New Concepts

WEBOS01 Current-Enhanced SASE Using an Optical Laser and its Application to the LCLS
Alexander A Zholents, William M. Fawley (LBNL/CBP, Berkeley, California), Paul J Emma, Zhirong Huang, Gennady Stupakov (SLAC, Menlo Park, California), Sven Reiche (UCLA/DPA, Los Angeles - California)

We propose a significant enhancement of the electron peak current entering a SASE undulator by inducing an energy modulation in an upstream wiggler magnet via resonant interaction with an optical laser, followed by micro-bunching of the energy-modulated electrons at the accelerator exit. This current enhancement allows a considerable reduction of the FEL gain length. The x-ray output consists of a series of uniformly spaced spikes, each spike being temporally coherent. The duration of this series is controlled by the laser pulse and in principle can be narrowed down to just a single, ~100-attosecond spike. Given potentially absolute temporal synchronization of the x-ray spikes to the energy-modulating laser pulse, this scheme naturally makes pump-probe experiments available to SASE FEL’s. We also study various detrimental effects related to the high electron peak current and discuss potential cures. We suggest a possible operational scenario for the LCLS optimized with respect to the choice of the modulating laser beam and electron beam parameters. Numerical simulations are provided.

WEBOS02 Generation of Terahertz Radiation by Modulating the Electron Beam at the Cathode
Jonathan Neumann, Ralph Fiorito, Patrick Gerard O'Shea (IREAP, College Park, Maryland), G.L. Carr (BNL, Upton, Long Island, New York), Henrik Loos, Timur Shaftan, Brian Sheehy, Yuzhen Shen, Zilu Wu (BNL/NSLS, Upton, Long Island, New York), Henry Freund (SAIC McLean, McLean)

A bunched electron beam can be used to generate coherent radiation in a particle accelerator. This experiment, a collaboration between the University of Maryland and the Source Development Laboratory at Brookhaven National Laboratory, uses a drive laser modulated at terahertz frequencies in an RF-photoinjecting electron accelerator to produce a bunched beam at the cathode. The experiment is designed to determine if such a scheme could be used to develop a compact, high power terahertz emitter. After acceleration to approximately 72 MeV, a mirror intercepts the beam. The backwards transition radiation from the mirror is measured with a bolometer. The experiment was conducted at various modulation frequencies and levels of charge.

WEBOS03 Suppression of Multipass, Multibunch Beam Breakup in Two Pass Recirculating Accelerators
Chris Tennant, David Douglas, Kevin Jordan, Lia Merminga, Eduard Pozdeyev (Jefferson Lab, Newport News, Virginia), Todd I. Smith (Stanford University, Stanford, Califormia)

Beam Breakup (BBU) occurs in all accelerators at sufficiently high currents. In recirculating accelerators, such as the energy recovery linacs used for high power FELs, the maximum current has historically been limited by multipass, multibunch BBU, a form that occurs when the electron beam interacts with the high order modes (HOMs) of an accelerating cavity on one pass and then again on the second pass. This effect is of particular concern in the designs of modern high average current energy recovery accelerators utilizing superconducting technology. In such two pass machines rotation of the betatron planes by 90°, first proposed by Smith and Rand in 1980 [1], should significantly increase the threshold current of the multibunch BBU. Using a newly developed 4-dimensional tracking code, we study the effect of rotation on the threshold current of the JLAB FEL Upgrade. We examine several optical rotator schemes based on quadrupoles and solenoids and evaluate their performance in terms of the instability threshold current increase and their effect on the FEL optics.

WEBOS04 Potential Use of eRHIC’s ERL for FELs and Light Sources
Vladimir N. Litvinenko, Ilan Ben-Zvi (BNL, Upton, Long Island, New York)

One of the design of future electron-hadron collider eRHIC* is based on a 5-10 GeV high current energy-recovery linac (ERL) with possible extension of its energy to 20 GeV. This ERL will operate with high brightness electron beams, which do naturally match requirements for X-ray FELs and other next generation light sources. In this paper we present a number of possible scenarios which use eRHIC ERL in parasitic and dedicated mode for SASE, HGHG and oscillator X-ray FELs. We explore a possibility of optic-free X-ray oscillator in detail.

WEBOS05 The Harmonically Coupled 2-Beam FEL
Brian W.J. McNeil, Gordon Robb (Strathclyde University, Glasgow), Mike Poole (CCLRC/DL/ASTeC, Daresbury, Warrington, Cheshire)

A 1-D model of a 2-beam Free Electron Laser amplifier is presented. The two co-propagating electron beams have different energies, chosen so that the fundamental resonant FEL interaction of the higher energy beam is at an harmonic of the lower energy beam. In this way, a coupling between the FEL interactions of the two beams occurs via the harmonic components of the electron bunching and radiation emission of the lower energy interaction. Such resonantly coupled FEL interactions may offer potential benefits over existing single beam FEL schemes. A simple example is presented where the lower energy FEL interaction only is seeded with radiation at its fundamental resonant wavelength. It is predicted that the coherence properties of this seed field are transfered via the resonantly coupled FEL interaction to the un-seeded higher energy FEL interaction, thereby improving its coherence properties over that of a SASE interaction alone. This method may offer an alternative seeding scheme for FELs operating in the XUV and x-ray regions of the spectrum.

THPOS01 Low Emittance Gun Project based on Field Emission
Romain Ganter, Arno Candel, Micha Dehler, Jens Gobrecht, Chris Gough, Gerhard Ingold, Simon C. Leemann, Kevin Shing Bruce Li, Martin Paraliev, Marco Pedrozzi, Jean-Yves Raguin, Leonid Rivkin, Volker Schlott, Harald Sehr, Andreas Streun, Albin Wrulich, Sasa Zelenika (PSI, Villigen)

The design of an electron gun capable of producing beam emittance one order of magnitude lower than current technology would reduce considerably the cost and size of a free electron laser emitting at 0.1nm. Field emitter arrays (FEAs) including a gate and a focusing layer are an attractive technology for such high brightness sources. Electrons are extracted from micrometric tips thanks to voltage pulses between gate and tips. The focusing layer should then reduce the initial divergence of each emitted beamlets. This FEA will be inserted in a high gradient diode configuration coupled with a radiofrequency structure. In the diode part very high electric field pulses (several hundreds of MV/m) will limit the degradation of emittance due to space charge effect. This first acceleration will be obtained with high voltage pulses (typically a megavolt in a few hundred of nanoseconds) synchronized with the low voltage pulses applied to the FEA (typically one hundred of volts in one nanosecond at frequency below kilohertz). This diode part will then be followed by an RF accelerating structure in order to bring the electrons to relativistic energies.

THPOS02 Field Emitter Arrays for a Free Electron Laser Application
Kevin Shing Bruce Li, Micha Dehler, Romain Ganter, Jens Gobrecht, Jean-Yves Raguin, Leonid Rivkin, Albin Wrulich (PSI, Villigen)

The development of a new electron gun with the lowest possible emittance would help reducing the total length and cost of a free electron laser. Field emitter arrays (FEAs) are an attractive technology for electron sources of ultra high brightness. Indeed, several thousands of microscopic tips can be deposited on a 1 mm diameter area. Electrons are then extracted by applying voltage to a first grid layer close to the tip apexes, the so called gate layer, and focused by a second grid layer one micrometer above the tips. The typical aperture diameter of the gate and the focusing layer is in the range of one micrometer. One challenge for such cathodes is to produce peak currents in the ampere range since the usual applications of FEAs require less than milliampere. Encouraging peak current performances have been obtained by applying voltage pulses at low frequency between gate and tips. In this paper we report on different tip materials available on the market: diamond FEAs from Extreme Devices Inc., ZrC single tips from Applied Physics Technologies Inc. and Mo FEAs from SRI International.

THPOS03 Undulators for the BESSY Soft-X-Ray FEL
Johannes Bahrdt, Winfried Frentrup, Andreas Gaupp, Bettina Kuske, Atoosa Meseck, Michael Scheer (BESSY GmbH, Berlin)

BESSY plans a linac based high gain harmonic generation FEL user facility with three FEL lines [1]. The modulators and most of the radiators are planar pure permanent magnet undulators. The last radiator and the final amplifier produce radiation of linear polarization with arbitrary orientation as well as elliptically or helically polarized light. They will have a modified APPLE II design which provides higher fields compared to a conventional APPLE II [2]. Detailed calculations for this design will be presented. FEL 3D-calculations provide information about the radiation field distribution at the end of the undulator. A beamline designer needs the information about the effective source size at the waist and the location of the waist. The electric fields calculated by GENESIS have been propagated and the source characteristics have been derived for various FEL parameters. The FEL process requires tight gap tolerances within and between modules. We present a new gap measurement system with a gap positioning accuracy of 1.5 microns.

THPOS13 Proposal of Laser-Driven Acceleration with Bessel Beam
Dazhi Li, Kazo Imasaki (ILT, 2-6 Yamada-oka, Suita, Osaka)

A possible approach of realizing multi-stage laser-driven acceleration with Bessel beam is explored. With using a set of annular slits, Bessel beam is truncated and structured into several separate sections during its transportation, leading to the possibility of accelerating electrons stage by stage. Bessel beam is regarded as diffraction-free beam, allowing a very long acceleration distance in contrast to the Gaussian beam. In this paper, the transportation characteristics of the structured Bessel beam is analyzed based on scalar diffraction theory, as well as the acceleration mechanism is demonstrated numerically.

THPOS16 Amplification of Short-Pulse Radiation from the Electron Undergoing Half-Cyclotron Rotation
Makoto R. Asakawa, H. Marusaki (OU-iFEL, Hirakata, Osaka), Kazo Imasaki (ILT, 2-6 Yamada-oka, Suita, Osaka), Nobuhisa Ohigashi (Kansai University, Osaka), Yoshiaki Tsunawaki (Osaka Sangyo University, Osaka)

Electrons undergoing half-cyclotron rotation emitts a half-cycle electromagnetic wave. A novel light source based on such short-pulse radiation is under developing at Institute of Free Electron Laser Osaka university. In this presentation, the experiments to amplify the radiation in the optical resonator will be discussed.

THPOS35 Reasearches of Thomson Scattering X-Ray Source at Tsinghua University
Wenhui Huang, Huaibi Chen, C. Cheng, Y. Cheng, Q. Du, Taibin Du, Y.Ch. Du, Y.Ch. Ge, X.Z. He, J.F. Hua, G. Huang, Y.Zh. Lin, Chuangxiang Tang, B. Xia, M.J. Xu, X.D. Yuan, Sh.X. Zheng (Tsinghua University, Beijing)

The bright and tunable short pulse X-ray sources are being widely used in various research fields including materials, chemistry, biology and solid physics. Thomson scattering source is one of the most promising approaches to short pulsed X-ray sources. Researches on Thomson scattering x-ray sources are being carried out in Tsinghua University. Some theoretical results and the preliminary experiment on the Thomson scattering between electron beams and laser pulses are described in this paper.

THPOS38 The Coherent Synchrotron Radiation Influence on the Storage Ring Longitudinal Beam Dynamics
Evgueni G. Bessonov, Rouslan Mikhailovich Feshchenko (LPI, Moscow), Vasily Ivanovich Shvedunov (MSU, Moscow)

We investigate influence on the storage ring beam dynamics of the coherent Synchrotron Radiation (SR) self fields produced by an electron bunch. We show that the maximum energy gain in the RF cavity must far exceed the energy loss of electrons due to the coherent SR.

THPOS45 Isochronous Bend for a High Gain Ring FEL
Alexander Nikolaevich Matveenko, Oleg Alexandrovich Shevchenko, Nikolai Aleksandrovich Vinokurov (BINP, Novosibirsk)

The recently proposed ring free electron laser (FEL) consists of several undulators with isochronous bends between them. Isochronous bends are necessary to preserve the beam bunching between undulators. Such FEL configuration may be used as an independent soft X-ray source or as a master oscillator for an X-ray FEL (high gain harmonic generator or other type). The lattice of the compact 500-MeV 60 degree bend for a soft X-ray (50 nm) FEL is proposed. Fundamental restrictions due to quantum fluctuations of synchrotron radiation and technically achievable fields to construct isochronous bends of a shorter wavelength ring FEL are discussed.

THPOS48 The Two-Beam Free Electron Laser Oscillator
Neil R. Thompson (CCLRC/DL/ASTeC, Daresbury, Warrington, Cheshire), Brian W.J. McNeil (Strathclyde University, Glasgow)

A one-dimensional model of a free-electron laser operating simultaneously with two electron beams of different energies [1] is extended to an oscillator configuration. The electron beam energies are chosen so that an harmonic of the lower energy beam is at the fundamental radiation wavelength of the higher energy beam. Potential benefits over a single-beam free-electron laser oscillator are discussed.

THPOS51 Harmonic Cascade FEL Designs for LUX, a Facilty for Ultrafast X-ray Science
Corlett John, William M. Fawley, Gregory Penn, Alexander A Zholents (LBNL/CBP, Berkeley, California), Weishi Wan (LBNL/ALS, Berkeley, California), Matthias Reinsch, Jonathan Wurtele (UC Berkeley, Berkeley)

LUX is a proposed facility for ultrafast X-ray science, based on an electron beam accelerated to ~3-GeV energy in a superconducting, recirculating linac.Included in the design are multiple FEL beamlines which use the harmonic cascade approach to produce coherent XUV & soft X-ray emission beginning with a strong input external laser seed at ~200 nm wavelength. Each cascade module generally operates in the low-gain regime and is composed of a radiator together with a modulator section, separated by a magnetic chicane. The chicane temporally delays the electron beam pulse in order that a "virgin" pulse region (with undegraded energy spread) be brought into synchronism with the radiation pulse. For each cascade, the output photon energy can be selected over a wide range by varying the seed laser wavelength and the field strength in the undulators. We present numerical simulation results, as well as those from analytical models, to examine predicted FEL performance. We also discuss lattice considerations pertinent to harmonic cascade FELs, as well as sensitivity studies and requirements on the electron beam.

THPOS53 Novel Method for Phase-Space Tomography of Rapidly Evolving E-beams
Kevin Chalut (Duke University, Durham, North Carolina), Vladimir N. Litvinenko (BNL, Upton, Long Island, New York), Igor V. Pinayev (BNL/NSLS, Upton, Long Island, New York)

Traditional tomographic methods based on Radon transformation require a full set projections covering full 180-degrees. This technique is applicable only to a stationary distribution of electrons, which do not evolve. In addition, this method can’t work with incomplete sub-set of data such as a few projections covering total angle of few degrees. We present novel method of tomography working with a limited number of non-degenerated linear projections. We present the description of the method, discuss its advantages as well as limitations. We compare the method with the Radon transformation. We present the application of this method to the study the dynamics of e-beam in longitudinal phase space using dual sweep streak-camera with psec resolution. This method allowed us to restore the evolution of e-beam during a giant pulse in a storage ring FEL when the distribution of the electrons changes completely during one synchrotron oscillation. We discuss another possible applications of this method in advanced FEL systems, where effects of space charge, nonlinearities or coherent synchrotron radiation impair traditional diagnostics methods.

THPOS57 Acceleration of Electrons in a Diffraction Dominated IFEL
Pietro Musumeci, Chan Joshi, Claudio Pellegrini, J. Ralph, James B Rosenzweig, C. Sung, Sergei Tochitsky, Gil Travish (UCLA, Los Angeles, California), Sergey Tolmachev, Alexander Varfolomeev, Alexander Varfolomeev Jr., Timofey Yarovoi (RRC Kurchatov Institute, Moscow), Salime Boucher, Adnan Doyuran, Robert England, Rodney Yoder (UCLA/DPA, Los Angeles - California)

We report on the observation of energy gain in excess of 20 MeV at the Inverse Free Electron Laser Accelerator experiment at the Neptune Laboratory at UCLA. A 14.5 MeV electron beam is injected ina 50 cm long undulator strongly tapered both in period and field amplitude. A CO2 10 μ m laser with power >300 GW is used as the IFEL driver. The Rayleigh range of the laser (1.8cm) is shorter than the undulator length so that the interaction is diffraction dominated. Few per cent of the injected particles are trapped in stable accelerating buckets and electrons with energies up to 35 MeV are detected on the magnetic spectrometers. Experimental results on the scaling of the accelerator characteristics versus input parameters like injection energy, laser focus position and laser power are discussed. Three dimensional simulations are in good agreement with the electron energy spectrums observed in the experiment and indicate that substantial energy exchange between laser and electron beam only occurs in the first 25-30 cm of the undulator. An energy gradient of >70 MeV is inferred. In the second section of the undulator higher harmonic IFEL interaction is observed.