Extreme X-ray Research Articles

Necklace-structured high-harmonic generation for low-divergence, soft x-ray harmonic combs with tunable line spacing.

The extreme nonlinear optical process of high-harmonic generation (HHG) makes it possible to map the properties of a laser beam onto a radiating electron wave function and, in turn, onto the emitted x-ray light. Bright HHG beams typically emerge from a longitudinal phased distribution of atomic-scale quantum antennae. Here, we form a transverse necklace-shaped phased array of linearly polarized HHG emitters, where orbital angular momentum conservation allows us to tune the line spacing and divergence properties of extreme ultraviolet and soft x-ray high-harmonic combs. The on-axis HHG emission has extremely low divergence, well below that obtained when using Gaussian driving beams, which further decreases with harmonic order. This work provides a new degree of freedom for the design of harmonic combs—particularly in the soft x-ray regime, where very limited options are available. Such harmonic beams can enable more sensitive probes of the fastest correlated charge and spin dynamics in molecules, nanoparticles, and materials.

Data-driven, time-dependent modeling of pre-eruptive coronal magnetic field configuration at the periphery of NOAA AR 11726

Context. Data-driven, time-dependent magnetofrictional modeling has proved to be an efficient tool for studying the pre-eruptive build-up of energy for solar eruptions, and sometimes even the ejection of coronal flux ropes during eruptions. However, previous modeling works have illustrated the sensitivity of the results on the data-driven boundary condition, as well as the difficulty in modeling the ejections with proper time scales. Aims. We aim to study the pre- and post-eruptive evolution of a weak coronal mass ejection producing eruption at the periphery of isolated NOAA active region (AR) 11726 using a data-driven, time-dependent magnetofrictional simulation, and aim to illustrate the strengths and weaknesses of our simulation approach. Methods. We used state-of-the-art data processing and electric field inversion methods to provide the data-driven boundary condition for the simulation. We analyzed the field-line evolution, magnetic connectivity, twist, as well as the energy and helicity budgets in the simulation to study the pre- and post-eruptive magnetic field evolution of the observed eruption from AR11726. Results. We find the simulation to produce a pre-eruptive flux rope system consistent with several features in the extreme ultraviolet and X-ray observations of the eruption, but the simulation largely fails to reproduce the ejection of the flux rope. We find the flux rope formation to be likely driven by the photospheric vorticity at one of the footpoints, although reconnection at a coronal null-point may also feed poloidal flux to the flux rope. The accurate determination of the non-inductive (curl-free) component of the photospheric electric field boundary condition is found to be essential for producing the flux rope in the simulation. Conclusions. Our results illustrate the applicability of the data-driven, time-dependent magnetofrictional simulations in modeling the pre-eruptive evolution and formation process of a flux rope system, but they indicate that the modeling output becomes problematic for the post-eruptive times. For the studied event, the flux rope also constituted only a small part of the related active region.

Light-curve Evolution of the Nearest Tidal Disruption Event: A Late-time, Radio-only Flare

Abstract Tidal disruption events (TDEs) occur when a star passes close enough to a galaxy’s supermassive black hole to be disrupted by tidal forces. We discuss new observations of IGRJ12580+0134, a TDE observed in NGC 4845 (d = 17 Mpc) in 2010 November, with the Karl G. Jansky Very Large Array (VLA 9 9 The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc. ). We also discuss a reanalysis of 2010–2011 Swift and XMM-Newton observations, as well as new, late-time Swift observations. Our JVLA observations show a decay of the nuclear radio flux until 2015, when a plateau was seen, and then a significant (factor ∼3) radio flare during 2016. The 2016 radio flare was also accompanied by radio spectral changes, but was not seen in the X-rays. We model the flare as resulting from the interaction of the nuclear jet with a cloud in the interstellar medium. This is distinct from late-time X-ray flares in a few other TDEs where changes in the accretion state and/or a fallback event were suggested, neither of which appears possible in this case. Our reanalysis of the Swift and XMM-Newton data from 2011 shows significant evidence for thermal emission from a disk, as well as a very soft power law. This, in addition to the extreme X-ray flux increase seen in 2010 (a factor of >100) bolsters the identification of IGRJ12580+0134 as a TDE, not an unusual active galactic nucleus variability event.

A solar coronal loop in a box: Energy generation and heating

Context. Coronal loops are the basic building block of the upper solar atmosphere as seen in the extreme UV and X-rays. Comprehending how these are energized, structured, and evolve is key to understanding stellar coronae. Aims. Here we investigate how the energy to heat the loop is generated by photospheric magneto-convection, transported into the upper atmosphere, and how the internal structure of a coronal magnetic loop forms. Methods. In a 3D magnetohydrodynamics model, we study an isolated coronal loop rooted with both footpoints in a shallow layer within the convection zone using the MURaM code. To resolve its internal structure, we limited the computational domain to a rectangular box containing a single coronal loop as a straightened magnetic flux tube. Field-aligned heat conduction, gray radiative transfer in the photosphere and chromosphere, and optically thin radiative losses in the corona were taken into account. The footpoints were allowed to interact self-consistently with the granulation surrounding them. Results. The loop is heated by a Poynting flux that is self-consistently generated through small-scale motions within individual magnetic concentrations in the photosphere. Turbulence develops in the upper layers of the atmosphere as a response to the footpoint motions. We see little sign of heating by large-scale braiding of magnetic flux tubes from different photospheric concentrations at a given footpoint. The synthesized emission, as it would be observed by the Atmospheric Imaging Assembly or the X-Ray Telescope, reveals transient bright strands that form in response to the heating events. Overall, our model roughly reproduces the properties and evolution of the plasma as observed within (the substructures of) coronal loops. Conclusions. With this model we can build a coherent picture of how the energy flux to heat the upper atmosphere is generated near the solar surface and how this process drives and governs the heating and dynamics of a coronal loop.

Application of a modified chirp-taper scheme for generation of attosecond pulses in extreme ultraviolet and soft x-ray free electron lasers

Typically, in Self-Amplified Spontaneous Emission Free Electron Laser (SASE FEL) based short-pulse schemes, pulse duration is limited by FEL coherence time. For hard X-ray FELs, coherence time is in a few hundred attosecond range while for XUV and soft X-ray FELs it is in the femtosecond regime. In this paper the modification of so-called chirp-taper scheme is developed that allows to overcome the coherence time barrier. Numerical simulations for XUV and soft X-ray FEL user facility FLASH demonstrate that one can generate a few hundred attosecond long pulses in the wavelength range 2 - 10 nm with peak power reaching hundreds of megawatts. With several thousand pulses per second this can be a unique source for attosecond science.

Single-shot experiments at the soft X-FEL FERMI using a back-side-illuminated scientific CMOS detector.

The latest Complementary Metal Oxide Semiconductor (CMOS) 2D sensors now rival the performance of state-of-the-art photon detectors for optical application, combining a high-frame-rate speed with a wide dynamic range. While the advent of high-repetition-rate hard X-ray free-electron lasers (FELs) has boosted the development of complex large-area fast CCD detectors in the extreme ultraviolet (EUV) and soft X-ray domains, scientists lacked such high-performance 2D detectors, principally due to the very poor efficiency limited by the sensor processing. Recently, a new generation of large back-side-illuminated scientific CMOS sensors (CMOS-BSI) has been developed and commercialized. One of these cost-efficient and competitive sensors, the GSENSE400BSI, has been implemented and characterized, and the proof of concept has been carried out at a synchrotron or laser-based X-ray source. In this article, we explore the feasibility of single-shot ultra-fast experiments at FEL sources operating in theEUV/soft X-ray regime with an AXIS-SXR camera equipped with the GSENSE400BSI-TVISB sensor. We illustrate the detector capabilities by performing a soft X-ray magnetic scattering experiment at the DiProi end-station of the FERMI FEL. These measurements show the possibility of integrating this camera for collecting single-shot images at the 50 Hz operation mode of FERMI with a cropped image size of 700× 700 pixels. The efficiency of the sensor at a working photon energy of 58 eV and the linearity over the large FEL intensity have been verified. Moreover, on-the-fly time-resolved single-shot X-ray resonant magnetic scattering imaging from prototype Co/Pt multilayer films has been carried out with a time collection gain of 30 compared to the classical start-and-stop acquisition method performed with the conventional CCD-BSI detector available at the end-station.

Ultra-precision machining and testing of reflector mirrors for extreme ultraviolet and X-ray

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Time-resolved X-ray and XUV based spectroscopic methods for nonadiabatic processes in photochemistry.

The photochemistry of numerous molecular systems is influenced by conical intersections (CIs). These omnipresent nonadiabatic phenomena provide ultra-fast radiationless relaxation channels by creating degeneracies between electronic states and decide over the final photoproducts. In their presence, the Born-Oppenheimer approximation breaks down, and the timescales of the electron and nuclear dynamics become comparable. Due to the ultra-fast dynamics and the complex interplay between nuclear and electronic degrees of freedom, the direct experimental observation of nonadiabatic processes close to CIs remains challenging. In this article, we give a theoretical perspective on novel spectroscopic techniques capable of observing clear signatures of CIs. We discuss methods that are based on ultra-short laser pulses in the extreme ultraviolet and X-ray regime, as their spectral and temporal resolution allow for resolving the ultra-fast dynamics near CIs.

Динамика и неравновесный ионный состав плазмы многозарядных ионов, создаваемой при взаимодействии мощного импульса лазерного излучения с цилиндрической плазменной мишенью

Based on the numerical simulation results in the framework of a nonstationary one-dimensional (1D) two-temperature (2T) radiative hydrodynamic (RHD) model, the main physical processes that determine the formation of cylindrical plasma bunches with a non-equilibrium charge containing high ionized ions at laser intensity QL≤10^14 W/cm^2 and energy EL~10 J/cm are analyzed. The results of the work are devoted to the creation of active media for lasers generating in the extreme ultraviolet (EUV) and soft X-ray (SXR) spectral ranges.

Investigation of microstructure and reflectivity of thermally annealed Mo/Be and W/Be multilayer mirrors

Periodic multilayers of Mo/Be and W/Be are important classes of mirrors operating in extreme ultraviolet and soft X-ray region. The phase and microstructural stability of these mirrors upon thermal annealing were investigated by Raman scattering and X-ray diffraction. Both the absorbers (Mo and W) and spacer (Be) layers in the periodic multilayers existed in the polycrystalline phase, which remains stable upon thermal annealing of temperature up to 573 K. The reflectivity value of the thermally annealed mirrors was fairly stable and similar as compared to as-deposited multilayer mirrors. This was explained by the phase and microstructural stability of multilayer mirrors upon thermal annealing.

Influence of shape resonances on the angular dependence of molecular photoionization delays

Characterizing time delays in molecular photoionization as a function of the ejected electron emission direction relative to the orientation of the molecule and the light polarization axis provides unprecedented insights into the attosecond dynamics induced by extreme ultraviolet or X-ray one-photon absorption, including the role of electronic correlation and continuum resonant states. Here, we report completely resolved experimental and computational angular dependence of single-photon ionization delays in NO molecules across a shape resonance, relying on synchrotron radiation and time-independent ab initio calculations. The angle-dependent time delay variations of few hundreds of attoseconds, resulting from the interference of the resonant and non-resonant contributions to the dynamics of the ejected electron, are well described using a multichannel Fano model where the time delay of the resonant component is angle-independent. Comparing these results with the same resonance computed in e-NO+ scattering highlights the connection of photoionization delays with Wigner scattering time delays.

Coronal Magnetic Field Measurements along a Partially Erupting Filament in a Solar Flare

Abstract Magnetic flux ropes are the centerpiece of solar eruptions. Direct measurements for the magnetic field of flux ropes are crucial for understanding the triggering and energy release processes, yet they remain heretofore elusive. Here we report microwave imaging spectroscopy observations of an M1.4-class solar flare that occurred on 2017 September 6, using data obtained by the Expanded Owens Valley Solar Array. This flare event is associated with a partial eruption of a twisted filament observed in Hα by the Goode Solar Telescope at the Big Bear Solar Observatory. The extreme ultraviolet (EUV) and X-ray signatures of the event are generally consistent with the standard scenario of eruptive flares, with the presence of double flare ribbons connected by a bright flare arcade. Intriguingly, this partial eruption event features a microwave counterpart, whose spatial and temporal evolution closely follow the filament seen in Hα and EUV. The spectral properties of the microwave source are consistent with nonthermal gyrosynchrotron radiation. Using spatially resolved microwave spectral analysis, we derive the magnetic field strength along the filament spine, which ranges from 600 to 1400 Gauss from its apex to the legs. The results agree well with the nonlinear force-free magnetic model extrapolated from the preflare photospheric magnetogram. We conclude that the microwave counterpart of the erupting filament is likely due to flare-accelerated electrons injected into the filament-hosting magnetic flux rope cavity following the newly reconnected magnetic field lines.

Solar Flares in Extreme Ultraviolet and X-Ray Bands Based on SDO and TIMED Satellite Data

Solar Flares in Extreme Ultraviolet and X-Ray Bands Based on SDO and TIMED Satellite Data

High-efficiency diffraction gratings for EUV and soft x-rays using spin-on-carbon underlayers

We report on the fabrication and characterization of high-resolution gratings with high efficiency in the extreme ultraviolet (EUV) and soft x-ray ranges using spin-on-carbon (SOC) underlayers. We demonstrate the fabrication of diffraction gratings down to 20 nm half-pitch (HP) on Si3N4 membranes with a bilayer of hydrogen silsesquioxane (HSQ) and spin-on-carbon and show their performance as a grating mask for extreme ultraviolet interference lithography (EUV-IL). High-resolution patterning of HSQ is possible only for thin films due to pattern collapse. The combination of this high-resolution resist with SOC circumvents this problem and enables the fabrication of high aspect ratio nanostructures. Rigorous coupled-wave analysis shows that the bilayer gratings exhibit higher diffraction efficiency than what is feasible with a grating made of HSQ. We also demonstrate a simple and accurate method to experimentally measure the diffraction efficiency of high-resolution gratings by measuring the relative ratio of the dose-to-clear curves of the photoresist. The measured diffraction efficiencies are in good agreement with the theoretically predicted values. Furthermore, we verify our calculations and measurements by printing line/space patterns in chemically amplified resists down to 10 nm HP with both HSQ and bilayer grating masks using EUV-IL. The improved diffraction efficiency of the bilayers is expected to have applications not only in gratings for interference lithography, but also in Fresnel zone plates and gratings for spectroscopy in the EUV and soft x-ray ranges.

Influence of the X-point location on edge plasma transport in the J-TEXT tokamak with a high-field-side single-null divertor

High-density experiments in the high-field-side mid-plane single-null divertor configuration have been performed for the first time on J-TEXT. The experiments show an increase in the highest central channel line-averaged density from to , while the X-point moves away from the target by increasing the divertor coil current. The corresponding Greenwald fraction rises from 0.50 to 0.79. For the impurity transport, the density normalized radiation intensity (absolute extreme ultraviolet and soft x-ray) of the central channel density decreased significantly (>50%) with an increase in the plasma density. To better understand the underlying physics mechanisms, the 3D edge Monte Carlo code coupled with EIRENE (EMC3- EIRENE) has been implemented for the first time on J-TEXT. The simulation results show good agreement with the experimental findings. As the X-point moves away from the target, the divertor power decay length drops and the scrape-off layer impurity screening effect is enhanced.

Microscopic Kinetics in Poly(Methyl Methacrylate) Exposed to a Single Ultra-Short XUV/X-ray Laser Pulse.

We study the behavior of poly(methyl methacrylate) (PMMA) exposed to femtosecond pulses of extreme ultraviolet and X-ray laser radiation in the single-shot damage regime. The employed microscopic simulation traces induced electron cascades, the thermal energy exchange of electrons with atoms, nonthermal modification of the interatomic potential, and a triggered atomic response. We identify that the nonthermal hydrogen decoupling triggers ultrafast fragmentation of PMMA strains at the absorbed threshold dose of ~0.07 eV/atom. At higher doses, more hydrogen atoms detach from their parental molecules, which, at the dose of ~0.5 eV/atom, leads to a complete separation of hydrogens from carbon and oxygen atoms and fragmentation of MMA molecules. At the dose of ~0.7 eV/atom, the band gap completely collapses indicating that a metallic liquid is formed with complete atomic disorder. An estimated single-shot ablation threshold and a crater depth as functions of fluence agree well with the experimental data collected.

Ultra-supercontinuum generation with high intense femtosecond pulses

Extreme spectral broadening has been theoretically simulated from the index of refraction changes from the envelope response to the fifth- and third-order susceptibilities from self-phase modulation (SPM) under the influence of an extremely high-intensity femtosecond laser pulse to potentially produce spectral broadening changes extending from extreme X-rays, UV, visible, THz down to DC. The theoretical results show that an extremely high-intensity pulse as high as on the order of 1014~1016 W/m2 can influence the fourth-order refractive index arising from fifth-order susceptibility large enough that the nonlinear n4I02 term to overtakes the n2I0 term from the third-order susceptibility to produce the ultra-supercontinuum broadening in the liquids such as CS2 and rare gas liquids and solids such as calcite, ZnO, Si, Argon, and Krypton. There has been experimental verification at lower intensities that the SC is symmetrical in frequency about laser frequency for say a Gaussian pulse extends from, UV visible, NIR, to MIR by many researchers using various states of matter. This paper provides an opportunity to extend the SPM model from X-rays to DC to form Ultra Supercontinuum Generation (USCG) using extreme intensity fs pulses in the four states of matter: gases, liquids, solids, and plasmas. At the highest intensity, a continuum with weaker intensity goes towards the X-ray region on the anti-Stokes side.

Confocal Fluorescence Microscopy and Confocal Raman Microspectroscopy of X-ray Irradiated LiF Crystals

Radiation-induced color centers locally produced in lithium fluoride (LiF) are successfully used for radiation detectors. LiF detectors for extreme ultraviolet radiation, soft and hard X-rays, based on photoluminescence from aggregate electronic defects, are currently under development for imaging applications with laboratory radiation sources, as well as large-scale facilities. Among the peculiarities of LiF-based detectors, noteworthy ones are their very high intrinsic spatial resolution across a large field of view, wide dynamic range, and versatility. LiF crystals irradiated with a monochromatic 8 keV X-ray beam at KIT synchrotron light source (Karlsruhe, Germany) and with the broadband white beam spectrum of the synchrotron bending magnet have been investigated by optical spectroscopy, laser scanning confocal microscopy in fluorescence mode, and confocal Raman micro-spectroscopy. The 3D reconstruction of the distributions of the color centers induced by the X-rays has been performed with both confocal techniques. The combination of the LiF crystal capability to register volumetric X-ray mapping with the optical sectioning operations of the confocal techniques has allowed performing 3D reconstructions of the X-ray colored volumes and it could provide advanced tools for 3D X-ray detection.

Anomalous dependence of ionization probability and electron angular distributions on orientation of molecular axis in photoionization of : effect of two-center interference

A theoretical and computational study of photoionization of the one-electron molecular ion initially in the 1σ u state is performed. The laser pulse is linearly polarized with the carrier wavelength in the extreme ultraviolet and soft x-ray regions. The electron wave function is obtained by solving the time-dependent Schrödinger equation with the help of the generalized pseudospectral method. The dependence of the total ionization probability and photoelectron spectra on the orientation of the molecular axis is analyzed. Within the wavelength interval 6 to 23 nm, anomalous behavior of the ionization probability is found, where it increases with the angle between the polarization vector of the external field and the molecular axis and reaches a maximum at the perpendicular orientation of the molecule. The phenomenon is explained as resulting from the two-center interference in the wave function of the emitted electron.

Tomography of a seeded free-electron laser focal spot: qualitative and quantitative comparison of two reconstruction methods for spot size characterization.

Performing experiments at free-electron lasers (FELs) requires an exhaustive knowledge of the pulse temporal and spectral profile, as well as the focal spot shape and size. Operating FELs in the extreme ultraviolet (EUV) and soft X-ray (SXR) spectral regions calls for designing ad-hoc optical layouts to transport and characterize the EUV/SXR beam, as well as tailoring its spatial dimensions at the focal plane down to sizes in the few micrometers range. At the FERMI FEL (Trieste, Italy) this task is carried out by the Photon Analysis Delivery and Reduction System (PADReS). In particular, to meet the different experimental requests on the focal spot shape and size, a proper tuning of the optical systems is required, and this should be monitored by means of dedicated techniques. Here, we present and compare two reconstruction methods for spot characterization: single-shot imprints captured via ablation on a poly(methyl methacrylate) sample (PMMA) and pulse profiles retrieved by means of a Hartmann wavefront sensor (WFS). By recording complementary datasets at and nearby the focal plane, we exploit the tomography of the pulse profile along the beam propagation axis, as well as a qualitative and quantitative comparison between these two reconstruction methods.