Paper  Title  Page 

MOD01 
Physics of PostSaturation Tapered FEL Towards SingleFrequency Terawatt Output Power  


Funding: This work is supported by the Fundamental Research Funds for the Central Universities under Project No. 5003131049 and the U.S. DOE under Contract No. DEAC0276SF00515 and No. FWP2013SLAC100164. TWlevel output power of a singlepass highgain xray FEL has recently attracted much attention. GWlevel power can be achieved out of beneficial FEL instability. The remaining factor of 50 before reaching TW poses great challenges, including sustaining power growth and retaining spectral purity in the postsaturation regime. Postsaturation FEL physics involves the sideband instability [1], radiation diffraction [2], undulator tapering [3] etc. In this talk I will introduce FEL sideband and diffraction effects, and give an estimate of individual effects to the overall performance. Undulator tapering is known an effective route to enhance power extraction efficiency. We recently proposed an efficiencyenhancement scheme based on preserving the longitudinal beam phase space, which ensures trapping of resonant particles in the ponderomotive bucket and meanwhile takes advantage of the increasing radiation amplitude to precipitate the particle deceleration process. Analysis shows that the optimal power efficiency based on the proposed scheme, together with a prebunched beam, can be greatly improved within a relative short taper length and the sideband effects effectively suppressed. [1] C.Y. Tsai et al., Phys. Rev. Accel. Beams 20, 120702 (2017) [2] C.Y. Tsai et al., Phys. Rev. Accel. Beams 21, 060702 (2018) [3] C.Y. Tsai et al., NIM A 913 (2019) 107119 

Slides MOD01 [8.530 MB]  
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MOD02 
Microbunch Rotation and Coherent Undulator Radiation from a Kicked Electron Beam  


Microbunches form perpendicular to the electron travel direction, and the conventional understanding is that they shear rather than rotate in response to a transverse kick, locking FEL facilities into a singleuser operating mode. We show that microbunches rotate toward the new direction of travel if the electron beam is kicked and defocused. We provide evidence that microbunch rotation explains the unexpectedly large amount of offaxis radiation observed during experiments at the Linac Coherent Light Source. We demonstrate that LCLS can be multiplexed into at least three soft Xray beams using this principle. Finally, we report on a more sophisticated scheme of offset quadrupoles that was used to produce two distinct hard Xray spots at LCLS.  
Slides MOD02 [25.347 MB]  
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MOD03 
Hanbury Brown and Twiss Interferometry at XFEL Sources  


The invention of optical lasers led to a revolution in the field of optics as well as to the birth of quantum optics. The reasons were the unique statistical and coherence properties of lasers. Shortwavelength freeelectron lasers (FELs) are sources of bright, coherent extremeultraviolet and Xray radiation with pulse duration on the order of tens of femtoseconds and are presently considered to be laser sources at these energies. Hanbury Brown and Twiss (HBT) interferometry that is based on intensity correlations allows fast and comprehensive analysis of the FEL statistical properties. We demonstrate that selfamplified spontaneous emission (SASE) FELs are highly spatially coherent to the firstorder, but despite their name, statistically behave as chaotic sources. HBT measurements performed at an externally seeded FEL FERMI showed that it behaves as a real laserlike source according to a Glauber definition.  
Slides MOD03 [8.298 MB]  
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MOD04 
PostSaturation Dynamics and Superluminal Propagation of a SuperRadiant Spike in a FreeElectrons Laser Amplifier  


The generation of a single Xray isolated spike of radiation with peak power at multiGW level and femtosecond temporal duration represents a unique opportunity in time resolved diffraction imaging of isolated molecules or nonperiodic structures for acquiring single shot images before the Coulomb explosion of the sample takes place. Such a condition is met by an FEL operating in superradiant regime for the possibility of simultaneously reducing the pulse duration while increasing the peak power and the pulse energy. We study the dynamics of an isolated spike of radiation in superradiant regime. We show that conditions exist where the pulse moves with a group velocity larger than one and is followed by a pedestal resulting from a complex postsaturation dynamic. The tail is constituted by a train of subpulses with both transverse and longitudinal coherence and decaying amplitudes. We analyze the dynamical conditions leading to the formation of the main pulse and the tail. We study the correlation of the tail structure with the longitudinal phase space of the electrons and provide a recipe to partially suppress this tail.  
Slides MOD04 [7.203 MB]  
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TUP019  Regime of MultiStage NonResonant Trapping in Free Electron Lasers  83 


Funding: This work is supported by the RFBR (grants #180240009, #180200765) and by the IAP RAS Project 003520190001. We describe three works united by the idea of the nonresonant regime [1] providing an effective trapping in a beam with a great energy spread. In this regime, the "bucket" corresponding to the resonant electronwave interaction passes through the electron layer on the energyphase plane and traps a fraction of electrons. (I) Operability of this regime was demonstrated in the highefficient 0.8 MeV Kaband FEMamplifier [2]. (II) In shortwavelength FELs the multistage trapping in several consecutive sections can be organized [3]. In each section a small ebeam fraction is trapped due to a weak electronwave interaction. However, repetition of this process from section to section involves in the interaction almost the whole ebeam. We describe efficiency enhancement and improving the frequency wave spectrum in multistage SASE FELs. (III) The multistage amplification of a singlefrequency wave signal can provide cooling of the electron bunch. In this regime, tapering of every section is provided such that the "bucket" goes from maximal initial electron energy down to the minimal one and moves down energies of trapped electrons. [1] A.Savilov et al., Nucl. Instr. Meth. A, vol. 507, p.158, 2003 [2] A.Kaminsky et al., Int. Conf. IRMMWTHz 2018, art. 4057938 [3] S.Kuzikov, A.Savilov, Phys. Plasmas, vol. 25, p.113114, 2018 

DOI •  reference for this paper ※ https://doi.org/10.18429/JACoWFEL2019TUP019  
About •  paper received ※ 14 August 2019 paper accepted ※ 27 August 2019 issue date ※ 05 November 2019  
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TUP023  Analytical and Numerical Comparison of Different Approaches to the Description of SASE in High Gain FELs  94 


Correlation function theory which has been developed recently gives rigorous statistical description of the SASE FEL operation. It directly deals with the values averaged over many shots. There are two other approaches which are based either on Vlasov equation or on direct solution of particle motion equations. Both of them use random functions which relate to single shot. To check the validity of these three approaches it might be interesting to compare them with each other. In this paper we present the results of such comparison obtained for the 1D FEL model. We show that twoparticle correlation function approximation is equivalent to the quasilinear approximation of the Vlasov equation approach. These two approximations are in a good agreement with the results of direct solution of particle motion equations at linear and early saturation stages. To obtain this agreement at strong saturation high order harmonics in Vlasov equation have to be taken into account which corresponds to taking into account of three and more particle correlations in the correlation function approach.  
DOI •  reference for this paper ※ https://doi.org/10.18429/JACoWFEL2019TUP023  
About •  paper received ※ 19 August 2019 paper accepted ※ 25 August 2019 issue date ※ 05 November 2019  
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TUP038  Axial Symmetry in Spontaneous Undulator Radiation for XFELO TwoBunch Experiment  134 


Funding: U.S. DOE, Office of Science, Office of BES, under Contract No. DEAC0206CH11357 and National Science Foundation under Award No. PHY1549132, the Center for Bright Beams. A well known discrepancy exists between 2D and 3D FEL simulation codes with respect to the radiation field intensity prior to the exponential gain regime [1]. This can be qualitatively explained by the fact that the 3D field representation preserves many more modes than does the axisymmetric field solved for by a 2D code. In this paper, we seek to develop an analytical model that quantifies this difference. We begin by expanding the spontaneous undulator radiation field as a multipole series, whose lowest order mode is axisymmetric. This allows us to calculate the difference in predicted intensity. Next, we confirm these results with numerical calculation and existing FEL codes GINGER and GENESIS. Finally, we discuss the implications of this study with respect to the XFELO twobunch experiment to be conducted at LCLSII. [1] Z. Huang and K.J. Kim, "Review of Xray freeelectron laser theory", Phys. Rev. STAB, vol. 10, p. 034801, 2007. 

DOI •  reference for this paper ※ https://doi.org/10.18429/JACoWFEL2019TUP038  
About •  paper received ※ 19 August 2019 paper accepted ※ 28 August 2019 issue date ※ 05 November 2019  
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TUP041  Xcos SCILAB Model for Simulation of Intensity and Gain of Planar Undulator Radiation  138 


SCILAB Xcos based model has been designed to simulate the Intensity and Gain of planar undulator radiation. Numerical approach has been used to determine the trajectories of an electron along x and z direction, traversing through a planar undulator. The present paper describes the technical details of the different blocks, parameters and possibility of combined model used for trajectory and intensity simulation Results are compared with the previous conventional syntax based codes.  
DOI •  reference for this paper ※ https://doi.org/10.18429/JACoWFEL2019TUP041  
About •  paper received ※ 01 August 2019 paper accepted ※ 27 August 2019 issue date ※ 05 November 2019  
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TUP042  Analysis of Undulator Radiations With Asymmetric Beam and NonPeriodic Magnetic Field  141 


Harmonic Undulator radiations at third harmonics with non periodic constant magnetic field has been analysed. Symmetric and asymmetric electron beam with homogeneous spread has been used to present viable solution for the resonance shift inherited in undulator with constant magnetic field. The radiation recovers shifts in resonance and regain its intensity with asymmetric electron beam and harmonic field
Harmonic undulator, energy spread 

Poster TUP042 [2.886 MB]  
DOI •  reference for this paper ※ https://doi.org/10.18429/JACoWFEL2019TUP042  
About •  paper received ※ 01 August 2019 paper accepted ※ 31 October 2019 issue date ※ 05 November 2019  
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TUP047  An Analysis of Optimal Initial Detuning for Maximum EnergyExtraction Efficiency  145 


For low gain free electron laser (FEL), the phase space evolutions of trapped electrons in the phase bucket are analyzed through calculating their synchrotron oscillation periods, which vary with the initial detuning and initial phase. The optimal initial detuning for the maximum energyextraction efficiency and the corresponding saturation length are given. The analysis demonstrated that for the low gain case the gain of the strong optical field is about a quarter of that of the weak optical field (small signal gain), and the saturation power larger than that of high gain FEL can be achieved in the resonator of oscillator FEL.  
DOI •  reference for this paper ※ https://doi.org/10.18429/JACoWFEL2019TUP047  
About •  paper received ※ 19 August 2019 paper accepted ※ 27 August 2019 issue date ※ 05 November 2019  
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TUP049  Simulating ShotNoise of ’Real’ Electron Bunches  149 


An algorithm and numerical code for the upsampling of a system of particles, from a smaller to a larger number, is described. The method introduces a Poissonian ’shotnoise’ to the upsampled distribution [1], typical of the noise statistics arising in a bunch of particles generated by a particle accelerator. The algorithm is applied on a phasespace distribution of relatively few simulation particles representing an electron beam generated by particle accelerator modelling software, for subsequent injection into an Free Electron Laser (FEL) amplifier which is used here to describe the model. A much larger number of particles is usually required to model the FEL lasing process than is required in the simulation models of the electron beam accelerators that drive it. A numerical code developed from the algorithm was then used to generate electron bunches for injection into to an unaveraged 3D FEL simulation code, Puffin [2]. Results show good qualitative and quantitative agreement with analytical theory. The program and usage manual is available to download from GitHub [3].
[1] B.W.J. McNeil, M.W. Poole and G.R.M. Robb, Physical Review Special Topics  Accelerators and Beams Vol 6, 070701 (2003). [2] L.T. Campbell and B.W.J. McNeil, Phys. Plasmas 19, 093119 (2012). [3] https://github.com/UKFELs/JDF 

Poster TUP049 [1.419 MB]  
DOI •  reference for this paper ※ https://doi.org/10.18429/JACoWFEL2019TUP049  
About •  paper received ※ 20 August 2019 paper accepted ※ 27 August 2019 issue date ※ 05 November 2019  
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TUP050  Comparison Between, and Validation Against an Experiment of, a Slowlyvarying Envelope Approximation Code and a ParticleinCell Simulation Code for FreeElectron Lasers  153 


Freeelectron laser simulation codes employ either the SlowlyVarying Envelope Approximation (SVEA) or a ParticleinCell (PiC) formulation. Maxwell’s equations are averaged over the fast time scale in the SVEA so that there is no need to resolve the wave period. In contrast, the fast oscillation is retained in PiC codes. As a result, the SVEA codes are much less computationally intensive and are used more frequently than PiC codes. While the orbit dynamics in PiC codes and some SVEA Codes (MEDUSA and MINERVA) use the full unaveraged Lorentz force equations, some SVEA codes use the KrollMortonRosenbluth (KMR) approximation (GENESIS, GINGER, FAST, and TDA3D). Steadystate simulation comparisons [1] have appeared in the literature between different codes using the averaged and unaveraged particle dynamics. Recently, a comparison between three KMR SVEA codes (GENESIS, GINGER, and FAST) and the PUFFIN PiC code in the timedependent regime has been reported [2]. In this paper, we present a comparison between the unaveraged PiC code PUFFIN, the unaveraged SVEA code MINERVA for the timedependent simulation of SASE freeelectron lasers with the experimental measurements from SPARC SASE FEL at ENEA Frascati.
[1] S.G. Biedron et al., NIMA 445, 110 (2000). [2] B. Garcia et al., paper presented at the 38th International Free Electron Laser Conference, Santa Fe, New Mexico, 20  25 August 2017. 

Poster TUP050 [0.908 MB]  
DOI •  reference for this paper ※ https://doi.org/10.18429/JACoWFEL2019TUP050  
About •  paper received ※ 02 August 2019 paper accepted ※ 28 August 2019 issue date ※ 05 November 2019  
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TUP053  An Investigation of Possible NonStandard Photon Statistics in a FreeElectron Laser I: Experiment  161 


Funding: Work supported by U.S. DOE, Office of Science, Office of BES, under Award No. DESC0018428. It was reported that the photon statistics of the seventh coherent spontaneous harmonic radiation of the MARK III FEL was subPoissonian [1], which concludes that Fano factor F (the ratio of photon number variance to the average photon number) is less than unity. Whether FEL light exhibits such nonstandard behavior is an important issue; if it does, our understanding of the FEL needs to be radically modified. In this paper, we reexamine the analyses of experimental data in Ref. [1]. We find that the observed value of F could be explained within the standard FEL theory if one combines the detector dead time effect with photon clustering arising from the FEL gain. We propose an improved experiment for a more definitive measurement of the FEL photon statistics. [1] T. Chen and J.M. Madey, J. Phys. Rev. Lett. 86, 5906 (2001). 

Poster TUP053 [0.929 MB]  
DOI •  reference for this paper ※ https://doi.org/10.18429/JACoWFEL2019TUP053  
About •  paper received ※ 21 August 2019 paper accepted ※ 12 September 2019 issue date ※ 05 November 2019  
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TUP054  An Investigation of Possible NonStandard Photon Statistics in a FreeElectron Laser II: Theory  165 


Funding: Work supported by U.S. DOE, Office of Science, Office of BES, under Award No. DESC0018428. In this paper we explore whether we can at present find a theoretical basis for nonstandard, subPoissonian photon statistics in the coherent spontaneous harmonic radiation of an FEL as was claimed to have been measured with the Mark III FEL [1]. We develop a one dimensional quantum FEL oscillator model of the harmonic radiation in the linear gain regime to calculate the photon statistics. According to our study, it seems unlikely that the photon statistics for an FEL oscillator starting from the noise could be subPoissonian. [1] T. Chen and J.M. Madey, J. Phys. Rev. Lett. 86, 5906 (2001). 

Poster TUP054 [0.386 MB]  
DOI •  reference for this paper ※ https://doi.org/10.18429/JACoWFEL2019TUP054  
About •  paper received ※ 21 August 2019 paper accepted ※ 16 September 2019 issue date ※ 05 November 2019  
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