M. Nishikino

Nonlinear increase of X-ray intensities from thin foils irradiated with a 200 TW femtosecond laser

We report, for the first time, that the energy of femtosecond optical laser pulses, E, with relativistic intensities Iu2009>u20091021u2009u2009W/cm2 is efficiently converted to X-ray radiation, which is emitted by “hot” electron component in collision-less processes and heats the solid density plasma periphery. As shown by direct high-resolution spectroscopic measurements X-ray radiation from plasma periphery exhibits unusual non-linear growth ~E4–5 of its power. The non-linear power growth occurs far earlier than the known regime when the radiation reaction dominates particle motion (RDR). Nevertheless, the radiation is shown to dominate the kinetics of the plasma periphery, changing in this regime (now labeled RDKR) the physical picture of the laser plasma interaction. Although in the experiments reported here we demonstrated by observation of KK hollow ions that X-ray intensities in the keV range exceeds ~1017u2009u2009W/cm2, there is no theoretical limit of the radiation power. Therefore, such powerful X-ray sources can produce and probe exotic material states with high densities and multiple inner-shell electron excitations even for higher Z elements. Femtosecond laser-produced plasmas may thus provide unique ultra-bright X-ray sources, for future studies of matter in extreme conditions, material science studies, and radiography of biological systems.

3D visualization of XFEL beam focusing properties using LiF crystal X-ray detector.

Here, we report, that by means of direct irradiation of lithium fluoride a (LiF) crystal, in situ 3D visualization of the SACLA XFEL focused beam profile along the propagation direction is realized, including propagation inside photoluminescence solid matter. High sensitivity and large dynamic range of the LiF crystal detector allowed measurements of the intensity distribution of the beam at distances far from the best focus as well as near the best focus and evaluation of XFEL source size and beam quality factor M2. Our measurements also support the theoretical prediction that for X-ray photons with energies ~10u2009keV the radius of the generated photoelectron cloud within the LiF crystal reaches about 600u2009nm before thermalization. The proposed method has a spatial resolution ~u20090.4–2.0u2009μm for photons with energies 6–14u2009keV and potentially could be used in a single shot mode for optimization of different focusing systems developed at XFEL and synchrotron facilities.

Dynamics of spallation during femtosecond laser ablation studied by time-resolved reflectivity with double pump pulses

The dynamics of photomechanical spallation during femtosecond laser ablation of fused silica was studied by time-resolved reflectivity with double pump pulses. Oscillation of reflectivity was caused by interference between the probe pulses reflected at the sample surface and the spallation layer, and was enhanced when the surface was irradiated with the second pump pulse within a time interval, Δτ, of several picoseconds after the first pump pulse. However, as Δτ was increased, the oscillation amplitude decreased with an exponential decay time of 10u2009ps. The oscillation disappeared when Δτ exceeded 20u2009ps. This result suggests that the formation time of the spallation layer is approximately 10u2009ps. A second pump pulse with Δτ shorter than 10u2009ps excites the bulk sample. The spallation layer that is photo-excited by the first and second pump pulses is separated afterward. In contrast, a pulse with Δτ longer than the formation time excites and breaks up the spallation layer that has already been separated from ...

Temporal behavior of unresolved transition array emission in water window soft x-ray spectral region from multiply charged ions

We have characterized the spectral structure and the temporal history of the laser-produced high-Z multi-charged ion plasmas for the efficient water window soft x-ray sources. Strong unresolved transition array emission was observed due to 4d–4f and 4f–5g transitions from Au, Pb, and Bi plasmas in the 280–700 eV photon energy region. The temporal behavior of the emission was essentially similar of that of the laser pulse with a slight delay between different transitions. These results provide feedback for accurate modeling of the atomic processes with the radiative hydrodynamic simulations.

Observation of dynamics and modification of solid surface using a picosecond soft x-ray laser

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

Construction of a magnetic bottle spectrometer and its application to pulse duration measurement of X-ray laser using a pump-probe method

To characterize the temporal evolution of ultrashort X-ray pulses emitted by laser plasmas using a pump-probe method, a magnetic bottle time-of-flight electron spectrometer is constructed. The design is determined by numerical calculations of a mirror magnetic field and of the electron trajectory in a flight tube. The performance of the spectrometer is characterized by measuring the electron spectra of xenon atoms irradiated with a laser-driven plasma X-ray pulse. In addition, two-color above-threshold ionization (ATI) experiment is conducted for measurement of the X-ray laser pulse duration, in which xenon atoms are simultaneously irradiated with an X-ray laser pump and an IR laser probe. The correlation in the intensity of the sideband spectra of the 4d inner-shell photoelectrons and in the time delay of the two laser pulses yields an X-ray pulse width of 5.7 ps, in good agreement with the value obtained using an X-ray streak camera.

Pump-Probe Experiment for Temporal Profile Measurement of Plasma X-Ray Laser

Temporal behavior of a soft x-ray laser pulse is measured by means of a pump-probe spectroscopy (cross correlation). In this scheme, first the innershell electron 4d of Xe atom is photoionized with 13.9 nm x-ray pulse (x-ray pump). Simultaneously, an ir probe laser pulse is irradiated to dress the electrons. By varying the time delay between both pulses, the sideband spectra associated with ir photon absorption/emission processes appear at a separation of the ir photon energy from the photo- and Auger main peaks. By analyzing the sideband intensity, we can evaluate the pulse width of the x-ray laser to be ~6.2 ps, which is in excellent agreement with the value measured by an x-ray streak camera

Low electron temperature in ablating materials formed by picosecond soft x-ray laser pulses

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.