Tatsuhiro Sakaiya

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.

Acceleration to high velocities and heating by impact using Nike KrF laser

The Nike krypton fluoride laser [S. P. Obenschain, S. E. Bodner, D. Colombant, et al., Phys. Plasmas 3, 2098 (1996)] is used to accelerate planar plastic foils to velocities that for the first time reach 1000 km/s. Collision of the highly accelerated deuterated polystyrene foil with a stationary target produces ∼Gbar shock pressures and results in heating of the foil to thermonuclear temperatures. The impact conditions are diagnosed using DD fusion neutron yield, with ∼106 neutrons produced during the collision. Time-of-flight neutron detectors are used to measure the ion temperature upon impact, which reaches 2–3 keV.

Shock Hugoniot and temperature data for polystyrene obtained with quartz standard

Equation-of-state data, not only pressure and density but also temperature, for polystyrene (CH) are obtained up to 510 GPa. The region investigated in this work corresponds to an intermediate region, bridging a large gap between available gas-gun data below 60 GPa and laser shock data above 500 GPa. The Hugoniot parameters and shock temperature were simultaneously determined by using optical velocimeters and pyrometers as the diagnostic tools and the α-quartz as a new standard material. The CH Hugoniot obtained tends to become stiffer than a semiempirical chemical theoretical model predictions at ultrahigh pressures but is consistent with other models and available experimental data.

Suppression of Rayleigh–Taylor instability due to radiative ablation in brominated plastic targets

Suppression of hydrodynamic instabilities is very crucial for the ultimate goal of inertial fusion energy (IFE). A high-Z doped plastic of CHBr (brominated polystyrene) ablator is a very promising candidate to suppress the ablative Rayleigh–Taylor (RT) instability in a directly laser-driven IFE target. When a CHBr target is irradiated by intense laser beams, bromine atoms in the corona plasma emit strong radiation. The strong radiation drives the radiative ablation front inside the CHBr targets. This radiative ablation in the high-Z doped plastic target has many advantages for the suppression of the growth of the RT instability in analogy to the indirect-drive approach, i.e., large mass ablation rate, long density scale length and low peak density. Two-dimensional (2D) hydrodynamic simulation shows significant suppression of the RT instability in a CHBr target compared to an undoped polystyrene (CH) target. RT growth rate, calculated theoretically using the Betti–Goncharov procedure with a one-dimensional...

Towards realization of hyper-velocities for impact fast ignition

A new ignition scheme, impact fast ignition (IFI), is studied, in which the compressed DT main fuel is to be ignited by impact with another fraction of separately imploded DT fuel, which is accelerated in the hollow conical target. The first and distinct milestone in the IFI scenario is the demonstration of such a hyper-velocity, of the order of 108 cm s−1. Two-dimensional hydrodynamic simulation results obtained in full geometry using plastic instead of DT fuel are presented, in which some key physical parameters for the impact shell dynamics, such as an implosion velocity of 108 cm s−1, a compressed density of 300–400 g cm−3 and a converted temperature greater than 5 keV, are demonstrated. A preliminary experimental result with a planar target is presented to show the highest velocity, 6 × 107 cm s−1, ever achieved.

Laser-shock compression and Hugoniot measurements of liquid hydrogen to 55 GPa

KYOKUGEN, Center for Quantum Science and Technology under Extreme Conditions,Osaka University, Toyonaka, Osaka 560-8531, Japan(Dated: January 7, 2011)The principal Hugoniot for liquid hydrogen was obtained up to 55 GPa under laser-driven shockloading. Pressure and density of compressed hydrogen were determined by impedance-matching toa quartz standard. The shock temperature was independently measured from the brightness of theshock front. Hugoniot data of hydrogen provide a good benchmark to modern theories of condensedmatter. The initial number density of liquid hydrogen is lower than that for liquid deuterium, andthis results in shock compressed hydrogen having a higher compression and higher temperature thandeuterium at the same shock pressure.

Self-similar ablative flow of nonstationary accelerating foil due to nonlinear heat conduction

Ablating plasma flow of an accelerating foil driven by nonlinear heat conduction is investigated theoretically. It is shown that the hydrodynamic system admits a new self-similar solution describing the nonstationary ablation process, through which the payload mass decreases to burn out at the end. In contrast to previous analyses based on stationary flow, the present solution provides a practical physical picture with a finite peak density and a distinct vacuum boundary at the front. The system is solved as a novel eigenvalue problem such that the acceleration and the heat conductivity are restrictive with each other under the self-similar evolution. Scaling laws are obtained to describe the temporal evolution for the shell acceleration and such ablation performances as the mass ablation rate and the ablation pressure.

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 ...

Rayleigh–Taylor instability growth on low-density foam targets

In recent laser fusion programs, foam cryogenic targets have been developed as promising targets which have a great potential to realize efficient nuclear fusion. The foam is porous plastic material having a microstructure inside. We observed the growth of the Rayleigh–Taylor (RT) instability on the foam target with initial surface perturbation for the first time. The measured RT growth rate on the foam target was clearly suppressed in comparison to that of normal-density polystyrene (CH) targets. The values of the RT growth rate for the low-density foam target and the normal-density CH target were 0.84±0.15 (1∕ns) and 1.33±0.1 (1∕ns), respectively.

Impact ignition as a track to laser fusion

In impact ignition, the compressed deuterium–tritium main fuel is ignited by impact with a separately imploded portion of fuel, which is accelerated in a hollow conical target to hyperspeeds of the order of 1000 km s−1. Its kinetic energy is directly converted into thermal energy corresponding to an ignition temperature of about 5 keV upon collision with the compressed fuel. The ignitor shell is irradiated by nanosecond pulses at intensities of between 1015 and 1016 W cm−2 with a wavelength of 0.25–0.35 µm, resulting in ablation pressures of several hundred mega-bars. Hydrodynamics-dominated physics and avoidance of ultra-intense petawatt lasers are notable features of this scheme. Experimental results for velocities exceeding 1000 km s−1, ion temperatures up to 3 keV, and neutron yield increases of 100-fold due to the impact effect indicate the potential of impact ignition for fusion energy production. The overall performance of impact ignition is reviewed with new analyses on the neutron yield and shell acceleration.