FEL2019 - List of Keywords (bunching)

Paper	Title	Other Keywords	Page
TUP023	Analytical and Numerical Comparison of Different Approaches to the Description of SASE in High Gain FELs	FEL, radiation, simulation, undulator	94
	O.A. Shevchenko, N.A. Vinokurov BINP SB RAS, Novosibirsk, Russia N.A. Vinokurov NSU, Novosibirsk, Russia
	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 1-D FEL model. We show that two-particle 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/JACoW-FEL2019-TUP023
About •	paper received ※ 19 August 2019 paper accepted ※ 25 August 2019 issue date ※ 05 November 2019
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TUP035	Sensitivity of LCLS Self-Seeded Pedestal Emission to Laser Heater Strength	laser, electron, free-electron-laser, experiment	126
	G. Marcus, D.K. Bohler, Y. Ding, W.M. Fawley, Y. Feng, E. Hemsing, Z. Huang, J. Krzywiński, A.A. Lutman, D.F. Ratner SLAC, Menlo Park, California, USA
	Measurements of the soft X-ray, self-seeding spectrum at the LCLS free-electron laser generally display a pedestal-like distribution around the central seeded wavelength that degrades the spectral purity. We have investigated the detailed experimental characteristics of this pedestal and found that it is comprised of two separate components: (1) normal SASE whose total strength is nominally insensitive to energy detuning and laser heater (LH) strength; (2) sideband-like emission whose strength positively correlates with that of the amplified seed and negatively with energy detuning and LH strength. We believe this latter, non-SASE component arises from comparatively long wavelength amplitude and phase modulations of the main seeded radiation line. Its shot-to-shot variability and LH sensitivity suggests an origin connected to growth of the longitudinal microbunching instability on the electron beam. Here, we present experimental results taken over a number of shifts that illustrate the above mentioned characteristics.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-FEL2019-TUP035
About •	paper received ※ 28 August 2019 paper accepted ※ 29 August 2019 issue date ※ 05 November 2019
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TUP049	Simulating Shot-Noise of ’Real’ Electron Bunches	micro-particles, electron, FEL, simulation	149
	P. Traczykowski, L.T. Campbell, B.W.J. MᶜNeil USTRAT/SUPA, Glasgow, United Kingdom L.T. Campbell, B.W.J. MᶜNeil, P. Traczykowski Cockcroft Institute, Warrington, Cheshire, United Kingdom L.T. Campbell, P. Traczykowski STFC/DL/ASTeC, Daresbury, Warrington, Cheshire, United Kingdom
	An algorithm and numerical code for the up-sampling of a system of particles, from a smaller to a larger number, is described. The method introduces a Poissonian ’shot-noise’ to the up-sampled distribution [1], typical of the noise statistics arising in a bunch of particles generated by a particle accelerator. The algorithm is applied on a phase-space 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/JACoW-FEL2019-TUP049
About •	paper received ※ 20 August 2019 paper accepted ※ 27 August 2019 issue date ※ 05 November 2019
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TUP051	Plasma Accelerator Driven Coherent Spontaneous Emission	electron, FEL, radiation, undulator	157
	B.M. Alotaibi, R. Altuijri PNU, Riyadh, Kingdom of Saudi Arabia B.M. Alotaibi, R. Altuijri, A.F. Habib, B. Hidding, B.W.J. MᶜNeil, P. Traczykowski USTRAT/SUPA, Glasgow, United Kingdom A.F. Habib, B. Hidding, B.W.J. MᶜNeil, P. Traczykowski Cockcroft Institute, Warrington, Cheshire, United Kingdom
	Plasma accelerators [1] are a potentially important source of high energy, low emittance electron beams with high peak currents generated within a relatively short distance. As such, they may have an important application in the driving of coherent light sources such as the Free Electron Laser (FEL) which operate into the X-ray region [2]. While novel plasma photocathodes [3] may offer orders of magnitude improvement to the normalized emittance and brightness of electron beams compared to Radio Frequency-driven accelerators, a substantial challenge is the energy spread and chirp of beams, which can make FEL operation impossible. In this paper it is shown that such an energy-chirped, ultrahigh brightness electron beam, with dynamically evolving current profile due to ballistic bunching at moderate energies, can generate significant coherent radiation output via the process of Coherent Spontaneous Emission (CSE)[4]. While this CSE is seen to cause some FEL-induced electron bunching at the radiation wavelength, the dynamic evolution of the energy chirped pulse dampens out any high-gain FEL interaction. [1] E. Esary et al., Reviews of Modern Physics p. 1229 (2009). [2] B. W. J. McNeil and N. R. Thompson, 2010 Nat. Photon.4 814-21 [3] B. Hidding et al., 2012 Phys. Rev. Lett. 108 035001 [4] L. T. Campbell and B. W. J. McNeil, 2012, in Proc. FEL2012
	Poster TUP051 [1.401 MB]
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-FEL2019-TUP051
About •	paper received ※ 20 August 2019 paper accepted ※ 27 August 2019 issue date ※ 05 November 2019
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TUP083	Energy Spread Impact on HGHG and EEHG FEL Pulse Energy	laser, FEL, electron, photon	250
	S. Spampinati, E. Allaria, L. Giannessi, P. Rebernik Ribič Elettra-Sincrotrone Trieste S.C.p.A., Basovizza, Italy L. Giannessi ENEA C.R. Frascati, Frascati (Roma), Italy P. Rebernik Ribič University of Nova Gorica, Nova Gorica, Slovenia
	VUV and X-ray free electron lasers (FELs) require a very bright electron beam. Seeded FEL harmonic generation is particularly sensible to energy spread and slice energy spread can limit the highest harmonic conversion factor at which coherent radiation can be produced. Different cas-cade schemes can have different sensibility to the slice energy spread. At FERMI we have evaluated the impact of the slice energy spread on the performance of high gain harmonic generation (HGHG) and of echo enable harmonic generation (EEHG) by measuring the FEL pulse energy as function of the electron beam slice energy spread. The measurements were done at different harmon-ics. The slice energy spread was varied trough the laser heater located in the linac that drives FERMI.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-FEL2019-TUP083
About •	paper received ※ 20 August 2019 paper accepted ※ 17 September 2019 issue date ※ 05 November 2019
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TUP090	Considerations on Implementing EEHG with a Strong Linear Chirp	electron, laser, FEL, simulation	262
	M.A. Pop, F. Curbis, W. Qin, S. Werin MAX IV Laboratory, Lund University, Lund, Sweden F. Curbis, S. Werin SLF, Lund, Sweden W. Qin Lund University, Lund, Sweden
	Funding: The work is supported by Knut and Alice Wallenberg foundation. The Soft X-ray Laser (SXL) currently being studied at MAX IV Laboratory is envisioned to produce coherent radiation in the 1-5 nm wavelength range. In this contribution, we present the results of simulations aimed at adding to the SXL an Echo Enabled Harmonic Generation scheme, which has been shown to increase the coherence of FELs in the Soft X-ray regime. Our work puts special emphasis on accommodating the positive energy chirp of the electron bunch coming out of the MAX IV Linac and on generating sufficient bunching at the high harmonics necessary for covering the full wavelength range.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-FEL2019-TUP090
About •	paper received ※ 20 August 2019 paper accepted ※ 28 August 2019 issue date ※ 05 November 2019
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WEP024	1.3 GHz Solid State Power Amplifier for the Buncher in CTFEL Facility	electron, experiment, FEL, cavity	371
	T.H. He, C.L. Lao, P. Li, X. Luo, L.J. Shan, K. Zhou CAEP/IAE, Mianyang, Sichuan, People’s Republic of China
	The THz Free Electron Laser facility (CAEP THz FEL, CTFEL) of the China Academy of Engineering Physics uses high quality electron beams to generate high average power terahertz radiations. A 1.3 GHz RF buncher is used in front of the superconducting linear accelerator of the CTFEL facility to improve the electron beams quality. The RF buncher is driven by a solid state power amplifier (SSPA), and the SSPA is feedback controlled by a low level RF (LLRF) control system to ensure the high stability of the amplitude and phase of the bunching field in the buncher cavity. The SSPA operates at 1.3 GHz and outputs 0 to 5 kW of continuous wave power. This paper mainly introduces the principle and composition of the SSPA, and presents some experiments on the RF buncher driven by the SSPA.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-FEL2019-WEP024
About •	paper received ※ 15 August 2019 paper accepted ※ 27 August 2019 issue date ※ 05 November 2019
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WEP035	NIR Spectrometer for Bunch-Resolved, Non-Destructive Studies of Microbunching at European XFEL	FEL, electron, laser, free-electron-laser	392
	S. Fahlström, M. Hamberg Uppsala University, Uppsala, Sweden C. Gerth, N.M. Lockmann, B. Steffen DESY, Hamburg, Germany
	At the European X-ray Free Electron Laser high brilliance femtosecond FEL radiation pulses are generated for user experiments. For this to be achieved electron bunches must be reliably produced within very tight tolerances. In order to investigate the presence of micro-bunching, i.e. charge density variation along the electron bunch with features in the micron range, a prism-based NIR spectrometer with an InGaAs sensor, sensitive in the wavelength range 900 nm to 1700 nm was installed. The spectrometer utilizes diffraction radiation (DR) generated at electron beam energies of up to 17.5 GeV. The MHz repetition rate needed for bunch resolved measurements is made possible by the KALYPSO line detector system, providing a read-out rate of up to 2.7 MHz. We present the first findings from commissioning of the NIR spectrometer, and measurements on the impact of the laser heater system for various bunch compression settings, in terms of amplitude and bunch-to-bunch variance of the NIR spectra as well as FEL pulse energy.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-FEL2019-WEP035
About •	paper received ※ 20 August 2019 paper accepted ※ 29 August 2019 issue date ※ 05 November 2019
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WEP041	Feasibility of Single-Shot Microbunching Diagnostics for a Pre-Bunched Beam at 266 nm	electron, laser, radiation, diagnostics	408
	A.H. Lumpkin AAI/ANL, Lemont, Illinois, USA D.W. Rule Private Address, Silver Spring, USA
	Funding: Work supported by U.S. Department of Energy, Office of Science, under Contract No. DE-AC02-06CH11357. Co-propagating a relativistic electron beam and a high-power laser pulse through a short undulator (modulator) provides an energy modulation which can be converted to a periodic longitudinal density modulation (or microbunching) via the R56 term of a chicane. Such pre-bunching of a beam at the resonant wavelength and the harmonics of a subsequent free-electron laser (FEL) amplifier seeds the process and results in improved gain. We describe potential characterizations of the resulting microbunched electron beams using coherent optical transition radiation (COTR) imaging techniques for transverse size (50 micron), divergence (sub-mrad), trajectory angle (0.1 mrad), spectrum (few nm), and pulse length (sub-ps). The transverse spatial alignment is provided with near-field imaging and the angular alignment is done with far-field imaging and two-foil COTR interferometry (COTRI). Analytical model results for a 266-nm wavelength COTRI case with a 10% microbunching fraction will be presented. COTR gains of 7 million were calculated for an initial charge of 300 pC which enables splitting the optical signal for single-shot measurements of all the cited parameters.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-FEL2019-WEP041
About •	paper received ※ 20 August 2019 paper accepted ※ 29 August 2019 issue date ※ 05 November 2019
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WEP063	The Preliminary Study of a Pre-Bunched Terahertz Free Electron Laser by a Velocity Bunching Scheme	radiation, electron, undulator, FEL	477
	R. Huang, Q.K. Jia, H.T. Li, Z. Zhao USTC/NSRL, Hefei, Anhui, People’s Republic of China
	Funding: Work supported by the National Natural Science Foundation of China Grant Number 11805200 Terahertz (THz) radiation has broad applications in biological sciences, materials imaging and radar communications and so on. High-power, frequency-adjustable THz radiation sources are desired. An electron beam, generated in a photoinjector and bunched at terahertz (THz) frequency, will excite a coherent THz radiation when entering an undulator. The radiation power mainly depends on the particle number and the bunching factor of the electron beam, which is limited by the space charge effect among the microbunches and the total rf phase width the macrobunch occupied. Previously we have designed a pre-bunched THz free electron laser (FEL) with the radiation frequency covering 0.5-5 THz. While the radiation intensity for the lower frequency (below 1~THz) is not very high because of the large energy spread and the low bunching factor. We will report a THz FEL by a velocity bunching scheme, which could realize more highly bunched beam especially in the low THz frequency region. The physical design of the electron source is described in detail.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-FEL2019-WEP063
About •	paper received ※ 19 August 2019 paper accepted ※ 28 August 2019 issue date ※ 05 November 2019
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WEP072	Expected Radiation Properties of the Harmonic Afterburner at FLASH2	undulator, radiation, polarization, simulation	492
	M. Mehrjoo, B. Faatz, G. Paraskaki, M. Tischer, P. Vagin DESY, Hamburg, Germany
	We discuss the afterburner option to upgrade the FLASH2 undulator line, at the FLASH facility in the Hamburg area, for delivering short wavelengths down to approximately 1.5 nm with variable polarization. This relatively straightforward upgrade enables us the study of the scientific cases in L- absorption edges of rare earth metals. The proposed afterburner setting with an energy upgrade to 1.35 GeV would potentially cover many of the community’s requests for the short wavelengths radiation and circular polarization. We also study the influence of reverse tapering on the radiation output. This contribution presents a series of simulations for the afterburner scheme and some of the technical choices made for implementation.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-FEL2019-WEP072
About •	paper received ※ 19 August 2019 paper accepted ※ 28 October 2019 issue date ※ 05 November 2019
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WEP101	Linear Polarisation via a Delta Afterburner for the CompactLight Facility	undulator, FEL, polarization, radiation	549
	H.M. Castañeda Cortés, D.J. Dunning, N. Thompson STFC/DL/ASTeC, Daresbury, Warrington, Cheshire, United Kingdom
	Funding: CompactLight is funded by the European Union’s Horizon 2020 research and innovation program under Grant Agreement No.777431. We studied the degree of polarisation of the FEL radiation from the diverted-beam scheme [1,2] using the layout of the CompactLight facility, which is in the process of being designed. To satisfy the polarisation requirements defined by the users [3] without compromising the aim of the facility to be compact, we studied a configuration comprising a helical Super Conductive Undulator (SCU) followed by a Delta afterburner (configured to generate linearly polarised light). The trade-offs between the SCU length, afterburner length, degree of polarisation and output power are presented and discussed. [1] E. A. Schneidmiller and M. V. Yurkov, Phys. Rev. ST Accel. Beams 16, 11702 (2013) [2] A. Lutman et al., Nature Photonics 10, 468(2016) [3] A. Mak et al., FREIA Report 2019/01, 2019
	Poster WEP101 [1.083 MB]
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-FEL2019-WEP101
About •	paper received ※ 16 August 2019 paper accepted ※ 25 August 2019 issue date ※ 05 November 2019
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THB04	Longitudinal Phase Space Study on Injector Beam of High Repetition Rate X-Ray FEL	linac, electron, cavity, laser	584
	Q. Gu SSRF, Shanghai, People’s Republic of China Z. Wang SARI-CAS, Pudong, Shanghai, People’s Republic of China
	The longitudinal phase space of the high repetition rate injector beam usually twisted and deteriorated by the space charge force. It causes the correlated energy spread and the local chirp within the beam, which could not compensated by the harmonic correction. As a consequence of this problem, one could not get ideal beam with a peak current more than kiloamperes. In this paper several approaches have been studied to relieve this effect and get the well compressed beam for the lasing.
	Slides THB04 [3.151 MB]
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-FEL2019-THB04
About •	paper received ※ 26 August 2019 paper accepted ※ 16 September 2019 issue date ※ 05 November 2019
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THP003	Arbitrary Order Perturbation Theory for a Time-Discrete Model of Micro-Bunching Driven by Longitudinal Space Charge	space-charge, FEL, simulation, electron	596
	Ph. Amstutz LBNL, Berkeley, California, USA M. Vogt DESY, Hamburg, Germany
	A well established model for studying the micro-bunching instability driven by longitudinal space charge in ultra-relativistic bunches in FEL-like beamlines can be identified as a time-discrete Vlasov system with general drift maps and Poisson type collective kick maps. Here we present an arbitrary order perturbative approach for the general system and the complete all-orders solution for a special example. For this example we benchmark our theory against our Perron-Frobenius tree-code.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-FEL2019-THP003
About •	paper received ※ 20 August 2019 paper accepted ※ 29 August 2019 issue date ※ 05 November 2019
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THP016	Study of Microbunching Instability in SHINE Linac	laser, linac, simulation, electron	629
	D. Huang SINAP, Shanghai, People’s Republic of China D. Gu SARI-CAS, Pudong, Shanghai, People’s Republic of China M. Zhang Shanghai Advanced Research Institute, Pudong, Shanghai, People’s Republic of China
	The SHINE project in China aims at the next generation high repetation rate and high power hard X-ray free electron laser facility. The high quality electron beam is thus requested in the linac to generate such a high quality FEL lasing. As the prerequisite, the microbuncing instability introduced by the nonlinear effects such as the LSC, CSR and wakefields in the bunch compressing process must be taken care of, otherwise the electron beam will not meet the requirements of lasing. In this article, the microbunching effects including the gain of the instability in the linac of SHINE are estimated, and several ways for the control of the instability are proposed.
	Poster THP016 [0.536 MB]
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-FEL2019-THP016
About •	paper received ※ 24 August 2019 paper accepted ※ 29 August 2019 issue date ※ 05 November 2019
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THP020	Microbunching Enhancement by Adiabatic Trapping	laser, storage-ring, emittance, injection	635
	X.J. Deng, W.-H. Huang, C.-X. Tang TUB, Beijing, People’s Republic of China A. Chao SLAC, Menlo Park, California, USA
	Storage ring based concept called steady-state microbunching was proposed years ago for high average power narrowband coherent radiation generation. There are now active efforts on-going by the SSMB collaboration established among Tsinghua University and several other institutes. In this paper we study the particle trap and filamentation process of beam in RF or Micro Bucket which is useful for understanding the injection beam dynamics of SSMB. One remarkable result is the steady-state current distribution after full filamentation has little dependence on the bucket height as long as it is several times larger than the energy spread. A discrete increase of bucket height can boost the bunching, with the sacrifice of emittance growth. An adiabatic change of bucket height from a smaller value to the final desired value is then proposed to boost the bunching while preserving the longitudinal emittance.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-FEL2019-THP020
About •	paper received ※ 20 August 2019 paper accepted ※ 28 August 2019 issue date ※ 05 November 2019
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THP069	Observations on Microbunching of Electrons in Laser-Driven Plasma Accelerators and Free-Electron Lasers	FEL, laser, electron, experiment	722
	A.H. Lumpkin Fermilab, Batavia, Illinois, USA M. Downer, M. LaBerge The University of Texas at Austin, Austin, Texas, USA D.W. Rule Private Address, Silver Spring, USA
	Funding: This manuscript has been authored by Fermi Research Alliance, LLC under Contract No. DE-AC02-07CH11359 with the U.S. Department of Energy, Office of Science, Office of High Energy Physics. The periodic longitudinal density modulation of relativistic electrons at the resonant wavelength (microbunching) is a fundamental aspect of free-electron lasers (FELs). In one case, microbunching fractions reached 20% at saturation of a self-amplified spontaneous emission (SASE) FEL resulting in gains of 1 million at 530 nm [1]. In that experiment the z-dependent gain of coherent optical transition radiation (COTR) was also measured. In laser-driven plasma accelerators (LPAs), microbunching at visible wavelengths has also been recently reported as evidenced by significant COTR enhancements measured in near-field and far-field images on a single shot for the first time [2]. An analytical model for COTR interferometry (COTRI) addresses both cases. In the FEL, one identified microbunched transverse cores of 25-100 microns while in the LPA the reported transverse sizes at the exit of the LPA were a few microns. In the latter case, signal enhancements of nearly 100, 000 and extensive fringes out to 30 mrad in angle space were recorded. The broadband microbunching observed in the LPA case could act as a seed for a SASE FEL experiment with tunability in principle over the visible regime. [1] A.H. Lumpkin et al., Phys. Rev. Lett. 88, No.23, 234801 (2002). [2] A.H. Lumpkin, M. LaBerge, D.W. Rule et al., Proceedings of AAC18, (IEEE), (2019).
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-FEL2019-THP069
About •	paper received ※ 20 August 2019 paper accepted ※ 28 August 2019 issue date ※ 05 November 2019
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Paper

Title

Other Keywords

Page

TUP023

Analytical and Numerical Comparison of Different Approaches to the Description of SASE in High Gain FELs

FEL, radiation, simulation, undulator

94

O.A. Shevchenko, N.A. Vinokurov
BINP SB RAS, Novosibirsk, Russia
N.A. Vinokurov
NSU, Novosibirsk, Russia

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 1-D FEL model. We show that two-particle 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.

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reference for this paper ※ https://doi.org/10.18429/JACoW-FEL2019-TUP023

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paper received ※ 19 August 2019 paper accepted ※ 25 August 2019 issue date ※ 05 November 2019

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TUP035

Sensitivity of LCLS Self-Seeded Pedestal Emission to Laser Heater Strength

laser, electron, free-electron-laser, experiment

126

G. Marcus, D.K. Bohler, Y. Ding, W.M. Fawley, Y. Feng, E. Hemsing, Z. Huang, J. Krzywiński, A.A. Lutman, D.F. Ratner
SLAC, Menlo Park, California, USA

Measurements of the soft X-ray, self-seeding spectrum at the LCLS free-electron laser generally display a pedestal-like distribution around the central seeded wavelength that degrades the spectral purity. We have investigated the detailed experimental characteristics of this pedestal and found that it is comprised of two separate components: (1) normal SASE whose total strength is nominally insensitive to energy detuning and laser heater (LH) strength; (2) sideband-like emission whose strength positively correlates with that of the amplified seed and negatively with energy detuning and LH strength. We believe this latter, non-SASE component arises from comparatively long wavelength amplitude and phase modulations of the main seeded radiation line. Its shot-to-shot variability and LH sensitivity suggests an origin connected to growth of the longitudinal microbunching instability on the electron beam. Here, we present experimental results taken over a number of shifts that illustrate the above mentioned characteristics.

DOI •

reference for this paper ※ https://doi.org/10.18429/JACoW-FEL2019-TUP035

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paper received ※ 28 August 2019 paper accepted ※ 29 August 2019 issue date ※ 05 November 2019

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TUP049

Simulating Shot-Noise of ’Real’ Electron Bunches

micro-particles, electron, FEL, simulation

149

P. Traczykowski, L.T. Campbell, B.W.J. MᶜNeil
USTRAT/SUPA, Glasgow, United Kingdom
L.T. Campbell, B.W.J. MᶜNeil, P. Traczykowski
Cockcroft Institute, Warrington, Cheshire, United Kingdom
L.T. Campbell, P. Traczykowski
STFC/DL/ASTeC, Daresbury, Warrington, Cheshire, United Kingdom

An algorithm and numerical code for the up-sampling of a system of particles, from a smaller to a larger number, is described. The method introduces a Poissonian ’shot-noise’ to the up-sampled distribution [1], typical of the noise statistics arising in a bunch of particles generated by a particle accelerator. The algorithm is applied on a phase-space 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]

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reference for this paper ※ https://doi.org/10.18429/JACoW-FEL2019-TUP049

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paper received ※ 20 August 2019 paper accepted ※ 27 August 2019 issue date ※ 05 November 2019

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TUP051

Plasma Accelerator Driven Coherent Spontaneous Emission

electron, FEL, radiation, undulator

157

B.M. Alotaibi, R. Altuijri
PNU, Riyadh, Kingdom of Saudi Arabia
B.M. Alotaibi, R. Altuijri, A.F. Habib, B. Hidding, B.W.J. MᶜNeil, P. Traczykowski
USTRAT/SUPA, Glasgow, United Kingdom
A.F. Habib, B. Hidding, B.W.J. MᶜNeil, P. Traczykowski
Cockcroft Institute, Warrington, Cheshire, United Kingdom

Plasma accelerators [1] are a potentially important source of high energy, low emittance electron beams with high peak currents generated within a relatively short distance. As such, they may have an important application in the driving of coherent light sources such as the Free Electron Laser (FEL) which operate into the X-ray region [2]. While novel plasma photocathodes [3] may offer orders of magnitude improvement to the normalized emittance and brightness of electron beams compared to Radio Frequency-driven accelerators, a substantial challenge is the energy spread and chirp of beams, which can make FEL operation impossible. In this paper it is shown that such an energy-chirped, ultrahigh brightness electron beam, with dynamically evolving current profile due to ballistic bunching at moderate energies, can generate significant coherent radiation output via the process of Coherent Spontaneous Emission (CSE)[4]. While this CSE is seen to cause some FEL-induced electron bunching at the radiation wavelength, the dynamic evolution of the energy chirped pulse dampens out any high-gain FEL interaction.
[1] E. Esary et al., Reviews of Modern Physics p. 1229 (2009).
[2] B. W. J. McNeil and N. R. Thompson, 2010 Nat. Photon.4 814-21
[3] B. Hidding et al., 2012 Phys. Rev. Lett. 108 035001
[4] L. T. Campbell and B. W. J. McNeil, 2012, in Proc. FEL2012

Poster TUP051 [1.401 MB]

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reference for this paper ※ https://doi.org/10.18429/JACoW-FEL2019-TUP051

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paper received ※ 20 August 2019 paper accepted ※ 27 August 2019 issue date ※ 05 November 2019

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TUP083

Energy Spread Impact on HGHG and EEHG FEL Pulse Energy

laser, FEL, electron, photon

250

S. Spampinati, E. Allaria, L. Giannessi, P. Rebernik Ribič
Elettra-Sincrotrone Trieste S.C.p.A., Basovizza, Italy
L. Giannessi
ENEA C.R. Frascati, Frascati (Roma), Italy
P. Rebernik Ribič
University of Nova Gorica, Nova Gorica, Slovenia

VUV and X-ray free electron lasers (FELs) require a very bright electron beam. Seeded FEL harmonic generation is particularly sensible to energy spread and slice energy spread can limit the highest harmonic conversion factor at which coherent radiation can be produced. Different cas-cade schemes can have different sensibility to the slice energy spread. At FERMI we have evaluated the impact of the slice energy spread on the performance of high gain harmonic generation (HGHG) and of echo enable harmonic generation (EEHG) by measuring the FEL pulse energy as function of the electron beam slice energy spread. The measurements were done at different harmon-ics. The slice energy spread was varied trough the laser heater located in the linac that drives FERMI.

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reference for this paper ※ https://doi.org/10.18429/JACoW-FEL2019-TUP083

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paper received ※ 20 August 2019 paper accepted ※ 17 September 2019 issue date ※ 05 November 2019

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TUP090

Considerations on Implementing EEHG with a Strong Linear Chirp

electron, laser, FEL, simulation

262

M.A. Pop, F. Curbis, W. Qin, S. Werin
MAX IV Laboratory, Lund University, Lund, Sweden
F. Curbis, S. Werin
SLF, Lund, Sweden
W. Qin
Lund University, Lund, Sweden

Funding: The work is supported by Knut and Alice Wallenberg foundation.
The Soft X-ray Laser (SXL) currently being studied at MAX IV Laboratory is envisioned to produce coherent radiation in the 1-5 nm wavelength range. In this contribution, we present the results of simulations aimed at adding to the SXL an Echo Enabled Harmonic Generation scheme, which has been shown to increase the coherence of FELs in the Soft X-ray regime. Our work puts special emphasis on accommodating the positive energy chirp of the electron bunch coming out of the MAX IV Linac and on generating sufficient bunching at the high harmonics necessary for covering the full wavelength range.

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reference for this paper ※ https://doi.org/10.18429/JACoW-FEL2019-TUP090

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paper received ※ 20 August 2019 paper accepted ※ 28 August 2019 issue date ※ 05 November 2019

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WEP024

1.3 GHz Solid State Power Amplifier for the Buncher in CTFEL Facility

electron, experiment, FEL, cavity

371

T.H. He, C.L. Lao, P. Li, X. Luo, L.J. Shan, K. Zhou
CAEP/IAE, Mianyang, Sichuan, People’s Republic of China

The THz Free Electron Laser facility (CAEP THz FEL, CTFEL) of the China Academy of Engineering Physics uses high quality electron beams to generate high average power terahertz radiations. A 1.3 GHz RF buncher is used in front of the superconducting linear accelerator of the CTFEL facility to improve the electron beams quality. The RF buncher is driven by a solid state power amplifier (SSPA), and the SSPA is feedback controlled by a low level RF (LLRF) control system to ensure the high stability of the amplitude and phase of the bunching field in the buncher cavity. The SSPA operates at 1.3 GHz and outputs 0 to 5 kW of continuous wave power. This paper mainly introduces the principle and composition of the SSPA, and presents some experiments on the RF buncher driven by the SSPA.

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reference for this paper ※ https://doi.org/10.18429/JACoW-FEL2019-WEP024

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paper received ※ 15 August 2019 paper accepted ※ 27 August 2019 issue date ※ 05 November 2019

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WEP035

NIR Spectrometer for Bunch-Resolved, Non-Destructive Studies of Microbunching at European XFEL

FEL, electron, laser, free-electron-laser

392

S. Fahlström, M. Hamberg
Uppsala University, Uppsala, Sweden
C. Gerth, N.M. Lockmann, B. Steffen
DESY, Hamburg, Germany

At the European X-ray Free Electron Laser high brilliance femtosecond FEL radiation pulses are generated for user experiments. For this to be achieved electron bunches must be reliably produced within very tight tolerances. In order to investigate the presence of micro-bunching, i.e. charge density variation along the electron bunch with features in the micron range, a prism-based NIR spectrometer with an InGaAs sensor, sensitive in the wavelength range 900 nm to 1700 nm was installed. The spectrometer utilizes diffraction radiation (DR) generated at electron beam energies of up to 17.5 GeV. The MHz repetition rate needed for bunch resolved measurements is made possible by the KALYPSO line detector system, providing a read-out rate of up to 2.7 MHz. We present the first findings from commissioning of the NIR spectrometer, and measurements on the impact of the laser heater system for various bunch compression settings, in terms of amplitude and bunch-to-bunch variance of the NIR spectra as well as FEL pulse energy.

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reference for this paper ※ https://doi.org/10.18429/JACoW-FEL2019-WEP035

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paper received ※ 20 August 2019 paper accepted ※ 29 August 2019 issue date ※ 05 November 2019

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WEP041

Feasibility of Single-Shot Microbunching Diagnostics for a Pre-Bunched Beam at 266 nm

electron, laser, radiation, diagnostics

408

A.H. Lumpkin
AAI/ANL, Lemont, Illinois, USA
D.W. Rule
Private Address, Silver Spring, USA

Funding: Work supported by U.S. Department of Energy, Office of Science, under Contract No. DE-AC02-06CH11357.
Co-propagating a relativistic electron beam and a high-power laser pulse through a short undulator (modulator) provides an energy modulation which can be converted to a periodic longitudinal density modulation (or microbunching) via the R56 term of a chicane. Such pre-bunching of a beam at the resonant wavelength and the harmonics of a subsequent free-electron laser (FEL) amplifier seeds the process and results in improved gain. We describe potential characterizations of the resulting microbunched electron beams using coherent optical transition radiation (COTR) imaging techniques for transverse size (50 micron), divergence (sub-mrad), trajectory angle (0.1 mrad), spectrum (few nm), and pulse length (sub-ps). The transverse spatial alignment is provided with near-field imaging and the angular alignment is done with far-field imaging and two-foil COTR interferometry (COTRI). Analytical model results for a 266-nm wavelength COTRI case with a 10% microbunching fraction will be presented. COTR gains of 7 million were calculated for an initial charge of 300 pC which enables splitting the optical signal for single-shot measurements of all the cited parameters.

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reference for this paper ※ https://doi.org/10.18429/JACoW-FEL2019-WEP041

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paper received ※ 20 August 2019 paper accepted ※ 29 August 2019 issue date ※ 05 November 2019

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WEP063

The Preliminary Study of a Pre-Bunched Terahertz Free Electron Laser by a Velocity Bunching Scheme

radiation, electron, undulator, FEL

477

R. Huang, Q.K. Jia, H.T. Li, Z. Zhao
USTC/NSRL, Hefei, Anhui, People’s Republic of China

Funding: Work supported by the National Natural Science Foundation of China Grant Number 11805200
Terahertz (THz) radiation has broad applications in biological sciences, materials imaging and radar communications and so on. High-power, frequency-adjustable THz radiation sources are desired. An electron beam, generated in a photoinjector and bunched at terahertz (THz) frequency, will excite a coherent THz radiation when entering an undulator. The radiation power mainly depends on the particle number and the bunching factor of the electron beam, which is limited by the space charge effect among the microbunches and the total rf phase width the macrobunch occupied. Previously we have designed a pre-bunched THz free electron laser (FEL) with the radiation frequency covering 0.5-5 THz. While the radiation intensity for the lower frequency (below 1~THz) is not very high because of the large energy spread and the low bunching factor. We will report a THz FEL by a velocity bunching scheme, which could realize more highly bunched beam especially in the low THz frequency region. The physical design of the electron source is described in detail.

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reference for this paper ※ https://doi.org/10.18429/JACoW-FEL2019-WEP063

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paper received ※ 19 August 2019 paper accepted ※ 28 August 2019 issue date ※ 05 November 2019

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WEP072

Expected Radiation Properties of the Harmonic Afterburner at FLASH2

undulator, radiation, polarization, simulation

492

M. Mehrjoo, B. Faatz, G. Paraskaki, M. Tischer, P. Vagin
DESY, Hamburg, Germany

We discuss the afterburner option to upgrade the FLASH2 undulator line, at the FLASH facility in the Hamburg area, for delivering short wavelengths down to approximately 1.5 nm with variable polarization. This relatively straightforward upgrade enables us the study of the scientific cases in L- absorption edges of rare earth metals. The proposed afterburner setting with an energy upgrade to 1.35 GeV would potentially cover many of the community’s requests for the short wavelengths radiation and circular polarization. We also study the influence of reverse tapering on the radiation output. This contribution presents a series of simulations for the afterburner scheme and some of the technical choices made for implementation.

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reference for this paper ※ https://doi.org/10.18429/JACoW-FEL2019-WEP072

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paper received ※ 19 August 2019 paper accepted ※ 28 October 2019 issue date ※ 05 November 2019

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WEP101

Linear Polarisation via a Delta Afterburner for the CompactLight Facility

undulator, FEL, polarization, radiation

549

H.M. Castañeda Cortés, D.J. Dunning, N. Thompson
STFC/DL/ASTeC, Daresbury, Warrington, Cheshire, United Kingdom

Funding: CompactLight is funded by the European Union’s Horizon 2020 research and innovation program under Grant Agreement No.777431.
We studied the degree of polarisation of the FEL radiation from the diverted-beam scheme [1,2] using the layout of the CompactLight facility, which is in the process of being designed. To satisfy the polarisation requirements defined by the users [3] without compromising the aim of the facility to be compact, we studied a configuration comprising a helical Super Conductive Undulator (SCU) followed by a Delta afterburner (configured to generate linearly polarised light). The trade-offs between the SCU length, afterburner length, degree of polarisation and output power are presented and discussed.
[1] E. A. Schneidmiller and M. V. Yurkov, Phys. Rev. ST Accel. Beams 16, 11702 (2013)
[2] A. Lutman et al., Nature Photonics 10, 468(2016)
[3] A. Mak et al., FREIA Report 2019/01, 2019

Poster WEP101 [1.083 MB]

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reference for this paper ※ https://doi.org/10.18429/JACoW-FEL2019-WEP101

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paper received ※ 16 August 2019 paper accepted ※ 25 August 2019 issue date ※ 05 November 2019

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THB04

Longitudinal Phase Space Study on Injector Beam of High Repetition Rate X-Ray FEL

linac, electron, cavity, laser

584

Q. Gu
SSRF, Shanghai, People’s Republic of China
Z. Wang
SARI-CAS, Pudong, Shanghai, People’s Republic of China

The longitudinal phase space of the high repetition rate injector beam usually twisted and deteriorated by the space charge force. It causes the correlated energy spread and the local chirp within the beam, which could not compensated by the harmonic correction. As a consequence of this problem, one could not get ideal beam with a peak current more than kiloamperes. In this paper several approaches have been studied to relieve this effect and get the well compressed beam for the lasing.

Slides THB04 [3.151 MB]

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reference for this paper ※ https://doi.org/10.18429/JACoW-FEL2019-THB04

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paper received ※ 26 August 2019 paper accepted ※ 16 September 2019 issue date ※ 05 November 2019

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THP003

Arbitrary Order Perturbation Theory for a Time-Discrete Model of Micro-Bunching Driven by Longitudinal Space Charge

space-charge, FEL, simulation, electron

596

Ph. Amstutz
LBNL, Berkeley, California, USA
M. Vogt
DESY, Hamburg, Germany

A well established model for studying the micro-bunching instability driven by longitudinal space charge in ultra-relativistic bunches in FEL-like beamlines can be identified as a time-discrete Vlasov system with general drift maps and Poisson type collective kick maps. Here we present an arbitrary order perturbative approach for the general system and the complete all-orders solution for a special example. For this example we benchmark our theory against our Perron-Frobenius tree-code.

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reference for this paper ※ https://doi.org/10.18429/JACoW-FEL2019-THP003

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paper received ※ 20 August 2019 paper accepted ※ 29 August 2019 issue date ※ 05 November 2019

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THP016

Study of Microbunching Instability in SHINE Linac

laser, linac, simulation, electron

629

D. Huang
SINAP, Shanghai, People’s Republic of China
D. Gu
SARI-CAS, Pudong, Shanghai, People’s Republic of China
M. Zhang
Shanghai Advanced Research Institute, Pudong, Shanghai, People’s Republic of China

The SHINE project in China aims at the next generation high repetation rate and high power hard X-ray free electron laser facility. The high quality electron beam is thus requested in the linac to generate such a high quality FEL lasing. As the prerequisite, the microbuncing instability introduced by the nonlinear effects such as the LSC, CSR and wakefields in the bunch compressing process must be taken care of, otherwise the electron beam will not meet the requirements of lasing. In this article, the microbunching effects including the gain of the instability in the linac of SHINE are estimated, and several ways for the control of the instability are proposed.

Poster THP016 [0.536 MB]

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reference for this paper ※ https://doi.org/10.18429/JACoW-FEL2019-THP016

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paper received ※ 24 August 2019 paper accepted ※ 29 August 2019 issue date ※ 05 November 2019

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THP020

Microbunching Enhancement by Adiabatic Trapping

laser, storage-ring, emittance, injection

635

X.J. Deng, W.-H. Huang, C.-X. Tang
TUB, Beijing, People’s Republic of China
A. Chao
SLAC, Menlo Park, California, USA

Storage ring based concept called steady-state microbunching was proposed years ago for high average power narrowband coherent radiation generation. There are now active efforts on-going by the SSMB collaboration established among Tsinghua University and several other institutes. In this paper we study the particle trap and filamentation process of beam in RF or Micro Bucket which is useful for understanding the injection beam dynamics of SSMB. One remarkable result is the steady-state current distribution after full filamentation has little dependence on the bucket height as long as it is several times larger than the energy spread. A discrete increase of bucket height can boost the bunching, with the sacrifice of emittance growth. An adiabatic change of bucket height from a smaller value to the final desired value is then proposed to boost the bunching while preserving the longitudinal emittance.

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reference for this paper ※ https://doi.org/10.18429/JACoW-FEL2019-THP020

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paper received ※ 20 August 2019 paper accepted ※ 28 August 2019 issue date ※ 05 November 2019

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THP069

Observations on Microbunching of Electrons in Laser-Driven Plasma Accelerators and Free-Electron Lasers

FEL, laser, electron, experiment

722

A.H. Lumpkin
Fermilab, Batavia, Illinois, USA
M. Downer, M. LaBerge
The University of Texas at Austin, Austin, Texas, USA
D.W. Rule
Private Address, Silver Spring, USA

Funding: This manuscript has been authored by Fermi Research Alliance, LLC under Contract No. DE-AC02-07CH11359 with the U.S. Department of Energy, Office of Science, Office of High Energy Physics.
The periodic longitudinal density modulation of relativistic electrons at the resonant wavelength (microbunching) is a fundamental aspect of free-electron lasers (FELs). In one case, microbunching fractions reached 20% at saturation of a self-amplified spontaneous emission (SASE) FEL resulting in gains of 1 million at 530 nm [1]. In that experiment the z-dependent gain of coherent optical transition radiation (COTR) was also measured. In laser-driven plasma accelerators (LPAs), microbunching at visible wavelengths has also been recently reported as evidenced by significant COTR enhancements measured in near-field and far-field images on a single shot for the first time [2]. An analytical model for COTR interferometry (COTRI) addresses both cases. In the FEL, one identified microbunched transverse cores of 25-100 microns while in the LPA the reported transverse sizes at the exit of the LPA were a few microns. In the latter case, signal enhancements of nearly 100, 000 and extensive fringes out to 30 mrad in angle space were recorded. The broadband microbunching observed in the LPA case could act as a seed for a SASE FEL experiment with tunability in principle over the visible regime.
[1] A.H. Lumpkin et al., Phys. Rev. Lett. 88, No.23, 234801 (2002).
[2] A.H. Lumpkin, M. LaBerge, D.W. Rule et al., Proceedings of AAC18, (IEEE), (2019).

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reference for this paper ※ https://doi.org/10.18429/JACoW-FEL2019-THP069

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paper received ※ 20 August 2019 paper accepted ※ 28 August 2019 issue date ※ 05 November 2019

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