Bartolini R

Dunning DJ

Geng H

Huang Z

McNeil BWJ

Thompson NR

English

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OPTIMISATION OF AN HHG-SEEDED HARMONIC CASCADE FELDESIGN FOR THE NLS PROJECTD.J. Dunning and N.R. Thompson, ASTeC & CI, STFC Daresbury Laboratory, UKB.W.J. McNeil, SUPA, University of Strathclyde, Scotland, UKR. Bartolini, Diamond Light Source and John Adams Institute, University of Oxford, UKZ. Huang and H. Geng, SLAC, Menlo Park, California, USAAbstractOptimisation studies of an HHG-seeded harmonic cas-cade FEL design for the UK’s proposed New Light Source(NLS) facility [1] are presented. Three separate FELs areplanned to meet the requirements for continuous coverageof the photon energy range 50-1000 eV with variable po-larisation, 20 fs pulse widths and good temporal coherence.The design uses an HHG seed source tuneable from 50-100 eV to provide direct FEL seeding in this range, and oneor two stage harmonic cascades to reach the higher photonenergies. Studies have been carried out to optimise a har-monic cascade FEL operating at 1 keV; topics investigatedinclude modulator configuration, seed power level and ef-fects of the HHG seed structure. FEL simulations usingrealistic electron beam distributions are presented and tol-erance to increased emittance has been considered.INTRODUCTIONThree FELs have been proposed [2] to meet the require-ments of the NLS science case [1], namely to provide con-tinuous coverage of the photon energy range 50-1000 eVwith variable polarisation, 20 fs pulse widths and good tem-poral coherence. The decision was made [1, 2] to seed theFELs with an HHG source, tuneable from 50-100 eV, withone or two stage harmonic casades to the required higherphoton energies. Details of the undulator and gap choiceand the consequent electron beam energy choice are givenin [1, 3]. Details on the optimisation of the accelerator de-signs are given in [4, 5]. In this paper an optimisation studyof the 430-1000 eV FEL (“FEL-3”), operating at its highestphoton energy of 1 keV is presented. Presented elsewherein these proceedings are an assessment of the performanceof FEL-3 in the presence of realistic electron beam jittersources and FEL intrinsic noise [6] and an assessment ofpolarization control using the crossed-planar undulator ap-proach [7] applied to the FEL-3 design.MODULATOR CONFIGURATIONThree alternative configurations considered for the mod-ulator setup are shown schematically in Fig. 1. For eachcase the FEL performance was optimised through choiceof modulator lengths, chicane strengths and beam match-ing, using the steady-state mode of Genesis 1.3 [8]. Theresults are given in Table 1.Figure 1: Schematic of the three alternative modulator con-figurations considered for FEL-3.Table 1: Comparison of FEL performance for the threemodulator configurations, |b0| is the modulus of the bunch-ing at the fundamental of the radiator and σγ is the energyspread (both at the entrance to the radiator).Scheme |b0| σγ Psat Lsat[GW] [m](a) 0.059 2.21 2.07 21.5(b) 0.050 2.31 1.74 23.0(c) 0.060 1.65 2.48 20.0Time-dependent simulations were carried out with agaussian seed of peak power 400 kW and FWHM 20 fs,and the electron beam parameters were set to have no lon-gitudinal variation. The results were found to match wellwith the steady-state and are shown in Fig. 2. Scheme (c),in which the second modulator length is twice that of thefirst modulator is found to give the best performance, withhigher bunching and lower energy spread attained at the ra-Figure 2: Radiation power as a function of distance throughthe radiator for time-dependent Genesis 1.3 simulations ofthe three modulator configurationsTUPE049 Proceedings of IPAC’10, Kyoto, Japan225402 Synchrotron Light Sources and FELsA06 Free Electron Lasersdiator entrance. The reason for this is thought to be thata higher degree of non-linearity in the beam is required inorder to produce significant bunching at higher harmonicnumbers. Since the first harmonic step is a factor of two,only a relatively short modulator-1 is required to give sig-nificant bunching, while a longer modulator-2 is neededto more effectively make the larger step to the fifth har-monic. Scheme (c) allows higher saturation power (P sat)and shorter saturation distance (Lsat) and was hence se-lected for use in further optimisation studies.START-TO-END SIMULATIONSTime-dependent simulations were carried out for FEL-3operating at 1 keV photon energy, using the tracked elec-tron bunch of [4]. The results are shown in Fig. 3, at thepoint where the seeded part of the bunch reaches satura-tion (16.5 m into the radiator) - where the bandwidth of theradiation pulse is minimum.Figure 3: Start-to-end FEL simulation results for FEL-3operating at 1 keV photon energy at saturation. Top leftshows the longitudinal profile of the radiation power andtop right shows the spectrum. Bottom left shows a log plotof the power profile to assess the contrast ratio and bot-tom right shows the radiation phase. The contrast ratio is12500 and time bandwidth product ΔνΔt  0.77.The longitudinal profile of the FEL radiation powershows improved smoothness and contrast ratio compared toprevious simulation results [1, 2]. Several reasons for thishave been identified, including the reduction of numericalnoise in simulations, reduced energy spread of the electronbunch resulting from accelerator optimisation, and optimi-sation of the modulator configuration.HHG FILTERINGIt has been shown in simulations that the attosecondstructure of an HHG seed washes out during amplificationin an FEL amplifier [9], and that the FEL saturation dis-tance is not significantly affected by filtering the seed toremove the structure [10]. Here the effect of the attosecondstructure of the seed is assessed for the harmonic cascadescheme. Two cases were considered with different valuesof spectral filtering bandwidth being applied to the seed,as shown in Fig. 4. With the filter half-bandwidth set toFigure 4: Normalised longitudinal profile, and spectrum(inset), of the input seed for the two cases of filtering.0.2 nm only the resonant harmonic is retained such thatthe attosecond structure is fully removed, while at 1.4 nm,three harmonics either side of the resonant frequency areretained. The harmonics are assumed to be exactly phase-matched which is expected to be the worst case in termsof impact on FEL performance. Time-dependent simula-tions of FEL-3 operating at 1 keV were carried out for bothcases, using the modulator configuration Scheme (c).The evolution of the radiation profile with distancethrough the radiator and the bunching at the start of theradiator, are shown in Fig. 5 for both cases. The attosecondstructure reduces through the modulators, however someFigure 5: Evolution of the normalised longitudinal radia-tion profile with distance through the radiator. Also shownis the bunching at the start of the radiator (inset left), andradiation profile at saturation (inset right).Proceedings of IPAC’10, Kyoto, Japan TUPE04902 Synchrotron Light Sources and FELsA06 Free Electron Lasers 2255structure in the bunching remains at the start of the radia-tor, and is not entirely washed out during amplification. Atsaturation the radiation bandwidth is 0.025% for the 0.2 nmcase, and 0.033% for the 1.4 nm case, indicating only aslight degradation in FEL performance. Further studiesare required to assess whether the harmonic scheme canbe optimised such that the attosecond structure is entirelywashed out. In practice an unfiltered seed would have sev-eral advantages, allowing lower demands on seed power(since no losses through filtering), and avoiding increasedcomplexity. The structure of the seed may also be reducedduring transport to the undulator.SEED POWER LEVELIn the simulation results of Fig. 3, the temporal profileof the radiation output retains some noisy structure whichis assumed to be a remnant of noise amplification in anharmonic system [11]. A study was carried out to deter-mine whether the profile could be smoothed using a higherpower seed. For direct seeding, this approach would be ex-pected to have the desired effect, however in the proposedharmonic scheme the effect of a higher power seed on FELperformance is not obvious.In [1, 2], criteria for the amplitude of the energy modula-tion applied to the beam in the modulators were used. Theinitial energy modulation generated, Δγ, must be greaterthan the natural RMS energy spread in the bunch σγ by afactor n, Δγ > nσγ , for there to be strong bunching upto the nth harmonic of the seed laser wavelength. To ra-diate strongly in the radiator, the total energy spread dueto the initial energy spread and imposed energy modula-tion must be small enough to satisfy: σγ,total = (σγ2 +(Δγ/√2)2)12 < ργ, where ρ is the fundamental FEL pa-rameter [12]. In order to satisfy both conditions, the fol-lowing criteria applies:nσγ < Δγ <√2(ρ2γ2 − σγ2) (1)and is plotted in Fig. 6 as a function of harmonic number,assuming the nominal NLS electron bunch parameters. ForFEL-3 where a step to the tenth harmonic is required, therange of appropriate energy modulation is 4 < Δγ < 6,therefore an increase in seed power necessitates a reduc-tion in modulator length. This has some advantages (lessexpensive undulator and less energy spread growth due tospontaneous emission) but it was found that even when in-creasing the seed power by a factor of 100 no improvementin pulse coherence or contrast ratio was obtained.TOLERANCE STUDIESThe performance of FEL-3 at 1 keV was assessed assum-ing a larger normalised emittance of εn = 0.6 mm-mrad.The saturation power was found to reduce to 0.62 of itsbaseline value and the saturation length increased by a fac-tor of 1.63, while the quality of the output pulse showedFigure 6: Energy modulation criteria for successful FELoperation, as a function of harmonic number.the seeding scheme still to be effective. These relativechanges are in extremely good agreement with the resultsof equivalent steady state simulations of the self-amplifiedspontaneous emission (SASE) power growth in the radiatoronly, for the two emittance values. The conclusion is madethat the harmonic upconversion still works effectively at thehigher emittance and the reduction in performance is onlydue to decreased FEL coupling in the radiator undulator.Assessments of the tolerance of the FEL design to wake-fields and seed/electron beam misalignment have beenmade and will be included in a future publication [13], to-gether with results showing the properties of the radiationoutput at harmonics of the fundamental.ACKNOWLEDGEMENTSThe authors would like to thank S. Reiche andA. Meseck for advice on simulation techniques andG. Penn and A. Zholents for useful advice regarding themodulator configurations.REFERENCES[1] J. Marangos et al., NLS Science Case & Outline FacilityDesign, http://www.newlightsource.org (2009).[2] N.R. Thompson et al., FEL ’09, Liverpool, August 2009,WEOD02, pp. 694-701 (2009).[3] J.A. Clarke et al., FEL ’09, Liverpool, August 2009,THOA03, pp. 722-725 (2009).[4] R. Bartolini et al., these proceedings, WEPEA065.[5] P.H. Williams et al., these proceedings, TUPEC035.[6] J.H. Rowland et al., these proceedings, TUPD063.[7] H. Geng et al., these proceedings, TUPE068.[8] S. Reiche, Nucl. Instrum. Meth. in Phys. Res. A, 429 (1999)243.[9] B.W.J. McNeil et al., New Journal of Physics, 9, 82 (2007).[10] L. Giannessi, FEL ’06, Berlin, August 2009, MOCAU05,pp. 248-251 (2006).[11] E.L. Saldin et al., Opt. Commun. 202, 169-187 (2002).[12] R. Bonifacio et al., Opt. Commun. 50, 373 (1984).[13] NLS Conceptual Design Report, http://www.newlightsource.org (2010).TUPE049 Proceedings of IPAC’10, Kyoto, Japan225602 Synchrotron Light Sources and FELsA06 Free Electron Lasers

Design Report, http://www. newlightsource.org (2010). TUPE049

FEL ’06,

these proceedings,

Optimisation of an HHG-seeded harmonic cascade FEL design for the NLS project

Optimisation of an HHG-seeded harmonic cascade FEL design for the NLS project

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