arXiv subjects

Gianluca Geloni

Publications and source records attributed to Gianluca Geloni.

At least 55 records · Page 3Linked to original sources

arXiv2013

Wake monochromator in asymmetric and symmetric Bragg and Laue geometry for self-seeding the European X-ray FEL

We discuss the use of self-seeding schemes with wake monochromators to produce TW power, fully coherent pulses for applications at the dedicated bio-imaging bealine at the European X-ray FEL, a concept for an upgrade of the facility beyond the baseline previously proposed by the authors. We exploit the asymmetric and symmetric Bragg and Laue reflections (sigma polarization) in diamond crystal. Optimization of the bio-imaging beamline is performed with extensive start-to-end simulations, which also take into account effects such as the spatio-temporal coupling caused by the wake monochromator. The spatial shift is maximal in the range for small Bragg angles. A geometry with Bragg angles close to pi/2 would be a more advantageous option from this viewpoint, albeit with decrease of the spectral tunability. We show that it will be possible to cover the photon energy range from 3 keV to 13 keV by using four different planes of the same crystal with one rotational degree of freedom.

physics.acc-ph↗

arXiv2012

Optimization of a dedicated bio-imaging beamline at the European X-ray FEL

We recently proposed a basic concept for design and layout of the undulator source for a dedicated bio-imaging beamline at the European XFEL. The goal of the optimized scheme proposed here is to enable experimental simplification and performance improvement. The core of the scheme is composed by soft and hard X-ray self-seeding setups. Based on the use of an improved design for both monochromators it is possible to increase the design electron energy up to 17.5 GeV in photon energy range between 2 keV and 13 keV, which is the most preferable for life science experiments. An advantage of operating at such high electron energy is the increase of the X-ray output peak power. Another advantage is that 17.5 GeV is the preferred operation energy for SASE1 and SASE2 beamline users. Since it will be necessary to run all the XFEL lines at the same electron energy, this choice will reduce the interference with other undulator lines and increase the total amount of scheduled beam time. In this work we also propose a study of the performance of the self-seeding scheme accounting for spatiotemporal coupling caused by the use of a single crystal monochromator. Our analysis indicates that this distortion is easily suppressed by the right choice of diamond crystal planes and that the proposed undulator source yields about the same performance as in the case for a X-ray seed pulse with no coupling. Simulations show that the FEL power reaches 2 TW in the 3 keV - 5 keV photon energy range, which is the most preferable for single biomolecule imaging.

physics.acc-ph↗

arXiv2012

Extension of self-seeding scheme with single crystal monochromator to lower energy < 5 keV as a way to generate multi-TW scale pulses at the European XFEL

We propose a use of the self-seeding scheme with single crystal monochromator to produce high power, fully-coherent pulses for applications at a dedicated bio-imaging beamline at the European X-ray FEL in the photon energy range between 3.5 keV and 5 keV. We exploit the C(111) Bragg reflection (pi-polarization) in diamond crystals with a thickness of 0.1 mm, and we show that, by tapering the 40 cells of the SASE3 type undulator the FEL power can reach up to 2 TW in the entire photon energy range. The present design assumes the use of a nominal electron bunch with charge 0.1 nC at nominal electron beam energy 17.5 GeV. The main application of the scheme proposed in this work is for single shot imaging of individual protein molecules.

physics.acc-ph↗

arXiv2012

Conceptual design of an undulator system for a dedicated bio-imaging beamline at the European X-ray FEL

We describe a future possible upgrade of the European XFEL consisting in the construction of an undulator beamline dedicated to life science experiments. The availability of free undulator tunnels at the European XFEL facility offers a unique opportunity to build a beamline optimized for coherent diffraction imaging of complex molecules, like proteins and other biologically interesting structures. Crucial parameters for such bio-imaging beamline are photon energy range, peak power, and pulse duration. Key component of the setup is the undulator source. The peak power is maximized in the photon energy range between 3 keV and 13 keV by the use of a very efficient combination of self-seeding, fresh bunch and tapered undulator techniques. The unique combination of ultra-high peak power of 1 TW in the entire energy range, and ultrashort pulse duration tunable from 2 fs to 10 fs, would allow for single shot coherent imaging of protein molecules with size larger than 10 nm. Also, the new beamline would enable imaging of large biological structures in the water window, between 0.3 keV and 0.4 keV. In order to make use of standardized components, at present we favor the use of SASE3-type undulator segments. The number segments, 40, is determined by the tapered length for the design output power of 1 TW. The present plan assumes the use of a nominal electron bunch with charge of 0.1 nC. Experiments will be performed without interference with the other three undulator beamlines. Therefore, the total amount of scheduled beam time per year is expected to be up to 4000 hours.

physics.acc-ph↗

arXiv2012

Pulse-front tilt caused by the use of a grating monochromator and self-seeding of soft X-ray FELs

Self-seeding is a promising approach to significantly narrow the SASE bandwidth of XFELs to produce nearly transform-limited pulses. The development of such schemes in the soft X-ray wavelength range necessarily involves gratings as dispersive elements. These introduce, in general, a pulse-front tilt, which is directly proportional to the angular dispersion. Pulse-front tilt may easily lead to a seed signal decrease by a factor two or more. Suggestions on how to minimize the pulse-front tilt effect in the self-seeding setup are given.

physics.optics↗

arXiv2012

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 [1,2]. 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 800 GW, that is about an order of magnitude higher than the SASE level at saturation (100 GW). The self-seeding setup studied in this work is extremely compact (about 5 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. [1] Y. Feng, J. Hastings, P. Heimann, M. Rowen, J. Krzywinski, and J. Wu, "X-ray Optics for soft X-ray self-seeding the LCLS-II", proceedings of 2010 FEL conference, Malmo, Sweden, (2010). [2] Y. Feng, P. Heimann, J. Wu, J. Krzywinski, M. Rowen, and J. Hastings, "Compact Grating Monochromator Design for LCLS-I Soft X-ray Self-Seeding", https://slacportal.slac.stanford.edu/sites/lcls_public/lcls_ii/Lists/LCLS_II_Calendar/Physics_Meetings.aspx, May 2011 and https://sites.google.com/a/lbl.gov/realizing-the-potential-of-seeded-fels-in-the-soft-x-ray-regime-workshop/talks, October 2011

physics.acc-ph↗

arXiv2012

On quantum effects in spontaneous emission by a relativistic electron beam in an undulator

Robb and Bonifacio (2011) claimed that a previously neglected quantum effect results in noticeable changes in the evolution of the energy distribution associated with spontaneous emission in long undulators. They revisited theoretical models used to describe the emission of radiation by relativistic electrons as a continuous diffusive process, and claimed that in the asymptotic limit for a large number of undulator periods the evolution of the electron energy distribution occurs as discrete energy groups according to Poisson distribution. We show that these novel results have no physical sense, because they are based on a one-dimensional model of spontaneous emission and assume that electrons are sheets of charge. However, electrons are point-like particles and, as is well-known, the bandwidth of the angular-integrated spectrum of undulator radiation is independent of the number of undulator periods. If we determine the evolution of the energy distribution using a three-dimensional theory we find the well-known results consistent with a continuous diffusive process. The additional pedagogical purpose of this paper is to review how quantum diffusion of electron energy in an undulator with small undulator parameter can be simply analyzed using the Thomson cross-section expression, unlike the conventional treatment based on the expression for the Lienard-Wiechert fields.

physics.acc-ph↗

arXiv2011

Scheme for generating and transporting THz radiation to the X-ray experimental hall at the European XFEL

The design of a THz edge radiation source for the European XFEL is presented. We consider generation of THz radiation from the spent electron beam downstream of the SASE2 undulator in the electron beam dump area. In this way, the THz output must propagate at least for 250 meters through the photon beam tunnel to the experimental hall to reach the SASE2 X-ray hutches. We propose to use an open beam waveguide such as an iris guide as transmission line. In order to efficiently couple radiation into the iris transmission line, generation of the THz radiation pulse can be performed directly within the iris guide. The line transporting the THz radiation to the SASE2 X-ray hutches introduces a path delay of about 20 m. Since THz pump/X-ray probe experiments should be enabled, we propose to exploit the European XFEL baseline multi-bunch mode of operation, with 222 ns electron bunch separation, in order to cope with the delay between THz and X-ray pulses. We present start-to-end simulations for 1 nC bunch operation-parameters, optimized for THz pump/X-ray probe experiments. Detailed characterization of the THz and SASE X-ray radiation pulses is performed. Highly focused THz beams will approach the high field limit of 1 V/atomic size.

physics.acc-ph↗

arXiv2011

Extension of self-seeding to hard X-rays >10 keV as a way to increase user access at the European XFEL

We propose to use the self-seeding scheme with single crystal monochromator at the European X-ray FEL to produce monochromatic, high-power radiation at 16 keV. Based on start to end simulations we show that the FEL power of the transform-limited pulses can reach about 100 GW by exploiting tapering in the tunable-gap baseline undulator. The combination of high photon energy, high peak power, and very narrow bandwidth opens a vast new range of applications, and includes the possibility to considerably increase the user capacity and fully exploit the high repetition rate of the European XFEL. In fact, dealing with monochromatic hard X-ray radiation one may use crystals as deflectors with minimum beam loss. To this end, a photon beam distribution system based on the use of crystals in the Bragg reflection geometry is proposed for future study and possible extension of the baseline facility. They can be repeated a number of times to form an almost complete (one meter scale) ring with an angle of 20 degrees between two neighboring lines. The reflectivity of crystal deflectors can be switched fast enough by flipping the crystals with piezo-electric devices similar to those for X-ray phase retarders at synchrotron radiation facilities. It is then possible to distribute monochromatic hard X-rays among 10 independent instruments, thereby enabling 10 users to work in parallel. The unmatched repetition rate of the European XFEL would be therefore fully exploited.

physics.acc-ph↗

arXiv2011

Analytical studies of constraints on the performance for EEHG FEL seed lasers

Laser seeding technique have been envisioned to produce nearly transform-limited pulses at soft X-ray FELs. Echo-Enabled Harmonic Generation (EEHG) is a promising, recent technique for harmonic generation with an excellent up-conversion to very high harmonics, from the standpoint of electron beam physics. This paper explores the constraints on seed laser performance for reaching wavelengths of 1 nm. We show that the main challenge in implementing the EEHG scheme at extreme harmonic factors is the requirement for accurate control of temporal and spatial quality of the seed laser pulse. For example, if the phase of the laser pulse is chirped before conversion to an UV seed pulse, the chirp in the electron beam microbunch turns out to be roughly multiplied by the harmonic factor. In the case of a Ti:Sa seed laser, such factor is about 800. For such large harmonic numbers, generation of nearly transform-limited soft X-ray pulses results in challenging constraints on the Ti:Sa laser. In fact, the relative discrepancy of the time-bandwidth product of the seed-laser pulse from the ideal transform-limited performance should be no more than one in a million. The generated electron beam microbunching is also very sensitive to distortions of the seed laser wavefront, which are also multiplied by the harmonic factor. In order to have minimal reduction of the FEL input coupling factor, it is desirable that the size-angular bandwidth product of the UV seed laser beam be very close to the ideal i.e. diffraction-limited performance in the waist plane at the middle of the modulator undulator.

physics.acc-ph↗

arXiv2011

Production of transform-limited X-ray pulses through self-seeding at the European X-ray FEL

An important goal for any advanced X-ray FEL is an option for providing Fourier-limited X-ray pulses. In this way, no monochromator is needed in the experimental hall. Self-seeding is a promising approach to significantly narrow the SASE bandwidth to produce nearly transform-limited pulses. These are important for many experiments including 3D diffraction imaging. We discuss the implementation of a single-crystal self-seeding scheme in the hard X-ray lines of the European XFEL. For this facility, transform-limited pulses are particularly valuable since they naturally support the extraction of more FEL power than at saturation by exploiting tapering in the tunable-gap baseline undulators. Tapering consists of a stepwise change of the undulator gap from segment to segment. Based on start-to-end simulations dealing with the up-to-date parameters of the European XFEL, we show that the FEL power reaches about 400 GW, or one order of magnitude higher power than the SASE saturation level (20 GW). This analysis indicates that our self-seeding scheme is not significantly affected by non-ideal electron phase-space distribution, and yields about the same performance as in the case for an electron beam with ideal parameters. The self-seeding scheme with a single crystal monochromator is extremely compact (about 5 m long), and cost estimations are low enough to consider adding it to the European XFEL capabilities from the very beginning of the operation phase.

physics.acc-ph↗

arXiv2011

Scheme for generating and transporting THz radiation to the X-ray experimental floor at the LCLS baseline

This paper describes a novel scheme for integrating a coherent THz source in the baseline of the LCLS facility. Any method relying on the spent electron beam downstream of the baseline undulator should provide a way of transporting the radiation up to the experimental floor. Here we propose to use the dump area access maze. In this way the THz output must propagate with limited size at least for one hundred meters in a maze, following many turns, to reach the near experimental hall. The use of a standard, discrete, open beam-waveguide formed by periodic reflectors, that is a mirror guide, would lead to unacceptable size of the system. To avoid these problems, in this paper we propose an alternative approach based on periodically spaced metallic screens with holes. This quasi-optical transmission line is referred to as an iris line. We present complete calculations for the iris line using both analytical and numerical methods, which we find in good agreement. We present a design of a THz edge radiation source based on the use of an iris line. The proposed setup takes almost no cost nor time to be implemented at the LCLS baseline, and can be used at other facilities as well. The edge radiation source is limited in maximally achievable field strength at the sample. An extension based on the use of an undulator in the presence of the iris line, which is feasible at the LCLS energies, is proposed as a possible upgrade of the baseline THz source.

physics.acc-ph↗

arXiv2011

Circular polarization control for the European XFEL in the soft X-ray regime

The possibility of producing highly circularly polarized X-ray radiation, especially in the soft X-ray region, is an important asset at XFELs. However, the baseline of the European XFEL, including the soft X-ray SASE3 line, foresees planar undulators only. The lowest-risk strategy for implementing polarization control at SASE3 involves adding an APPLE II-type undulator at the end of the planar undulator, to exploit the microbunching from the baseline FEL. 5m-long APPLE II undulators are standard devices at synchrotrons. However, the choice of a short helical radiator leads to the problem of background suppression. The driving idea of our proposal is that the background radiation can be suppressed by spatial filtering. Slits can be inserted behind the APPLE II radiator, where the linearly-polarized radiation spot size is about 30 times larger than the radiation spot size from the helical radiator. The last 7 cells of the SASE3 undulator are left with an open gap in order to provide a total 42 m drift section for electron beam and radiation. The linearly-polarized radiation spot size increases along the drift, and the linearly polarized background radiation can be suppressed by the slits. At the same time, the microbunch structure is preserved. 100GW level coherent radiation is emitted in the helical radiator. The filtering setup consists of a pair of water cooled slits for X-ray beam filtering and of a 5m-long magnetic chicane, creating an offset for slit installation immediately behind the helical radiator. Electrons and X-rays are separated before the slits by the chicane, and the electron beam can pass by the filtering setup without perturbations. Based on start-to-end simulations we present calculations from the SASE3 undulator entrance up to the radiator exit including the modulated electron beam transport by the FODO focusing system in the low charge (20pC) mode of operation.

physics.acc-ph↗

arXiv2011

Microbunch preserving in-line system for an APPLE II helical radiator at the LCLS baseline

In a previous work we proposed a scheme for polarization control at the LCLS baseline, which exploited the microbunching from the planar undulator. After the baseline undulator, the electron beam is transported through a drift by a FODO focusing system, and through a short helical radiator. The microbunching structure can be preserved, and intense coherent radiation is emitted in the helical undulator at fundamental harmonic. The driving idea of this proposal is that the background linearly-polarized radiation from the baseline undulator is suppressed by spatial filtering. Filtering is achieved by letting radiation and electron beam through Be slits upstream of the helical radiator, where the radiation spot size is about ten times larger than the electron beam transverse size. Several changes considered in the present paper were made to improve the previous design. Slits are now placed immediately behind the helical radiator. The advantage is that the electron beam can be spoiled by the slits, and narrower slits width can be used for spatial filtering. Due to this fundamental reason, the present setup is shorter than the previous one. The helical radiator is now placed immediately behind the SHAB undulator. It is thus sufficient to use the existing FODO focusing system of the SHAB undulator for transporting the modulated electron beam. This paper presents complete GENESIS code calculations for the new design, starting from the baseline undulator entrance up to the helical radiator exit including the modulated electron beam transport by the SHAB FODO focusing system.

physics.acc-ph↗

arXiv2011

The effects of betatron motion on the preservation of FEL microbunching

In some options for circular polarization control at X-ray FELs, a helical radiator is placed a few ten meters distance behind the baseline undulator. If the microbunch structure induced in the baseline (planar) undulator can be preserved, intense coherent radiation is emitted in the helical radiator. The effects of betatron motion on the preservation of micro bunching in such in-line schemes should be accounting for. In this paper we present a comprehensive study of these effects. It is shown that one can work out an analytical expression for the debunching of an electron beam moving in a FODO lattice, strictly valid in the asymptote for a FODO cell much shorter than the betatron function. Further on, numerical studies can be used to demonstrate that the validity of such analytical expression goes beyond the above-mentioned asymptote, and can be used in much more a general context. Finally, a comparison with Genesis simulations is given.

physics.acc-ph↗

arXiv2011

Gas-filled cell as a narrow bandwidth bandpass filter in the VUV wavelength range

We propose a method for spectrally filtering radiation in the VUV wavelength range by means of a monochromator constituted by a cell filled with a resonantly absorbing rare gas. Around particular wavelengths, the gas exhibits narrow-bandwidth absorbing resonances following the Fano profile. In particular, within the photon energy range 60 eV - 65 eV, the correlation index of the Fano profiles for the photo-ionization spectra in Helium is equal to unity, meaning that the minimum of the cross-section is exactly zero. For sufficiently large column density in the gas cell, the spectrum of the incoming radiation will be attenuated by the background cross-section of many orders of magnitude, except for those wavelengths close to the point where the cross-section is zero. Remarkable advantages of a gas monochromator based on this principle are simplicity, efficiency and narrow-bandwidth. A gas monochromator installed in the experimental hall of a VUV SASE FEL facility would enable the delivery of a single-mode VUV laser beam. The design is identical to that of already existing gas attenuator systems for VUV or X-ray FELs. We present feasibility study and exemplifications for the FLASH facility in the VUV regime.

physics.acc-ph↗

arXiv2011

Self-seeding scheme with gas monochromator for narrow-bandwidth soft X-ray FELs

Self-seeding schemes, consisting of two undulators with a monochromator in between, aim at reducing the bandwidth of SASE X-ray FELs. We recently proposed to use a new method of monochromatization exploiting a single crystal in Bragg-transmission geometry for self-seeding in the hard X-ray range. Here we consider a possible extension of this method to the soft X-ray range using a cell filled with resonantly absorbing gas as monochromator. The transmittance spectrum in the gas exhibits an absorbing resonance with narrow bandwidth. Then, similarly to the hard X-ray case, the temporal waveform of the transmitted radiation pulse is characterized by a long monochromatic wake. In fact, the FEL pulse forces the gas atoms to oscillate in a way consistent with a forward-propagating, monochromatic radiation beam. The radiation power within this wake is much larger than the equivalent shot noise power in the electron bunch. Further on, the monochromatic wake of the radiation pulse is combined with the delayed electron bunch and amplified in the second undulator. The proposed setup is extremely simple, and composed of as few as two simple elements. These are the gas cell, to be filled with noble gas, and a short magnetic chicane. The installation of the magnetic chicane does not perturb the undulator focusing system and does not interfere with the baseline mode of operation. In this paper we assess the features of gas monochromator based on the use of He and Ne. We analyze the processes in the monochromator gas cell and outside it, touching upon the performance of the differential pumping system as well. We study the feasibility of using the proposed self-seeding technique to generate narrow bandwidth soft X-ray radiation in the LCLS-II soft X-ray beam line. We present conceptual design, technical implementation and expected performances of the gas monochromator self-seeding scheme.

physics.acc-ph↗

arXiv2011

Improvement of the crossed undulator design for effective circular polarization control in X-ray FELs

The production of X-ray radiation with a high degree of circular polarization constitutes an important goal at XFEL facilities. A simple scheme to obtain circular polarization control with crossed undulators has been proposed so far. In its simplest configuration the crossed undulators consist of pair of short planar undulators in crossed position separated by an electromagnetic phase shifter. An advantage of this configuration is a fast helicity switching. A drawback is that a high degree of circular polarization (over 90%) can only be achieved for lengths of the insertion devices significantly shorter than the gain length, i.e. at output power significantly lower than the saturation power level. The obvious and technically possible extension considered in this paper, is to use a setup with two or more crossed undulators separated by phase shifters. This cascade crossed undulator scheme is distinguished, in performance, by a fast helicity switching, a high degree of circular polarization (over 95%) and a high output power level, comparable with the saturation power level in the baseline undulator at fundamental wavelength. We present feasibility study and exemplifications for the LCLS baseline in the soft X-ray regime.

physics.acc-ph↗