Keisuke Shigemori

Hugoniot measurement of diamond under laser shock compression up to 2 Tpa

Hugoniot data of diamond was obtained using laser-driven shock waves in the terapascal range of 0.5–2TPa. Strong shock waves were generated by direct irradiation of a 2.5ns laser pulse on an Al driver plate. The shock wave velocities in diamond and Al were determined from optical measurements. Particle velocities and pressures were obtained using an impedance matching method and known Al Hugoniot. The obtained Hugoniot data of diamond does not show a marked difference from the extrapolations of the Pavlovskii Hugoniot data in the TPa range within experimental errors.

Reduction of the Rayleigh-Taylor instability growth with cocktail color irradiation

A novel method for reducing the Rayleigh-Taylor instability (RTI) growth in inertial confinement fusion (ICF) targets is reported. It is well known that high-density compression of ICF targets is potentially prevented by the RTI. Previous studies [K. Shigemori et al., Phys. Rev. Lett. 78, 250 (1997), S. G. Glendinning et al., Phys. Rev. Lett. 78, 3318 (1997), and H. Azechi et al., Phys. Plasmas 4, 4079 (1997)] have indicated that nonlocal electron heat transport enhances the effect on the ablative stabilization of the RTI growth with long wavelength laser irradiation. Planar target experiments, using a small fraction of a long wavelength laser (λ=0.53 or 1.05μm) in addition to the main drive laser (λ=0.35μm), were conducted to verify the RTI reduction by inducing the effect of the nonlocal electron heat transport. The measured RTI growth rate for this “cocktail-color” laser irradiation was clearly reduced from that for the “single-color” short-wavelength laser irradiation. The experimental growth factors ...

Measurement of heating laser injection time to imploded core plasma by using x-ray framing camera.

A simultaneous measurement of imploded core plasma and injection time of heating laser is conducted by using an x-ray framing camera (XFC). The experiments are performed using Gekko XII laser system for implosion of the deuterated polystyrene (CD) plastic shell target and Peta Watt (PW) laser system for heating. The time of PW laser injection is observed as the bright zone in the XFC image. The measured x-ray intensity profiles fit the Gaussian profiles well. The calculations of microchannel plate by using dynode model explain these broadened temporal profiles qualitatively. The peak position of fitted x-ray intensity profile is almost in agreement with the time when the high energy x ray is observed by x-ray streak camera. Moreover, the peak position is delayed corresponding to the delayed setting of PW laser injection time. From these results, it is concluded that we can estimate the heating laser injection time with resolution of the order of 10 ps by using XFC.

Shock Pyrometry of Laser-Irradiated Foils Below 1 eV

An improved technique for measuring the temperature at the rear surface of laser-irradiated foils is presented. We observed the optical emission at the rear surface of laser-irradiated foils using a spectrometer equipped with an optical streak camera. Time-resolved temperature below 1 eV was obtained from the black-body spectra.

Multiple shock compression of diamond foils with a shaped laser pulse over 1 TPa

Experiments on shock compression of diamond foils with intense laser are reported. In order to explore high-pressure and low-temperature region of the carbon phase-diagram, we irradiate single-crystal (Type-Ib) crystal foils with a shaped (ramp) pulse at the pressure of over 1TPa. We observed time-resolved reflectivity from the rear surface of the diamond during the shock compression with velocity interferometer system for any reflector (VISAR). Simultaneous observation of optical measurements and x-ray diffraction has been tested. Preliminary results on decompression of the diamond crystal due to x-ray heating were observed.

Streaked x-ray backlighting with twin-slit imager for study of density profile and trajectory of low-density foam target filled with deuterium liquid

Low-density plastic foam filled with liquid deuterium is one of the candidates for inertial fusion target. Density profile and trajectory of 527 nm laser-irradiated planer foam-deuterium target in the acceleration phase were observed with streaked side-on x-ray backlighting. An x-ray imager employing twin slits coupled to an x-ray streak camera was used to simultaneously observe three images of the target: self-emission from the target, x-ray backlighter profile, and the backlit target. The experimentally obtained density profile and trajectory were in good agreement with predictions by one-dimensional hydrodynamic simulation code ILESTA-1D.

Non-dimensional scaling of impact fast ignition experiments

Recent experiments at the Osaka University Institute for Laser Engineering (ILE) showed that Impact Fast Ignition (IFI) could increase the neutron yield of inertial fusion targets by two orders of magnitude [1]. IFI utilizes the thermal and kinetic energy of a laser-accelerated disk to impact an imploded fusion target. ILE researchers estimate a disk velocity of 108 cm/sec is needed to ignite the fusion target [2]. To be able to study the IFI concept using lasers different from that at ILE, appropriate non-dimensionalization of the flow should be done. Analysis of the rocket equation gives parameters needed for producing similar IFI results with different lasers. This analysis shows that a variety of laboratory-scale commercial lasers could produce results useful to full-scale ILE experiments.

Measurements of Sound Velocity of Laser‐Irradiated Iron Foils Relevant to Earth Core Condition

We have developed a novel method to measure sound velocity of laser‐irradiated iron foils by side‐on x‐ray radiograph technique. Iron foils were irradiated with two‐stepped laser pulse to reach the earth’s core condition. We obtained not only the sound velocity but also temperature, pressure, shock velocity, compressibility, and particle velocity of the laser‐irradiated iron. The experimental results are in good agreements with previous experimental results and with one‐dimensional simulation results.