FEL Theory
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MOC01
Using Pipe With Corrugated Walls for a Sub-Terahertz FEL  
 
  • G. Stupakov
    SLAC, Menlo Park, California, USA
 
  Funding: This work was supported by Department of Energy contract DE-AC03-76SF00515.
A metallic pipe with corrugated walls supports propagation of a high-frequency mode that is in resonance with a relativistic beam propagating along the axis of the pipe. This mode can be excited by a beam whose length is a fraction of the wavelength. In this work, we discuss another option of excitation of the resonant mode'via the mechanism of the free electron laser instability. This mechanism works if the bunch length is much longer than the wavelength of the radiation, and hence does not require bunch compression. It provides an alternative to excitation by short bunches that can be realized with relatively low energy and low peak-current electron beams.
 
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MOC02 Optimization of a High Efficiency Free Electron Laser Amplifier 17
 
  • E. Schneidmiller, M.V. Yurkov
    DESY, Hamburg, Germany
 
  Technique of undulator tapering in the post-saturation regime is used at the existing X-ray FELs for increasing the radiation power. We present comprehensive analysis of the problem in the framework of one-dimensional and three-dimensional theory. We find that diffraction effects essentially influence on the choice of the tapering strategy. Our studies resulted in a general law of the undulator tapering for a seeded FEL amplifier as well as for SASE FEL.  
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MOC03 Estimate of Free-Electron Laser Gain Length in the Presence of Electron Beam Collective Effects 24
 
  • S. Di Mitri
    Elettra-Sincrotrone Trieste S.C.p.A., Basovizza, Italy
  • S. Spampinati
    Cockcroft Institute, Warrington, Cheshire, United Kingdom
 
  A novel definition for the three-dimensional free electron laser gain length is proposed*, which takes into account the increase of electron beam projected emittance as due, for example, to geometric transverse wakefield and coherent synchrotron radiation developing in linear accelerators. The analysis shows that the gain length is affected by an increase of the electron beam projected emittance, even though the slice (local) emittance is preserved, and found to be in agreement with Genesis code simulation results. It is then shown that the minimum gain length and the maximum of output power may notably differ from the ones derived when collective effects are neglected. The proposed model turns out to be handy for a parametric study of electron beam six-dimensional brightness and FEL performance as function, e.g., of bunch length compression factor, accelerator alignment tolerances and optics design.
* S. Di Mitri, S. Spampinati, Phys. Rev. Special Topics Accel. Beams, 17, 110702 (2014)
 
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MOC04 Operating of SXFEL in a Single Stage High Gain Harmonic Generation Scheme 29
 
  • G.L. Wang, D.X. Dai, G.R. Wu, X.M. Yang, W.Q. Zhang
    DICP, Dalian, People's Republic of China
 
  The beam energy spread at the entrance of undulator system is of paramount importance for efficient density modulation in high-gain seeded free-electron lasers (FELs). In this paper, the dependencies of high harmonic bunching efficiency in the high-gain harmonic generation (HGHG) schemes on the electron energy spread distribution are studied. Theoretical investigations and multi-dimensional numerical simulations are applied to the cases of uniform and saddle beam energy distributions and compared to a traditional Gaussian distribution. It shows that the uniform and saddle electron energy distributions significantly enhance the performance of HGHG-FELs. A numerical example demonstrates that, with the saddle distribution of sliced beam energy spread controlled by a laser heater, the 30th harmonic radiation can be directly generated by a single-stage seeding scheme for a soft x-ray FEL facility.  
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MOP004 Influence of Horizantal Constant Magnetic Field on Harmonic Undulator Radiations and Gain 34
 
  • H. Jeevakhan, P.K. Purohit
    NITTTR, Bhopal, India
  • G. Mishra
    Devi Ahilya University, Indore, India
 
  Harmonic undulators has been analyzed in the presence of constant magnetic field along the direction perpendicular to the main undulator field. Effect of constant magnetic field magnitude on trajectory of electron beam , intensity of radiation and FEL gain at fundamental and third harmonics has been evaluated. Performance of harmonic undulator in the presence horizontal component of earth's magnetic field is the practical realization of the suggested scheme.  
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MOP008 Theoretical Computation of the Polarization Characteristics of an X-Ray Free-Electron Laser with Planar Undulator 38
 
  • G. Geloni
    XFEL. EU, Hamburg, Germany
  • V. Kocharyan, E. Saldin
    DESY, Hamburg, Germany
 
  We show that radiation pulses from an X-ray Free-Electron Laser (XFEL) with a planar undulator, which are mainly polarized in the horizontal direction, exhibit a suppression of the vertical polarization component of the power at least by a factor λw2/(4 pi Lg)2, where λw is the length of the undulator period and Lg is the FEL field gain length. We illustrate this fact by examining the XFEL operation under the steady state assumption. In our calculations we considered only resonance terms: in fact, non resonance terms are suppressed by a factor λw3/(4 pi Lg)3 and can be neglected. While finding a situation for making quantitative comparison between analytical and experimental results may not be straightforward, the qualitative aspects of the suppression of the vertical polarization rate at XFELs should be easy to observe. We remark that our exact results can potentially be useful to developers of new generation FEL codes for cross-checking their results.  
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MOP009 Efficient Electron Sources of Coherent Spontaneous Radiation with Combined Helical and Uniform Magnetic Fields 43
 
  • N. Balal, V.L. Bratman, E. Magori
    Ariel University, Ariel, Israel
  • V.L. Bratman
    IAP/RAS, Nizhny Novgorod, Russia
 
  We discuss two methods to mitigate repulsion of particles in dense electron bunches from photo-injectors and to enhance the power of terahertz radiation. First, the repulsion may be reduced with both very short bunches and undulator periods, reducing the length of a radiation section. According to simulations bunches with duration (50-100) fs, charge (50-200) pC, and energy 6 MeV could fairly effectively radiate at frequencies up to (10-20) THz. The undulator for such a source can be formed by means of redistribution of a solenoid field by a non-magnetized iron helix and a permanently magnetized helix. The second source is based on an idea proposed by A.V. Savilov for electron bunching under conditions when the cyclotron electron frequency is larger than the undulator frequency, and an increase of particle energy in the bunch Coulomb field leads to a decrease in longitudinal momentum and attraction of particles (this effect is analogous to the cyclotron negative-mass instability). A large value of the required uniform field can be used to easily obtain the needed undulator field by placing a simple iron helix inside a pulsed solenoid. Simulations confirm that the corresponding particle attraction can provide a powerful and narrowband radiation at the frequencies (1-3) THz.  
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TUP001 THz Photo-Injector FEM Based on Spontaneous Coherent Emission from a Bunch of Negative-Mass Electrons 317
 
  • A.V. Savilov, I.V. Bandurkin
    IAP/RAS, Nizhny Novgorod, Russia
 
  The use of short dense electron bunches produced form a photo-injector gun is attractive for realization of a THz FEL based on spontaneous coherent emission from such bunches. This type of emission is realized, when the axial length of bunches is shorter that the wavelength of the radiated wave. Therefore, the length of the operating region of such a FEL is strictly limited by degradation (an increase in the axial size) of the e-bunch caused by both the Coulomb repulsion and the velocity spread. The use of the regime of the 'negative mass' can be a way to provide stabilization of the axial size of e-bunches. Such a regime is realized in a magnetostatic undulator with a guiding homogeneous axial magnetic field. If the electron cyclotron frequency (corresponding to the guiding magnetic field) exceeds the bounce-frequency of electron oscillations in the periodic undulator field, then the abnormal dependence of the axial velocity of electrons on their energy takes place (an increase in the energy leads to a decrease in the axial velocity). In such regime, axial Coulomb repulsion of the electrons leads to their mutual attraction which slows down bunch degradation. The use of this regime results in a substantial increase in the length of the spontaneous coherent emission, and, therefore, in an increase in both the duration and the power of the output radiation pulse.
The work was supported by the Russian Scientific Foundation (RSCF), Project no. 14-19-01723
 
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TUP002 Further Studies of Undulator Tapering in X-Ray FELs 321
 
  • A. Mak, F. Curbis, S. Werin
    MAX-lab, Lund, Sweden
 
  We further the studies of the model-based optimization of tapered free-electron lasers presented in a recent publication [Phys. Rev. ST Accel. Beams 18, 040702 (2015)]. Departing from the ideal case, wherein the taper profile is a smooth and continuous function, we consider the more realistic case, with individual undulator segments separated by break sections. Using the simulation code GENESIS, we apply our taper optimization method to a case, which closely resembles the FLASH2 facility in Hamburg, Germany. By comparing steady-state and time-dependent simulations, we examine how time-dependent effects alter the optimal taper scenario. From the simulation results, we also deduce that the "traditional" empirical method, whereby the intermediate radiation power is maximized after closing every undulator gap, does not necessarily produce the highest final power at the exit of the undulator line.  
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TUP003 Threshold of a Mirror-less Photonic Free Electron Laser Oscillator Pumped by One or More Electron Beams 327
 
  • P.J.M. van der Slot, K.-J. Boller, A. Strooisma
    Mesa+, Enschede, The Netherlands
 
  Funding: This research is supported by the Dutch Technology Foundation STW, which is part of the Netherlands Organisation for Scientific Research, and which is partly funded by the Ministry of Economic Affairs
Transmitting electrons through a photonic crystal can result in stimulated emission and the generation of coherent Cerenkov radiation. Here we consider a photonic-crystal slab consisting of a two-dimensional, periodic array of bars inside a rectangular waveguide. By appropriately tapering the bars at both ends of the slab, we numerically show that an electromagnetic wave can be transmitted through the waveguide filled with the photonic-crystal slab with close to zero reflection. Furthermore, the photonic-crystal slab allows transmission of electrons in the form of one or more beams. By appropriately designing the photonic-crystal slab, we obtain a backward wave interaction at low electron-beam energy of around 15 kV, that results in distributed feedback of the radiation on the electrons without any external mirrors being present. Here we discuss the dynamics of the laser oscillator near threshold and numerically show that the threshold current can be distributed over multiple electron beams, resulting in a lower current per beam.
 
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TUP004 Three-dimensional, Time-dependent Simulation of Free-Electron Lasers 331
 
  • P.J.M. van der Slot
    Mesa+, Enschede, The Netherlands
  • H. Freund, P.J.M. van der Slot
    CSU, Fort Collins, Colorado, USA
 
  Minerva is a simulation code that models the interaction of electrons with an optical field inside an undulator. Minerva uses a modal expansion for the optical field and the full Newton-Lorentz force equation to track the particles through the optical and magnetic fields. To allow propagation of the optical field outside the undulator and interact with optical elements, MINERVA interfaces with the optical propagation code OPC to mode, for example, FEL oscillators. As there exists a large variety of FELs ranging from long-wavelength oscillators to soft and hard X-ray FELs that are either seeded or starting from noise, a simulation code, such as Minerva, should be capable of modelling this huge variety of FEL configurations. Here we present a validation of the Minerva code against experimental data for various FEL configurations, ranging from long wavelength FEL oscillators to hard X-ray SASE FEL.  
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TUP005 A Mirror-Less, Multi-Beam Photonic Free-Electron Laser Oscillator Pumped Far Beyond Threshold 334
 
  • P.J.M. van der Slot, K.-J. Boller, A. Strooisma
    Mesa+, Enschede, The Netherlands
 
  Funding: This research is supported by the Dutch Technology Foundation STW, which is part of the Netherlands Organisation for Scientific Research, and which is partly funded by the Ministry of Economic Affairs
In a photonic free-electron laser electrons are transmitted through a photonic crystal in the form of one or multiple electron beams to generate coherent Cerenkov radiation. Here we consider a photonic-crystal slab consisting of a two-dimensional, periodic array of bars inside a rectangular waveguide, with both ends tapered to provide complete transmission of an electromagnetic wave. By appropriately designing the photonic-crystal slab, we obtain a backward wave interaction at low electron beam energy of around 15 kV, that results in distributed feedback of the radiation on the electrons without any external mirrors being present. Here we numerically study the dynamics of the laser oscillator when pumped far beyond threshold with one or multiple electron beams. We show that using multiple beams with the same total current provide better suppression of higher-order modes and can produce more output power, compared to the laser pumped by a single beam of the same total current.
 
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TUP006 Quantum Nature of Electrons in Classical X-ray FELs 338
 
  • P.M. Anisimov
    LANL, Los Alamos, New Mexico, USA
 
  X-ray FELs built to date are well described by the classical theory. This theory in its simplest form is expressed as a system of pendulum equations for electrons coupled to the electromagnetic field. The FEL interaction requires bunching of the electrons on a scale less than radiation wavelength. The progress in the development of FELs and the need to reach even shorter laser radiation wavelength with low energy electrons require that the quantum characteristic of the FEL interaction to be properly considered. Quantum theories have been already proposed by a number of authors. These theories, however, have been developed for regimes that are not relevant for modern/planned X-ray FELs. Here, we focus on quantum effects in modern/future X-ray FELs and stop treating an electron as a point-particle. This results in quantum reduction of the bunching! Starting with the analysis of the free space dispersion for the electron wave packet, we will present a modified 1D FEL theory that takes into account the quantum uncertainty of the electron position in X-ray FELs. This theory allows for a unified classification of FELs with respect to the wave nature of an electron that shows a planned FEL at Los Alamos National Lab to be most affected. The Genesis simulation code has been modified in order to include quantum reduction of the bunching that lead to interesting results. LA-UR-15-26276  
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TUP007 High Fidelity Start-to-end Numerical Particle Simulations and Performance Studies for LCLS-II 342
 
  • G. Marcus, Y. Ding, P. Emma, Z. Huang, T.O. Raubenheimer, L. Wang
    SLAC, Menlo Park, California, USA
  • J. Qiang, M. Venturini
    LBNL, Berkeley, California, USA
 
  High fidelity numerical particle simulations that leverage a number of accelerator and FEL codes have been used to analyze the LCLS-II FEL performance. Together, the physics models that are included in these codes have been crucial in identifying, understanding, and mitigating a number of potential hazards that can adversely affect the FEL performance, some of which are discussed in papers submitted to this conference[*, **]. Here, we present a broad overview of the LCLS-II FEL performance, based on these start-to-end simulations, for both the soft X-ray and hard X-ray undulators including both SASE and self-seeded operational modes.
* M. Venturini, et al., The microbunching instability and LCLS-II lattice design: lessons learned, FEL'15
** Z. Zhang, et al., Microbunching-induced sidebands in a seeded free-electron laser, FEL'15
 
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TUP008 High-Gain FEL in the Space-Charge Dominated Raman Limit 347
 
  • I.I. Gadjev, C. Emma, A. Nause, J.B. Rosenzweig
    UCLA, Los Angeles, USA
 
  While FEL technology has reached the EUV and X-ray regime at existing machines such as LCLS and SACLA, the scale of these projects is often impractical for research and industrial applications. Sub-millimeter period undulators can reduce the size of a high-gain EUV FEL, but will impose stringent conditions on the electron beam. In particular, a high-gain EUV FEL based on undulators with a sub-millimeter period will require electron beam currents upwards of 1 kA at energies below 100 MeV. Coupled with the small gap of such undulators and their low undulator strengths, K < 0.1, these beam parameters bring longitudinal space-charge effects to the foreground of the FEL process. When the wavelength of plasma oscillations in the electron beam becomes comparable to the gain-length, the 1D theoretical FEL model transitions from the Compton to the Raman limit. In this work, we investigate the behavior of the FEL's gain-length and efficiency in these two limits. The starting point for the analysis was the one-dimensional FEL theory including space-charge forces. The derived results were compared to numerical results of Genesis 1.3 simulations. This theoretical model predicts that in the Raman limit, the gain-length scales as the beam current to the -1/4th power while the efficiency plateaus to a constant.  
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TUP009 Coherent Thomson Scattering Using the PEHG 351
 
  • S. Chen, K. Ohmi, D. Zhou
    KEK, Ibaraki, Japan
 
  Electron beam is density modulated by the phase-merging effect to obtain ultra-short longitudinal structures in the phase space. Coherent radiations are then generated by the coherent Thomson scattering between the phase-merged beam and a long wavelength laser pulse.  
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WEP004 Energy Spread Constraints on Field Suppression in a Reverse Tapered Undulator 597
 
  • J.P. MacArthur, Z. Huang, A.A. Lutman, A. Marinelli, H.-D. Nuhn
    SLAC, Menlo Park, California, USA
 
  A 3.2 m variable polarization Delta undulator[1] has been installed at the end of the LCLS undulator line. The Delta undulator acts an an afterburner in this configuration, using bunching from upstream planar undulators to produce radiation with arbitrary polarization. To optimize the degree of polarization from this device, a reverse taper[2] has been proposed to suppress background radiation produced in upstream undulators while still microbunching the beam. Here we extend previous work on free electron lasers with a slowly varying undulator parameter[3] to show there is a strong energy spread dependence to the maximum allowable detune from resonance. At LCLS, this energy spread limitation keeps the reverse taper slope in the slowly varying regime and limits the achievable degree of circular polarization.
[1] A. B. Temnykh, PRST-AB, 11, 120702, (2008).
[2] E. A. Schneidmiller and M. V. Yurkov, PRST-AB, 16, 110702, (2013).
[3] Z. Huang and G. Stupakov, PRST-AB, 8, 040702, (2005).
 
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WEP008 Four-Dimensional Models of FEL Amplifiers and Oscillators 607
 
  • J. Blau, K. R. Cohn, W.B. Colson
    NPS, Monterey, California, USA
 
  Funding: This work has been supported by the High Energy Laser Joint Technology Office.
New four-dimensional models of free electron lasers (FELs) are described, for both amplifier and oscillator configurations. Model validation and benchmarking results are shown, including comparisons to theoretical formulas and experiments.
 
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WEP084 Microbunching-Instability-Induced Sidebands in a Seeded Free-Electron Laser 741
 
  • Z. Zhang
    TUB, Beijing, People's Republic of China
  • Y. Ding, W.M. Fawley, Z. Huang, J. Krzywinski, A.A. Lutman, G. Marcus, A. Marinelli, D.F. Ratner
    SLAC, Menlo Park, California, USA
 
  The measured, self-seeded soft X-ray radiation spectrum corresponding to multiple effective undulator lengths of the LCLS exhibits a pedestal-like distribution around the seeded frequency. In the absence of a post-undulator monochromator, this contamination limits the spectral purity and may seriously degrade certain user applications. In general for either externally- or self-seeded FELs, such pedestals may originate with any time-varying property of the electron beam that can modulate the complex gain function. In this paper we specifically focus on the contributions of electron beam microbunching prior to the undulator. We show that both energy and density modulations can induce sidebands in a seeded FEL configuration. Analytic FEL theory and numerical simulations are used to analyze the sideband content relative to the amplified seeded signal, and to compare with experimental results.  
poster icon Poster WEP084 [1.263 MB]  
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WEP085 Conceptual Theory of Spontaneous and Taper-Enhanced Superradiance and Stimulated Superradiance 746
 
  • A. Gover, R. Ianconescu
    University of Tel-Aviv, Faculty of Engineering, Tel-Aviv, Israel
  • C. Emma, P. Musumeci
    UCLA, Los Angeles, USA
  • A. Friedman
    Ariel University, Ariel, Israel
 
  Funding: We acknowledge partial support by the U.S. Israel Binational Science Foundation (BSF)Jerusalem, Israel
In the context of radiation emission from an electron beam Dicke's superradiance (SR) is the enhanced radiation emission from a pre-bunched beam. Stimulated Superradiance (ST-SR) is the further enhanced emission of the bunched beam in the presence of a phase-matched radiation wave. These processes were analyzed for Undulator radiation in the framework of radiation field mode-excitation theory[1]. In the nonlinear saturation regime the synchronism of the bunched beam and an injected radiation wave may be sustained by wiggler tapering [2]. Same processes are instrumental also in enhancing the radiative emission in the tapered wiggler section of seeded FEL[3]. In a long tapered wiggler the diffraction of the emitted radiation wave is not negligible even at Angstroms wavelengths (as in LCLS). A Fresnel diffraction model was provided in [4] for the SR process only. Here we outline the fundamental physical concepts of Spontaneous Superradiadce (SR), Stimulated Superradiance (ST-SR), Taper-Enhanced Superradiance (TES) and Taper-Enhanced Stimulated Superradiance Amplification (TESSA), and compare their Fourier and Phasor formulations in the radiation mode expansion and free-diffraction models. Detailed further analysis can provide better design concepts of high power FELs and improved tapering strategy for enhancing the power of seeded short wavelength FELs
1. A. Gover, PR ST-AB 8, (030701) ; (030702) (2005)
2. J. Duris et al., arxiv 2015.
3. Y. Jiao et al., PR ST-AB 15 050704 2012
4. E.A. Schneidmiller, M.V. Yurkov, PR ST-AB 18, 030705 (2015)
 
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WEP087 Smith-Purcell Radiation from Microbunched Beams Modulated after Passing the Undulators in FELs 752
 
  • A. Potylitsyn
    TPU, Tomsk, Russia
  • D.Yu. Sergeeva, M.N. Strikhanov, A.A. Tishchenko
    MEPhI, Moscow, Russia
 
  We suggest using the Smith-Purcell effect from microbunched beams modulated after passing the undulators in FELs as an extra source of monochromatic radiation. We investigate theoretically characteristics of Smith-Purcell radiation in THz and X-ray frequency regions for two types of distribution of the particles in the beam. The expression for spectral-angular distribution of such radiation is obtained and analyzed, both for fully and partially modulated beams. The intensity of Smith-Purcell radiation is shown to be able to increase both due to the periodicity of the beam and the periodicity of the target. The numerical results prove that such radiation source can be an effective instrument for different FEL users, supplementary for the main FEL source.  
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