Spontaneous Emission High Gain Harmonic Generation Free Electron Laser Chuanxiang Tang *, Qingzi Xing, Chao Feng * Tang.xuh@tsinghua.edu.cn Presented at Mini-Workshop on Present and Future FEL Schemes December 11, 2008, Shanghai, China
Content Introduction Spontaneous Emission High Gain Harmonic Generation Free Electron Laser (SEHG) SXFEL based on SEHG Summary
R. Bonifacio, C. Pellegrini, L. M.Narducci. Opt. Commun, 1984, 50: 373-378 J. Murphy, C. Pellegrini, R. Bonifacio, Opt. Commun., 53, 197 (1985) SASE
HGHG R. Bonifacio, L. de Salvo Souza, P. Pierini, E. T. Scharlemann,Nucl. Instrum. Methods,Phys. Res. A 296,787 (1990). L.-H.Yu, et al, High- Gain Harmonic- Generation Free- Electron Laser, 11 AUGUST 2000 VOL 932 289 SCIENCE
Cascaded HGHG L.-H. Yu, et al. High-Gain Harmonic-Generation Free-Electron LaserScience 289, 932 (2000); Li-Hua Yu*, Ilan Ben-Zvi, High-gain harmonic generation of soft X-rays with the fresh bunch technique Nuclear Instruments and Methods in Physics Research A 393 (1997) 96-99 J. Knobloch, et al, STARS AN FEL TO DEMONSTRATE CASCADED HIGH- GAIN HARMONIC GENERATION Proceedings of FEL 2007, Novosibirsk, Russia
Spontaneous Emission High Gain Harmonic Generation Free Electron Laser
proper pre-bunching can be viewed not only as an equivalent input seed, but also as a means to provide a more clean spectrum of the output radiation. From: G. Dattoli, A. Doria, L. Giannessi, et al., Bunching and exotic undulator configurations in SASE FELs, NIM A 507 (2003) 388-391
Frequency doubler scheme for SASE proposed by J. Feldhaus, et al.: It consists of an undulator tuned to the first harmonic, a dispersion section, and a tapered undulator tuned to the second harmonic. The maximum energy modulation of the electron beam at the first undulator exit is about equal to the local energy spread, but still far away from saturation. From: J. Feldhaus, M. KÖrfer, T. MÖller, et al., Efficient frequency doubler for the soft X-ray SASE FEL at the TESLA Test Facility, NIM A 528 (2004) 471-475
(Simulated by time-dependent FEL code FAST)
The result of our study is that the TTF FEL would be able to produce radiation down to 3 nm wavelength with an output peak power in the GW range, and with excellent spectral properties. From: J. Feldhaus, M. KÖrfer, T. MÖller, et al., Efficient frequency doubler for the soft X-ray SASE FEL at the TESLA Test Facility, NIM A 528 (2004) 471-475
Higher-order Harmonics Nonlinear Generation: Semianalytical expressions developed Design of cascaded undulators: ( without dispersion section) 1) Induced relative energy spread: σ ( z) 3C i Az ( ) 1+ 9 B A( z) 1 2) Length of the 1 st section: Z 0.94Z 2.44Lg b s [ ] A good criterion is that of cutting the first section when the induced energy spread is just ρ /2, which corresponds to a condition far from the saturation with a fairly strong bunching which may provide a conspicuous amount of coherently generated power at higher harmonics. From: G. Dattoli, P.L. Ottaviani, and S. Pagnutti, Nonlinear harmonic generation in high-gain free-electron lasers, J. Applied Physics 97, 113102 (2005)
Optical klystron concept invented by Vinokurov and Skrinsky in 1977. (The device works only on the fundamental harmonic) Later I. Boscolo and V. Stagno pointed out that higher-harmonic operation is feasible in 1980. The scheme of the converter working on the fundamental frequency and with an optical cavity placed on the system looks exactly like the klystron. Since the force pattern either in the buncher or in the radiator is due to transverse fields, we call the device transverse optical klystron (TOK). From: N.A. Vinokurov and A.N. skrinsky, Preprint of INP 77-59, Novosibirsk, 1977 I. Boscolo and Y. Stagno, The converter and the transverse optical klystron, IL Nuovo Cimento 58B, 267-285
Scheme of the converter: three separate elements: buncher, drift system and the radiator. Buncher: producing a phase-dependent momentum modulation; Drift region: transforming of the electron momentum modulation into the density modulation; Radiator: all the bunches radiate coherently. If the radiator wiggler has a period length λ r =λ w /mβ i, an efficient generation of the m-th harmonic of the guided input wave is obtainable.
Distributed Optical klystrons proposed by H.P. Freund and G.R. Neil The principal advantage of the nonlinear harmonics is in a shorter saturation length. the enhanced bunching in a chicane gives rise to amplification in the next wiggler segment that is initially faster than exponential but falls off to the expected exponential rate after about one gain length. Therefore, if the wiggler segments are shorter than a gain length, then the interaction is faster than exponential and the saturation length can be considerably shortened. (Simulated by MEDUSA based on LCLS parameters) From: H.P. Freund and G.R. Neil. Nonlinear harmonic generation in distributed optical klystrons, NIM A 475 (2001)373-376
The Distributed Optical klystrons have also been studied by Y.T. Ding, P. Emma, Z.R. Huang, et al. as Optical klystron enhancement to SASE FEL, based on LCLS parameters. (harmonic generation is not adopted) (Simulated by GENESIS 1.3 based on LCLS parameters) From: Y.T. Ding, P. Emma, Z.R. Huang, et al., Optical klystron enhancement to self-amplified spontaneous emission free electron lasers, PRST, 9, 070702 (2006)
SASE operation with the option of harmonic up-conversion ---- under investigations on SPARX project If the SASE amplifier is interrupted when the bunching is maximum at roughly 80% of saturation the bunched beam will contain large Fourier components of the current at the harmonics of the SASE fundamental frequency. If this bunched beam is immediately injected into a radiator (undulator) that is tuned to the harmonic the beam will emit strong, coherent synchrotron radiation at the harmonic. From: L. Palumbo, et al. Status of SPARX Project, Proc. EPAC08, 121-123
SIHG (Self-Induced Harmonic Generation) ---- several schemes of high-gain FELs exploiting SIHG are discussed In a single undulator, the power of the fundamental is always significantly larger than that of the harmonics. Generally speaking, the third harmonic power at saturation is in the range of one percent of the fundamental. To enhance the power of the SIHG: the second part of the undulator has a period exactly 1/3 of that of the first section but with the same K parameter. From: S.G. Biedron, G. Dattoli, H.P. Freund, et al, Physics of, and Science with, the X-Ray Free-Electron Laser, AIP Conference Proceedings no.581, 203-210
Free Electron Laser Afterburner The FEL afterburner employs a free-electron laser as the electron beam buncher and a slow-wave output structure to couple the bunched beam for the generation of intense rf radiation. The bunching process results from the FEL mechanism and the radiating process come from the coupling of the bunches with a longitudinal field. From: C.B. Wang and A.M. Sessler. Efficient microwave power source the free-electron laser afterburner, J. Appl. Phys. 74(8), (1993)4840-4844
SXFEL based on cascaded HGHG
SXFEL based on SEHG 17m 16m 45nm 9nm
Bunching factors at different location of the undulator, for different power of the seed laser
Electron Bunch Energy Modulation in SEHG Longitudinal phase space estimated by 1D theory assuming a zero initial energy spread. The red line and blue line are at the optimized undulator length of 17 m (from GENESIS) and 18.6 m (saturation length from Equation (2)) for the SEHG scheme respectively, with no seed laser. The black line is at end of the second modulator in the cascaded HGHG scheme with the optimized parameters of SXFEL, undulator length of 1 m and the power of the seed laser 240 MW.
SXFEL based on SEHG scheme The wavelength of SEHG scheme is tunable λ λ 2γ ( 1 a ) u u 1 s1 1 2 2 0 λ 2γ u = + λ 2 ( 2 ) s2= 1+ au2 2 0 λ s1 λ = s2 const Tune the energy of the electron can tune the radiated wavelength
The wavelength tuning of SXFEL based on SEHG
The radiation spectrum of SEHG at a different location: (a) z=0.5m, (b) z=16m (a) (b)
Radiation Power of SXFEL SASE Cascaded HGHG SEHG
Radiation Spectra of SXFEL SASE? Cascaded HGHG SEHG
SEHG radiation according to time
Comparison of the Optimized parameters of SXFEL with SASE, HGHG and SEHG Electron beam: Energy 840MeV, Current 600A, Energy spread 0.1-0.15% Normalized emittance 2.0mm.mrad, rms beam length(fwhm)1.6 ps SASE HGHG SEHG first stage second stage Seed laser Power/ MW 0 200 240 0 λ s /nm / 270 45 / Undulator modulator radiator modulator radiator 1st undulator 2nd undulator λ u /cm 2.6 5.8 3.8 3.8 2.6 3.8 2.6 a u 0.93 4.91 2.32 2.32 0.93 2.32 0.93 L u /m 50 1 6 1 14 17 16 disperser B c /T / 0.7 0.33 0.8 L d /m / 0.08 0.08 0.15 FEL λ r /nm 9 45 9 9 P sat /MW 425 710 420 420
Comparison between SASE, HGHG and SEHG schemes for SXFEL SASE HGHG SEHG Seed laser no yes no Frequency tuning yes little yes Saturation Power/MW 425 420 420 Kinds of Undulator 1 3 2 Total length of undulators/m 50 22 32 Spectra width 1%? 0.15% 0.2%
Summary SEHG scheme uses two undulators and one dispersion section. It has advantages over SASE and cascaded HGHG: Shorter undulator length and better spectrum than that of SASE Tunable radiation wavelength and no need for seed laser in comparison with the cascaded HGHG. Nevertheless SEHG scheme requires longer undulator than HGHG, two undulators and an extra dispersion section comparing with SASE. Moreover, since the radiation wavelength is no longer limited by the seed laser (no need for seed laser in SEHG), it is possible to generate coherent hard X ray radiation using only one stage of SEHG.
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