Saturable absorption is a phenomenon readily seen in the optical and infrared wavelengths. It has never been observed in core-electron transitions owing to the short lifetime of the excited states involved and the high intensities of the soft X-rays needed. We report saturable absorption of an L-shell transition in aluminium using record intensities over 10 16 W cm −2 at a photon energy of 92 eV. From a consideration of the relevant timescales, we infer that immediately after the X-rays have passed, the sample is in an exotic state where all of the aluminium atoms have an L-shell hole, and the valence band has approximately a 9 eV temperature, whereas the atoms are still on their crystallographic positions. Subsequently, Auger decay heats the material to the warm dense matter regime, at around 25 eV temperatures. The method is an ideal candidate to study homogeneous warm dense matter, highly relevant to planetary science, astrophysics and inertial confinement fusion. Saturable absorption, the decrease in the absorption of light with increasing intensity, is a well-known effect in the visible and near-visible region of the electromagnetic spectrum 1 , and is a widely exploited phenomenon in laser technology. Although there are many ways to induce this effect, in the simplest two-level system it will occur when the population of the lower, absorbing level is severely depleted, which requires light intensities sufficiently high to overcome relaxation from the upper level. Here, we report on the production of saturable absorption of a metal in the soft X-ray regime by the creation of highly uniform warm dense conditions, a regime that is of great interest in high-pressure science 2,3 , the geophysics of large planets 4,5 , astrophysics 6 , plasma production and inertial confinement fusion 7 . Furthermore, the process by which the saturation of the absorption occurs will lead, after the X-ray pulse, to the storage of about 100 eV per atom, which in turn evolves to a warm dense state. This manner of creation is unique as it requires intense, subpicosecond, soft X-rays. As such, it has not hitherto been observed in this region of the spectrum, owing both to the lack of high-intensity sources, and the rapid recombination times associated with such high photon energies. However, with the advent of new fourth-generation X-ray light sources, including the free-electron laser in Hamburg 8 (FLASH), soft X-ray intensities that have previously remained the province of high-power optical lasers can now be produced. Experiments at such high intensities using gas jets have already exhibited novel absorption phenomena 9 , and the possibility of irradiating solid samples with intense soft and hard X-rays has aroused interest as a possible means of producing warm dense matter (WDM) at known atomic densities 10,11 .We present the first measurements of the absorption coefficient of solid samples subject to subpicosecond soft X-ray pulses with intensities up to and in excess of 10 16 W cm −2 , two orders of magnitude higher than could ...

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We have carried out an optical Thomson scatter study of a KrF laser-ablated Mg plume. The evolution of the electron temperature and density at distances 2 -5 mm from the target surface has been studied. We have observed that the electron density falls more rapidly than the atomic density and believe that this is a result of rapid dielectronic recombination. A comparison of the electron density profile and evolution with simple hydrodynamic modeling indicates that there is a strong absorption of the laser in the plasma vapor above the target, probably due to photoionization. We also conclude that an isothermal model of expansion better fits the data than an isentropic expansion model. Finally, we compared data obtained from Thomson scatter with those obtained by emission spectroscopy under similar conditions. The two sets of data have differences but are broadly consistent.

The first high-resolution (spatial and temporal) 2D images of the density region close to the ablation surface of laser-irradiated targets are obtained using a novel x-ray imaging technique. These show that laser intensity nonuniformities imprint themselves onto the cold target surface, even if these exist only on short -10 ps time scales as is the case with a laser beam smoothed by induced spatial incoherence. This imprint phenomenon is interpreted in terms of the poor thermal smoothing under the conditions prevailing at the beginning of the pulse. It appears that the problem can be combatted by preforming a uniform plasma before interacting with the optical drive.PACS numbers: 52.50.Jm, 52.40.Nk For laser-irradiated targets, a uniform ablation surface is of great importance for several reasons. Nonuniformities are susceptible to unstable growth by a large number of thermomagnetic [l], hydrodynamic [2], and radiation-rate-dependent [3] instability mechanisms. Further, nonuniformities in the blowoff plasma (often originating from the ablation front) may cause increased filamentation and self-focusing [4] of the laser drive. All of these processes serve to reduce drive symmetry and efficiency and are detrimental to inertial-confinement fusion (ICF). To reduce such nonuniformities, much effort has recently been invested in the development of laser beam smoothing techniques, such as induced spatial incoherence (ISO [5], smoothing by spectral dispersion (SSD) [6], and random phase plates (RPP) [7]. These operate by breaking up the spatial and temporal coherence of the beam to produce smoother time-integrated focal spots. However, so far these techniques have not eliminated the problem of laser beam nonuniformities on shorter (-laser coherence time) time scales, which remain detrimental to the drive uniformity. A recent computational study [8] predicts the formation of large laser-induced areal mass density (pr) perturbations even under ISI-type irradiance conditions, where these arise due to transient effects in the so-called "start-up" phase. Previous studies of plasma blowoff uniformity [9] have used the technique of optical probing to investigate the subcritical density region and clear jetlike structures were observed. It is, however, more important for the drive symmetry that the supercritical, high-density region close to the ablation front is uniform.This Letter describes an experiment where a novel xray imaging technique with submicron spatial resolution and approximately 100-ps temporal resolution was used to investigate drive uniformity in laser-irradiated targets. Time-resolved, high-resolution images at densities close to the ablation surface are presented for the first time. The images show that the gross nonuniformities present even in ISI-smoothed focal spots on time scales of the order of the laser coherence time immediately form an imprint on the cold target surface at the beginning of the laser pulse. The density nonuniformities thus seeded persist throughout the rest of the pulse with no evidence o...

We have studied a solid-to-plasma transition by irradiating Al foils with the FLASH free electron laser at intensities up to 10(16)  W/cm(2). Intense XUV self-emission shows spectral features that are consistent with emission from regions of high density, which go beyond single inner-shell photoionization of solids. Characteristic features of intrashell transitions allowed us to identify Auger heating of the electrons in the conduction band occurring immediately after the absorption of the XUV laser energy as the dominant mechanism. A simple model of a multicharge state inverse Auger effect is proposed to explain the target emission when the conduction band at solid density becomes more atomiclike as energy is transferred from the electrons to the ions. This allows one to determine, independent of plasma simulations, the electron temperature and density just after the decay of crystalline order and to characterize the early time evolution.