Deep learning for laser beam imprinting.

Methods of ablation imprints in solid targets are widely used to characterize focused X-ray laser beams due to a remarkable dynamic range and resolving power. A detailed description of intense beam profiles is especially important in high-energy-density physics aiming at nonlinear phenomena. Complex interaction experiments require an enormous number of imprints to be created under all desired conditions making the analysis demanding and requiring a huge amount of human work. Here, for the first time, we present ablation imprinting methods assisted by deep learning approaches. Employing a multi-layer convolutional neural network (U-Net) trained on thousands of manually annotated ablation imprints in poly(methyl methacrylate), we characterize a focused beam of beamline FL24/FLASH2 at the Free-electron laser in Hamburg. The performance of the neural network is subject to a thorough benchmark test and comparison with experienced human analysts. Methods presented in this Paper pave the way towards a virtual analyst automatically processing experimental data from start to end.

Investigation of femtosecond collisional ionization rates in a solid-density aluminium plasma.

The rate at which atoms and ions within a plasma are further ionized by collisions with the free electrons is a fundamental parameter that dictates the dynamics of plasma systems at intermediate and high densities. While collision rates are well known experimentally in a few dilute systems, similar measurements for nonideal plasmas at densities approaching or exceeding those of solids remain elusive. Here we describe a spectroscopic method to study collision rates in solid-density aluminium plasmas created and diagnosed using the Linac Coherent light Source free-electron X-ray laser, tuned to specific interaction pathways around the absorption edges of ionic charge states. We estimate the rate of collisional ionization in solid-density aluminium plasmas at temperatures ~30 eV to be several times higher than that predicted by standard semiempirical models.

In situ focus characterization by ablation technique to enable optics alignment at an XUV FEL source.

In situ focus characterization is demonstrated by working at an extreme ultraviolet (XUV) free-electron laser source using ablation technique. Design of the instrument reported here allows reaching a few micrometres resolution along with keeping the ultrahigh vacuum conditions and ensures high-contrast visibility of ablative imprints on optically transparent samples, e.g., PMMA. This enables on-line monitoring of the beam profile changes and thus makes possible in situ alignment of the XUV focusing optics. A good agreement between focal characterizations retrieved from in situ inspection of ablative imprints contours and from well-established accurate ex situ analysis with Nomarski microscope has been observed for a typical micro-focus experiment.

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.

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.

Resonant Kα spectroscopy of solid-density aluminum plasmas.

The x-ray intensities made available by x-ray free electron lasers (FEL) open up new x-ray matter interaction channels not accessible with previous sources. We report here on the resonant generation of Kα emission, that is to say the production of copious Kα radiation by tuning the x-ray FEL pulse to photon energies below that of the K edge of a solid aluminum sample. The sequential absorption of multiple photons in the same atom during the 80 fs pulse, with photons creating L-shell holes and then one resonantly exciting a K-shell electron into one of these holes, opens up a channel for the Kα production, as well as the absorption of further photons. We demonstrate rich spectra of such channels, and investigate the emission produced by tuning the FEL energy to the K-L transitions of those highly charged ions that have transition energies below the K edge of the cold material. The spectra are sensitive to x-ray intensity dependent opacity effects, with ions containing L-shell holes readily reabsorbing the Kα radiation.

Damage mechanisms of MoN/SiN multilayer optics for next-generation pulsed XUV light sources.

We investigated the damage mechanism of MoN/SiN multilayer XUV optics under two extreme conditions: thermal annealing and irradiation with single shot intense XUV pulses from the free-electron laser facility in Hamburg - FLASH. The damage was studied "post-mortem" by means of X-ray diffraction, interference-polarizing optical microscopy, atomic force microscopy, and scanning transmission electron microscopy. Although the timescale of the damage processes and the damage threshold temperatures were different (in the case of annealing it was the dissociation temperature of Mo2N and in the case of XUV irradiation it was the melting temperature of MoN) the main damage mechanism is very similar: molecular dissociation and the formation of N2, leading to bubbles inside the multilayer structure.

Electronic structure of an XUV photogenerated solid-density aluminum plasma.

By use of high intensity XUV radiation from the FLASH free-electron laser at DESY, we have created highly excited exotic states of matter in solid-density aluminum samples. The XUV intensity is sufficiently high to excite an inner-shell electron from a large fraction of the atoms in the focal region. We show that soft-x-ray emission spectroscopy measurements reveal the electronic temperature and density of this highly excited system immediately after the excitation pulse, with detailed calculations of the electronic structure, based on finite-temperature density functional theory, in good agreement with the experimental results.

Single shot damage mechanism of Mo/Si multilayer optics under intense pulsed XUV-exposure.

We investigated single shot damage of Mo/Si multilayer coatings exposed to the intense fs XUV radiation at the Free-electron LASer facility in Hamburg - FLASH. The interaction process was studied in situ by XUV reflectometry, time resolved optical microscopy, and "post-mortem" by interference-polarizing optical microscopy (with Nomarski contrast), atomic force microscopy, and scanning transmission electron microcopy. An ultrafast molybdenum silicide formation due to enhanced atomic diffusion in melted silicon has been determined to be the key process in the damage mechanism. The influence of the energy diffusion on the damage process was estimated. The results are of significance for the design of multilayer optics for a new generation of pulsed (from atto- to nanosecond) XUV sources.

Spot size characterization of focused non-Gaussian X-ray laser beams.

We present a new technique for the characterization of non-Gaussian laser beams which cannot be described by an analytical formula. As a generalization of the beam spot area we apply and refine the definition of so called effective area (A(eff)) [1] in order to avoid using the full-width at half maximum (FWHM) parameter which is inappropriate for non-Gaussian beams. Furthermore, we demonstrate a practical utilization of our technique for a femtosecond soft X-ray free-electron laser. The ablative imprints in poly(methyl methacrylate) - PMMA and amorphous carbon (a-C) are used to characterize the spatial beam profile and to determine the effective area. Two procedures of the effective area determination are presented in this work. An F-scan method, newly developed in this paper, appears to be a good candidate for the spatial beam diagnostics applicable to lasers of various kinds.

Soft x-ray free electron laser microfocus for exploring matter under extreme conditions.

We have focused a beam (BL3) of FLASH (Free-electron LASer in Hamburg: lambda = 13.5 nm, pulse length 15 fs, pulse energy 10-40 microJ, 5 Hz) using a fine polished off-axis parabola having a focal length of 270 mm and coated with a Mo/Si multilayer with an initial reflectivity of 67% at 13.5 nm. The OAP was mounted and aligned with a picomotor controlled six-axis gimbal. Beam imprints on poly(methyl methacrylate) - PMMA were used to measure focus and the focused beam was used to create isochoric heating of various slab targets. Results show the focal spot has a diameter of < or =1 microm. Observations were correlated with simulations of best focus to provide further relevant information.

Non-thermal desorption/ablation of molecular solids induced by ultra-short soft x-ray pulses.

We report the first observation of single-shot soft x-ray laser induced desorption occurring below the ablation threshold in a thin layer of poly (methyl methacrylate)--PMMA. Irradiated by the focused beam from the Free-electron LASer in Hamburg (FLASH) at 21.7 nm, the samples have been investigated by atomic-force microscope (AFM) enabling the visualization of mild surface modifications caused by the desorption. A model describing non-thermal desorption and ablation has been developed and used to analyze single-shot imprints in PMMA. An intermediate regime of materials removal has been found, confirming model predictions. We also report below-threshold multiple-shot desorption of PMMA induced by high-order harmonics (HOH) at 32 nm. Short-time exposure imprints provide sufficient information about transverse beam profile in HOH's tight focus whereas long-time exposed PMMA exhibits radiation-initiated surface ardening making the beam profile measurement infeasible.