Haut PDF Proposal for a Raman X-ray Free Electron Laser

Proposal for a Raman X-ray Free Electron Laser

Proposal for a Raman X-ray Free Electron Laser

4.2 Effect of electron velocity mismatch The energy dispersion of the incident electron bunch is a major concern for X-ray free electron lasers. In partic- ular, all the simulations on the FEL effect with optical undulators, in the Compton regime, demonstrate that a remarkable value of mono-energeticity is required, typi- cally of the order of 10 −4 [15] to few 10 −4 for electron energies of few tens of MeV [16]. Indeed, in the Compton regime, amplification occurs throughout the laser undu- lator length only if δγ/γ < 1/2N , N being the number of undulator periods over the whole amplification length [33]. The Doppler frequency shift is therefore limited to the emission linewidth due to the finite emission time. This very stringent condition on the electron energy dis- persion is obviously one of the major reasons why this op- tical undulator scheme has not been demonstrated up to now. How the proposed Raman scheme for a X-ray FEL copes with the electron energy dispersion is therefore a major issue; however, a detailed study of Raman ampli- fication with a spread of electron energies is beyond the scope of the present study, leading us to restrict ourselves to discuss the spectral broadening induced the electron energy spread, and the amplification regime between a monochromatic X-ray field, and an out-of-resonance elec- tron population.
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X-ray amplification from a Raman Free Electron Laser

X-ray amplification from a Raman Free Electron Laser

FIG. 1. X-ray Raman scattering geometry in a reference sys- tem moving with the electron bunch. A conceptually different new scheme considers a rel- ativistic electron bunch injected into the overlap region between two transversally incident, counter-propagating intense lasers beams [ 9 ]. The setup is depicted in Fig. 1, directly in the reference frame of the electron bunch. The interference between the laser beams forms an optical lattice, and induces a spatially corrugated ponderomo- tive potential for the incident electrons that is trapping them transversely. The electron dynamics then consists of high frequency oscillations induced by the two lasers along the laser polarization direction, and of low fre- quency oscillations along the interference direction, simi- lar to betatron oscillations, with a characteristic angular frequency Ω. Light is hence scattered at the betatron fre- quency, and on the Stokes and anti-Stokes lines around the laser frequency. In the laboratory frame, this scat- tering is Doppler up-shifted by 2γ 2 , where γ is the elec- tron Lorentz factor. Scattering is spontaneous as long as the electron motions are uncorrelated; however, elec- trons may also exhibit a collective low-frequency oscilla- tory behaviour, so that we can expect a stimulated Ra- man instability and coherent emission of X-ray radiation in the forward direction. This Raman-type scattering should be distinguished from the known Raman instabili- ties in conventional long wavelength Free Electron Lasers, where the system oscillations are the Langmuir plasma waves [ 10 ]. This new Raman instability dominates if the bounce frequency Ω is greater than the electron beam plasma frequency ω p .
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Compton and Raman free electron laser stability properties for a cold electron beam propagating through a helical magnetic wiggler

Compton and Raman free electron laser stability properties for a cold electron beam propagating through a helical magnetic wiggler

From Figs. VI, the CDR growth rate curve provides an adequate approximation to that of the FDR only over the interval of k extending from the downshifted peak to somewhat b[r]

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Compton and Raman free electron laser stability properties for a warm electron beam propagating through a helical magnetic wiggler

Compton and Raman free electron laser stability properties for a warm electron beam propagating through a helical magnetic wiggler

Results of these comparisons indicate that the validity condition for the Compton approx- imation developed in §9 are also applicable to the Gaussian dispersion rel[r]

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Modeling of the interaction of an x-ray free-electron laser with large finite samples

Modeling of the interaction of an x-ray free-electron laser with large finite samples

collisional secondary processes coupling different atomic configurations, are involved during and after the x-ray pulse. This specific aspect as well as the completness of the configura- tion list is discussed elsewhere [7, 8]. From the deposited energy, electronic temperature is obtained consistently with the population kinetics through a linearization procedure. The model depends on external physical constants such that shear modulus, yield strength, ther- mal conduction coefficient and electron-ion equilibration time. Throughout a simulation these parameters are needed for a broad range of temperature and density. This point can be a problem to address in independent studies. The computer tool described here allows one to imagine pump-probe experiments where the dynamics of an adequately pumped sam- ple can be probed, then modeled and fitted as a function of rather unknown parameters in the WDM regime (electron-ion collision frequency, conductivity, shear modulus, etc). The values chosen or given to these parameters are a way of testing microscopic theories or ab- initio derived thermophysical properties [9–11]. Furthermore, the development of such a code could be of crucial help for optimizing the design of optics for x-ray radiation sources. Most of the current beamlines use flat mirrors to steer the beam, and Kirkpatrick-Baez mirrors to achieve high focusing. X-ray mirrors are made of a substrate (most of the time Si, or SiO2) coated with a thin layer (between 50 to 100 nm) of a reflective element. Other solutions like Fresnel zone plates [12], or Berylium lens [13] are also used to focus the beam. These latter ones are used at normal incidence angle, while mirrors are set up at very low grazing incidence. To sum up, this article is of interest for both fundamental studies of X-ray interaction with matter and applied studies important for X-ray optics.
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Megahertz data collection from protein microcrystals at an X-ray free-electron laser

Megahertz data collection from protein microcrystals at an X-ray free-electron laser

formative capabilities, demand for beam time at XFELs is very high. For this reason, MHz repetition rate XFELs have been awaited eagerly, since they can deliver X-ray pulses with an up to ~10,000-fold higher maximum repetition rate than the first hard X-ray FEL that came online in 2009 11 . An increase in pulse rate is expected to speed up data collection, thereby accommodating more users and allowing the collection of enough data to study systems with very weak signals. Also, high pulse rates make far better use of the often highly valuable samples that are generally delivered continuously into the X-ray beam by means of liquid jets, aerosols or molecular beams. However, data collection at MHz rates brings with it many new challenges including the rapid delivery of samples to present fresh material for each pulse, and the development of high frame rate detectors, allowing fast data acquisition and storage 12 .
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Two-colour generation in a chirped seeded free-electron laser: a close look

Two-colour generation in a chirped seeded free-electron laser: a close look

In this paper, we characterize the performance of a seeded FEL where frequency chirping on the seed laser is instead exploited to generate two independent pulses spectrally and temporally separated in an externally controllable fashion. The generation of multiple pulses of different colours in FEL’s is opening new opportunities for pump and probe experiments in spectral regions ranging from vacuum-ultraviolet to hard X-rays with unprecedented brightness. Re- cently, the LCLS group has reported the generation of pairs of temporally and spectrally sep- arated soft X-ray FEL pulses via a double undulator scheme [ 9 ]. Similar studies are ongoing at SPARC [ 10 ], where two distinct electron bunches at different energies generate independent pulses of radiation. At FERMI@Elettra, multiple pulses have also been obtained recently with a different technique, based on the simultaneous injection of two different seed pulses on the same electron bunch [ 11 ]. In this case the two pulses minimum separation is limited by the seed pulse length. The method we propose here allows to reduce this distance by using a single seed pulse and exploiting saturation to generate the pulse splitting [ 12 , 13 ].
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Electron bunch diagnostics for laser-plasma accelerators, from THz to X-rays

Electron bunch diagnostics for laser-plasma accelerators, from THz to X-rays

3.2 Off-axis colliding pulse injection Decoupling injection from acceleration is a key challenge to achieving compact, reli- able, tunable laser-plasma accelerators (LPAs) [9, 12]. Although capillary-guided LPAs have demonstrated high-quality electron beams at 1 GeV [10] with 2.5% r.m.s. energy spread, most of present LPAs [39, 41, 40] still rely on transverse wavebreaking effects [19] of highly nonlinear waves [57] to inject electrons into the accelerating phase of the electron density wave. In this scheme, injection and acceleration are coupled, limiting control of the acceleration structure which is essential for LPAs’ applications such as free-electron lasers [127], THz [77, 70] and X-ray radiation sources [84, 85, 128]. Elec- trons injected at different longitudinal positions behind the laser pulse, i.e., different phases of the periodic structure of the accelerating wakefield, experience different elec- tric fields, due for example to beam loading effects (Sec. 2.6), which can lead to a large energy spread. Several methods to control trapping of the electrons have been pro- posed and demonstrated: external injection of an electron beam from a conventional accelerator [16, 17, 18], triggering injection in plasma density gradients with density decreasing in the laser propagation direction [63, 2], and using additional laser pulses [20, 66, 129, 22, 67].
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Single-shot time-resolved magnetic x-ray absorption at a free-electron laser

Single-shot time-resolved magnetic x-ray absorption at a free-electron laser

Further development of the employed zone plate optic holds significant potential to further increase the capabilities of this novel technique. First of all, improving the manufactur- ing process will reduce the artifacts currently hampering the achievable signal quality. Indeed, we are now able to optimize the fabrication conditions in a way to suppress stitching arti- facts almost completely. In addition, the fabrication of off-axis zone plates with finer structures will enable to extend the time window. For example, the current capabilities to manufacture, in a reasonable amount of time, efficient off-axis zone plate optics for time-streaking experiments in the EUV will allow us to access time windows up to 3.3 ps at the iron M edge (55 eV). Last but not least, more complex zone plate schemes are currently designed to extend the scientific possibilities. Specifically, we plan to fabricate off-axis zone plates with (i) two focus spots to have a beam copy transmitted through an unpumped region of the sample and projected onto the same camera for a reliable shot-to-shot reference with the same magnification, and (ii) a segmentation of our zone plate patterns to probe several absorption edges simultaneously.
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Ultrafast myoglobin structural dynamics observed with an X-ray free-electron laser.

Ultrafast myoglobin structural dynamics observed with an X-ray free-electron laser.

visible pulses. The timing tool data show that over the B6 h of data collection the X-ray/laser timing has been stable to about B150 fs and that the jitter was about 200 fs. The time resolution of the experiment is also affected by the group velocity mismatch between the X-ray and optical pulses propagating inside the 300 mm thick protein solution. By taking into account both effects and the duration of photolysis pulses (B250 fs), we estimate an overall time resolution of B500 fs. Calculation of scattering difference patterns . Two-dimensional scattering images recorded with a Rayonix SX165 detector were azimuthally averaged to give one-dimensional scattering patterns. The scattering angle y was converted to momentum transfer q using the formula q ¼ 4p/lsin(y/2), where l ¼ 1.377 Å is the X-rays wavelength. Since the changes induced in the scattering patterns by optical photoexcitation account for less than a percent of the absolute signal, data were normalized 30 at 1.4±0.1 Å  1 before calculating difference patterns
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Interferometry for full temporal reconstruction of laser-plasma accelerator-based seeded free electron lasers

Interferometry for full temporal reconstruction of laser-plasma accelerator-based seeded free electron lasers

1. Introduction Free Electron Lasers (FELs) [ 1 , 2 ] deliver ultrashort, narrow-band and ultrabright pulses down to the hard x-ray range [ 3 – 5 ], enabling breakthrough experiments in chemical, physical, and biological sciences. These light sources rely on relativistic electron beams wiggling in the periodic magnetic field of an undulator as gain medium. Interacting with the spontaneous radiation of the undulator or an external seed, the electrons experience an energy modulation at the resonance wavelength which is further transformed into a density modulation by dispersive elements. After this ‘lethargy’ [ 6 ] phase, the beam density modulation allows the emission of a coherent radiation which can then be exponentially amplified. A saturation is reached when the electrons energy loss is such that the resonance condition is violated, causing a red spectral shift of the FEL line [ 7 , 8 ].
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1-kHz table-top ultrashort hard x-ray source for time-resolved x-ray protein

1-kHz table-top ultrashort hard x-ray source for time-resolved x-ray protein

The x-ray source consists of a copper wire running through a small vacuum chamber (see Fig. 2). The wire, issued from a spool, is first flattened and moved by a motorized rolling mill to expose a flat and fresh surface to each laser shot. It then crosses the cham- ber through two Teflon guides that maintain an air pressure of 100 Pa and is guided by two free-rotating bearings. The wire tension is controlled by a pair of toothed wheels that pulled the wire out from the chamber. The jitter of the wire motion is still reduced by a small metallic finger positioned between the bearings. The laser beam is focused with an 18 cm focal-length lens and hits the wire with an incidence angle of 60°. The laser intensity on the target is esti- mated to 3 ⫻ 10 16 W / cm 2 , which is thought to be op-
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Nondipole effects in helium ionization by intense soft x-ray laser pulses

Nondipole effects in helium ionization by intense soft x-ray laser pulses

tion from 1s. The situation is quite different at I = 10 19 W/cm 2 (left figure). Now the contributions of TDSE-DA- A 2 and TDSE-AP are of the same order of magnitude. The TDSE-AP contribution has increased linearly with the in- tensity I, in agreement with the one-photon absorption scheme explained above. In parallel with the resolution of the TDSE-AP we have performed time-dependent calcu- lations where H (1) RET is treated in first-order PT, they are in good agreement with the TDSE-AP PPS (l = 2, m = 1) for both intensities, this confirms that it is a one-photon transition in the perturbative regime. If we consider now the TDSE-DA-A 2 contribution, we have identified two pathways involving one- and two-photon resonances. First a one-photon dipole coupling from 1s to the channel (l = 1, m = 0), followed by a Compton-like two-photon tran- sition from the channel (l = 1, m = 0) to the PPS (l = 2, m = ±1) around the one-photon resonance (at energy ∼ 19.1 a.u.). The latter transition is induced by the nondipole coupling term H RET (2) (associated with A 2 ) according to the selection rule (∆l = ±1; ∆m ± 1), it is a two-photon absorption-emission process. The second process relies on a direct two-photon coupling (through H (2) RET ) from 1s to the channel (l = 1, m = ±1) at energy ∼ 39.1 a.u., fol- lowed by a dipole transition to the channel (l = 2, m = ±1) at energy ∼ 19.1 a.u. The TDSE does not allow to discriminate between these two pathways, but the second one involves a continuum-continuum dipole coupling with high energy continua (in the region of peak 1 and peak 2), this type of coupling is usually very small and we can therefore surmise that the first pathway dominates. Both
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Addition of X-ray fluorescent tracers into polymers, new technology for automatic sorting of plastics : proposal for selecting some relevant tracers

Addition of X-ray fluorescent tracers into polymers, new technology for automatic sorting of plastics : proposal for selecting some relevant tracers

3. Tracing: a new concept for plastic identification and automatic sorting As shown previously, the existing technologies of sorting do not provide the versatility or flexibility needed for separating the dark plastics into monopolymeric fractions which are the essential pre- requisite materials for any efficient recycling process. In the end of 1990s, Simmons et al. (1998) and Ahmad (2000) proposed a new concept on identification of plastics by marking them with a binary combination of fluorescent tracers detectable by UV (ultra- violet) spectroscopy. The use of a tracer system could provide high purity of the sorted materials, separation by polymer grade as well as polymer type, separation by additive system, high speed posi- tive identification and high speed sorting. This project, founded by the European Economic Community, was focused on the sorting of rigid plastics from packaging household waste for demonstrating the concept. They concluded that the speed and purity of sort- ing were limited by the mechanical singulation inadequacy of the conveyor system at high speed for the clear plastics and that the presence of pigments reduced the fluorescence yield. In the case of black pigments, the reduction was too drastic to allow identifi- cation. Aside from these limitations, UV spectroscopy is a surface detection method and this may imply a “clean” surface for tracer identification. The use of “tags” for plastic identification by UV/Vis spectroscopy has also been studied by Corbett et al. (1994) . They showed that the addition of phosphor luminescent “tags” to dif- ferent sort of polymers is viable. However, the detection can be disturbed by the stability of the organic “tags” during the repro- cessing of polymers and contaminants, which may be luminescent. The specialized companies in magnetic sorting, as Eriez ( Mankosa and Luttrell, 2005 ), also proposed a magnetic sorting pro- cess of polymers in which a magnetic substance was dispersed. The main advantage of the magnetic detection was the lack of sensitiv- ity with respect to the additives contained in polymers. However, the magnetic tracer system provided only binary separation and required high tracer amount.
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The X-ray Integral Field Unit (X-IFU) for Athena

The X-ray Integral Field Unit (X-IFU) for Athena

to meet the background requirements stated in Tab. 1. In addition to the instrumental component, the background includes an X-ray diffuse component of various origins. A diffuse X-ray emission observed in every directions is produced at high energies mostly by the unresolved emission of AGNs, and below 1 keV by line emission from hot diffuse gas in the galactic halo and the local hot bubble, with contributions from Solar Wind Charge Exchange in the 3/4 keV band. This latter component is highly variable, and a model representative of typical high galactic latitude fields had to be defined, as well as a study of its variations with time and pointing direction. Details on our modeling of these components are given in Ref. 35. The total background for X-IFU in the focal plane of Athena is plotted in Fig. 8 (right), showing that the particle background dominates only for energies above 2-3 keV.
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Enhanced X-ray emission arising from laser-plasma confinement by a strong transverse magnetic field

Enhanced X-ray emission arising from laser-plasma confinement by a strong transverse magnetic field

Discussion In summary, our experiment demonstrates the ability to strongly impact the dynamics of a laser-driven plasma flow from a solid target by applying an external transverse magnetic field. In particular, we have shown that the magnetic field leads to a reduction of the overall plasma flow; and with a more precise analysis, we reveal the increased separation of the plasma into two components: a dense, slow one, increasingly confined against the target as the magnetic field increases, and a low-density, fast one, which is redirected on axis by the magnetic field. The compression applied onto the plasma by the magnetic field in the plane transverse to the latter (the XY plane here) induces an increase of the local plasma density and temperature, both of which drive an increase in the plasma X-ray emission. We have shown here results obtained with given laser parameters, but in the Appendix, one can also see that a similar conclusion can be reached in another (lower) intensity regime. Our observations of such increase of X-ray emission (of an hohlraum wall) could constitute additional interest for magnetized ICF.
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Exploring phase contrast imaging with a laser-based K α x-ray source up to relativistic laser intensity

Exploring phase contrast imaging with a laser-based K α x-ray source up to relativistic laser intensity

The interest of developing new x-ray sources and/or improving their performances in terms of brightness, sta- bility, and compactness is still growing since decades. This is strongly motivated by applications of x-ray sources for imaging and related applied developments to biology, medicine and material science. In particular, the advent of synchrotron radiation sources in the seventies as well as the development of optical components for x-rays, definitively allowed to transfer the phase contrast imaging (PCI) techniques from the visible spectral range to the x-ray one. Phase contrast x-ray imaging is sensitive to phase shift induced by an object placed in the x-ray path and does not rely on its absorption. Thus, it can image weakly absorbing materials, such as carbon-based materi- als and biological objects. In addition, it should be noted that the sensitivity of absorption contrast decreases as the photon energy (E) increases 1 as E –3 , whereas that of phase contrast methods decreases only as E –2 . Therefore,
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Electron Rephasing in a Laser-Wakefield Accelerator

Electron Rephasing in a Laser-Wakefield Accelerator

DOI: 10.1103/PhysRevLett.115.155002 PACS numbers: 52.38.Kd, 41.75.Jv, 52.38.Ph Laser-wakefield accelerators allow the production of relativistic electron beams over a short acceleration dis- tance (millimeter to centimeter scale) by focusing a high- intensity laser pulse in an underdense plasma [1–3] . The maximum attainable energy is limited by three processes: laser pulse depletion, laser defocusing, and dephasing. Each of theses processes occurs after a characteristic propagation length and the final electron energy is deter- mined by the process that sets in first. First, the depletion length is the distance over which the laser pulse transfers most of its energy to the wakefield and subsequently cannot sustain the wakefield any further. Increasing the energy transfer in a depletion-limited accelerator would require increasing the laser energy [4,5] . Second, diffraction of the laser during propagation will reduce the intensity. This effect is generally mitigated by self-focusing. However, self-focusing is not efficient over an arbitrarily long distance because the laser power decreases during the propagation, due to pump depletion, eventually becoming smaller than the critical power for self-focusing. Therefore, accelerating the electron beam over long lengths requires plasma waveguides [6,7] . Pump depletion and defocusing determine the distance over which the wakefield structure can be maintained. Yet, the excitation of a wakefield is not sufficient to guarantee that the electron beam is accelerated, because of dephasing. Actually, as the laser group velocity and thus the wake velocity are smaller than the electron beam velocity, the electron beam outruns the plasma wave during the acceleration and reaches a phase of the wake where the field is decelerating. This effect is an important limiting factor in a considerable range of experimental conditions.
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The X-ray universe

The X-ray universe

This publication could be one of several versions: author’s original, accepted manuscript or the publisher’s version. / La version de cette publication peut être l’une des suivantes : la version prépublication de l’auteur, la version acceptée du manuscrit ou la version de l’éditeur. For the publisher’s version, please access the DOI link below./ Pour consulter la version de l’éditeur, utilisez le lien DOI ci-dessous.

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Effect of X-ray irradiation on ancient DNA in sub-fossil bones - Guidelines for safe X-ray imaging

Effect of X-ray irradiation on ancient DNA in sub-fossil bones - Guidelines for safe X-ray imaging

48. Jonsson, H., Ginolhac, A., Schubert, M., Johnson, P. L. & Orlando, L. mapDamage2.0: fast approximate Bayesian estimates of ancient DNA damage parameters. Bioinformatics 29, 1682–1684 (2013). Acknowledgements We thank the ESRF BM05 beamline to have provided access to beamtime, as well as Thierry Brochard for his help with the dosimetry experiments. The computational work was performed on the computational resource bwGRiD Cluster Tübingen funded by the Ministry of Science, Research and the Arts Baden-Württemberg and the Universities of the State of Baden-Württemberg, Germany, within the framework program bwHPC. We want to thank Marek Dynowski for providing access to the computational resource bwGRiD Cluster Tübingen and his technical support. We are grateful to Smiths Heimann GmbH, Andreas Frank, Arno Folkerts and Christian Rauth (Wiesbaden, Germany) for sharing information about inspection devices in airports, as well as to Anne Bonnin (Paul Scherrer Institut, Switzerland). We want to acknowledge Morten Allentoft for his help and advice on applying the λ- exponential fragment length decline model to our data. This research was partially funded by the European Research Council Starting Grant APGREID (to A.I., A.H., K.B. and J.K.). We want to thank James Fellows Yates for proofreading the manuscript and improving the language. We thank Cosimo Posth and Maria Spyrou for their helpful comments. We also wish to acknowledge Michel Toussaint for providing access to the Engis 2 Neandertal material. We are grateful to Svante Pääbo for his help and support at an earlier stage of this project.
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