We present an overview of our systematic studies of the surface modifications resulting from the interactions of both single and multiple picosecond soft x-ray laser (SXRL) pulses with materials, such as gold (Au), copper (Cu), aluminum (Al), and lithium fluoride (LiF). We show experimentally the possibility of the precise nanometer size structures (~10–40 nm) formation on their surfaces by ultra-low (~10–30 mJ/cm2 ) fluencies of single picosecond SXRL pulse. Comparison experimental results with the atomistic model of ablation, which was developed for the single SXRL shot interaction with dielectrics and metals, is provided. Theoretical description of surface nanostructures is considered and is shown that such structures are formed after laser illumination in a process of mechanical spallation of ultrathin surface layer of molten metal. Spallation is accompanied by a strong foaming of melt, breaking of foam, and freezing of foam remnants. Those remnants form chaotic nanostructures, which are observed in experiments. Our measurements show that electron temperature of matter under irradiation of SXRL was lower than 1 eV. The model calculation also predicts that the ablation induced by the SXRL can create the significant low electron temperature. Our results demonstrate that tensile stress created in LiF and metals by short SXRL pulse can produce spallative ablation of target even for drastically small fluencies, which open new opportunities for material nano processing.
To study the ablation process induced by the soft x-ray laser pulse, we investigated the electron temperature of the ablating material. Focused soft x-ray laser pulses having a wavelength of 13.9 nm and duration of 7 ps were irradiated onto the LiF, Al, and Cu surfaces, and we observed the optical emission from the surfaces by use of an optical camera. On sample surfaces, we could confirm damage structures, but no emission signal in the visible spectral range during ablation could be observed. Then, we estimated the electron temperature in the ablating matter. To consider the radiation from a heated layer, we supposed a black-body radiator as an object. The calculation result was that the electron temperature was estimated to be lower than 1 eV and the process duration was shorter than 1000 ps. The theoretical model calculation suggests the spallative ablation for the interaction between the soft x-ray laser and materials. The driving force for the spallation is an increasing pressure appearing in the heated layer, and the change of the surface is considered to be due to a splash of a molten layer. The model calculation predicts that the soft x-ray laser with the fluence around the ablation threshold can create an electron temperature around 1 eV in a material. The experimental result is in good accordance with the theoretical prediction. Our investigation implies that the spallative ablation occurs in the low electron temperature region of a non-equilibrium state of warm dense matter.
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Short pulse x-ray sources are widely used as probing beams for new material development and non-destructive x-ray imaging. The high quality soft x-ray laser (SXRL) source enables us to achieve quite high spatial-resolution as a probe and quite intense x-ray as a pump. As an application using the SXRL, we have observed the spallative ablation process by the interaction with SXRL or femto-second (fs) laser. The dynamical processes of the SXRL and/or the fs laserinduced surface modifications come to attract much attention for the micro processing. However, it is difficult to observe the spallative ablation dynamics, because of non-repetitive, irreversible and rapid phenomena in a small feature size. In the case with SXRL irradiation (13.9 nm, 7ps, ~50 mJ/cm2), we have observed the damage structures and the optical emission from the ablated materials. When focused SXRL pulses were have been irradiated onto the metal surface, we have confirmed damage structures, however no optical emission signal during SXRL ablation could be observed. The electron temperature is estimated to be around a few eV at the ablated surface. In the case with fs laser irradiation (795 nm, 80fs, ~1.5 J/cm2), we have observed the surface morphology of fs laser ablation by the SXRL interferometer and SXRL reflectometer. The time resolved image of nano-scaled ablation dynamics of tungsten surface was observed. The numerical simulation study is underway by using a molecular dynamics code. These results lead not only to understanding the full process of the interaction with the SXRL and/or fs laser, but also to candidate the material of the first wall of magnetic confinement fusion reactors. We also described a preliminary study of radiation effect on culture cells irradiated with the SXRL. Our study demonstrated for the first time that the SXRL induced the DNA double strand breaks
To study the interactions between a soft x-ray laser (SXRL) and various materials, we irradiated Al, Au, Cu, and Si with
the SXRL beam pulses having a wavelength of 13.9 nm and duration of 7 ps. Following the irradiation, the induced
structures were observed using a scanning electron microscope and an atomic force microscope. With single pulse
irradiation, conical structures were observed on the Al surface, and ripple-like structures were formed on the Au and Cu
surfaces. The conical structures were destroyed under multiple SXRL pulse irradiation. On the other hand, the
developments of modified structures were observed after multiple pulse irradiations on the Au and Cu surfaces. On the
Si surface, deep holes, that seemed to be molten structures induced by the accumulation of multiple pulse irradiations,
were found. Therefore, it is concluded that the SXRL pulse irradiations of various material surfaces cause different
types of surface modifications, and the changes in the surface behaviors are attributed to the differences in the elemental
properties of each materials, such as the melting point and the attenuation length of x-rays.
We demonstrate a new ion diagnosis method for high energy ions by utilizing a combination of a single CR-39 detector and plastic plates, which enables to detect high energy ions beyond the detection threshold limit of the CR-39. This detection method coupled with a magnetic spectrometer is applied to identify high energy ions of 50 MeV per nucleon in laser-driven ion acceleration experiments using cluster-gas targets.
Review of results, obtained by using recently proposed new imaging detector, based on formation of color centers in LiF
crystal and LiF film, for in situ high performance measurements of near-field and far-field properties of soft X-ray lasers
(SXRL) beams is presented. Experiments have been carried out with laser-driven transient-collision plasma SXRL and
free electron SXRL beams. It was demonstrated that due to favorable combination of high spatial resolution, high
dynamic range and wide field of view this technique allows measuring not only intensity distribution across the full
beam and in local areas, but also permits to evaluate coherence and spectral distribution of radiation across the beam.
Experimental diffraction patterns in the images of periodical structures are analyzed by comparison with the modeled
ones in the last case. The estimated accuracy of measurements is between 10-20%.
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