Following three different types of high power lasers at Kansai Photon Science Institute are overviewed and controlling
the laser damages in these laser systems are described: (1) PW-class Ti:sapphire laser for high field science, (2) zig-zag
slab Nd:glass laser for x-ray laser pumping, and (3) high-repetition Yb:YAG thin-slab laser for THz generation. Also
reported is the use of plasma mirror for characterization of short-wavelength ultrashort laser pulses. This new method
will be useful to study evolution of plasma formation which leads to laser damages.
We developed high-resistant anti-reflection (AR) coating by using Al<sub>2</sub>O<sub>3</sub>/SiO<sub>2</sub> multilayer for Yb:YAG thin disk
amplifier. The AR coating was designed both for 940 nm of pump laser at an incident angle of 30 degrees and for 1030
nm of seed laser at 5 degrees. The Al<sub>2</sub>O<sub>3</sub>/SiO<sub>2</sub> multilayer was deposited by using the electron beam evaporation
technique on a fused silica substrate and then the laser induced damage threshold was evaluated. The sample was
irradiated by 1030 nm laser with 520 ps duration delivered from the Yb:YAG thin-disk regenerative amplifier. The
measured damage threshold of the Al<sub>2</sub>O<sub>3</sub>/SiO<sub>2</sub> AR coating was 75 J/cm<sup>2</sup>.
We have developed a soft x-ray laser (SXRL) interferometer capable of the single-shot imaging of nano-scaled
structure dynamics. The interferometer consisted of the reflection optics including double Lloyd's mirrors and focusing
optics, and the interference fringes are produced on the detector surface. The depth and lateral resolutions of the
interferometer were about 1 nm and 1.8 μm, respectively. By using this interferometer, the initial stage (~50 ps) of the
ablation process of the Pt surface pumped by a 70 fs Ti:Sapphire laser pulse was observed. The expansion speed of the
surface estimated from the result (34 nm/50 ps) indicated that the nano-bubble structures were formed in the initial stage
of the ablation. In order to observe the detailed dynamics, the temporal synchronization between the pump and probe
pulses was improved to be 3 ps by adopting a portion of the SXRL and pump beams as the time fiducials, to which the
pump and probe timing was adjusted by using the x-ray streak camera.
This paper gives an overview of recent progress of laser-driven plasma x-ray lasers in Japan Atomic Energy Agency (JAEA). Fully spatial coherent plasma x-ray laser (XRL) at 13.9 nm with 0.1 Hz repetition rate has been developed using new driver laser system TOPAZ, and the succeeding optimization of the pumping condition has realized more efficient generation of the coherent x-ray pulse. The 0.1 Hz XRL is now routinely used in the wide variety of the
application experiments: The highlights of these applications are the study of fluctuation in the atomic structure of ferroelectric substances under the phase transition using the double XRL probe beam technique and the construction of new x-ray laser interferometer to observe nano-scale dynamics of materials.
This paper gives an overview of recent progress of x-ray laser research in Japan Atomic Energy Research Institute (JAERI). In the development of high quality x-ray laser beam, the progress includes the improvement of output energy of fully spatial coherent x-ray laser beam at a wavelength of 13.9 nm and generation of temporally coherent x-ray laser at 26.9 nm by use of seed x-ray injection technique. Beam stability is greatly improved to be better than 0.5 mrad by introducing new designed target chamber and target alignment system. In the application of the 13.9 nm laser, an experiment by use of x-ray speckle technique reveals firstly the existence of polarization clusters in ferroelectric substance. For the purpose of further application experiments, 0.1 Hz-repetition rate x-ray laser driver is being developed, which is based on an OPCPA pre-amplifier and a Nd:glass zigzag slab amplifier with two beam lines, and each line provides 10 Joules 1 ps pulse on target.
Experiments on the Equation of State (EOS) of Carbon were performed at the PALS and LULI laboratories. We used Carbon samples with two different value of initial density, in order to explore a wider region of the phase diagram. We obtain experimental data for carbon Hugoniot at Megabar pressures induced by laser-driven shock waves. The target rear side emissivity due to the shock unloading was recorded from (two-materials two-steps) targets (Al-C) with
space and time resolution. By applying the impedance mismatch method, a direct determination of relative EOS points was obtained. Experimental data are compared with previous experiments and with theoretical models. Our results indicate a higher compressibility of carbon at Megabar pressures compared to theoretical models.
Here we discuss the results of the experiments performed using the Prague Asterix Laser System (PALS) of wavelength 0.44 μm (3ω of Iodine laser) and energy ≈ 250 J in 450 ps (FWHM). Two sets of experiments were carried out, firstly, generation of high quality shocks which were steady in time and uniform in space using Phase Zone Plates (PZP), to establish the scaling laws of shock pressure Vs. laser intensity for aluminum foil target of thickness 8 μm. Our results show a good agreement with the delocalized laser absorption model. Secondly, measurements of the Equation of State of carbon compressed by shocks at megabars of pressure have been realized. Equation of State were obtained for carbon using the impedance mismatch technique. Step targets allowed the simultaneous measurements of shock velocity in two different materials. Aluminum was used as a reference material and relative EOS data for carbon have been obtained up to ≈ 14 Mbar pressure.
We have succeeded in developing a laser-pumped x-ray laser with full spatial coherence at 13.9 nm. A highly directed x-ray laser beam with the divergence of 0.2 mrad was generated from the double target experiment, where a seeding light from the first laser medium was amplified in the second medium. The observed divergence is close to the diffraction limited value within a factor of two. The seeding light was amplified in the second medium without refraction influence and the gain coefficient was about 8 cm<sup>-1</sup>. The gain region of the second medium was far away from the target surface compared with that of the first medium and located in the considerably low density region. From the measurement of visibility, it was found that the spatial coherent length is longer than the beam diameter.
An imploded plasma core is irradiated by a 100 ps laser pulse in a model experiments of fast ignition. Additional laser pulses for drilling and heating are introduced co- axially with the laser beams for the implosion. The preformed imploded core is created by the 12 beams of 0.53 micrometers laser with the total energy of 800 J. The additional heating pluses contain 100 ps pulses separated by 300 ps at the wavelength of 1.06 micrometers with the total energy of 320J. The first pulse is intended for drilling the coronal pulses surrounding the core and the second is for addition heating of the core. We measured the imploded core additionally heated with 100 ps pulses.
In the direct-drive scheme implosion of the inertial confinement fusion, the hot spark formation is critically affected by laser irradiation non-uniformities and subsequent hydrodynamic instabilities. Influence of the low- modal irradiation non-uniformities on the hot spark formation was investigated by means of the time- and space- resolved x-ray spectroscopic measurements. Experimental results were compared with post-processed hydro-code simulations by the aid of x-ray spectrum analysis code.
IN order to directly observe low-mode implosion nonuniformities, especially of l equals 1, which prevents stable formation of a hot spark in the compressed core plasma at the final stage of the implosion, a series of direct-drive implosion experiments has been performed at the Gekko-XII glass laser facility by using gas-filled plastic- shell targets. Partially coherent light (PCL) was used as a drive laser to suppress middle- to higher-modes of the irradiation nonuniformity down to approximately 1 percent. A clear shift from the center of the chamber and a slight crosswise structure were seen in the time- and space- resolved shape of the shell in the accelerating phase with an x-ray framing camera and absorbed laser intensity were also estimated by using a rocket equation. In order to confirm the property of these results, separate experiments under similar laser conditions were performed by using Au- coated sapphire spheres as surrogate target. The x-ray intensity distribution on the circumference of the target in the XFCs image, which is strongly dependent on the drive nonuniformity in the accelerating shell resulted from the l equals 1 drive nonuniformity. In our experiments, the l equals 1 drive nonuniformity due to some reproducible factors was found to be of the order of approximately 10-20 percent.