Session: MOPOS - 30 Aug 2004
Single Pass FELs and FEL Theory

MOPOS03 Start-To-End Injector and Linac Tolerance Studies for the BESSY FEL
Michael Abo-Bakr, Michael von Hartrott, Jens Knobloch, Bettina Kuske, Atoosa Meseck (BESSY GmbH, Berlin)

BESSY is proposing a Soft X-ray FEL user facility in Berlin, delivering short and stabile photon pulses in the wavelength range of 62 nm < λ < 1.2 nm by applying up to four cascaded High Gain Harmonic Generation (HGHG) stages. For optimization of the FEL performance of the cascaded HGHG stages extensive Start-to-End (S2E) simulations have been carried out. In order to test the quality of the chosen configuration with respect to the sensitivity against various error sources tolerance studies from the injector to the linac end have been performed. Procedures and results of these studies will be presented.

MOPOS04 A Multistage HGHG-Scheme for the BESSY Soft X-ray FEL Multiuser Facility
Dieter Krämer (BESSY GmbH, Berlin)

BESSY recently finalized the Technical Design Report for a multi-user FEL-facility in the VUV to soft X-ray spectral range. A cascaded HGHG-approach has been adopted, allowing to generate stable, reproducible multi-GW pulses at selectable pulse duration down to <20 fs. Three independent FEL-lines with three beamlines each are proposed. The FELs are driven by a most flexible superconducting CW linac based on the TESLA 9-cell cavities[1]. 18 modules of 8 cavities are needed to accelerate the beam to a final energy of 2.3 GeV at a modest field of < 16 MV/m. A 1 kHz repetition frequency rf-photoinjector, similar to the PITZ-gun [2], will be used in the beginning, to be replaced later on by a CW-photoinjector to produce arbitrary pulse patterns as needed by the experimenters. A summary of the project plans and present activities will be given.

MOPOS05 Impact Studies of bunch parameter variations on the performance of the BESSY HGHG FEL
Bettina Kuske, Michael Abo-Bakr, Atoosa Meseck (BESSY GmbH, Berlin)

BESSY is proposing a linac-based High Gain Harmonic Generation FEL multi user facility covering the VUV to soft X-ray spectral range. A photoinjector and a superconducting 2.3 GeV CW linac will feed three independent multi-stage HGHG lines consisting of APPLE II type undulators. Tolerance studies have been performed for the FEL process, including variations in the electron beam parameters like current, emittance, and energy spread. The influence of undulator field errors has been investigated and start to end calculations, taking varying conditions for each slice into account, have been performed. The investigations show, that the behaviour in the presence of errors is quite different from the SASE case, and depends heavily on the specific design and optimization of the different stages.

MOPOS06 The Influence of the Seed Pulse Shape on the Output Performance of the BESSY Multi-stage HGHG-FEL
Atoosa Meseck, Michael Abo-Bakr, Bettina Kuske (BESSY GmbH, Berlin)

The BESSY soft X-ray FEL is planned as a High Gain Harmonic Generation (HGHG) FEL multi-user facility covering the VUV to soft X-ray spectral range. In the HGHG scheme, the properties of the radiation output are dominated by the characteristics of the laser seed. In this connection, the influence of the laser pulse shape on the output, in particular on the output spectrum is of interest. Simulation studies for the BESSY-HGHG-FELs are presented and the output performance for different shapes of the laser pulse is discussed.

MOPOS07 Velocity Bunching Simulation for the DESY VUV FEL
Jean-Paul Carneiro, Bart Faatz, Klaus Floettmann (DESY, Hamburg)

The TESLA Test Facility FEL is currently being upgraded to reach in its final stage a SASE radiation wavelength of 6 nm. After a brief description of layout and status of the TTF-FEL, the paper will present start-to-end simulations studying velocity bunching as a scenario for first operations of the accelerator.

MOPOS08 A 3D Self-Consistent, Analytical Model for Longitudinal Plasma Oscillation in a Relativistic Electron Beam
Gianluca Geloni, Evgeny L. Saldin, Evgeny A. Schneidmiller, Mikhail V. Yurkov (DESY, Hamburg)

Longitudinal plasma oscillations are becoming a subject of great interest for XFEL physics in connection with LSC microbunching instability [1] and certain pump-probe synchronization schemes [2]. In the present paper we developed the first exact analytical treatment for longitudinal oscillations within an axis-symmetric, (relativistic) electron beam, which can be used as a primary standard for benchmarking space-charge simulation codes. Also, this result is per se of obvious theoretical relevance as it constitutes one of the few exact solutions for the evolution of charged particles under the action of self-interactions.

MOPOS09 Benchmark of ASTRA with Analytical Solution for the Longitudinal Plasma Oscillation Problem
Gianluca Geloni, Evgeny L. Saldin, Evgeny A. Schneidmiller, Mikhail V. Yurkov (DESY, Hamburg)

During the design of X-FELs, space-charge codes are required to simulate the evolution of longitudinal plasma oscillation within an electron beam in connection with LSC microbunching instability [1] and certain pump-probe synchronization schemes [2]. In the paper [3] we presented an analytical solution to the initial value problem for longitudinal plasma oscillation in an electron beam. Such a result, besides its theoretical importance, allows one to benchmark space-charge simulation programs against a self-consistent solution of the evolution problem. In this paper we present a comparison between our results [3] and the outcomes of the simulation code ASTRA.

MOPOS10 The Potential for the Development of the X-ray Free Electron Laser: Generation of SASE Radiation
Evgeny L. Saldin, Evgeny A. Schneidmiller, Mikhail V. Yurkov (DESY, Hamburg)

We present a concept of a universal FEL beamline covering continuously wavelength range from 0.1 to 1.6 nm at a fixed energy of the electron beam. FEL beamline accommodates three undulators (SASE1-3) installed one after another. The first undulator, SASE1, is optimized for operation at the wavelength range 0.1-0.15 nm. Our study shows that such tunability range almost does not affect operation at the shortest wavelength of 0.1 nm. Operation of two other FELs (SASE2 and SASE3) is not so critical, and nominal tunability range is chosen to be by a factor of two (2-4 nm, and 8-16 nm, respectively). The length of the undulators is chosen such that continuous wavelength tunability can be provided by means of extra opening the undulator gaps, or by tuning to the frequency doubler mode of operation. Changing of undulator gaps in different parts of SASE2 and SASE3 undulators allows one to tune the modes with high output power (sub-TW level), or for effective generation of the second harmonic. The latter feature might be important for future pump-probe experiments. Also, recently proposed attosecond SASE FEL scheme is foreseen for implementation.

MOPOS11 Attosecond X-ray Source for Light-Triggered Time-Resolved Experiments Associated with the X-ray SASE FEL
Evgeny L. Saldin, Evgeny A. Schneidmiller, Mikhail V. Yurkov (DESY, Hamburg)

We propose a technique for the production of attosecond X-ray pulses which is based on the use of X-ray SASE FEL combined with a femtosecond laser system. A few-cycle optical pulse from a Ti:sapphire laser interacts with the electron beam in a two-period undulator resonant to 800 nm wavelength and produces energy modulation within a slice of the electron bunch. Following the energy modulator the electron beam enters the X-ray undulator and produces SASE radiation. Due to energy modulation the frequency is correlated to the longitudinal position within the few-cycle-driven slice of SASE radiation pulse. The largest frequency offset corresponds to a single-spike pulse in the time domain which is confined to one half-oscillation period near the central peak electron energy. The selection of single-spike pulses is achieved by using a crystal monochromator after the X-ray undulator. Our studies show that the proposed technique is capable to produce 300 attoseconds long single pulses with GW-level output power in the 0.1 m wavelength range, and is applicable to the European X-Ray Laser Project XFEL and the Linac Coherent Light Source at SLAC.

MOPOS12 The Potential for the Development of the X-ray Free Electron Laser: Multi-User Photon Distribution System for XFEL Laboratory
Evgeny L. Saldin, Evgeny A. Schneidmiller, Mikhail V. Yurkov (DESY, Hamburg)

X-ray photon beam from a SASE FEL undulator is in principle a single user tool, just like an optical laser. Therefore, the operation and amortization cost cannot be easily spread over many simultaneous experiments. To avoid prohibitive cost for each experiment, a new XFEL laboratory scheme is proposed. A photon beam distribution system based on movable multilayer X-ray mirrors can provide an efficient way to generate a multi-user facility. Distribution of photons is achieved on the basis of pulse trains and it is possible to partition the photon beam among a few tens independent beamlines thereby obtaining many users working in parallel. The second way to increase the number of simultaneous experiments is based on the working with a series of perfect crystals in transmission (Laue) geometry. The later concept is the basic idea of the Troika beamline at ESRF. In principle, a hundred of photon beamlines with different experiments can be served by a single XFEL source.

MOPOS14 Sub-Terawatt Mode of Operation of X-ray SASE FEL
Evgeny L. Saldin, Evgeny A. Schneidmiller, Mikhail V. Yurkov (DESY, Hamburg)

Application of dispersion section in combination with undulator tapering is an effective tool for achieving extremely high output power of XFEL. In the first part of the undulator the gap is fixed, and amplification process is developed as in usual SASE FEL. When energy modulation of the electron beam becomes to be comparable with local energy spread, the electron bunch passes via dispersion section resulting in an effective compression of the electron bunch. Then bunched electron beam enters the second half of the undulator where the gap is tapered for effective extraction of the energy from the electron bunch. Our studies shows that output radiation power can reach a sub-TW level in Angstrom wavelength range.

MOPOS15 Design Formulas for VUV and X-Ray FELs
Evgeny L. Saldin, Evgeny A. Schneidmiller, Mikhail V. Yurkov (DESY, Hamburg)

Simple formulas for optimization of VUV and X-ray SASE FELs are presented. The FEL gain length and the optimal beta-function are explicitly expressed in terms of the electron beam and undulator parameters. The FEL saturation length is estimated taking into account energy diffusion due to quantum fluctuations of the undulator radiation. Examples of the FEL optimization are given. Parameters of a SASE FEL, operating at the Compton wavelength, are suggested.

MOPOS16 The Free Electron Laser Klystron Amplifier Concept
Evgeny L. Saldin, Evgeny A. Schneidmiller, Mikhail V. Yurkov (DESY, Hamburg)

We consider optical klystron with a high gain per cascade pass. In order to achieve high gain at short wavelengths, conventional FEL amplifier require electron beam peak current of a few kA. This is achieved by applying longitudinal compression using a magnetic chicane. In the case of klystron things are quite different and gain of klystron does not depend on the bunch compression in the injector linac. A distinguishing feature of the klystron amplifier is that maximum of gain per cascade pass at high beam peak current is the same as at low beam peak current without compression. Second important feature of the klystron configuration is that there are no requirements on the alignment of the cascade undulators and dispersion sections. This is related to the fact that the cascades, in our (high gain) case, do not need the radiation phase matching. There are applications, like XFELs, where unique properties of high gain klystron FEL amplifier are very desirable. Such a scheme allows one to decrease the total length of magnetic system. On the other hand, the saturation efficiency of the klystron is the same that of conventional XFEL.

MOPOS17 The Potential for Extending the Spectral Range Accessible to the European X-ray Free Electron Laser in the Direction of Longer Wavelengths
Evgeny L. Saldin, Evgeny A. Schneidmiller, Mikhail V. Yurkov (DESY, Hamburg)

The baseline specifications of European XFEL give a range of wavelengths between 0.1 nm and 2 nm. This wavelength range at fixed electron beam energy 17.5 GeV can be covered by operating the SASE FEL with three undulators which have different period and tunable gap. A study of the potential for the extending the spectral range accessible to the XFEL in the direction of longer wavelengths is presented. The extension of the wavelength range to 6 nm would be cover the water window in the VUV region, opening the facility to a new class of experiments. There are at least two possible sources of VUV radiation associated with the X-ray FEL; the "low (2.5 GeV) energy electron beam dedicated" and the " 17.5 GeV spent beam parasitic" (or "after-burner") source modes. The second alternative, "after-burner undulator" is the one we regard as most favorable. It is possible to place an undulator as long as 80 meters after 2 nm undulator. Ultimately, VUV undulator would be able to deliver output power approaching 100 GW. A beam from this device could be run in pump-probe mode with X-ray FEL.

MOPOS18 Start-To-End Simulations for PAL XFEL Project
Yujong Kim (DESY, Hamburg), Y. Kim, Dongchul Son (CHEP, Daegu), Moo-Hyun Cho, In-Soo Ko, Jong-Seok Oh (PAL, Pohang), Won Namkung (POSTECH, Pohang, Kyungbuk)

At the Pohang Accelerator Laboratory (PAL), there is a 2.5 GeV S-band linac which is under operating as a full energy injector for the Pohang Light Source (PLS) storage ring. By installing a new electron gun, a new 0.5 GeV linac, and two new bunch compressors, the PLS linac can be used as an FEL driver for the PAL XFEL project. To saturate 0.3 nm wavelength PAL XFEL SASE source, we should supply high quality electron beams to a 60 m long undulator. In this paper, we describe a new linac layout for the PAL XFEL project and its various start-to-end simulations from the cathode to the end of the linac.

MOPOS21 Seeding High Gain Harmonic Generation with Laser Harmonics Produced in Gases
Guillaume Lambert, Bertrand Carré, Yann Mairesse, Pascal Salières (CEA/Saclay, Gif-sur-Yvette), Marie-Emmanuelle Couprie, David Garzella (CEA/SPAM, Gif-sur-Yvette), Andrea Doria, Luca Giannessi (ENEA C.R. Frascati, Frascati - Roma), Toru Hara, Hideo Kitamura, Tsumoru Shintake (RIKEN Spring-8, Hyogo)

Free electron Lasers employing High Gain Harmonic generation (HGHG) schemes are very promising coherent ligth sources for the soft X-ray regime. They offer both transverse and longitudinal coherence, while Self Amplified Spontaneous Emission schemes have a longitudinal coherence limited. We propose here to seed HGHG with high harmonics produced by a Ti:Sa femtosecond laser focused on a gas jet, tuneable in the 100-10 nm spectral region. Specifities concerning the implementation of this particular laser source as a seed for HGHG are investigated. Semi analytical , numerical 1D and 3D calculations are given, for the cases of the SCSS, SPARC and ARC-EN-CIEL projects.

MOPOS22 Off-Axis Orbits in Realistic Helical Wigglers: Fixed Points and Time Averaged Dynamical Variables
John Thomas Donohue (CENBG, Gradignan), Jean-Luc Rullier (CESTA, Le Barp)

Many years ago Fajans, Kirkpatrick and Bekefi studied off-axis orbits in a realistic helical wiggler, both experimentally and theoretically. They found that as the distance from the axis of symmetry to the guiding center increased, both the mean axial velocity and the precession frequency of the guiding center varied. . They proposed a clever semi-empirical model which yielded an excellent description of both these variations. We point out that a approximate model proposed by us several years ago can be made to predict these delicate effects correctly, provided we extend our truncated quadratic Hamiltonian to include appropriate cubic terms. We develop an argument similar to the virial theorem to compare time averaged and fixed-point values of dynamical variables. Illustrative comparisons of our model with numerical calculation are presented.

MOPOS23 Analytic Electron Trajectories in an Extremely Relativistic Helical Wiggler: an Application to the Proposed SLAC E166 Experiment.
John Thomas Donohue (CENBG, Gradignan)

The proposed experiment SLAC E166 intends to generate circularly polarized gamma rays of energy 10 MeV by passing a 15 GeV electron beam through a meter long wiggler with approximately 400 periods. Using an analytic model formulated by Rullier and me, I present calculations of electron trajectories. At this extremely high energy the trajectories are described quite well by the model, and an extremely simple picture emerges, even for trajectories that that fail to encircle the axis of the wiggler. Our calculations are successfully compared with standard numerical integration of the Lorentz force equations of motion. In addition, the calculation of the spectrum and angular distribution of the radiated photons is easily carried out.

MOPOS25 Electron beam simulations for the FERMI project at ELETTRA
Simone Di Mitri, Rene Johan Bakker, Carlo Joseph Bocchetta, Paolo Craievich, Gerardo D'Auria, Giovanni De Ninno, Bruno Diviacco, Lidia Tosi, Victor Verzilov (Elettra, Basovizza, Trieste)

FERMI at ELETTRA is a project aims at the construction of a single-pass user facility for the spectral range from 100 nm to 10 nm. Starting point is the existing 1.2 GeV, 3-GHz linac. Downstream of the linac two undulator beamlines will serve the wavelength range from 100 nm to 40 nm, and 40 nm to 10 nm, respectively. The former beamline will be based on a single-stage HGHG scheme while for the latter a double stage HGHG scheme is foreseen. This paper discusses results from start-to-end simulations for the former beamline, including the option of HGHG. Care has been taken to include realistic models for the injector, accelerating structures, and undulator beamline.

MOPOS26 Harmonic Generation and Linewidth Narrowing in Seeded FELs
Luca Giannessi (ENEA C.R. Frascati, Frascati - Roma)

The process of harmonic generation in a seeded single pass Free Electron Laser are studied in the time/frequency domain. The linewidth narrowing of the fundamental harmonic is correlated to the amplitude of the input seed. The spectral evolution of the harmonics is studied within a self consistent time dependent model.

MOPOS27 Status Report on SPARC Project
Alberto Renieri, Mariano Carpanese, Franco Ciocci, Giuseppe Dattoli, Antonio Di Pace, Andrea Doria, Francesco Flora, Gian Piero Gallerano, Luca Giannessi, Emilio Giovenale, Giovanni Messina, Luca Mezi, Pier Luigi Ottaviani, Simonetta Pagnutti, Giovanni Parisi, Luigi Picardi, Marcello Quattromini, Giuseppe Ronci, Concetta Ronsivalle, Elio Sabia, Mauro Sassi, Alberto Zucchini (ENEA C.R. Frascati, Frascati - Roma), Carlo Joseph Bocchetta, Miltcho B. Danailov, Gerardo D'Auria, Mario Ferianis (Elettra, Basovizza, Trieste), F. Alessandria, A. Bacci, Ilario Boscolo, F. Broggi, S. Cialdi, C. De Martinis, D. Giove, C. Maroli, V. Petrillo, M. Romè, Luca Serafini (INFN Milano, Milano), D. Levi, Mario Mattioli, G. Medici, Pietro Musumeci (INFN Roma, Roma), L. Catani, E. Chiadroni, Sergio Tazzari (INFN-Roma2, Roma), David Alesini, M. Bellaveglia, Sergio Bertolucci, M.E. Biagini, Caterina Biscari, R. Boni, Manuela Boscolo, Michele Castellano, A. Clozza, G. Di Pirro, A. Drago, A. Esposito, Massimo Ferrario, D. Filippetto, V. Fusco, A. Gallo, A. Ghigo, Susanna Guiducci, M. Incurvati, C. Ligi, F. Marcellini, Mauro Migliorati, C. Milardi, Luigi Palumbo, L. Pellegrino, Miro Preger, P. Raimondi, R. Ricci, C. Sanelli, Mario Serio, F. Sgamma, Bruno Spataro, A. Stecchi, A. Stella, Franco Tazzioli, Cristina Vaccarezza, Mario Vescovi, C. Vicario, M. Zobov (INFN/LNF, Frascati (Roma)), A. Cianchi, A. D'Angelo, R. Di Salvo, A. Fantini, D. Moricciani, Carlo Schaerf (Rome University Tor Vergata, Roma), D. H. Dowell, Paul J Emma, C. Limborg-Deprey, D. T. Palmer (SLAC, Menlo Park, California), James B Rosenzweig, Gil Travish (UCLA, Los Angeles, California), Sven Reiche (UCLA/DPA, Los Angeles - California)

We review the status of FEL source activity of the on going SPARC FEL experiment, developed within the framework of a collaboration among ENEA, CNR, INFN, INFM, Sincrotrone Trieste and University of Rome Tor Vergata. The project is aimed at realising a SASE FEL source, operating in the visible (around 500 nm), with an extended range of tunability down to the VUV (100nm) by the use of the mechanism of non-linear harmonic generation. The development of the relevant activities foresees the realisation of an advanced 150 MeV photo-injector source, aimed at producing a high brightness electron beams, needed to drive a SASE-FEL experiment and a 12 m long undulator. We present the status of the design and construction of the injector, of the undulator and of the e-beam transport line. In particular we discuss the choice of the project parameters, their optimisation and the sensitivity of the SPARC performance to any parameter variation. We will show, using start to-end simulations, what is the impact of the e-beam and of the undulator parameters on the characteristics of the output laser field and in particular on the amount of the non linearly generated power at higher harmonics.

MOPOS29 Analytical Solution of Phase Space Evolution of Electrons in a SASE FEL
Nobuyuki Nishimori (JAERI/FEL, Ibaraki-ken)

The phase space evolution of electrons in a self-amplified spontaneous emission (SASE) FEL operating in the linear regime before saturation is solved analytically from the one dimensional (1D) FEL equaiton. The analytical solution agrees well with a time-dependent numerical simulation which solves the 1D FEL equation in an exact manner and includes a shot-noise effect. The radiation field reaches saturation when the optimum bunching of the electrons is formed on the scale of the resonant wavelength. The optimum bunching leads to the well known saturation condition ρ Nw ~ 1.

MOPOS30 Analysis of Intensity Fluctuations of SASE using the AR Model
Ryukou Kato, S. Isaka, Goro Isoyama, Shigeru Kashiwagi, C. Okamoto, Shoji Suemine, Tamotsu Yamamoto (ISIR, Osaka), Hironao Sakaki (J-PARC, Ibaraki-ken)

We are conducting experimental study on Self-Amplified Spontaneous Emission (SASE) in the far-infrared region using the L-band linac at the Institute of Scientific and Industrial Research (ISIR), Osaka University. The intensity of SASE fluctuates intrinsically because the number of coherent optical pulses generated in an electron bunch is limited. In the actual system, however, another factor producing intensity fluctuations also shows up, which is instability of the linac. Generally speaking, it is difficult to distinguish contributions of these two factors in measured intensity fluctuations. We have applied the autoregressive (AR) model, which is one of the techniques of statistical analysis, to exclude contributions of linac instability from measured data. In the AR model, the present data can be expressed with a linear combination of the past data plus white noise. In the analysis, contributions of the linac instability are identified with the AR model and can be subtracted from the measured data of SASE, so that white noise remains, which is due to intrinsic fluctuations of SASE. In this paper, we will report results of analysis of intensity fluctuations of SASE measured at ISIR, Osaka University, using the AR model.

MOPOS32 Behaviour of Electron Beam in Combined a Self-Generated Field and a Reversed Guide Field in the Helical Wiggler
Soon-Kwon Nam, Ki-Bum Kim (Kangwon National University, Kangwon-Do)

We have studied behaviour of electron beam in combined a self-generated field and a reversed axial-guide field in the tapered helical wiggler. The divergence of electron beam is caused by a three-dimensional effect on the off-axis electrons and self-generated fields. In order to reduce the electron beam loss, we employ a tapered and reversed axial-guide magnetic field. Evolution of energy spread and emittance of electron beam are investigated using three dimensional simulation. Beam cross-section, transverse momentum variation and cross-section view of electron beam are calculated along the z axis. The electron beam loss is reduced by optimizing the magnetic field strength and tapering parameter of reversed axial guide field.

MOPOS33 Evolution of Electron Beam in the Tapered Planar Wiggler
Soon-Kwon Nam, Ki-Bum Kim (Kangwon National University, Kangwon-Do)

We have investigated the evolution of electron beam in the tapered planar wiggler field with a axial-guide field, self-electric fields, and self-magnetic fields. In order to suppress the divergence of emittance and energy spread of the electron beam by the three-dimensional effects on the off-axis electrons and a self-generated field effect,the wiggler field was tapered and the reversed axial-guide magnetic field was applied. We calculate the emittance, transverse trajectories and Fourier transfomation of electron beam using three dimensional simulation by optimizing the magnetic field strength, a tapering parameter and self-field parameters. This method could be expected to enhance the gain and efficiency compared to those of a uniform axial-guide field and a untapered wiggler in a free-electron laser.

MOPOS34 Linac Lattice and Beam Dynamics for X-ray FEL at PAL
Eun-San Kim, D.E. Kim, Sungju J. Park, Moohyun Yoon (PAL, Pohang)

Lattice optimization of 3 GeV linac and studies on the beam dynamics for X-ray FEL at PAL are presented. Investigation of microbunching instability due to coherent synchrotron radiation and wake fields in the PAL linac is also given.

MOPOS36 Design Study on 0.3-nm PAL-XFEL
Jong-Seok Oh (PAL, Pohang), Yujong Kim (DESY, Hamburg), In-Soo Ko, Won Namkung (POSTECH, Pohang, Kyungbuk)

PAL is operating a 2.5-GeV electron linac as a full-energy injector to the PLS storage ring. The PAL linac can be converted to a SASE-XFEL facility (PAL-XFEL) that supplies coherent X-rays down to 0.3-nm wavelength. It requires a 3-GeV driver linac and a 60-m long in-vacuum undulator with a 3-mm gap and a 12.5-mm period to realize a hard X-ray SASE-FEL. The linac should supply highly bright beams with emittance of 1.5 mm-mrad, a peak current of 4 kA, and a low energy spread of 0.02%. FEL performance is very sensitive to electron beam parameters. The beam quality is degraded along the undulator trajectory due to the energy loss and the wake field. Also the FEL gain is reduced by errors in the undulator fields and beam trajectories. The preliminary design details for the 0.3-nm PAL-XFEL are presented with parametric analysis.

MOPOS37 Design Study of Low-Emittance Injector for SASE-XFEL at Pohang Accelerator Laboratory
Sungju J. Park, Sunggi G. Baik, Jung Yun Huang, D.E. Kim, Eun-San Kim, In-Soo Ko, Sanghoon Nam, Jong-Seok Oh (PAL, Pohang), Won Namkung, Jangho H Park, Moohyun Yoon (POSTECH, Pohang, Kyungbuk)

We report on the design study of the low-emittance injector for the SASE-XFEL that is being considered as a possible choice for the next-generation light sources at the Pohang Accelerator Laboratory, POSTECH. Using the PARMELA code, beam dynamics simulations were performed aiming to achieve the invariant-envelope matching at booster entrance, and to insure beam emittance < 1 mm.mrad (at 1-nC bunch charge) at the injector end. We also utilized the MAGIC code for analyzing beam dynamics inside the RF-gun cavities and to confirm the part of PARMELA simulations. Hardware design was done with possible implementation of high-Q.E. photocathode, which could reduce burdens imposed on laser system, thus improving overall system stability and reliability.

MOPOS38 Optimization of a Soft X-Ray SASE-FEL Parameters at Pohang Accelerator Laboratory
Moohyun Yoon, J. E. Han (POSTECH, Pohang, Kyungbuk), Eun-San Kim (PAL, Pohang)

A free-electron laser (FEL) based on self-amplified spontaneous emission has been designed. This FEL is utilizing the existing 2.5 GeV electron linear accelerator at Pohang Accelerator Laboratory (PAL). The radiation wavelength was chosen to be in the water-window region 3-4 nm which can be used for biological imaging. In this paper, it is shown that the PAL is particularly suited for this wavelength if the existing linear accelerator is employed without having major modification. For 4 nm wavelength, the saturated power is shown to be 12 GW with a saturation length of about 25 m.

MOPOS39 The Effect of Liner Induced Phase Fluctuations on the Gain of a Cerenkov FEL
Isabel de la Fuente, Klaus Boller, Peter van der Slot (UTWENTE, Enschede)

It is well known that wiggler errors can degrade the gain of a FEL. A similar process takes places in a Cerenkov FEL where the phase velocity of the amplified EM wave is determined by parameters of the dielectric liner. Thus deviations in the inner radius (Rd) of the liner will result in fluctuations of the phase velocity of the EM wave. E.g., for a quartz liner (εr=4.4) with Rd = 1.5 mm and outer radius of 4 mm, the derivative of the longitudinal wavenumber with respect to Rd is about 105 m-2 for a frequency of 50 GHz. Thus fluctuations in the Rd induce phase fluctuations that may degrade the gain of a Cerenkov FEL. In contrast to the undulator FEL, where the relative phase fluctuations are a result from deviations in the electron trajectories, the phase fluctuations are a result of pertubations in the radiation field for a Cerenkov FEL. As an example of the influence of pertubations in the inner radius of the liner on the gain we will present results of an analysis for a 100 kV Cerenkov FEL operating at a frequency of 50 GHz.

MOPOS40 Simulation Investigation of the Detuning Curve
Xiaojian Shu, Yuhuan Dou, Yuanzhang Wang (IAP, Beijing)

The detuning curves of free-electron laser oscillators are calculated with the help of our three-dimensional code, which are compared with those from one-dimensional simulations and supermode theory, and the experiments. The influence of the optical guiding and other three-dimensional effects on the detuning curve is studied. It is found that the length of the detuning curve from three-dimemsional simulations is shorter than that from one-dimensional simulations and supermode theroy.

MOPOS41 Harmonic Amplifier Free Electron Laser
Qika Jia (USTC/NSRL, Hefei, Anhui)

The evolution of the optical field for an optical klystron(OK) free electron laser is described analytically for both low gain case and high gain case. The harmonic optical klystron (HOK) in which the second undulator is resonant on the higher harmonic of the first undulator is analysed as a harmonic amplifier. The optical field evolution equation of the HOK is derived analytically for both CHG mode (Coherent Harmonic Generation, the quadratic gain regime) and HGHG mode (High Gain Harmonic Generation, the exponential gain regime), the effects of energy spread, energy modulation and dispersion in the whole process are considered. For the quadratic gain regime, the analytical formula of CHG mode is given and compared with that of OK case. For the exponential gain regime, the linear theory of HGHG mode is given and discussed.

MOPOS42 Parameter Analysis For A High-Gain Harmonic Generation FEL By Numerical Calculation Based On 1D Theory
Yuhui Li, Qika Jia, Shancai Zhang (USTC/NSRL, Hefei, Anhui)

The high-gain harmonic generation (HGHG) free-electron laser (FEL) is an important candidate for a fourth-generation light source. Lots of theoretical work has been performed. Recently a further 1D theory about HGHG FEL has been developed. It considers the effects of different parameters for the whole process. An initial program based on this theory has been made. In this paper, a brief comparison of the results from this 1D program and from TDA (3D code) is discussed. It also analyses the parameters for Shanghai deep ultra violate free-electron laser source (SDUV-FEL), including electron beam energy spread, seed laser power, strength of dispersion section etc.

MOPOS47 Simulating FEM Amplifiers: Features in Various Regimes
Artem Elzhov, Alim Kaminsky, Elkuno Perelstein, Sergey Sedykh, Anatoly Sergeev (JINR, Dubna, Moscow Region)

The results of previous experiments with free electron maser amplifiers in high-gain Compton and Raman regimes are analyzed by numerical simulation. Compact quasi-one-dimensional models in a helical trajectory approximation were employed. A tare on the stationary trajectories due to radiation was inserted. Reasonable agreement between simulation and experimental results get in the regimes with reversed guide magnetic field. A handy means for determination of the basic amplification parameters for possible future experiment is available.

MOPOS49 Improving Selectivity of 1D Bragg Resonator Using Coupling of Propagating and Trapped Waves
Naum Samuilovich Ginzburg, Andrey Malkin, Nikolay Peskov, Alexander Sergeevich Sergeev (IAP RAS, Nizhny Novgorod), Artem Elzhov, Alim Kaminsky, Sergey Sedykh, Anatoly Sergeev (JINR, Dubna, Moscow Region)

A novel 1D Bragg resonator based on coupling propagated and locked (quasi cut-off) modes should be tested in a JINR- IAP FEM-oscillator to improve selectivity over the transverse mode index. In this scheme the electron beam interacts with only propagating wave, and the latter is coupled with a quasi cut-off mode. This coupling can be realized by either helical or azimuthally-symmetric corrugation. The quasi cut-off mode provides the feedback in the system leading to the absolute instability and the self-excitation of the whole system while efficiency in the steady-state regime of generation is almost completely determined by the propagating mode, synchronous to the beam. Analytical consideration and numerical simulation show that the efficiency of such an FEM can be rather high. The main advantage of this scheme is provision of higher selectivity over the transverse mode index than traditional scheme of Bragg FEL that encourage increasing operating frequency for fixed transverse size of the interaction space.

MOPOS51 Design considerations for the coherent radiator, FEL, in the MAX IV proposal
Sverker Werin, Åke Andersson, Mathias Brandin, Mikael Eriksson, Tue Norman Hansen, Jörgen Larsson, Lars-Johan Lindgren, Hamed Tarawneh (MAX-lab, Lund)

The MAX IV proposal is a project for the next Swedish synchrotron radiation source. Currently a design study is produced with funding from the Swedish research council (VR). One half of the project will be a double storage ring (at 3 and 1.5 GeV respectively) but the other half will be a coherent radiator, FEL, based on the 3 GeV injector. The basic FEL solution will consist of a normal conducting linac at 3 GeV which feeds three seeded cascaded optical klystrons and a radiator undulator to produce radiation down to 1.5-3 nm at GW powers. To assure synchronisation the gun laser and the seed laser share initial laser and amplifier. This basic system can be enhanced in various ways which will be discussed .

MOPOS57 Analytic Model of Harmonic Generation in the Low-Gain FEL Regime
Gregory Penn (LBNL/CBP, Berkeley, California), Matthias Reinsch (LBNL, Berkeley, California), Jonathan Wurtele (LBNL, Berkeley, California; UC Berkeley, Berkeley)

Harmonic generation using free electron lasers (FELs) requires two undulators: the first generates energy modulation through the inverse FEL effect; the second undulator uses the subsequently bunched beam to radiate at a higher harmonic. These processes are currently evaluated using extensive calculations or simulation codes which can be slow to evaluate and difficult to set up. We describe a simple algorithm to predict the output of a harmonic generation beamline in the low-gain FEL regime based on trial functions for the output radiation. Full three-dimensional effects are included. This method has been implemented as a Mathematica script which runs rapidly and can be generalized to include effects such as asymmetric beams and misalignments. This method is compared with simulation results using the FEL code GENESIS, both for single stages of harmonic generation and to model the design of LUX, a proposed facility for ultra-short X-ray pulses, where multiple stages upshift the input laser frequency by factors of up to 200.

MOPOS59 Coherent Radiation Effects in the LCLS Undulator
Sven Reiche (UCLA/DPA, Los Angeles - California), Zhirong Huang (SLAC, Menlo Park, California)

For X-ray Free-Electron Lasers, a change in the electron energy while amplifying the FEL radiation can shift the resonance condition out of the bandwidth of the FEL. The largest sources of energy loss is incoherent undulator radiation. Because the loss per electron depends only on the undulator parameters and the beam energy, which are fixed for a given resonant wavelength, the average energy loss can be compensated for by a fixed taper of the undulator. Coherent radiation has a strong enhancement proportional to the number of electrons in the bunch for wavelengths comparable to or longer than the bunch dimension or bunch sub-structures. If the coherent loss is comparable to that of the incoherent the required taper depends on the bunch charge and the applied compression scheme and a change of these parameters would require a change of the taper. This imposes a limitation on the operation of FELs, where the taper can only be adjusted manually. In this presentation we analyze the coherent emission of undulator radiation and transition undulator radiation for LCLS, and estimate the effect of the energy spread by the coherent synchrotron radiation within the undulator.

MOPOS60 Spontaneous Radiation Background Calculation for LCLS
Sven Reiche (UCLA/DPA, Los Angeles - California)

The intensity of undulator radiation, not amplified by the FEL interaction, can be larger than the maximum FEL signal in the case of an X-ray FEL. In the commissioning of a SASE FEL it is essential to extract an amplified signal early to diagnose eventual misalignment of undulator modules or errors in the undulator field strength. We developed a numerical code to calculate the radiation pattern at any position behind a multi-segmented undulator with arbitrary spacing and field profiles. The output can be run through numerical spatial and frequency filters to model the radiation beam transport and diagnostic. In this presentation we estimate the expected background signal for the FEL diagnostic and at what point along the undulator the FEL signal can be separated from the background. We also discusses how much information on the undulator field and alignment can be obtained from the incoherent radiation signal itself.

MOPOS63 Generation of GW-level, sub-Angstrom Radiation in the LCLS using a Second-Harmonic Radiator
Zhirong Huang (SLAC, Menlo Park, California), Sven Reiche (UCLA/DPA, Los Angeles - California)

Electron beams are strongly microbunched near the high-gain FEL saturation with a rich harmonic content in the beam current. While the coherent harmonic emission is possible in a planar undulator, the third harmonic radiation typically dominates with about 1% of the fundamental power at saturation. In this paper, we discuss the second harmonic emission in the main undulator induced by effects of finite beam size and angular spread. We show that by a suitable design of an second-stage undulator with its fundamental wavelength tuned to the second harmonic of the main undulator, coherent second harmonic radiation much more intense than the third harmonic is emitted. Numerical simulations and applications to the LCLS project aiming at generating GW-level and sub-Angstrom x-ray pulses are presented.

MOPOS64 Emittance and Quantum Efficiency Measurements from a 1.6 cell S-band Photocathode RF Gun with Mg Cathode
John F. Schmerge, John M. Castro, D. H. Dowell, Steve M. Gierman (SLAC/LCLS, Menlo Park, California), Jym E. Clendenin (SLAC/AD, Menlo Park, California), Rowan O. Hettel (SLAC/SSRL, Menlo Park, California)

Previously we had shown transverse slice and longitudinal emittance measurements using a 1.6 cell S-band rf gun with a Cu cathode [1]. Slice emittances at low charge (15 pC) set an upper limit of 0.6 microns per mm radius. Previous measurements were limited to a maximum of 600 pC total charge due to the relatively low quantum efficiency (5 10-5) of the copper cathodes. In order to increase the available charge to the nC level, we have installed a Mg cathode. The lower work function of Mg compared to Cu increases the quantum efficiency for a fixed laser wavelength but also potentially increases the thermal emittance. Emittance measurements with the Mg cathode will be presented and compared with previous Cu cathode measurements.

MOPOS65 Short Rayleigh Length Free Electron Laser Simulations in Expanding Coordinates
Robert L. Armstead, Joseph Blau, William B. Colson (NPS, Monterey, CA)

For compact short-Rayleigh length FELs, the area of the optical beam can be thousands of times greater at the mirrors than at the beam waist. A fixed numerical grid of sufficient resolution to represent the narrow mode at the waist and the broad mode at the mirrors would be prohibitively large. To accommodate this extreme change of scale with no loss of information, we employ a coordinate system that expands with the diffracting optical mode. The simulation using the new expanding coordinates has been validated by comparison to analytical cold-cavity theory, and is now used to simulate short-Rayleigh length FELs.

MOPOS66 Optical Mode Distortion in a Short Rayleigh Length Free Electron Laser
Joseph Blau, William B. Colson, Robb P. Mansfield, Sean P. Niles, Brett W. Williams (NPS, Monterey, CA)

A short-Rayleigh length FEL will operate primarily in the fundamental mode with a Gaussian profile that is narrow at the waist and broad at the mirrors. The gain medium will distort the optical mode profile and produce higher-order modes that will expand more rapidly than the fundamental. Wavefront propagation simulations are used to study the higher-order modes, as the cavity length, Rayleigh length, electron beam current and radius, undulator taper, and the focus positions of the optical mode and electron beam are varied.

MOPOS67 The Upgrade of the DUV-FEL Facility at the BNL
Xijie Wang, Henrik Loos, James Murphy, George Rakowsky, James Rose, Brian Sheehy, Yuzhen Shen, John Skaritka, Zilu Wu, Li-Hua Yu (BNL/NSLS, Upton, Long Island, New York)

The DUV-FEL at BNL, is the world’s only facility dedicated to laser-seeded FEL R&D and its applications. The HGHG at the DUV-FEL reached saturation at 266 nm with 800 nm seeding [1] in 2002. Since then, the first chemical science experiment – ion pair imaging, was successfully completed [2].The DUV-FEL linac is being upgraded from 200 to 300 MeV to enable the HGHG FEL to produce 100 μJ pulses of 100 nm light. This will establish the DUV FEL as a premier user facility for XUV radiation. The upgraded facility will also enable several critical R&Ds for a future X-ray FEL based on HGHG, such as cascaded HGHG and higher harmonic HGHG (n>5). The upgraded HGHG will operate at the 4th harmonic with the seed laser at 400nm. The increase of the electron beam energy will be accomplished by installing a 5th linac cavity and two 45 MW klystrons. New modulator and dispersion sections vacuum chambers will be manufactured to accommodate new matching optics and 8th harmonic HGHG. The status of the DUV-FEL upgrade and other FEL R&D opportunities will be discussed.

MOPOS69 Wiggler Effects on the Growth Rate of a Raman Free-electron Laser with Axial Magnetic Field or Ion-Channel Guiding
Behrouz Maraghechi (IPM, Tehran; AUT, Tehran), Hossein Aghahosseini (AUT, Tehran), Amir Kordbacheh (AUT, Tehran; IPM, Tehran)

A relativistic theory for Raman backscattering in the beam frame of electrons is presented and is used to find the growth rate of a free-electron laser (FEL), in the Raman regime. A one dimensional helical wiggler and an axial magnetic field are considered. The effects of static self-electric and self-magnetic fields, induced by the steady-state charge density and currents of the non-neutral electron beam, are taken into account to find the steady-state trajectories. The wiggler effects on the linear dispersion relations of the space-charge wave and radiation are included in the analysis. A numerical computation is conducted to compare the growth rate of the excited waves with nonrelativistic treatment. It was found that self-field effects increase the growth rate in the group II orbits and decrease it in the group I orbits. However, the wiggler effects on growth rate are stronger and increase the growth rate on both group I and group II orbits. The discontinuity, due to the cyclotron resonance with the radiation, is removed by including the self-fields or wiggler effects.