• 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.

• 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).

• J.Y. Huang, Y.S. Bae, M.-H. Chun, Y.J. Han, S.-H. Jeong, H.-S. Kang, D.T. Kim, S.H. Kim, S.-C. Kim, I.S. Ko, H.J. Park, I.-S. Park, S.J. Park, Y.J. Park, S.Y. Rah, J.-H. Suh
  PAL, Pohang, Kyungbuk
• J.H. Hong, C. Kim
  POSTECH, Pohang, Kyungbuk

Funding: Work supported by Ministry of Science and Technology (MOST)

Beam diagnostics for PAL-XFEL physics calls for precision of femto-second in time structure and sub-micrometer in beam position measurement(BPM). Existing instruments can be used for standard diagnostics such as single bunch charge measurement, wire scanner or optical transition radiator for beam size measurement. Instead, major R&D efforts should be focused on the measurement of femto-second bunch structure using electro-optic crystal, coherent radiation and transverse deflecting cavity. Nanometer BPM technique being developed in collaboration with linear collider group will also be utilized for sub-micrometer BPM. Overall plan and the ongoing R&D activities will be presented.

• G.M. wang, J.-E. Chen, Y.T. Ding, J. Hao, S.L. Huang, L. Lin, X.Y. Lu, S.W. Quan, L.F. Wang, R. Xiang, B.C. Zhang, K. Zhao, F. Jiao, D. Xie, L. Yang, F. Zhu, C. Liu, F. Wang, W. Xu, Z. Liu
  PKU/IHIP, Beijing
• Y.G. Wang
  Peking University, School of Physics, Beijing

Funding: NSFC, MOST of China

Beam loading experiments on DC-SC photoinjector test facility have been finished at 4.4 K. Upon the present experiments, the gradient of 6 MV/m is achieved. The maximum energy gain is 1.1 MeV at 4.4 K. With average beam current of 270 mA, the measured rms emittance is about 5 mm-mrad at the beam energy of 500 keV. Experiments on the test facility has validated that the DC-SC photoinjector is a good choice to provide moderate average current electron beams with low bunch charge and very high repetition rate.

• 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.

• 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

• 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.

• 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.-Y. Raguin, R.J. Bakker, K.S.B. Li, M. Pedrozzi
  PSI, Villigen

In the Low Emittance Gun (LEG) under development at PSI an extremely bright electron beam is produced from a field emission array and then rapidly accelerated in a diode configuration up to 1 MeV with gradients of the order of 250 to 500 MV/m. The electronic emission from such a cold cathode allows normalized intrinsic emittance below 0.1 mm.mrad well suited for X-ray FELs or linear collider applications. The diode is followed by an L-band RF-gun like cavity to further accelerate the beam. A third harmonic field is superposed to the fundamental [1] 1.5 GHz pi-mode field to minimize the RF emittance. We report here on the design of such a two-frequency RF cavity with some details on the RF coupling and possible tuning mechanisms. Beam dynamics studies, performed with PARMELA and the fully self-consistent code MAFIA, are presented and compared with the results obtained for an RF cavity excited with the fundamental frequency only.

[1] D.H. Dowell et al., Nucl. Instr. and Meth. A 528 (2004) 316.

• 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.

• S. Zhang, D. Hardy, 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 Air Force Research Laboratory, and by DOE Contract DE-AC05-84ER40150.

We report the characterization and performance of a diode-pumped, high-power, picosecond laser system designed for high-current photo-cathode accelerator injector at repetition rates of both 75MHz and 750MHz. Our characterization includes measurement of the system's amplitude stability, beam quality, pulsewidth, and phase noise for both frequencies.

• 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.

• 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.

• 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.

• E. Allaria, G. De Ninno
  ELETTRA, Basovizza, Trieste
• A. Antoniazzi, D. Fanelli
  Universita di Firenze, Florence
• F.T. Arecchi
  UNIFI, Sesto Fiorentino (FI)
• R. Meucci
  INOA, Firenze

We numerically investigate the effect of a delayed control method on the stabilization of the dynamics of the Elettra storage-ring free-electron laser in Trieste (Italy). Simulations give evidence of a significant reduction of the typical large oscillations of the laser intensity. Results are compared with numerical and experimental data obtained with a derivative feedback. The possibility of an experimental implementation of the proposed method is also discussed.

• 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.

• A.A. Shcherbakov, V.P. Androsov, E.V. Bulyak, A. Dovbnya, I.V. Drebot, P. Gladkikh, V.A. Grevtsev, Yu.N. Grigor'ev, A. Gvozd, V.A. Ivashchenko, I.M. Karnaukhov, V.P. Kozin, V. Lapshin, V.P. Lyashchenko, V. Markov, N.I. Mocheshnikov, V.B. Molodkin, A. Mytsykov, I.M. Necklyudov, F.A. Peev, A.V. Rezaev, A. Shpak, V.L. Skirda, V. Skomorokhov, Y.N. Telegin, V.I. Trotsenko, A.Y. Zelinsky, O.D. Zvonarjova, N. kovalyova
  NSC/KIPT, Kharkov
• A. Agafonov, A.N. Lebedev
  LPI, Moscow
• J.I.M. Botman
  TUE, Eindhoven
• R. Tatchyn
  SLAC, Menlo Park, California

The sources of the X-rays based on Compton scattering of intense Nd:YAG laser beam on electron beam circulating in a storage ring with beam energy 43 - 225 MeV is under construction in NSC KIPT. In the paper the progress in development and construction of Kharkov X-ray generator NESTOR is presented. The current status of the main facility system design and development are described. New scheme and main parameters of injection system are presented. The facility is going to be in operation in the middle of 2007 and generated X-rays flux is expected to be of about 10(13) phot/s.

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

Funding: JTO, ONR, NAVSEA

The short-Rayleigh length FEL configuration leaves the optical resonator near the cold-cavity stability limit. Studies show that the electron beam interaction stabilizes the optical modes and establishes limits to the vibrations of mirrors and the electron beam. Several types of vibrations are considered.

• 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

• 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.

• M. Roehrs, A. Bolzmann, M. Huening, H. Schlarb
  DESY, Hamburg
• K. Honkavaara
  Uni HH, Hamburg

Among the very critical parameters for the operation of the VUV-FEL at DESY are the slice-emmittance and beam optics matching of the current peak in the electron bunch. Conventional tools for measuring the beam size are sensitive to the projected properties of the bunch only and hence suffer from mixing of different parts of the bunch. A combination of streaking with a transverse deflecting rf structure (LOLA) and a quadrupole scan allowed to measure the spike separate from the rest of the bunch. Indeed significant differences in terms of emmittance and optical functions have been found.

• S. Schreiber
  DESY, Hamburg

The VUV-FEL is a free electron laser user facility being commissioned at DESY in Hamburg. In the current configuration, the linac accelerates an electron beam up to 800 MeV. The injector is a crucial part of the linac, since it has to generate and maintain a high brightness electron beam required for SASE operation. The injector includes a laser driven RF gun, a booster section, a bunch compressor, and diagnostic sections. The good performance of the injector was crucial for the first lasing of the VUV-FEL at a wavelength of 32 nm in January 2005. We report on the present layout of the injector, the properties of the electron beam and on upgrade plans scheduled in the near future.

• A. Oppelt, K. Abrahamyan, G. Asova, J.W. Baehr, G. Dimitrov, U. Gensch, H.-J. Grabosch, J.H. Han, S. Khodyachykh, G. Klemz, M. Krasilnikov, S. Liu, V. Miltchev, B. Petrosyan, S. Riemann, L. Staykov, F. Stephan
  DESY Zeuthen, Zeuthen
• W. Ackermann, W.F.O. Muller, S. Schnepp, T. Weiland
  TEMF, Darmstadt
• D. Alesini, M. Boscolo, G. Di Pirro, M. Ferrario, D. Filippetto, L. Palumbo, C. Vicario
  INFN/LNF, Frascati (Roma)
• V. Boccone
  Humboldt Universität zu Berlin, Berlin
• L. Catani, E. Chiadroni, A. Cianchi
  INFN-Roma II, Roma
• K. Floettmann, S. Schreiber
  DESY, Hamburg
• T. Garvey
  LAL, Orsay
• M.V. Hartrott, E. Jaeschke, D. Kraemer, D. Lipka, F. Marhauser, R. Richter
  BESSY GmbH, Berlin
• P. Michelato, L. Monaco, C. Pagani, D. Sertore
  INFN/LASA, Segrate (MI)
• V.V. Paramonov
  RAS/INR, Moscow
• N. Pavel
  Humboldt University Berlin, Institut für Physik, Berlin
• J.R. Roensch, J. Rossbach
  Uni HH, Hamburg
• W. Sandner, I. Will
  MBI, Berlin
• I. Tsakov
  INRNE, Sofia

Funding: This work has been partly supported by the European Community, contract numbers RII3-CT-2004-506008 and 011935, and by the 'Impuls- und Vernetzungsfonds" of the Helmholtz Association, contract number VH-FZ-05.

Since December 2004, the photo injector test facility at DESY in Zeuthen (PITZ) has been upgraded. A normal conducting copper booster cavity has been installed and the diagnostics beamline has been strongly modified. An extended water cooling system has been installed and was successfully taken into operation. Actually, the new diagnostics elements are being commissioned. After the installation of the new 10 MW klystron in June/July, the gun can be conditioned towards higher average power, and the whole beamline including the booster will be taken into operation. First results from the commissioning phase including gun and booster conditioning are reported.