Sciences, under contract No. W-31-109-ENG-38

We present a one-dimensional time-dependent analysis and simulation of Smith-Purcell (SP) free-electron lasers (FELs). The coupled Maxwell-Lorentz equations for SP-FELs are set up, and the details of numerical simulation are presented. For an SP-FEL based on low energy (~35 keV) DC (~1 mA) electron beam from a scanning electron microscope, we show that around 100 mW of power can be generated at THz. frequency in the form of a surface electromagnetic wave due to FEL interaction. We propose a two-grating configuration where the electron beam gets bunched in the first grating and radiates copiously in the second grating due to coherent spontaneous emission, if the parameters of the second grating are optimized suitably.

• R. Tatchyn
  SLAC, Menlo Park, California

The unusually long insertion devices being prepared for Angstrom-wavelength Free Electron Lassers (FELs) will generate spectral-angular distributions in the proposed experimental areas substantially different from those conventionally calculated for the far field. In this paper we report on computational simulations of near vs. far field distributions for the SLAC linac Coherent Light Source (LCLS) undulator, an insertion device approximateely 140 meters long. The properties of the coherent radiation as a limiting case of the near-field emission, for the special condition of a microbunched beam radiating along the undulator axis, are reviewed.

• U. Lehnert, P. Michel, W. Seidel, D. Stehr, J. Teichert, D. Wohlfarth, R. Wuensch
  FZR, Dresden

First lasing of the mid-infrared free-electron laser at ELBE was achieved on May 7, 2004. Since then stable lasing has been achieved in the IR range from 4 to 22~μm using electron beam energies from 15 to 35~MeV. At all wavelengths below 20~μm a cw optical power higher than 1~W can be produced with an electron beam of 50~pC bunch charge or less. The optical pulse width at its minimum (2.2~ps measured at 17~μm) resembles the typical electron bunch length of 2~ps without bunch compression but can be increased by detuning the optical cavity. The optical bandwidth was in all cases close to the fourier limit.

• W. Seidel, E. Grosse, U. Lehnert, P. Michel, R. Schlenk, U. Willkommen, D. Wohlfarth, R. Wuensch
  FZR, Dresden
• A. Wolf
  MPI-K, Heidelberg

After successfully commissioning the mid-infrared FEL (U27) and adjoining a second accelerator unit (up to 35 MeV) at ELBE we have modified our plan how to produce radiation in the far infrared.To ensure the continuous variation of the wavelength up to 150 microns we want to complement the U27 undulator by a permanent magnet undulator with a period of 100 mm (U100). The minimum gap of 24 mm and the hybrid construction consisting of Sm/Co magnets and soft iron poles ensures sufficient radiation resistance and allows rms undulator parameters up to 2.7. The large field variation allows us to cover the whole wavelength range by only two different electron energies (e.g. 20 and 35 MeV). To reduce the transverse beam size we use a partial waveguide which is 10 mm high and wide enough to allow free propagation in horizontal direction. It spans from the last quadrupole in front of the undulator up to the downstream mirror and is somewhat longer than 8 m. To minimize the coupling losses between free propagation and the waveguide mode appropriate bifocal resonator mirrors will be used. Detailed calculations and computer simulations predict an outcoupled laser power of roughly 35 W around 40 microns and 20 W at 150 microns.

• A. Yahalom, Yu. Lurie, Y. Pinhasi
  CJS, Ariel
• A. Eliran, A. Gover
  University of Tel-Aviv, Faculty of Engineering, Tel-Aviv

We have measured spontaneous FEL radiation in the Israeli Electrostatic Accelerator FEL (EA-FEL) . The measurements were repeated numerous times in order to get information of statistical significance. The distribution of the radiation power measurements and various statistical moments derived from this distribution contain information on the electron beam statistics and its interaction with the wiggler magnetic field and the resonator that contain the radiation. In this work we present both the measured radiation power statistical data and its connection to the statistics of the electron beam through numerical and analytical models.

• A. Gover, A. Faingersh, M. Kanter, B. Kapilevich, B. Litvak, S. Peleg, Y. Socol, M. Volshonok
  University of Tel-Aviv, Faculty of Engineering, Tel-Aviv
• M. Einat, Yu. Lurie, Y. Pinhasi, A. Yahalom
  CJS, Ariel

The Israeli Electrostatic Accelerator FEL (EA-FEL) is now being upgraded towards long pulse (1005s) operation and ultra-high resolution (10(-6)) single pulse coherent spectroscopy. We present quantitative estimations regarding the applications of controlled radiation chirp for spectroscopic applications with pulse-time Fourier Transform limited spectral resolution. Additionally, we describe a novel extraction-efficiency-improving scheme based on increase of accelerating voltage (boosting) after saturation is achieved. The efficiency of the proposed scheme is confirmed by theoretical and numerical calculations. The latter are performed using software, based on 3D space-frequency domain model. The presentation provides an overview of the upgrade status: the high-voltage terminal is being reconfigured to accept the accelerating voltage boost system; a new broad band low-loss resonator is being manufactured; multi-stage depressed collector is assembled.

• R. Hajima, H. Iijima, N. Kikuzawa, E.J. Minehara, R. Nagai, T. Nishitani, M. Sawamura
  JAEA/FEL, Ibaraki-ken
• S. Okuda
  Osaka Prefecture University, Sakai
• T. Takahashi
  KURRI, Osaka

An electron beam with high-average current and short bunch length can be accelerated by energy-recovery linac. Coherent synchrotron radiation (CSR) from such an electron beam will be a useful light source around millimeter wavelength. We report results from a preliminary measurement of CSR emitted from a bending magnet of JAERI-ERL. Possible enhancement of CSR power by FEL micro-bunching is also discussed.

• E.J. Minehara, R. Hajima, H. Iijima, N. Kikuzawa, R. Nagai, N. Nishimori, T. Nishitani, M. Sawamura, T. Yamauchi
  JAEA/FEL, Ibaraki-ken

The JAERI high power ERL-FEL has been extended to the more powerful and efficient free-electron laser (FEL) than 10kW for nuclear energy industries, and other heavy industries like defense, shipbuilding, chemical industries, environmental sciences, space-debris, and power beaming and so on. In order to realize such a tunable, highly-efficient, high average power, high peak power and ultra-short pulse FEL, we need the efficient and powerful FEL driven by the JAERI compact, stand-alone and zero boil-off super-conducting RF linac with an energy-recovery geometry. Our discussions on the ERL-FEL will cover the current status of the 10kW upgrading and its applications of non-thermal peeling, cutting, and drilling to decommission the nuclear power plants, and to demonstrate successfully the proof of principle prevention of cold-worked stress-corrosion cracking failures in nuclear power reactors under routine operation using small cubic low-Carbon stainless steel samples.

• J. Blau, O.E. Bowlin, W.B. Colson, R. Vigil, T. Voughs, B.W. Williams
  NPS, Monterey, California

Funding: JTO, ONR, NAVSEA

After successfully lasing at 10 kW of average power at a wavelength of 6 &mu;m, a new electromagnetic wiggler has been installed at Jefferson Lab, which will be used to achieve high power at shorter wavelengths. Wavefront propagation simulations are used to predict system performance for weak-field gain and steady-state extraction, as the bunch charge, pulse length, electron beam radius, Rayleigh length, and mirror output coupling are varied.

• 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).

• T. Kanai, K. Awazu, S. Yoshihashi-Suzuki
  Osaka University, Suita

Free Electron Laser (FEL) at Osaka University can be continuously varied in the range of 5.0-20.0 &mu;m. A FEL has a double pulse structure. The structure consists of a train of macropulses of the pulse width 15 &mu;s, and each macropulse contains a train of 330 micropulses of the pulse width 5 ps. The tunability and short pulse afford new medical applications such as investigation of protein dynamics and ablation of soft tissues. Precise control of micropulse train is very important for medical applications using FEL because macropulse with long pulse duration sometimes leads to undesirable thermal effects. FEL pulse control system using an acousto optic modulators (AOM) was developed in order to investigate of non-thermal effect between the FEL and tissue. This system provide a very good efficiency (~60 %) and a fast switching speed (>200 ns). A phosphorylated protein was irradiated with FEL that controlled the pulse. These result confirmed that the thermal effect is controlled by pulse duration. This system will be expected as a novel tool for investigation of interaction between the FEL and normal tissue.

• D. Li
  ILT, Suita, Osaka

The conventional cavity for a free-electron laser (FEL) oscillator forms Gaussian optical beam, transversely spreading along the interaction region. The transverse divergence will induce reduction of the FEL gain by three aspects: degenerating filling factor, causing diffraction loss and limiting the effective interaction distance. Bessel optical beam has been experimentally demonstrated diffraction-free characteristic, providing a possibility of improvement of FEL gain. In this paper, we present a conceptual design of a Bessel beam cavity for the free-electron laser. This cavity generates nondiffracting optical beam in the wiggler, which can improve the filling factor, decrease the diffraction loss and elongate the effective interaction distance.

• H. Ogasawara, D. Nordlund
  SLAC, Menlo Park, California
• A. Nilsson
  New Affiliation Request Pending, Menlo Park, California

The microscopic understanding of surface chemistry requires a detailed understanding of the dynamics of elementary processes at surfaces. The ultrashort electron pulse obtained in the linear accelerator to feed the FEL can be used for generation of coherent synchrotron radiation in the low energy THz regime. With the current parameters for LCLS this corresponds to radiation with energy corresponding to excitations of low-energy vibrational modes of molecules on surfaces or phonons in substrates. The coherent radiation can coherently manipulate atoms or molecules on surfaces. In this respect a chemical reaction can be initiated by coherent atomic motion along a specific reaction coordinate. Since the THz radiation is generated from the same source as the FEL radiation full-time synchronization for pump-probe experiments will be possible. The possibility to perform time-resolved X-ray Emission Spectroscopy (XES) and X-ray Photoelectron Spectroscopy (XPS) measurements as a probe of chemical dynamics is an exciting prospect. The combination of THz and soft x-ray spectroscopy could be a unique possibility for low repetition FEL facilities for ultrafast surface chemistry studies.

• W.B. Colson, R. Vigil, T. Voughs
  NPS, Monterey, California

Funding: JTO, ONR, NAVSEA

Twenty-eight years after the first operation of the short wavelength free electron laser (FEL) at Stanford University, there continue to be many important experiments, proposed experiments, and user facilities around the world. Properties of FELs in the infrared, visible, UV, and x-ray wavelength regimes are listed and discussed.

• R. Bonifacio
  INFN-Milano, Milano
• N. Piovella
  Universita' degli Studi di Milano, MILANO

We formulate a quantum linear theory of the N-particle free-electron laser Hamiltonian model, quantizing both the radiation field and the electron motion, in the steady state regime. Quantum effects such as frequency shift, line narrowing, quantum limitation for bunching and energy spread and minimum uncertainty states are described. Using a second quantization formalism we demonstrate quantum entanglement between the recoiling electrons and the radiation field.

• R. Bonifacio
  INFN-Milano, Milano

We derive a simple analytical solution for the non linear regime in the quasi steady state situation of the high gain Free Electron Laser (FEL) and Collective Atomic Recoil Lasing (CARL) model which up to now have been described only numerically. We show that the system can be described by an exact reduced Hamiltonian which does not contain the field explicitly. We give simple analytical expressions for the field amplitude, frequency shift, bunching factor, particle average momentum and momentum spread, as well as the period of oscillations around the quasi steady state solution, in very good agreement with the numerical values.

• S.-H. Lee, S. Sasaki, I. Vasserman, D.R. Walters
  ANL, Argonne, Illinois
• D.E. Kim
  PAL, Pohang, Kyungbuk

Funding: Work supported by DOE under contract no. W-31-109-Eng-38.

A prototype vacuum chamber is being designed for use in the Linac Coherent Light Source at Stanford Linear Accelerator Center under development at the Advanced Photon Source. The chamber will be fabricated from the austenite stainless steels. In general, the magnetic properties of austenite stainless steels are affected by their compositions, processing methods and physical conditions. Austenite stainless steels are generally regarded as non-magnetic in the annealed condition and not attracted significantly by a magnet. However, cold working or welding will change their magnetic properties. This paper presents measurements use to choose a proper chamber material for LCLS undulator, to examine the fabrication processes, and to investigate the relative magnetic permeabilities of the stainless steels such as 316LN, 20Cb-3, Nitronic 33, Nitronic 40 and 310S. This paper presents the results of fabricating of 3"-long vacuum chambers along with their permeability measurements. In addition, the magnetic field variations with/without vacuum chamber under APS undulator A and numerical studies of magnetic field to the permeability of the flat/cylindrical chambers are presented.

• S. Bielawski, C. Bruni, C. Szwaj
  PhLAM/CERCLA, Villeneuve d'Ascq Cedex
• M.-E. Couprie, D. Garzella
  CEA/DSM/DRECAM/SPAM, Gif-sur-Yvette
• G. De Ninno, B. Diviacco, M. Trovo
  ELETTRA, Basovizza, Trieste
• D. Fanelli
  Universita di Firenze, Florence
• M. Hosaka, M. Katoh, A. Mochihashi
  UVSOR, Okazaki
• G.L. Orlandi
  ENEA C.R. Frascati, Frascati (Roma)
• Y. Takashima
  Nagoya University Graduate School of Engineering, Nagoya

Dynamical instabilities lead to unwanted full-scale power oscillations in many classical lasers and FEL oscillators. For a long time, applications requiring stable operation were typically performed by working outside the problematic parameter regions. A breakthrough occurred in the nineties [1], when emphasis was made on the practical importance of unstable states (stationary or periodic) that coexist with unwanted oscillatory states. Indeed, although not observable in usual experiments, unstable states can be stabilized, using a feedback control involving arbitrarily small perturbations of a parameter. This observation stimulated a set of works leading to successful suppression of dynamical instabilities (initially chaos) in lasers, sometimes with surprisingly simple feedback devices [2]. We will review a set of key results, including in particular the recent works on the stabilization of mode-locked lasers, and of the super-ACO, ELETTRA and UVSOR FELs [3].

[1] Ott et al. Phys. Rev. Lett., 64, 1196 (1990). [2] Bielawski et al. Phys. Rev. A 47, 327 (1993). [3] Bielawski et al. Phys. Rev. E. 69, 045502 (2004), De Ninno & Fanelli, Phys. Rev. Lett. 92, 094801 (2004), Bruni et al., proc. EPAC 2004.

• M. Hosaka, M. Katoh, A. Mochihashi
  UVSOR, Okazaki
• M.-E. Couprie, M. Labat
  CEA/Saclay, Gif-sur-Yvette
• Y. Takashima
  Nagoya University Graduate School of Engineering, Nagoya

In a Storage Ring Free Electron Laser (FEL), the saturation mechanism results from the so-called "bunch heating” phenomenon. The periodic interaction between the electron bunch and the laser pulse within the cavity is responsible of the enhancement of the energy spread of the bunch correlated with a bunch lengthening. Recently, new electron beam optics aiming at achieving low emittance with distributed dispersive function  revealed a particular interest for the FEL. In the undulator straight section, these optics lead also to an increase of the transverse sizes of the beam, and to a significant change of the Touschek lifetime. Experimental results obtained on the Super-ACO and UVSORII FELs illustrating the change of saturation process according to the chromatic or achromatic optics will be given. They will be compared with simulations performed with the LAS model, which has been modified to represent this new saturation process.

• Y.K. Wu, J. Li, S. Mikhailov, V. Popov
  DU/FEL, Durham, North Carolina
• N. Gavrilov, G. Kulipanov, O.A. Shevchenko, N. Vinokurov, P. Vobly
  BINP SB RAS, Novosibirsk

Funding: This work is supported by the U.S. AFOSR MFEL grant F49620-001-0370 and by U.S. DOE grant DE-FG05-91ER40665.

To improve the capability and performance of its light sources, the Duke FEL lab (DFELL) is upgrading its storage ring based FEL by replacing the existing linearly polarized OK-4 FEL with the next generation OK-5 FEL which is capable of delivering both linearly and circularly polarized light. To reduce and manage the risk associated with this project, the FEL upgrade is carried out in three phases. In the second phase of upgrade in 2005, two OK-5 wigglers are installed in a specially designed lattice where OK-4 wigglers remain, forming a distributed optical klystron FEL with hybrid wiggler magnets. In this paper, we report our commissioning experience of this distributed optical klystron FEL, including its first lasing in visible wavelengths and measured lasing spectra and power. We will also present our first experimental results on the FEL polarization manipulation using OK-4 and OK-5 wigglers. In addition, we report the performance enhancement of the Compton gamma-ray source driven by this FEL and initial FEL operation experience for user applications.

• K. Honkavaara
  Uni HH, Hamburg

The VUV-FEL being commissioned at DESY Hamburg is a user facility for SASE FEL radiation in the VUV wavelength range. The quality of the high brightness electron beam driving the VUV-FEL plays an important role for the performance of the facility. Prior to installation, the electron photo-injector of the VUV-FEL has been fully tested and characterized at the PITZ photo injector test facility at DESY Zeuthen, dedicated to develop high brightness electron sources for FEL projects like the VUV-FEL and the XFEL. We summarize the results on transverse emittance optimization at PITZ and report on the upgrade of the PITZ facility presently under construction. Results on transverse emittance optimization and measurements at the VUV-FEL are presented. Projected emittances around 1.4 mm mrad for 90% of a 1 nC bunch have been regularly measured. In addition, recent measurements of the longitudinal bunch profile after compression using a transverse deflecting cavity are presented.

• M. Goldstein, S.H. Lee, Y.A. Shroff, P.J. Silverman, D. Williams
  Intel, Santa Clara, California
• R. Pantell
  Stanford University, Stanford, Califormia
• H. Park, M.A. Piestrup
  Adelphi Technology, Inc., San Carlos, California

Funding: Intel Research

Semiconductor industry growth has largely been made possible by regular improvements in lithography. State of the art lithographic tools cost upwards of twenty five million dollars and use 0.93 numerical aperture projection optics with 193nm wavelengths to pattern features for 45 nm node development. Scaling beyond the 32 nm feature size node is expected to require extreme ultraviolet (EUV) wavelength light. EUV source requirements and equipment industry plasma source development efforts are reviewed. Exploratory research on a novel hybrid klystron and high gain harmonic generation FEL with oblique laser seeding will be disclosed. The opportunity and challenges for FELs to serve as a second generation (year 2011-2013) source technology in the semiconductor industry are presented.

• 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.

• B.W.J. McNeil, G.R.M. Robb
  Strathclyde University, Glasgow
• M.W. Poole, N. Thompson
  CCLRC/DL/ASTeC, Daresbury, Warrington, Cheshire

Funding: We acknowledge the support of the European Framework Programme 6 EUROFEL Design Study, CCLRC, and the Scottish Universities Physics Alliance.

Recent proof-of-principle simulations have demonstrated a method that allows a planar undulator FEL to lase so that the interaction with an odd harmonic of the radiation field dominates that of the fundamental [1]. This harmonic lasing of the FEL is achieved by disrupting the interaction between the fundamental radiation field and electrons as they propagate through the undulator while allowing the n-th harmonic interaction to evolve unhindered. The disruption of the interaction at the fundamental is achieved by a series of relative phase changes between electrons and the fundamental ponderomotive potential of 2k pi/n (k = 1, 2, 3, . . . ; k not equal to n). The corresponding phase change with the ponderomotive potential of the n-th harmonic is then 2k pi which, at least in a simple steady-state FEL model, will have no deleterious effect upon the harmonic interaction. Such phase changes are relatively easy to implement and indeed some current FEL designs would not require any structural modification. We present a more detailed analysis of harmonic lasing and use this to discuss potential benefits and applications in extending the operational bandwidth of FELs to shorter wavelengths.

[1] B.W.J. McNeil, G.R.M. Robb and M.W. Poole, Proceedings of Particle Accelerator Conference, Knoxville, USA (2005)

• C. Pellegrini, J.B. Rosenzweig, G. Travish
  UCLA, Los Angeles, California
• V.A. Dolgashev, C.D. Nantista, S.G. Tantawi
  SLAC, Menlo Park, California

Funding: US Department of Energy

We describe a proposed high-gain FEL using an X-band microwave undulator and operating at a wavelength of about 0.5 μm. The FEL electron beam energy is 65 MeV. The beam is produced by the NLCTA X-band linac at SLAC, using an S-band high-brightness photoinjector. The undulator consists of a circular waveguide with an rf wave counter-propagating with respect to the electron beam. The undulator is powered with two high-power X-band klystrons and a dual-moded pulse compressor recently developed at SLAC. This system is capable of delivering flat-top rf pulses of up to 400 ns and a few hundred megawatts. The equivalent undulator period is 1.4 cm, the radius of the circular pipe is 1 cm, and the undulator parameter is about 0.4 for a helical undulator configuration, obtained using two cross-polarized TE modes, or larger for a planar configuration, using one rf polarization. The undulator is about four meters long. The FEL will reach saturation within this distance when operated in a SASE mode. We describe the FEL performance parameters, the undulator characteristics and tolerances. One main goal of the experiment is to demonstrate the feasibility of an rf undulator for high gain FELs.

• G. Dattoli, A. Renieri
  ENEA C.R. Frascati, Frascati (Roma)

The existing soft and hard X-Ray FEL Projects are discussed, along with the underlying design and technological strategies. We consider two main categories: large facilities, which will involve the joint efforts of big laboratories and will take benefit from the heritage of high energy Physics facilities and smaller devices, which are aimed at exploiting high quality accelerators with modest e-beam energy to reach shorter wavelengths with alternative schemes. We will discuss advantages and drawbacks of the different conceptions and make an outlook to the future developments, with particular attention to combinations of different solutions like exotic undulators, seeding and so on, aimed not only at improving X-ray beam qualities but also at reducing device complexity and cost.

• D. Garzella
  CEA/Saclay, Gif-sur-Yvette
• C. Bruni
  PhLAM/CERCLA, Villeneuve d'Ascq Cedex
• M.-E. Couprie
  CEA/DSM/DRECAM/SPAM, Gif-sur-Yvette
• G. De Ninno, B. Diviacco, M. Marsi, M. Trovo
  ELETTRA, Basovizza, Trieste

The ELETTRA Storage Ring FEL succeded in operating in the Ultraviolet range, around 350 nm, with an etalon Fabry Perot inserted in the optical cavity. The high vacuum vessel, integrating a totally motorized control system for the principal degrees of freedom of the silica plate, allowed to obtain the laser oscillation, showing a reduction of the spectral linewidth by more than an order of magnitude. Temporal analysis by a double sweep streak camera showed also a broadening of the temporal pulse width. These major results are here exposed and compared with a numerical analysis and the Storage Ring FEL dynamics theory.

• M. Labat, M.-E. Couprie
  CEA/DSM/DRECAM/SPAM, Gif-sur-Yvette
• M. Hosaka, M. Katoh, A. Mochihashi
  UVSOR, Okazaki
• Y. Takashima
  Nagoya University Graduate School of Engineering, Nagoya

Storage Ring Free-Electron Laser dynamics and behaviour can be explored versus the detuning, i.e. a small difference between the frequencies of revolution of the electron bunches, and of the optical pulse circulating into the optical cavity. In fact, it provides situations ranging from the maximum initial gain over losses conditions to threshold ones. Systematic measurements of the UVSOR2 detuning curves have been performed. A complete detuning curve gives the intensity of the FEL versus the detuning. On such a plot, one can distinguish five distinct zones: three corresponding to continuous modes of emission for the FEL, and two pulsed modes. Each zone can then be described with its width and period for the pulsed modes. Streak camera also provides a full characterisation of the FEL versus detuning: position of the centre of mass of the laser, bunch lengthening. The energy spread is deduced from the electron beam transverse sizes. The analysis of the FEL behaviour versus detuning is compared with simulations performed with LAS. The detuning behaviour is then illustrated under different cases (current, control of the pulsed zone, chromatic or achromatic electron-beam optics).

• F. Curbis, F. Curbis
  Universita degli Studi di Trieste, Trieste
• G. De Ninno
  ELETTRA, Basovizza, Trieste

Coherent harmonic generation can be obtained by means of frequency up-conversion of a high-power external laser focused into the first undulator of an optical klystron. The standard configuration is based on a single-pass device, where the seed laser is synchronized with an electron beam entering the first undulator of the optical klystron after being accelerated using a linear accelerator. As an alternative, the optical klystron may be installed on a storage ring, where it is normally used as interaction region for an oscillator free-electron laser. In this case, removing the optical cavity and using an external seed, one obtains a configuration which is similar to the standard one but also presents some peculiar characteristics. In this paper we investigate the possibility of harmonic generation using the Elettra storage-ring optical klystron. We explore different experimental set-ups varying the beam energy, the seed characteristics and the strength of the optical-klystron dispersive section. We also study the performance sensitivity to fluctuations of some beam parameters and the coherent/incoherent signal ratio for different harmonics. Numerical simulations are performed using different 3-D numerical codes.

• K. Hayakawa, Y. Hayakawa, K. Nakao, K. Nogami, T. Sakai, I. Sato, T. Tanaka
  LEBRA, Funabashi

Funding: "Academic Frontier" Project for Private Universities: matching fund subsidy from MEXT (Ministry of Educatin, Culture, Sports, Science and Technology), 2000-2004

In general, maximum gain and maximum power of a free-electron laser (FEL) oscillator are not simultaneously satisfied at an identical length of the optical resonator. Use of a short bunch electron beam, therefore, can cause a large fluctuation of gain and saturated power of the FEL due to only a small change in the resonator length. If the length of the resonator can be adjusted at the middle in the macropulse duration of the electron beam, both maximizing conditions will be satisfied simultaneously, which will result in a large FEL output power compared with a normal operation. Since it is difficult to change the length of the resonator during the macro pulse, modulation of the bunch interval has been attempted for the LEBRA FEL system by modulating the phase of the accelerating rf of the electron linac, which has an equivalent effect to change of the resonator length. The modulation of the rf phase has resulted in intensification of the output energy per macro pulse by approximately twice compared with that in normal operation, which is consistent with numerical simulation.

• H. Ogawa, N. Sei, K.W. Watanabe, K. Y. Yamada, M.Y. Yasumoto
  AIST, Tsukuba, Ibaraki

Funding: This study was financially supported by the Budget for Nuclear Research of the Ministry of Education, Culture, Sports, Science and Technology, based on the screening and counseling by the Atomic Energy Commission, Japan.

At AIST, we have been making an effort to obtain FELs with an ultra-wide wavelength range from the vacuum ultraviolet (VUV) to the infrared (IR) on a compact storage ring NIJI-IV. Recently, performance of the NIJI-IV FEL was improved at the deep UV (DUV) around 200 nm and it became possible to make real-time observation of chemical reactions on the transition metal surfaces using the photoelectron emission microscopy (PEEM) combined with the DUV FELs. To observe dynamic behavior of the chemical reactions in detail, the FEL-PEEM system is being improved by optimizing experimental conditions. The performance of the system and the experimental results will be presented.

• N. Sei, H. Ogawa, K.W. Watanabe, K. Y. Yamada, M.Y. Yasumoto
  AIST, Tsukuba, Ibaraki

Funding: This work was supported by the Budget for Nuclear Research of the Ministry of Education, Culture, Sports, Science and Technology of Japan.

Free electron lasers (FELs) are being developed in a broad wavelength region from the VUV to the IR with the compact storage ring NIJI-IV at AIST. In the DUV and VUV regions, the FEL is used as an intense light source for real-time surface observation with the photoelectron emission microscopy. To extend the application field of the NIJI-IV FEL, for example to the structural analysis of proteins, experiments to obtain FEL oscillations at the wavelength below 195 nm are going on. In addition, a 3.6-m optical klystron, ETLOK-III, for developing infrared FELs has been installed in the north straight section of the NIJI-IV. Fundamental and higher harmonic spontaneous emissions from the ETLOK-III were observed in the visible and near-infrared regions. It was expected that the FEL gain for the 3rd harmonics exceed 5%. In the presentation, we will report the recent results of the VUV and IR FEL experiments.

• G. De Ninno, E. Allaria, F. Curbis, M.B. Danailov, B. Diviacco, M. Marsi, M. Trovo
  ELETTRA, Basovizza, Trieste
• M. Coreno
  CNR - IMIP, Trieste
• S. Günster, D. Ristau
  Laser Zentrum Hannover, Hannover

Thanks to an intensive technological effort in the framework of the EEC Contract HPRI CT-2001-50025 (EUFELE), the European FEL at ELETTRA was able to break the previous record for the shortest wavelength of an FEL oscillator. Novel solutions were adopted for multilayer mirrors to allow FEL operation in the wavelength region between 160 and 190 nm, which is one of the main targets of the project. The characteristics of the FEL pulses measured at 176 nm (spectral profiles, high intensity, meV bandpass, MHz repetition rate) make it a competitive light source for spectroscopy, in particular for fluorescence studies in the VUV spectral range. Proof of principle experiments have been performed on different types of silica glasses, yielding information on the mechanisms of light absorption in this material.

• K.S.B. Li, A. Adelmann, A. Anghel, R.J. Bakker, M. Boge, A.E. Candel, M. Dehler, R. Ganter, C. Gough, G. Ingold, S.C. Leemann, M. Pedrozzi, J.-Y. Raguin, L. Rivkin, V. Schlott, A. Streun, A. Wrulich
  PSI, Villigen

The Paul Scherrer Institute (PSI) in Switzerland currently develops a Low-Emittance electron-Gun (LEG) based on field-emitter technology [1]. The target is a normalized transverse emittance of 5 10(-8) m rad or less. Such a source is particularly interesting for FELs that target wavelengths below 0.3 nm since it permits a reduction of the required beam-energy and hence, a reduction of the construction- and operational costs of X-ray FELs. That is, for the case that this initial low emittance can be maintained throughout the accelerator. Here we present a concept for a 0.1 nm X-FEL based on LEG, which can be located close to the Swiss Light Source (SLS). Special attention goes to the maintenance of the emittance during the process of acceleration and bunch-compression, in particular in the regimes where either space-charge forces or coherent-synchrotron radiation are of importance.

[1] R. Ganter et al, Proceedings of the 2004 FEL Conference, Trieste, Italy, p. 602 (2004)

• M.B. Danailov
  ELETTRA, Basovizza, Trieste
• F.O. Ilday, F.X. Kaertner
  MIT, Cambridge, Massachusetts

Laser systems will undoubtedly be one of the key factors determining the performance of VUV and X-ray FELs. In particular, harmonic generation scheme based FELs require at least three mutually synchronized solid-state laser systems: photoinjector laser, seeding laser, end station lasers. In addition, a laser heater is also included in recent FEL designs. It is therefore very important to consider the possibility of integrating these systems to a maximum possible degree. In this paper we consider a promising approach to the integration of the above specified laser systems for the FERMI@ Elettra FEL, based on the distribution of a fiber laser generated seed signal at 1550 nm. This signal, after further amplification and frequency doubling, is used as a seed for Ti:Sapphire amplifiers at the different locations. The paper presents a general layout of the system, the main pulse parameters (i.e. pulse energy and duration) needed in different parts of the system and discusses possible technical solutions

• 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.

• T.-Y. Lee, J. Choi, H.-S. Kang
  PAL, Pohang, Kyungbuk

PAL-XFEL is the newly announced SASE FEL project that is going to achieve 0.3 nm wavelength radiation with 3.7 GeV electron beam. To overcome the relatively low energy of 3.7 GeV, short period and small gap in-vacuum undulator will be adopted. Wake field effects of this in-vacuum undulator on the SASE process is studied in this paper.

• B.W.J. McNeil, G.R.M. Robb
  Strathclyde University, Glasgow
• C. Gerth
  DESY, Hamburg
• J.K. Jones, M.W. Poole, N. Thompson
  CCLRC/DL/ASTeC, Daresbury, Warrington, Cheshire

Funding: We acknowledge the support of the European Framework Programme 6 EUROFEL Design Study, CCLRC, and the Scottish Universities Physics Alliance.

An XUV Free-Electron Laser operating in the photon energy range 10-100eV is a key component of the proposed 4th Generation Light Source (4GLS) at Daresbury Laboratory in the UK. The current design proposal is an amplifier FEL seeded by a Higher Harmonic Generation (HHG) source. In this paper we present and discuss the considerations that led to the current design. We also present 3D simulation results that illustrate the potential radiation output characteristics.

• 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).

• Y. Kim, K. Floettmann, S. Schreiber
  DESY, Hamburg
• D. Son
  CHEP, Daegu

Funding: for the TTF2 VUV-FEL Team

By the help of nonlinearity in the longitudinal phase space, the VUV-FEL at the TESLA Test Facility phase 2 (TTF2) is under operating in the femtosecond (fs) FEL mode which generates coherent and ultra-bright SASE source with photon pulse duration time of around 20 fs (FWHM) and wavelength of around 32 nm. For the fs FEL mode operation, bunch length of electron beams should be compressed by two bunch compressors to have a leading spike in the longitudinal beam density distribution or peak current. The required peak current at the spike is higher than about 1.0 kA, and the spike length is shorter than around 200 fs (FWHM). In this paper, we describe our commissioning experiences to optimize two TTF2 bunch compressors for the fs FEL mode operation.

• 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).

• J. Teichert, A. Arnold, H. Buettig, D. Janssen, U. Lehnert, P. Michel, K. Moeller, P. Murcek, Ch. Schneider, R. Schurig, F. Staufenbiel, R. Xiang
  FZR, Dresden
• T.  Kamps, D. Lipka, F. Marhauser
  BESSY GmbH, Berlin
• W.-D. Lehmann
  IfE, Dresden
• J. Stephan
  IKST, Drsden
• V. Volkov
  BINP SB RAS, Novosibirsk
• I. Will
  MBI, Berlin

A superconducting rf photo electron injector (SRF gun) is under development at the Forschungszentrum Rossendorf. The project aims at several issues: improvement of the beam quality for the ELBE superconducting electron linac, demonstration of feasibility of this gun type, investigation of critical components, and parameter studies for future application (BESSY-FEL, 4GLS). In 2005, a substantial progress has been made. The two 3.5-cell niobium cavities for the gun have been delivered from the company ACCEL. The main parts for gun cryostat like vacuum vessel, cryogenic and magnetic shields are ready. Test benches for the cathode cooling system and the cavity tuner are being assembled. The photo cathode preparation lab has been arranged, and the diagnostic beam line has been designed (see T. Kamps et al., this conference). After delivering the gun cavities, their rf properties are being measured at room temperature and the warm tuning is being carried out. The set-up for this treatment and measurement as well as the results will be presented.

• P. Craievich, S. Di Mitri
  ELETTRA, Basovizza, Trieste

The FEL project FERMI@ELETTRA will use the existing Linac, upgraded to 1.2 GeV, to produce VUV radiation between 100-10 nm. FEL operations require a high quality beam in terms of the bunch energy spread and emittance. In this paper we present an analytical study based on a continuum model to describe the transverse motion of a single bunch. Such a study allows predicting the emittance growth under the combined influence of short-range transverse wakefields, injection offset, initial emittance and misaligned accelerating sections. We also report a comparison between analytical and numerical (tracking code) results.

• S. Zhang, S.V. Benson, D. Douglas, D. Hardy, C. Hernandez-Garcia, K. Jordan, G. Neil, M.D. Shinn
  Jefferson Lab, Newport News, Virginia

Funding: This work supported by the Office of Naval Research, the Joint Technology Office, the Commonwealth of Virginia, the Army Night Vision Laboratory, the Air Force Research Laboratory, and by DOE Contract DE-AC05-84ER40150.

The design and construction of an optical transport that brings synchrotron radiation from electron bunches to a fast streak camera in a remote area has become a useful tool for online observation of bunch length and stability. This paper will report on the temporal measurements we have done, comparison with simulations, and the on-going work for another imaging optical transport system that will make possible the direct measurement of the longitudinal phase space by measuring the bunch length as a function of energy.

• 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.

• R. Bonifacio, R. Bonifacio
  Universidade Federal de Alagoas, Maceio
• N. Piovella
  Universita' degli Studi di Milano, MILANO
• G.R.M. Robb
  Strathclyde University, Glasgow

We present a proof of principle of the novel regime of quantum SASE with propagation effects. Using a self-consistent system of Schrodinger-Maxwell equations, we show that the dynamics of the system is determined by a properly defined "quantum FEL-parameter", ρ', which rules the number of photons emitted per electron, as well as the electron recoil in units of ћk. In the limit ρ'>>1 the quantum model reproduces the classical SASE regime with random spiking behavior and broad spectrum. In this limit we show that the equation for the Wigner function reduces to the classical Vlasov equation. In the opposite limit, ρ'<1, we demonstrate "quantum purification" of SASE: the classical spiking behavior disappears and the power spectrum becomes very narrow so that the temporal coherence of the SASE spectrum is dramatically improved. Photon statistics, electron-photon entangled states, minimum uncertainty states and quantum limitations on bunching and energy spread will be discussed.

• 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.

• I. Will
  MBI, Berlin
• S. Düsterer, J. Feldhaus, E. Plönjes, H. Redlin
  DESY, Hamburg

Funding: This work was funded by the European Commission under Contract no. HPRI-CT-1999-50009

The VUV-FEL at DESY provides ultra-short pulses with pulse durations below 50 fs. To explore a wider field of time resolved experiments a complex laser system has been installed delivering 150 fs pulses at a wavelength of 800 nm with 50 μJ pulse energy at 1MHz repetition rate during the FEL burst (of 800 μs). In order to perform two color pump-probe experiments the laser has to be synchronized to the FEL. To ensure precise and reliable synchronized operation of the laser, various diagnostic experiments have been developed. Concepts as well as first results of the synchronization will be shown.

• J. Bahrdt
  BESSY GmbH, Berlin

The design of beamlines for VUV and x-ray FEL facilities requires a detailed knowledge of the coherent radiation source. Time dependent simulations with FEL codes like GENESIS provide the electric field distribution at the end of the FEL which represents the complete information. Ray tracing codes used to transform the light from the source to the sample are generally based on geometrical optics and do not include directly the coherent properties of the FEL radiation. On the other hand Fourier optic techniques are usually applied to the propagation across normal incidence optics. We present an algorithm based on physical optics which permits the propagation of wavefronts across grazing incidence optics including interference effects, diffraction, polarization variation and pulse lengthening. Some examples are given for the proposed BESSY soft x-ray FEL.

• 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.