46 research outputs found
SCHEME FOR GENERATING AND TRANSPORTING THZ RADIATION TO THE X-RAY EXPERIMENTAL HALL AT THE EUROPEAN XFEL
Abstract We consider generation of THz radiation from the spent electron beam downstream of the SASE2 undulator in the electron beam dump area. 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
Cooperative effects in nuclear excitation with coherent x-ray light
The interaction between super-intense coherent x-ray light and nuclei is
studied theoretically. One of the main difficulties with driving nuclear
transitions arises from the very narrow nuclear excited state widths which
limit the coupling between laser and nuclei. In the context of direct
laser-nucleus interaction, we consider the nuclear width broadening that occurs
when in solid targets, the excitation caused by a single photon is shared by a
large number of nuclei, forming a collective excited state. Our results show
that for certain isotopes, cooperative effects may lead to an enhancement of
the nuclear excited state population by almost two orders of magnitude.
Additionally, an update of previous estimates for nuclear excited state
population and signal photons taking into account the experimental advances of
the x-ray coherent light sources is given. The presented values are an
improvement by orders of magnitude and are encouraging for the future prospects
of nuclear quantum optics.Comment: 22 pages, 4 figures, 5 tables; updated to the published version, one
additional results tabl
Scheme for generation of highly monochromatic X-rays from a baseline XFEL undulator
One goal of XFEL facilities is the production of narrow bandwidth X-ray
radiation. The self-seeding scheme was proposed to obtain a bandwidth narrower
than that achievable with conventional X-ray SASE FELs. A self-seeded FEL is
composed of two undulators separated by a monochromator and an electron beam
bypass that must compensate for the path delay of X-rays in the monochromator.
This leads to a long bypass, with a length in the order of 40-60 m, which
requires modifications of the baseline undulator configuration. As an attempt
to get around this obstacle, together with a study of the self-seeding scheme
for the European XFEL, here we propose a novel technique based on a pulse
doubler concept. Using a crystal monochromator installed within a short
magnetic chicane in the baseline undulator, it is possible to decrease the
bandwidth of the radiation well beyond the XFEL design down to 10^(-5). The
magnetic chicane can be installed without any perturbation of the XFEL focusing
structure, and does not interfere with the baseline mode of operation. We
present a feasibility study and we make exemplifications with the parameters of
the SASE2 line of the European XFEL
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
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
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.Comment: 24 pages, no figure
A simple method for controlling the line width of SASE X-ray FELs
This paper describes a novel single-bunch self-seeding scheme for generating
highly monochromatic X-rays from a baseline XFEL undulator. Previous schemes
made use of a four-crystal fixed-exit monochromator in Bragg geometry. In such
monochromator the X-ray pulse acquires a cm-long path delay, which must be
compensated. For a single-bunch self-seeding scheme this requires a long
electron beam bypass, implying modifications of the baseline undulator
configuration. To avoid this problem, a double bunch self-seeding scheme based
on a special photoinjector setup was recently proposed. At variance, here we
propose a new time-domain method of monochromatization exploiting a single
crystal in the transmission direction, thus avoiding the problem of extra-path
delay for the X-ray pulse. The method can be realized using a temporal
windowing technique, requiring a magnetic delay for the electron bunch only.
The proposed setup is extremely simple and composed of as few as two simple
elements. These are the crystal and the short magnetic chicane, which
accomplishes three tasks by itself. It creates an offset for crystal
installation, it removes the electron micro-bunching produced in the first
undulator, and it acts as a delay line for temporal windowing. Using a single
crystal installed within a short magnetic chicane in the baseline undulator, it
is possible to decrease the bandwidth of the radiation well beyond the XFEL
design down to 10E-5. The installation of the magnetic chicane does not perturb
the undulator focusing system and does not interfere with the baseline mode of
operation. We present feasibility study and exemplifications for the SASE2 line
of the European XFEL
Cascade self-seeding scheme with wake monochromator for narrow-bandwidth X-ray FELs
Three different approaches have been proposed so far for production of highly
monochromatic X-rays from a baseline XFEL undulator: (i) single-bunch
self-seeding scheme with a four crystal monochromator in Bragg reflection
geometry; (ii) double-bunch self-seeding scheme with a four-crystal
monochromator in Bragg reflection geometry; (iii) single-bunch self-seeding
scheme with a wake monochromator. A unique element of the X-ray optical design
of the last scheme is the monochromatization of X-rays using a single crystal
in Bragg-transmission geometry. A great advantage of this method is that the
monochromator introduces no path delay of X-rays. This fact eliminates the need
for a long electron beam bypass, or for the creation of two precisely
separated, identical electron bunches, as required in the other two
self-seeding schemes. In its simplest configuration, the self-seeded XFEL
consists of an input undulator and an output undulator separated by a
monochromator. In some experimental situations this simplest two-undulator
configuration is not optimal. The obvious and technically possible extension is
to use a setup with three or more undulators separated by monochromators. This
amplification-monochromatization cascade scheme is distinguished, in
performance, by a small heat-loading of crystals and a high spectral purity of
the output radiation. This paper describes such cascade self-seeding scheme
with wake monochromators. We present feasibility study and exemplifications for
the SASE2 line of the European XFEL
The potential for extending the spectral range accessible to the European XFEL down to 0.05 nm
Specifications of the European XFEL cover a range of wavelengths down to 0.1
nm. The baseline design of the European XFEL assumes standard (SASE) FEL mode
for production of radiation i.e. only one photon beam at one fixed wavelength
from each baseline undulator with tunable gap. Recent developments in the field
of FEL physics and technology form a reliable basis for an extensions of the
mode of operation of XFEL facilities. This paper explores how the wavelength of
the output radiation can be decreased well beyond the European XFEL design,
down to 0.05 nm. In the proposed scheme, which is based on the use "fresh
bunch" technique, simultaneous operation at two different wavelengths possible.
It is shown that one can generate simultaneously, in the same baseline
undulator with tunable gap, high intensity radiation at 0.05 nm at saturation,
and high intensity radiation around 0.15 nm according to design specifications.
We present a feasibility study and we make exemplifications with the parameters
of SASE2 line of the European XFEL