Opportunities for Two-color Experiments at the SASE3 undulator line of the European XFEL
Gianluca Geloni, Vitali Kocharyan, Tommaso Mazza, Michael Meyer, Evgeni Saldin, Svitozar Serkez
DDEUTSCHES ELEKTRONEN-SYNCHROTRON
Ein Forschungszentrum der Helmholtz-Gemeinschaft
DESY 17-068May 2017
Opportunities for Two-color Experiments at the SASE3undulator line of the European XFEL
Gianluca Geloni, Tommaso Mazza, Michael Meyer and Svitozar Serkez
European XFEL GmbH, Hamburg
Vitali Kocharyan and Evgeni Saldin
Deutsches Elektronen-Synchrotron DESY, Hamburg
ISSN 0418-9833
NOTKESTRASSE 85 - 22607 HAMBURG a r X i v : . [ phy s i c s . acc - ph ] J un pportunities for Two-color Experiments at theSASE3 undulator line of the European XFEL Gianluca Geloni , Vitali Kocharyan , Tommaso Mazza , MichaelMeyer , Evgeni Saldin , and Svitozar Serkez European XFEL GmbH, Hamburg, Germany Deutsches Elektronen-Synchrotron (DESY), Hamburg, Germany
Abstract
X-ray Free Electron Lasers (XFELs) have been proven to generate short andpowerful radiation pulses allowing for a wide class of novel experiments. If anXFEL facility supports the generation of two X-ray pulses with di ff erent wave-lengths and controllable delay, the range of possible experiments is broadenedeven further to include X-ray-pump / X-ray-probe applications. In this work wediscuss the possibility of applying a simple and cost-e ff ective method for produc-ing two-color pulses at the SASE3 soft X-ray beamline of the European XFEL. Thetechnique is based on the installation of a magnetic chicane in the baseline undu-lator and can be accomplished in several steps. We discuss the scientific interestof this upgrade for the Small Quantum Systems (SQS) instrument, in connectionwith the high-repetition rate of the European XFEL, and we provide start-to-endsimulations up to the radiation focus on the sample, proving the feasibility of ourconcept. The simplest way currently available to enable the generation of two closely separated(in the order of 50 fs) pulses of di ff erent wavelengths (later - colors) at X-ray Free-Electron lasers consists of inserting a magnetic chicane between two undulator partsas suggested in [1] and experimentally proven in [2, 3]. The scheme is illustrated inFigure 1-a. We propose to split the baseline SASE3 soft X-ray undulator into two partswith a magnetic chicane. Both parts act as independent undulators and will be referredfurther as U U
2. The nominal electron beam enters the first undulator U
1, tuned tothe resonant wavelength λ . After passing through U
1, both electron beam and emittedradiation enter the chicane. This magnetic chicane has two functions: first, it introducesa suitable delay between the electron beam and the radiation generated in U
1. Delaysfrom zero up to the picosecond level can be obtained with a compact magnetic chicaneof several meters length. Second, due to dispersion, the passage of the electron beam In our case, due to radiation slippage in the subsequent undulators, the e ff ective minimum delaybetween the two pulses of di ff erent colors is of the order of several femtoseconds. a) Chicane only(b) Chicane and optical delay line Figure 1: (color online) Schematic illustration of a simple two-color FEL techniquewithout (top) and with the addition of a compact optical delay line (bottom).through the magnetic chicane smears out the microbunching at wavelength λ . As aresult, when the -after the magnetic chicane- delayed electron beam enters the secondundulator U
2, the Self-Amplified Spontaneous Emission (SASE) process starts fromshot-noise again. Therefore, if the undulator U λ ,then at the undulator exit one obtains a first radiation pulse at wavelength λ followedby a second one with wavelength λ delayed by a time interval that can be varied bychanging the strength of the chicane magnets.One must ensure that the electron beam quality at the entrance of the secondundulator U U
1. In particular, the amplificationprocess there should not reach saturation. Optimization of the maximum power alsoposes limits on the wavelengths choices The wavelength separation between the twopulses can theoretically span across the entire range made available by the undulatorsystem, in the case of SASE3 between about 250 eV and 3000 eV. However, the impactof the FEL process on the electron beam quality depends on the radiation wavelength.Therefore, in order to maximize the combined radiation power that can be extracted,especially at large wavelength separations, the first pulse to be produced should beat the shortest wavelength. Moreover, the magnetic chicane strength should be largeenough to smear out the microbunching at λ , unless the separation between λ and λ is larger than the FEL bandwidth.An easy way to increase the flexibility of the scheme is to introduce a compactoptical delay line to have full control on the relative temporal separation betweenthe two pulses as shown in Figure 1-b. Since the photon beam transverse size at3he position of the magnetic chicane is, roughly speaking, as small as the electronbeam, i.e. a few tens of microns, the length of each mirror can be as short as severalcentimeters. In order to simplify the design of the mirror delay line, one may fix theoptical delay to a few hundred femtoseconds, thus avoiding the use of moving mirrors,and subsequently tune the delay by changing the current in the magnetic chicane coils.Therefore, the introduction of an optical delay line would allow one to sweep betweennegative and positive delays at a cost of a smaller delay tuneability, caused by a lowerlimit of the chicane magnetic field (while the optical delay is inserted).It is also worth noting that the scheme proposed here can, in principle, be upgradedto a self-seeding setup in the soft X-ray range (SXRSS), by inserting a SXRSS monochro-mator in place of the optical delay line, and by properly choosing the lengths the U U ff erent colors.Even the simplest way of generating two-color pulses at the SASE3 beamline of theEuropean XFEL, in combination with the high-repetition rate capabilities of the facilityis expected to enable novel exciting science at the two soft X-ray instruments: SmallQuantum Systems (SQS) [4] and Spectroscopy & Coherent Scattering (SCS) [5].In the following sections we limit ourselves to the analysis of one science case forthe SQS instrument, including FEL simulations and wavefront propagation studies upto the sample, while leaving further studies for the SQS and the SCS instruments forthe future. The two-color operation mode enables a large number of scientific applications basedon a pump-probe excitation scheme with two individually controllable X-ray pulses.In the following a concrete example is discussed, which will make use of site-specificexcitation in molecules possible at SQS scientific instrument of the European XFEL [4].The excitation of a specific atomic site is enabled using soft X-ray pulses, since theradiation e ffi ciently couples to the strongly bound core electrons, which are localizedat the atomic site. Tuning the wavelength of the pump pulse to a specific threshold,a molecule can be excited at a well-defined atomic position. Using then the probeat another wavelength, which is connected to another core hole excitation, possiblechanges induced by the first pulse at a di ff erent site in the molecule are measured.Finally, the variation of the time delay between the two pulses provide access tothe dynamics of this process, i.e. on the time, which is necessary to transport theinformation from one position in the molecule to another.As illustrating example, we discuss charge transfer processes in a linear molecule,4 HC HHC …. ….
I Cl n+ h ν ν …. Charge migra*on
Figure 2 (color online): Schematic representation of the two-color pump-probe processin molecular I − C n − H n − Cl .such as I − C n − H n − Cl , composed of long carbon chain with two di ff erent halogen atoms,e.g. iodine and chlorine, at both ends (Figure 2). In the wavelength range accessiblewith the SASE3 undulator the 2 p core electron of chlorine (threshold at 210 eV) as wellas the 3 d core electron of iodine (threshold at 630 eV) can be ionized. Considering a firstpulse (pump) at a photon energy of 250 eV, the perturbation introduced by the XUVphoton will be localized at the chlorine site, since core electrons of the other atoms ( I and C ) are still not in reach and cross section for valence ionization is weak. In thesame way, choosing for the probe pulse the second photon energy at 630 eV assuresthat preferentially (i.e. most e ffi ciently) the 3 d electron at the iodine site is excited orionized.An e ffi cient and informative experimental method to monitor the intramolecularprocesses is given by high-resolution Auger spectroscopy, ideally in combination withion spectroscopy performed in a coincidence arrangement. The 3 d Auger spectrum ofiodine is located in the kinetic energy range around 400-500 eV arising mainly fromthe most prominent transitions to doubly charged states with electron configurations4 d − and 4 d − p − [6]. These lines are well separated from the corresponding Cl p Auger spectrum at kinetic energies between 165 and 175 eV [7] and other ionizationprocesses taking place at the photon energies considered here. An illustration of allpossible ionization processes, including also the valence ionization of all atoms at bothwavelengths is given in Figure 3.Due to the high intensity of the FEL pulses sequential ionization processes arepossible and likely to happen. As a consequence, the electron spectrum of the neutralparent molecule (as depicted in Figure 3) will be overlaid with emission lines arisingfrom the ionization of the ionic species and of the dissociation fragments. In order toseparate the emission from di ff erent species coincidences between electrons and ionicfragment can be used for a more detailed analysis. In fact, coincidence experiments willbe one of the major experimental tools available at the SQS instrument, and are feasibledue to the high number of X-ray pulses (up to 27 000 per second) at the European XFEL.This high repetition rate allows one to record data of high statistics for coincidentmeasurements between electrons and ionic fragments coming unambiguously fromthe same molecule.In a typical experimental scenario, first the Auger spectra would be recorded atthe individual wavelength 250 eV and 630 eV, respectively, in order to obtain the one-5igure 3 (color online): Schematic representation of the electron spectrum recordedupon ionization with two pulses at photon energies at 250 and 630 eV, respectivelyphoton reference spectra. In addition, electron-ion coincidence will provide chargeand fragment resolved electron spectra would at these photon energies. In a secondstep, the Iodine 3 d Auger spectrum – caused by the 630 eV photon pulse – will bemonitored in the presence of the additional the 250 eV pulse. When the 250 eV pulsecomes after the 630 eV pulse, the spectrum will be unchanged compared to the singlecolor spectrum. When both pulses are overlapping or the 250 eV comes earlier, theobservation of the iodine Auger spectrum for di ff erent delays between both pulsesprovide the information about the intermolecular processes. Changes of the kineticenergy position and of the intensity distribution within the I d Auger spectrum are themonitor to follow charge migration processes inside the molecule, i.e. to determinee.g. the time required to transmit the information about the creation of a 2 p core holeon the chlorine site to the iodine atom. For small molecules this time scale is in theorder to a few femtoseconds [8], so probably di ffi cult to access with pulses of about2 fs duration each. For longer carbon chains the time scale is expected to increase toabout 10 fs or more and therefore well suited to be studied with the set-up at the SQSinstrument.Furthermore, by selecting in coincidence mode a fragment containing the iodineatom or the iodine atom itself, also information on charge transfer processes is madeavailable. Compared to earlier work using an optical laser to initiate the fragmenta-tion [9], the pump can be used in a very selective way, changing for example betweenexcitations of the chlorine and the carbon atom. In this way a more versatile and de-tailed analysis of the complex intra-molecular interaction and on the related electronand nuclear dynamics will become possible. For the particular science case at the SQS instrument discussed in the previous section,two fs-order-long X-ray pulses with a tunable relative delay are required. In whatfollows we consider a simulation scenario where a magnetic chicane and an opticaldelay line are installed at SASE3, see Figure 4.Figure 5 shows the result of start-to-end simulations for the electron beam through6igure 4 (color online): The SASE3 undulator, consisting of 21 undulator segments.It is e ff ectively separated into two parts U1 and U2 by the magnetic chicane with anoptical delay line.Figure 5 (color online): Nominal 20pC electron beam at the entrance to the SASE3undulator 7he European XFEL linac to the entrance of the SASE3 beamline based on [10]. In orderto deliver the electron beam to the SASE3 undulator line, it should pass through theSASE1 line. It was shown [11], that lasing in the SASE1 undulator can be inhibited, butdue to quantum fluctuations [12, 13] and synchrotron radiation the electron parametersdeteriorate. Also, the influence of the resistive wake in the SASE1 undulator a ff ectsthe electron beam energy distribution. Therefore, we simulated the electron beampropagation through the SASE1 undulator, accounting for all these e ff ects.The electron beam obtained in this way is sent through the first part of the SASE3undulator U U ff erent energies per pulse (top right), power along the FEL pulses (bottom left)and their spectra (bottom right). Mean energy per pulse is 50 µ J, corresponding to5 × photons per pulse on average.The photon beam then passes through the fixed optical delay line, while the electronbeam goes through the magnetic chicane. The delay in the magnetic chicane can beadjusted with sub-fs accuracy, and can be set to under- or over-compensate the optical8igure 7: The radiation properties from U ff erent energies per pulse (top right), power along the FEL pulses(bottom left) and their spectra (bottom right plot). Mean energy per pulse is 70 µ J,corresponding to 1 . × photons per pulse on average.delay. In this way any delay between the two photon pulses, positive or negative,can be set. If the desired color separation is smaller than 1%, i.e. when both colorsare within the SASE amplification bandwidth, we should set the magnetic chicaneto have a dispersion strength large enough to destroy the microbunching developedin the first undulator part. We work under the assumption that X-ray di ff ractione ff ects in the optical delay line are negligible and that its mirrors do not sensiblymodify the radiation wavefront distribution. Moreover, we estimate that a variabledelay up to several hundreds femtoseconds with the before-mentioned accuracy of afraction of femtosecond can be provided between the two radiation pulses. Due to theintroduction of the optical delay line into the magnetic chicane, the 630 eV radiationpulse generated in the U U U U
2. The proper shotnoise is automatically introduced into the electron beam by the Genesis simulationcode. The second undulator U ff and the remaining seven lasing at 250 eV. The results of a U ff set mirrors and two KB mirrors, Figure 9.For simplicity, in this example we chose the same incident angle of 9 mrad for allmirrors, though 12 mrad is possible for the o ff set mirrors. The reflectivity is then fixed,see Figure 10. We assume 1 nm RMS height errors for the mirror surfaces.The finite length of the mirrors, together with the grazing incidence angle andthe long propagation distance are potentially responsible for clipping of the radiationpulse along the transverse directions and, consequently, for di ff raction e ff ects. In factwe treat the finite length of mirrors as e ff ective apertures through which radiation is10igure 9 (color online): Optical elements relevant to SQS and their positions assumedfor calculationsFigure 10 (color online): Reflectivity of B4C on Si substrate at 9 mrad, as a function ofthe photon energy [15].propagated. We did not observe any visible e ff ects originating from the o ff set mirrors,while the KB mirror pair introduces a significant clipping, illustrated on Figure 11. Itshould be remarked here that the use of the intermediate focus that can be generatedby the second o ff set mirror would reduce the beam footprint on the second KB andthereby increase the geometric transmission. The performances of this option are not11igure 11 (color online): Illustration of radiation clipping due to the limited length ofthe refocusing KB mirrors. The example is based on 630eV photon energy. Nearly 80%of the radiation does not pass the KB mirror pair.calculated in the present work.Figure 12 (color online): Illustration of the two spatially separated sources S S ff erent position.After passing through the entire beamline, the photon beam can be focused at thesample position. One issue concerning the optimal focusing is related to the presenceof the two separate sources for the two pulses, Figure 12. In our case study the distancebetween sources S S ff ectively focused at the sample position (see Figure 13).In order to ease the issue, one may rely on the flexibility of our setup and openthe gap of the last seven undulator sections, instead of the first seven as was shownin Figure 4. In this case, the distance between the sources S S S ff erent focal lengths.Therefore, if we choose to obtain an image of S S ff erent positions in thehorizontal and vertical planes. In other words, if we decide to tune the KB mirrors toimage one of the two sources at one particular position, the image of the other will notonly appear shifted in space, but will also be a subject to astigmatism.12igure 13 (color online): Intensity distributions according to Figure 12 at the locationsof the two image planes. Values of the maximum photon flux are provided.Figure 14 (color online): Illustration of astigmatism e ff ects when imaging the twosources: only one source may be reimaged simultaneously in both vertical and hori-zontal planes.It is also possible to select an intermediate, imaginary source position between S S
2, which we call S .
5, as in Figure 15, and image it by properly tuning the KBmirror system. In this case, the image of S . I .
5. Thisyields a good compromise in terms of beam sizes at the sample, Figure 16. Based on oursimulations, the resulting photon fluxes would be su ffi cient to conduct the experimentproposed in Section 2.Since the distance between I I S .
5, located between sources S S
2, thenthe images at planes I I ff er from aberration. However this solution allowsone to obtain a good radiation quality at I . ff erent photon energies at various image planes from Figure 15. Peak photon densityis provided above the plots. The method of an intermediate source reimaging wouldallow one to obtain comparable radiation distribution size as well as the photon flux.14 Conclusions
There is a great scientific interest to deliver two pulses of di ff erent wavelengths with acontrollable delay to the experimental samples. In this paper we explore a method toconduct two-color pump-probe experiments at the SASE3 soft X-ray beamline of theEuropean XFEL with minimal hardware modifications.The scheme, originally proposed at DESY and the European XFEL has been alreadyexperimentally tested in LCLS and SACLA and has proven to be robust, easy to set-upand intrinsically resistant to the time delay jitter between the pulses. Both radiationpulses can be satisfactorily focused onto the sample with the baseline KB mirror system.The relative intensity can be varied either by modifying the undulators setup or bychanging the settings of the focusing system. Radiation wavelengths can be easily andindependently tuned by changing the K value of the undulators.This scheme can be technically implemented in two steps: first - installation of anelectromagnetic chicane; second - upgrade of the chicane with an introduction of anoptical delay line. The second step allows for an increased flexibility of the setup, as itenables to scan through positive and negative delays between the two wavelengths λ and λ . An option to combine the optical delay with a SXRSS within a single chicaneset-up may be also a subject of a future study.The two-color setup described in this paper can serve both instruments at SASE3,Small Quantum System (SQS) and Spectroscopy & Coherent Scattering (SCS). In thiswork we limited ourselves to illustrate our proposal by selecting and analyze one pos-sible application for the SQS instrument, namely the study of charge transfer processesin a linear molecule. We presented the scientific case and we performed start-to-end(s2e) simulations up to the sample position. We started our work from s2e simulationsfor the electron beam at the entrance of the undulator and we simulated the radiationpulses from our setup. Further on, we computed the propagation of the radiationthrough the optical beamline up to the focus in the SQS instrument.While simulations presented in this paper were performed only for the purpose ofillustrating the capabilities and the flexibility of the proposed setup, the same com-putational techniques produce results that may serve as a starting point for detailedsimulation of the interaction between radiation and matter, and can be used to defineand prepare experiments in great detail. Acknowledgments
We thank Joachim Pfl ¨uger, Andreas Scherz and Alexander Yaroslavtsev for usefuldiscussions and Serguei Molodtsov for his interest in this work.
References [1] G. Geloni, V. Kocharyan, and E. Saldin. “Scheme for femtosecond-resolutionpump-probe experiments at XFELs with two-color ten GW-level X-ray pulses”.In: arXiv.org
January (2010), pp. 10–004. arXiv: .152] A. A. Lutman et al. “Experimental Demonstration of Femtosecond Two-ColorX-Ray Free-Electron Lasers”. In:
Phys. Rev. Lett. issn :0031-9007. doi : .[3] T. Hara et al. “Two-colour hard X-ray free-electron laser with wide tunability”.In: Nat. Commun. issn : 2041-1723. doi : .[4] T Mazza, H Zhang, and M Meyer. “Scientific Instrument SQS”. In: Tech. Des. Rep.
December (2012). doi : .[5] A Scherz et al. “Scientific Instrument Spectroscopy and Coherent Scattering(SCS)”. In: Concept. Des. Rep. (2013). doi : .[6] V Jonauskas et al. “Auger cascade satellites following 3d ionization in xenon”.In: J. Phys. B At. Mol. Opt. Phys. issn : 0953-4075. doi : .[7] M. Kivilompolo et al. “The gas phase L2,3VV Auger electron spectra of chlorinein XCl (X = H, D, Li, Na, K) molecules”. In:
J. Chem. Phys. issn : 0021-9606. doi : .[8] N. V. Golubev and A. I. Kule ff . “Control of charge migration in molecules byultrashort laser pulses”. In: Phys. Rev. A issn : 1050-2947. doi : .[9] B. Erk et al. “Imaging charge transfer in iodomethane upon x-ray photoab-sorption”. In: Science (80-. ). issn : 0036-8075. doi : .[10] I. Zagorodnov. DESY MPY Start-to-End Simulations page: http: // / fel-beam / s2e / xfel.html . url : .[11] R. Brinkmann, E. Schneidmiller, and M. Yurkov. “Possible operation of the Euro-pean XFEL with ultra-low emittance beams”. In: Nucl. Instruments Methods Phys.Res. Sect. A Accel. Spectrometers, Detect. Assoc. Equip. issn :01689002. doi : . arXiv: .[12] J. Rossbach et al. “Interdependence of parameters of an X-ray FEL”. In: Nucl.Instruments Methods Phys. Res. Sect. A Accel. Spectrometers, Detect. Assoc. Equip. issn : 01689002. doi : .[13] J. Rossbach et al. “Fundamental limitations of an X-ray FEL operation due toquantum fluctuations of undulator radiation”. In: Nucl. Instruments Methods Phys.Res. Sect. A Accel. Spectrometers, Detect. Assoc. Equip. issn : 01689002. doi : .[14] S. Reiche. “Genesis 1.3”. 2004. url : http://genesis.web.psi.ch/download/documentation/genesis_manual.pdf .[15] E. Gullikson. X-Ray database . Berkeley, California. url : http://henke.lbl.gov/optical_constants/http://henke.lbl.gov/optical_constants/