Dielectronic satellite emission from a solid-density Mg plasma: Relationship to models of ionization potential depression.

We report on experiments where solid-density Mg plasmas are created by heating with the focused output of the Linac Coherent Light Source x-ray free-electron laser. We study the K-shell emission from the helium- and lithium-like ions using Bragg crystal spectroscopy. Observation of the dielectronic satellites in lithium-like ions confirms that the M-shell electrons appear bound for these high charge states. An analysis of the intensity of these satellites indicates that when modeled with an atomic-kinetics code, the ionization potential depression model employed needs to produce depressions for these ions which lie between those predicted by the well known Stewart-Pyatt and Ecker-Kroll models. These results are largely consistent with recent density functional theory calculations.

Author Correction: Nanofocusing of hard X-ray free electron laser pulses using diamond based Fresnel zone plates.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Clocking Femtosecond Collisional Dynamics via Resonant X-Ray Spectroscopy.

Electron-ion collisional dynamics is of fundamental importance in determining plasma transport properties, nonequilibrium plasma evolution, and electron damage in diffraction imaging applications using bright x-ray free-electron lasers (FELs). Here we describe the first experimental measurements of ultrafast electron impact collisional ionization dynamics using resonant core-hole spectroscopy in a solid-density magnesium plasma, created and diagnosed with the Linac Coherent Light Source x-ray FEL. By resonantly pumping the 1s→2p transition in highly charged ions within an optically thin plasma, we have measured how off-resonance charge states are populated via collisional processes on femtosecond time scales. We present a collisional cross section model that matches our results and demonstrates how the cross sections are enhanced by dense-plasma effects including continuum lowering. Nonlocal thermodynamic equilibrium collisional radiative simulations show excellent agreement with the experimental results and provide new insight on collisional ionization and three-body-recombination processes in the dense-plasma regime.

Direct experimental observation of the gas density depression effect using a two-bunch X-ray FEL beam.

The experimental observation of the depression effect in gas devices designed for X-ray free-electron lasers (FELs) is reported. The measurements were carried out at the Linac Coherent Light Source using a two-bunch FEL beam at 6.5 keV with 122.5 ns separation passing through an argon gas cell. The relative intensities of the two pulses of the two-bunch beam were measured, after and before the gas cell, from X-ray scattering off thin targets by using fast diodes with sufficient temporal resolution. At a cell pressure of 140 hPa, it was found that the after-to-before ratio of the intensities of the second pulse was about 17% ±â€…6% higher than that of the first pulse, revealing lower effective attenuation of the gas cell due to heating by the first pulse and subsequent gas density reduction in the beam path. This measurement is important in guiding the design and/or mitigating the adverse effects in gas devices for high-repetition-rate FELs such as the LCLS-II and the European XFEL or other future high-repetition-rate upgrades to existing FEL facilities.

Generating Single-Spike Hard X-Ray Pulses with Nonlinear Bunch Compression in Free-Electron Lasers.

A simple method for generating single-spike hard x-ray pulses in free-electron lasers (FELs) has been developed at the Linac Coherent Light Source (LCLS). This is realized by nonlinear bunch compression using 20-pC bunch charge, demonstrated in the hard x-ray regime at 5.6 and 9 keV, respectively. Measurements show about half of the FEL shots containing a single-spike spectrum. At 5.6-keV photon energy, the single-spike shots have a mean pulse energy of about 10 µJ with 70% intensity fluctuation and the pulse full width at half maximum is evaluated to be at 200-as level.

Observation of Reverse Saturable Absorption of an X-ray Laser.

A nonlinear absorber in which the excited state absorption is larger than the ground state can undergo a process called reverse saturable absorption. It is a well-known phenomenon in laser physics in the optical regime, but is more difficult to generate in the x-ray regime, where fast nonradiative core electron transitions typically dominate the population kinetics during light matter interactions. Here, we report the first observation of decreasing x-ray transmission in a solid target pumped by intense x-ray free electron laser pulses. The measurement has been made below the K-absorption edge of aluminum, and the x-ray intensity ranges are 10^{16} -10^{17} W/cm^{2}. It has been confirmed by collisional radiative population kinetic calculations, underscoring the fast spectral modulation of the x-ray pulses and charge states relevant to the absorption and transmission of x-ray photons. The processes shown through detailed simulations are consistent with reverse saturable absorption, which would be the first observation of this phenomena in the x-ray regime. These light matter interactions provide a unique opportunity to investigate optical transport properties in the extreme state of matters, as well as affording the potential to regulate ultrafast x-ray free-electron laser pulses.

Measurements of the K-Shell Opacity of a Solid-Density Magnesium Plasma Heated by an X-Ray Free-Electron Laser.

We present measurements of the spectrally resolved x rays emitted from solid-density magnesium targets of varying sub-µm thicknesses isochorically heated by an x-ray laser. The data exhibit a largely thickness-independent source function, allowing the extraction of a measure of the opacity to K-shell x rays within well-defined regimes of electron density and temperature, extremely close to local thermodynamic equilibrium conditions. The deduced opacities at the peak of the Kα transitions of the ions are consistent with those predicted by detailed atomic-kinetics calculations.

Measurement of high-dynamic range x-ray Thomson scattering spectra for the characterization of nano-plasmas at LCLS.

Atomic clusters can serve as ideal model systems for exploring ultrafast (∼100 fs) laser-driven ionization dynamics of dense matter on the nanometer scale. Resonant absorption of optical laser pulses enables heating to temperatures on the order of 1 keV at near solid density conditions. To date, direct probing of transient states of such nano-plasmas was limited to coherent x-ray imaging. Here we present the first measurement of spectrally resolved incoherent x-ray scattering from clusters, enabling measurements of transient temperature, densities, and ionization. Single shot x-ray Thomson scattering signals were recorded at 120 Hz using a crystal spectrometer in combination with a single-photon counting and energy-dispersive pnCCD. A precise pump laser collimation scheme enabled recording near background-free scattering spectra from Ar clusters with an unprecedented dynamic range of more than 3 orders of magnitude. Such measurements are important for understanding collective effects in laser-matter interactions on femtosecond time scales, opening new routes for the development of schemes for their ultrafast control.

The sex locus is tightly linked to factors conferring sex-specific lethal effects in the mosquito Aedes aegypti.

In many taxa, sex chromosomes are heteromorphic and largely non-recombining. Evolutionary models predict that spread of recombination suppression on the Y chromosome is fueled by the accumulation of sexually antagonistic alleles in close linkage to the sex determination region. However, empirical evidence for the existence of sexually antagonistic alleles is scarce. In the mosquito Aedes aegypti, the sex-determining chromosomes are homomorphic. The region of suppressed recombination, which surrounds the male-specific sex-determining gene, remains very small, despite ancient origin of the sex chromosomes in the Aedes lineage. We conducted a genetic analysis of the A. aegypti chromosome region tightly linked to the sex locus. We used a strain with an enhanced green fluorescent protein (EGFP)-tagged transgene inserted near the male-determining gene to monitor crossing-over events close to the boundary of the sex-determining region (SDR), and to trace the inheritance pattern of the transgene in relation to sex. In a series of crossing experiments involving individuals with a recombinant sex chromosome we found developmental abnormalities leading to 1:2 sex biases, caused by lethality of half of the male or female progeny. Our results suggest that various factors causing sex-specific lethal effects are clustered within the neighborhood of the SDR, which in the affected sex are likely lost or gained through recombination, leading to death. These may include genes that are recessive lethal, vital for development and/or sexually antagonistic. The sex chromosome fragment in question represents a fascinating test case for the analysis of processes that shape stable boundaries of a non-recombining region.

Measurements of continuum lowering in solid-density plasmas created from elements and compounds.

The effect of a dense plasma environment on the energy levels of an embedded ion is usually described in terms of the lowering of its continuum level. For strongly coupled plasmas, the phenomenon is intimately related to the equation of state; hence, an accurate treatment is crucial for most astrophysical and inertial-fusion applications, where the case of plasma mixtures is of particular interest. Here we present an experiment showing that the standard density-dependent analytical models are inadequate to describe solid-density plasmas at the temperatures studied, where the reduction of the binding energies for a given species is unaffected by the different plasma environment (ion density) in either the element or compounds of that species, and can be accurately estimated by calculations only involving the energy levels of an isolated neutral atom. The results have implications for the standard approaches to the equation of state calculations.

Hetero-site-specific X-ray pump-probe spectroscopy for femtosecond intramolecular dynamics.

New capabilities at X-ray free-electron laser facilities allow the generation of two-colour femtosecond X-ray pulses, opening the possibility of performing ultrafast studies of X-ray-induced phenomena. Particularly, the experimental realization of hetero-site-specific X-ray-pump/X-ray-probe spectroscopy is of special interest, in which an X-ray pump pulse is absorbed at one site within a molecule and an X-ray probe pulse follows the X-ray-induced dynamics at another site within the same molecule. Here we show experimental evidence of a hetero-site pump-probe signal. By using two-colour 10-fs X-ray pulses, we are able to observe the femtosecond time dependence for the formation of F ions during the fragmentation of XeF2 molecules following X-ray absorption at the Xe site.

Experimental demonstration of a soft x-ray self-seeded free-electron laser.

The Linac Coherent Light Source has added a self-seeding capability to the soft x-ray range using a grating monochromator system. We report the demonstration of soft x-ray self-seeding with a measured resolving power of 2000-5000, wavelength stability of 10(-4), and an increase in peak brightness by a factor of 2-5 across the photon energy range of 500-1000 eV. By avoiding the need for a monochromator at the experimental station, the self-seeded beam can deliver as much as 50-fold higher brightness to users.

Saturable absorption of an x-ray free-electron-laser heated solid-density aluminum plasma.

High-intensity x-ray pulses from an x-ray free-electron laser are used to heat and probe a solid-density aluminum sample. The photon-energy-dependent transmission of the heating beam is studied through the use of a photodiode. Saturable absorption is observed, with the resulting transmission differing significantly from the cold case, in good agreement with atomic-kinetics simulations.

Spectral encoding method for measuring the relative arrival time between x-ray/optical pulses.

The advent of few femtosecond x-ray light sources brings promise of x-ray/optical pump-probe experiments that can measure chemical and structural changes in the 10-100 fs time regime. Widely distributed timing systems used at x-ray Free-Electron Laser facilities are typically limited to above 50 fs fwhm jitter in active x-ray/optical synchronization. The approach of single-shot timing measurements is used to sort results in the event processing stage. This has seen wide use to accommodate the insufficient precision of active stabilization schemes. In this article, we review the current technique for "measure-and-sort" at the Linac Coherent Light Source at the SLAC National Accelerator Laboratory. The relative arrival time between an x-ray pulse and an optical pulse is measured near the experimental interaction region as a spectrally encoded cross-correlation signal. The cross-correlation provides a time-stamp for filter-and-sort algorithms used for real-time sorting. Sub-10 fs rms resolution is common in this technique, placing timing precision at the same scale as the duration of the shortest achievable x-ray pulses.

Multicolor operation and spectral control in a gain-modulated x-ray free-electron laser.

We show that the spectral properties of a self-amplified spontaneous emission x-ray free-electron laser can be controlled by modulating the gain in magnetic undulators, thus producing one or several spectral lines within a single few femtosecond pulse. By varying the magnetic field along the undulator and the electron beam transport line, the system we demonstrate can tailor the x-ray spectrum to optimally meet numerous experimental requirements for multicolor operation.

Experimental demonstration of femtosecond two-color x-ray free-electron lasers.

With an eye toward extending optical wave-mixing techniques to the x-ray regime, we present the first experimental demonstration of a two-color x-ray free-electron laser at the Linac Coherent Light Source. We combine the emittance-spoiler technique with a magnetic chicane in the undulator section to control the pulse duration and relative delay between two intense x-ray pulses and we use differently tuned canted pole undulators such that the two pulses have different wavelengths as well. Two schemes are shown to produce two-color soft x-ray pulses with a wavelength separation up to ∼1.9% and a controllable relative delay up to 40 fs.

Direct measurements of the ionization potential depression in a dense plasma.

We have used the Linac Coherent Light Source to generate solid-density aluminum plasmas at temperatures of up to 180 eV. By varying the photon energy of the x rays that both create and probe the plasma, and observing the K-α fluorescence, we can directly measure the position of the K edge of the highly charged ions within the system. The results are found to disagree with the predictions of the extensively used Stewart-Pyatt model, but are consistent with the earlier model of Ecker and Kröll, which predicts significantly greater depression of the ionization potential.

Ultrafast transitions from solid to liquid and plasma states of graphite induced by x-ray free-electron laser pulses.

We used photon pulses from an x-ray free-electron laser to study ultrafast x-ray-induced transitions of graphite from solid to liquid and plasma states. This was accomplished by isochoric heating of graphite samples and simultaneous probing via Bragg and diffuse scattering at high time resolution. We observe that disintegration of the crystal lattice and ion heating of up to 5 eV occur within tens of femtoseconds. The threshold fluence for Bragg-peak degradation is smaller and the ion-heating rate is faster than current x-ray-matter interaction models predict.

The soft x-ray instrument for materials studies at the linac coherent light source x-ray free-electron laser.

The soft x-ray materials science instrument is the second operational beamline at the linac coherent light source x-ray free electron laser. The instrument operates with a photon energy range of 480-2000 eV and features a grating monochromator as well as bendable refocusing mirrors. A broad range of experimental stations may be installed to study diverse scientific topics such as: ultrafast chemistry, surface science, highly correlated electron systems, matter under extreme conditions, and laboratory astrophysics. Preliminary commissioning results are presented including the first soft x-ray single-shot energy spectrum from a free electron laser.

Creation and diagnosis of a solid-density plasma with an X-ray free-electron laser.

Matter with a high energy density (>10(5) joules per cm(3)) is prevalent throughout the Universe, being present in all types of stars and towards the centre of the giant planets; it is also relevant for inertial confinement fusion. Its thermodynamic and transport properties are challenging to measure, requiring the creation of sufficiently long-lived samples at homogeneous temperatures and densities. With the advent of the Linac Coherent Light Source (LCLS) X-ray laser, high-intensity radiation (>10(17) watts per cm(2), previously the domain of optical lasers) can be produced at X-ray wavelengths. The interaction of single atoms with such intense X-rays has recently been investigated. An understanding of the contrasting case of intense X-ray interaction with dense systems is important from a fundamental viewpoint and for applications. Here we report the experimental creation of a solid-density plasma at temperatures in excess of 10(6) kelvin on inertial-confinement timescales using an X-ray free-electron laser. We discuss the pertinent physics of the intense X-ray-matter interactions, and illustrate the importance of electron-ion collisions. Detailed simulations of the interaction process conducted with a radiative-collisional code show good qualitative agreement with the experimental results. We obtain insights into the evolution of the charge state distribution of the system, the electron density and temperature, and the timescales of collisional processes. Our results should inform future high-intensity X-ray experiments involving dense samples, such as X-ray diffractive imaging of biological systems, material science investigations, and the study of matter in extreme conditions.