Self-seeding scheme for the soft X-ray line at the European XFEL
This paper discusses the potential for enhancing the capabilities of the European FEL in the soft X-ray regime. A high longitudinal coherence will be the key to such performance upgrade. In order to reach this goal we study a very compact soft X-ray self-seeding scheme originally designed at SLAC FENG , FENG2 . The scheme is based on a grating monochromator, and can be straightforwardly installed in the SASE3 undulator beamline at the European XFEL. For the European XFEL fully-coherent soft X-ray pulses are particularly valuable since they naturally support the extraction of more FEL power than at saturation by exploiting tapering in the tunable-gap SASE3 undulator. Tapering consists of a stepwise change of the undulator gap from segment to segment. Based on start-to-end simulations we show that soft X-ray FEL power reaches about GW, that is about an order of magnitude higher than the SASE level at saturation ( GW). The self-seeding setup studied in this work is extremely compact (about m long), and cost-effective. This last characteristic may justify to consider it as a possible addition to the European XFEL capabilities from the very beginning of the operation phase.
Ein Forschungszentrum der Helmholtz-Gemeinschaft
Self-seeding scheme for the soft X-ray line at the European XFEL
European XFEL GmbH, Hamburg
Vitali Kocharyan and Evgeni Saldin
Deutsches Elektronen-Synchrotron DESY, Hamburg
NOTKESTRASSE 85 - 22607 HAMBURG
The quality of the output radiation of X-ray SASE FELs is far from ideal. SASE X-ray beams are characterized by nearly full transverse coherence but only limited longitudinal coherence LCLS2 -tdr-2006 . However, many experiments require both transverse and longitudinal coherence. In principle, one can create a longitudinal coherent source by using a monochromator located in experimental hall, but this is often undesirable because of intensity losses. An important goal for any advanced XFEL facility is the production of X-ray radiation pulses with minimum allowed photon energy width for a given pulse length, that is Fourier-limited pulses. In this way, no monochromator is needed in the experimental hall. Self-seeding is a promising approach to significantly narrow the SASE bandwidth and to produce nearly transform-limited pulses SELF -WUFEL2 . Considerable effort has been invested in theoretical investigation and RD at LCLS leading to this capability CDRL2 .
In general, a self-seeding setup consists of two undulators separated by a photon monochromator and an electron bypass, normally a four-dipole chicane. The two undulators are resonant to the same radiation wavelength. The SASE radiation generated by the first undulator passes through the narrow-band monochromator. A transform-limited pulse is created, which is used as a coherent seed in the second undulator. Chromatic dispersion effects in the bypass chicane smear out the microbunching in the electron bunch, produced by the SASE lasing in the first undulator. The electrons and the monochromatized photon beam are recombined at the entrance of the second undulator, and radiation is amplified by the electron bunch until saturation is reached. The required seed power at the beginning of the second undulator must dominate over the shot noise power within the gain bandpass, which is order of a kW in the soft X-ray range.
For soft X-ray self-seeding, a monochromator usually consists of a grating SELF . Recently, a very compact soft X-ray self-seeding scheme has appeared, based on a grating monochromator FENG , FENG2 . The proposed monochromator is composed of only three mirrors and a rotational VLS grating. A preliminary design of the gratings adopts a constant focal-point mode in order to have fixed slit location. When tuning the photon energy in the range between and eV, the variation in the optical delay is limited to of the nominal value of ps. A short magnetic chicane delays the electron bunch accordingly, so that the photon beam passing through the monochromator system recombines with the same electron bunch. The chicane provides a dispersion strength of about mm in order to match the optical delay and also smears out the SASE microbunching generated in the first undulator.
In this article we study the performance of the above-described scheme for the European XFEL upgrade. The installation of the chicane does not perturb the undulator focusing system and allows for a safe return to the baseline mode of operation. The inclusion of a chicane is not expensive and may find many other applications MEAS , PUMP .
With the radiation beam monochromatized down to the Fourier transform limit, a variety of very different techniques leading to further improvement of the X-ray FEL performance become feasible. In particular, the most promising way to extract more FEL power than that at saturation is by tapering the magnetic field of the undulator TAP1 -TAP4 . A significant increase in power is achievable by starting the FEL process from monochromatic seed rather than from noise, FAWL -WANG . In this paper we propose a study of the performance of the soft X-ray self-seeding scheme for the European XFEL, based on start-to-end simulations for an electron beam with 0.1 nC charge S2ER . Simulations show that the FEL power of the transform-limited soft X-ray pulses may be increased up to 800 GW by properly tapering the baseline undulator. In particular, it is possible to create a source capable of delivering fully-coherent, fs (FWHM) soft X-ray pulses with photons per pulse at the wavelength of 1.5 nm.
2 Possible self-seeding scheme with grating monochromator for the baseline undulator SASE3
Any self-seeding setup should be compact enough to fit one undulator segment. The elements of the adopted electron bypass design are four m-long dipole magnets of rectangular shape. Under the constraints imposed by space and strength of the magnetic field it is only possible to operate at an electron beam energy of GeV. The first dipole deflects the beam by degrees. After m, the second dipole deflects the beam back in a direction parallel to the straight beampath at a distance of about 3 cm, which is sufficient for the installation of the optical elements of the monochromator. The total elongation of the electron beampath is approximately mm. The layout of the bypass is shown in Fig. 2 and Fig. 3.
The monochromator design should be compact enough to fit with this magnetic chicane design. In particular, the optical delay should be matched to that induced by the magnetic chicane on the electron beam. The design adopted in this paper is the novel one by Y. Feng et al. FENG , FENG2 , and is based on a planar VLS grating. It is equipped only with an exit slit. Such design includes four optical elements, a cylindrical and spherical focusing mirrors, a VLS grating and a plane mirror in front of the grating. The specifications of the monochromator are summarized below:
The total optical beamline length, from the source to the image, is about 6 m.
The total length between first and last optical components is about 2.3 m
The delay of the photons is about ps.
The monochromator is continuously tunable in the photon energy range eV.
The resolving power is about at the wavelength around nm.
The photon beam size and divergence at the entrance of the 2nd undulator are close to those at the exit of the first undulator.
The transmission of the beamline is close to for wavelengths around nm.
The optical layout of the monochromator is schematically shown in Fig. 4. The first optical component in the monochromator, , is a cylindrical mirror which focuses the input photon beam in the sagittal plane at the entrance point, and deflects it in the dispersion plane of about degree. This mirror has a long focal length of about m. The last optical component, , is a collimation mirror that refocuses the photon beam at the entrance point. It introduces a deflection angle in the dispersion plane of about . This mirror has a short focal distance of about cm. The second mirror, , is plane mirror set in front of the grating. The function of this mirror is to illuminate the grating at the proper angle of incidence, which is chosen so that the correct wavelength is approximately focused at the exit slit. The monochromator scanning is performed by rotating and translating the pre-mirror and by rotating the grating. The translation of the mirror implies that the optical path is wavelength dependent. Evidently, the scanning will result on a wavelength dependent optical path. The tunability of the path length in the magnetic chicane will be required to compensate for the change in the optical path in the range of mm. The last optical component is a planar VLS grating. A monochromator resolving power of about (at nm) is obtained with the grating having a line spacing of micron, a groove height of nm and using a microns exit slit FENG , FENG2 . The estimated grating efficiency is about . Reflectivity of the grazing incident mirrors and grating are close to .
3 FEL simulations
With reference to Fig. 1, we performed a feasibility study with the help of the FEL code GENESIS 1.3 GENE  running on a parallel machine. We will present a feasibility study for the SASE3 FEL line of the European XFEL, based on a statistical analysis consisting of runs. The overall beam parameters used in the simulations are presented in Table 1.
|Periods per cell||-||77|
|K parameter (rms)||-||4.2|
|Total number of cells||-||21|
The expected beam parameters at the entrance of the SASE3 undulator, and the resistive wake inside the undulator are shown in Fig. 5, S2ER . The evolution of the transverse electron bunch dimensions are plotted in Fig. 6.
The SASE pulse power and spectrum after the first undulator in Fig. 1 is shown in Fig. 7. This pulse goes through the grating monochromator. We assume a monochromator with a Gaussian shape. At the exit of the monochromator, one obtains the seed pulse, Fig. 8. As explained before, the monochromator introduces only a short optical delay of about ps, which can be easily compensated by the electron chicane. The chicane also washes out the electron beam microbunching. As a result, at the entrance of the second (output) undulator the electron beam and the radiation pulse can be recombined.
If the output undulator is not tapered, one needs sections to reach saturation. The best compromise between power and spectral bandwidth are reached after sections, Fig. 9. In this case, the evolution of the energy per pulse and of the energy fluctuations as a function of the undulator length are shown in Fig. 10. The pulse now reaches the GW power level, with an average relative FWHM spectral width narrower than . Finally, the transverse radiation distribution and divergence at the exit of the output undulator are shown in Fig. 11.
The most promising way to increase the output power is via post-saturation tapering. Tapering consists in a slow reduction of the field strength of the undulator in order to preserve the resonance wavelength, while the kinetic energy of the electrons decreases due to the FEL process. The undulator taper could be simply implemented as a step taper from one undulator segment to the next, as shown in Fig. 12. The magnetic field tapering is provided by changing the undulator gap. A further increase in power is achievable by starting the FEL process from the monochromatic seed, rather than from noise. The reason is the higher degree of coherence of the radiation in the seed case, thus involving, with tapering, a larger portion of the bunch in the energy-wavelength synchronism. Using the tapering configuration in Fig. 12, one obtains the output characteristics, in terms of power and spectrum, shown in Fig. 13. The output power is increased of about a factor ten, allowing one to reach about one TW. The spectral width remains almost unvaried, with an average relative bandwidth (FWHM) narrower than . The evolution of the energy per pulse and of the energy fluctuations as a function of the undulator length are shown in Fig. 10. Finally, the transverse radiation distribution and divergence at the exit of the output undulator are shown in Fig. 15. By comparison with Fig. 11 one can see that the divergence decrease is accompanied by an increase in the transverse size of the radiation spot at the exit of the undulator.
In this paper we showed that monochromatization of soft X-ray pulses is of great importance for the European XFEL upgrade program. In fact, aside for an improvement of the longitudinal coherence, highly monochromatized pulses open up many possibilities to enhance the capacity of the European XFEL.
In particular we showed that monochromatization effectively allows one to use undulator tapering techniques enabling a TW-power mode of operation in the soft X-ray wavelength regime.
The progress which can be achieved with the methods considered here is based on the novel soft X-ray self-seeding scheme with grating monochromator FENG , FENG2 , which is extremely compact and can be straightforwardly realized at the European XFEL baseline undulator SASE3. Preliminary estimates seem to indicate that a monochromator efficiency close to can be achieved. However, detailed efficiency calculations are required to determine if this is indeed the case. Another problem with the operation of this scheme can be the mismatching of the seed radiation pulse with the electron bunch. However, the safety margin of the proposed design is large enough: even if the nominal seed power (200 kW) is degraded of an order of magnitude due to non-ideal effects, one would still obtain a reasonable seed signal (20 kW) compared to the equivalent shot noise power (1 kW).
We are grateful to Massimo Altarelli, Reinhard Brinkmann, Serguei Molodtsov and Edgar Weckert for their support and their interest during the compilation of this work.
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