FEL2011 - List of Authors (Geloni, G.)

Paper

Title

Page

Improvement of the Crossed Undulator Design for Effective Circular Polarization

61

G. Geloni
European XFEL GmbH, Hamburg, Germany
V. Kocharyan, E. Saldin
DESY, Hamburg, Germany

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. Here we propose 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.

MOPB26

Self-seeded Operation of the LCLS Hard X-ray FEL in the Long-bunch Mode of Operation

65

G. Geloni
European XFEL GmbH, Hamburg, Germany
V. Kocharyan, E. Saldin
DESY, Hamburg, Germany

Self-seeding options for the LCLS baseline were recently investigated using a scheme which relies on a single-crystal monochromator in Bragg-transmission geometry. The LCLS low-charge (0.02 nC) mode of operation was considered in order to demonstrate the feasibility of the proposed scheme. The wakefield effects from the linac and from the undulator vacuum chamber are much reduced at such low charge, and can be ignored. In this paper we extend our previous investigations to the case of the LCLS mode of operation with nominal charge. Based on the LCLS start-to-end simulation for an electron beam charge of 0.25 nC, and accounting for the wakefields from the undulator vacuum chamber we demonstrate that the same simplest self-seeding system (two undulators with a single-crystal monochromator in between) is appropriate not only for short (few femtosecond) bunches, but for longer bunches too.

MOPB27

Circular Polarization Control for the LCLS Baseline in the Soft X-ray Regime

69

G. Geloni
European XFEL GmbH, Hamburg, Germany
V. Kocharyan, E. Saldin
DESY, Hamburg, Germany

Several schemes have been discussed to obtain soft-polarization control in the context of the LCLS. We propose a novel method to generate 10 GW level power at the fundamental harmonic with 99% degree of circular polarization from the LCLS baseline. Its merits are low cost, simplicity and easy implementation. As in previously proposed methods, the microbunching of the planar undulator is used here as well. After the baseline undulator, the electron beam is sent through a 40 m long straight section, and subsequently through a short helical (APPLE II) radiator. The microbunching is easily preserved, and intense coherent radiation is emitted in the helical radiator. The background radiation from the baseline undulator can be suppressed by letting radiation and electrons through horizontal and vertical slits upstream the helical radiator, where the radiation spot size is about ten times larger than the electron bunch transverse size. Thin Be foils for the slits will preserve from electron losses. Other facilities e.g. LCLS II or the European XFEL may benefit from this work as well, due to availability of sufficiently long free space at the end of undulator tunnel.

MOPB28

Gas-filled Cell as a Narrow Bandwidth Bandpass Filter in the VUV Wavelength Range

73

G. Geloni
European XFEL GmbH, Hamburg, Germany
V. Kocharyan, E. Saldin
DESY, Hamburg, Germany

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. Within the photon energy range 60-65 eV, the correlation index of the Fano profiles for the photo-ionization spectra in He 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 existing gas attenuator systems for VUV or X-ray FELs. We present feasibility study and exemplifications for the FLASH facility in the VUV regime.

MOPB29

Generation of Doublet Spectral Lines at Self-seeded X-ray FELs

77

G. Geloni
European XFEL GmbH, Hamburg, Germany
V. Kocharyan, E. Saldin
DESY, Hamburg, Germany

We propose to extend our recently proposed single-crystal monochromatization setup to the case when two or more crystals are arranged in a series to spectrally filter the SASE radiation at two or more closely-spaced wavelengths within the FEL gain band. This allows for the production of doublet or multiplet spectral lines. We present simulation results for the LCLS baseline operating at two closely spaced wavelengths. We show that we can produce fully coherent radiation shared between two longitudinal modes. Mode spacing can be easily tuned within the FEL gain band. The proposed scheme allows for a modulation of the electron bunch at optical frequencies without a seed quantum laser. In fact, the XFEL output intensity contains an oscillating "mode-beat" component whose frequency is related to the frequency difference between the pair of longitudinal modes considered. At saturation one obtains FEL-induced optical modulations of energy loss and energy spread in the electron bunch, which can be converted into density modulation with a weak chicane behind the baseline undulator. Powerful coherent radiation, synchronized with the X-ray pulses, can then be generated with an OTR station.

MOPB30

The Effects of Betatron Motion on the Preservation of FEL Microbunching

81

G. Geloni
European XFEL GmbH, Hamburg, Germany
V. Kocharyan, E. Saldin
DESY, Hamburg, Germany

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 ofμbunching 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.

MOPB31

Self-seeding Scheme with Gas Monochromator for Narrow-Bandwidth Soft X-Ray

85

G. Geloni
European XFEL GmbH, Hamburg, Germany
V. Kocharyan, E. Saldin
DESY, Hamburg, Germany

We propose an extension of our recently-proposed single-crystal self-seeding scheme 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, whose power is much larger than the equivalent shot noise power in the electron bunch. 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: a 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.

TUOAI1

Hard X-ray Self-seeding for XFELs: Towards Coherent FEL Pulses

148

G. Geloni
European XFEL GmbH, Hamburg, Germany
V. Kocharyan, E. Saldin
DESY, Hamburg, Germany

Start-up from shot noise limits the longitudinal coherence of typical SASE XFEL pulses. Self-seeding schemes provide an elegant solution to this problem. However, their applicability to the baseline of already working or designed XFELs is subject to constraints, including minimal changes to the baseline design and possibility to recover the baseline mode of operation. Here we discuss a recently proposed single-bunch self-seeding scheme for hard X-rays. The physical principles of this scheme can be extended to soft X-rays as well. The method is based on a particular kind of monochromator, which relies on the use of a single crystal in Bragg-transmission geometry. In its simplest configuration, the setup consists of an input undulator and an output undulator separated by such monochromator. Several, more advanced configurations can be considered. For example, for high repetition rates of the X-ray pulses, or when a high spectral purity of the output radiation is requested, the simplest two-undulator configuration is not optimal: three or more undulators separated by monochromators can then be used. Exemplifications, based on facilities working or under construction will be discussed.

Slides TUOAI1 [2.818 MB]

Paper	Title	Page
MOPB25	Improvement of the Crossed Undulator Design for Effective Circular Polarization	61
	G. Geloni European XFEL GmbH, Hamburg, Germany V. Kocharyan, E. Saldin DESY, Hamburg, Germany
	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. Here we propose 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.

MOPB26	Self-seeded Operation of the LCLS Hard X-ray FEL in the Long-bunch Mode of Operation	65
	G. Geloni European XFEL GmbH, Hamburg, Germany V. Kocharyan, E. Saldin DESY, Hamburg, Germany
	Self-seeding options for the LCLS baseline were recently investigated using a scheme which relies on a single-crystal monochromator in Bragg-transmission geometry. The LCLS low-charge (0.02 nC) mode of operation was considered in order to demonstrate the feasibility of the proposed scheme. The wakefield effects from the linac and from the undulator vacuum chamber are much reduced at such low charge, and can be ignored. In this paper we extend our previous investigations to the case of the LCLS mode of operation with nominal charge. Based on the LCLS start-to-end simulation for an electron beam charge of 0.25 nC, and accounting for the wakefields from the undulator vacuum chamber we demonstrate that the same simplest self-seeding system (two undulators with a single-crystal monochromator in between) is appropriate not only for short (few femtosecond) bunches, but for longer bunches too.

MOPB27	Circular Polarization Control for the LCLS Baseline in the Soft X-ray Regime	69
	G. Geloni European XFEL GmbH, Hamburg, Germany V. Kocharyan, E. Saldin DESY, Hamburg, Germany
	Several schemes have been discussed to obtain soft-polarization control in the context of the LCLS. We propose a novel method to generate 10 GW level power at the fundamental harmonic with 99% degree of circular polarization from the LCLS baseline. Its merits are low cost, simplicity and easy implementation. As in previously proposed methods, the microbunching of the planar undulator is used here as well. After the baseline undulator, the electron beam is sent through a 40 m long straight section, and subsequently through a short helical (APPLE II) radiator. The microbunching is easily preserved, and intense coherent radiation is emitted in the helical radiator. The background radiation from the baseline undulator can be suppressed by letting radiation and electrons through horizontal and vertical slits upstream the helical radiator, where the radiation spot size is about ten times larger than the electron bunch transverse size. Thin Be foils for the slits will preserve from electron losses. Other facilities e.g. LCLS II or the European XFEL may benefit from this work as well, due to availability of sufficiently long free space at the end of undulator tunnel.

MOPB28	Gas-filled Cell as a Narrow Bandwidth Bandpass Filter in the VUV Wavelength Range	73
	G. Geloni European XFEL GmbH, Hamburg, Germany V. Kocharyan, E. Saldin DESY, Hamburg, Germany
	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. Within the photon energy range 60-65 eV, the correlation index of the Fano profiles for the photo-ionization spectra in He 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 existing gas attenuator systems for VUV or X-ray FELs. We present feasibility study and exemplifications for the FLASH facility in the VUV regime.

MOPB29	Generation of Doublet Spectral Lines at Self-seeded X-ray FELs	77
	G. Geloni European XFEL GmbH, Hamburg, Germany V. Kocharyan, E. Saldin DESY, Hamburg, Germany
	We propose to extend our recently proposed single-crystal monochromatization setup to the case when two or more crystals are arranged in a series to spectrally filter the SASE radiation at two or more closely-spaced wavelengths within the FEL gain band. This allows for the production of doublet or multiplet spectral lines. We present simulation results for the LCLS baseline operating at two closely spaced wavelengths. We show that we can produce fully coherent radiation shared between two longitudinal modes. Mode spacing can be easily tuned within the FEL gain band. The proposed scheme allows for a modulation of the electron bunch at optical frequencies without a seed quantum laser. In fact, the XFEL output intensity contains an oscillating "mode-beat" component whose frequency is related to the frequency difference between the pair of longitudinal modes considered. At saturation one obtains FEL-induced optical modulations of energy loss and energy spread in the electron bunch, which can be converted into density modulation with a weak chicane behind the baseline undulator. Powerful coherent radiation, synchronized with the X-ray pulses, can then be generated with an OTR station.

MOPB30	The Effects of Betatron Motion on the Preservation of FEL Microbunching	81
	G. Geloni European XFEL GmbH, Hamburg, Germany V. Kocharyan, E. Saldin DESY, Hamburg, Germany
	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 ofμbunching 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.

MOPB31	Self-seeding Scheme with Gas Monochromator for Narrow-Bandwidth Soft X-Ray	85
	G. Geloni European XFEL GmbH, Hamburg, Germany V. Kocharyan, E. Saldin DESY, Hamburg, Germany
	We propose an extension of our recently-proposed single-crystal self-seeding scheme 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, whose power is much larger than the equivalent shot noise power in the electron bunch. 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: a 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.

TUOAI1	Hard X-ray Self-seeding for XFELs: Towards Coherent FEL Pulses	148
	G. Geloni European XFEL GmbH, Hamburg, Germany V. Kocharyan, E. Saldin DESY, Hamburg, Germany
	Start-up from shot noise limits the longitudinal coherence of typical SASE XFEL pulses. Self-seeding schemes provide an elegant solution to this problem. However, their applicability to the baseline of already working or designed XFELs is subject to constraints, including minimal changes to the baseline design and possibility to recover the baseline mode of operation. Here we discuss a recently proposed single-bunch self-seeding scheme for hard X-rays. The physical principles of this scheme can be extended to soft X-rays as well. The method is based on a particular kind of monochromator, which relies on the use of a single crystal in Bragg-transmission geometry. In its simplest configuration, the setup consists of an input undulator and an output undulator separated by such monochromator. Several, more advanced configurations can be considered. For example, for high repetition rates of the X-ray pulses, or when a high spectral purity of the output radiation is requested, the simplest two-undulator configuration is not optimal: three or more undulators separated by monochromators can then be used. Exemplifications, based on facilities working or under construction will be discussed.
	Slides TUOAI1 [2.818 MB]