Takeshi Watari

Fast ignition integrated experiments with Gekko and LFEX lasers

Based on the successful result of fast heating of a shell target with a cone for heating beam injection at Osaka University in 2002 using the PW laser (Kodama et al 2002 Nature 418 933), the FIREX-1 project was started in 2004. Its goal is to demonstrate fuel heating up to 5 keV using an upgraded heating laser beam. For this purpose, the LFEX laser, which can deliver an energy up to10 kJ in a 0.5–20 ps pulse at its full spec, has been constructed in addition to the Gekko-XII laser system at the Institute of Laser Engineering, Osaka University. It has been activated and became operational since 2009. Following the previous experiment with the PW laser, upgraded integrated experiments of fast ignition have been started using the LFEX laser with an energy up to 1 kJ in 2009 and 2 kJ in 2010 in a 1–5 ps 1.053 µm pulse. Experimental results including implosion of the shell target by Gekko-XII, heating of the imploded fuel core by LFEX laser injection, and increase of the neutron yield due to fast heating compared with no heating have been achieved. Results in the 2009 experiment indicated that the heating efficiency was 3–5%, much lower than the 20–30% expected from the previous 2002 data. It was attributed to the very hot electrons generated in a long scale length plasma in the cone preformed with a prepulse in the LFEX beam. The prepulse level was significantly reduced in the 2010 experiment to improve the heating efficiency. Also we have improved the plasma diagnostics significantly which enabled us to observe the plasma even in the hard x-ray harsh environment. In the 2010 experiment, we have observed neutron enhancement up to 3.5 × 107 with total heating energy of 300 J on the target, which is higher than the yield obtained in the 2009 experiment and the previous data in 2002. We found the estimated heating efficiency to be at a level of 10–20%. 5 keV heating is expected at the full output of the LFEX laser by controlling the heating efficiency.

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.

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...

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-driven shock waves and thermonuclear neutron generation

Impact-driven shock waves, thermonuclear plasma and neutron yield were investigated. The results of 2D numerical simulations and Gekko/HIPER laser experiments on the collision of a laser-accelerated disk-projectile with a massive target, both containing (CD)n-material, are discussed. A two-temperature model of the non-equilibrium plasma created by impact-driven shock waves due to the collision of a laser-accelerated planar projectile with a massive target was developed and used for analysis of the numerical and experimental results. The model defines the characteristics of shock waves and plasmas (including their lifetime) as well as neutron yields in both the colliding objects as functions of velocity, density and mass of the projectile–impactor just before collision. The neutron yield generated during the period of laser-driven acceleration of the impactor was also determined.Two effects were discovered that exert a substantial influence on the plasma parameters and neutron yield. The first of them relates to the formation of the pre-impact state of the impactor. It decreases the projectile density due to thermal expansion of its matter through a free boundary during the period of laser-driven acceleration. The other relates to the formation of impact-produced plasma. Predominant heating of the ion component of plasma leads to the existence of a non-equilibrium two-temperature plasma during the period of electron–ion relaxation.

Direct measurement of chemical composition of SOx in impact vapor using a laser gun

The final chemical composition of vapor clouds created by the impacts higher than 10 km/s is important to investigate the evolution of the planetary surface environment and life. However, no previous experimental study has observed directly it because of experimental difficulties. In this study, we conducted hypervelocity impact experiments using a laser gun and measured the chemical compositions of the impact-generated sulfuric oxides directly. The result clearly shows that the sulfur oxides released by hypervelocity impacts are dominated by SO3, not SO2.

Fast response neutron scintillation detector for FIRE-X

We have been developing fast responding neutron detectors with a view to revealing the effect of additional laser heating in FIREX-I (Fast Ignition Realization Experiment) by measuring the burn time with a time resolution under 100 ps at the relatively low neutron yield (about 106). The detector is constructed with a thin plastic-scintillaotor (BC-422), a micro-channel-plate photomultiplier tube (MCP-PMT) and a bundle optical fiber as a light-guide. The rise time of a neutron signal from the detector is measured to be 220 ps. The time-determination error for measuring burn time is estimated to be less than ± 46 ps from the data of characterization experiments measuring the transit time distribution of signals, and calculated values of Doppler broadening and the uncertainty of the scintillation time due to the thickness of the scintillator. In the future with more neutron yield, we will construct a scintillation-fiber-streak camera to detect burn history.

Development of TOF neutron spectrometer for the measurement of degenerated plasma in fast ignition experiment

There are two essential requirements to achieve the Fast Ignition. One is high density compression of main fuel and the other one is additional heating of main fuel by ultra intense laser. Then the experimental determination of both the density of the main fuel before heating laser irradiation and ion temperature after heating laser irradiation is necessary. In the experiment of the Central Ignition, we developed both ion temperature measurement and density measurement, which is based on theoretical degenerated plasma model, by using Time-Of-Flight neutron spectrometer. The main problem in applying this measurement in Fast Ignition experiment is huge amount of high energy x ray, which is generated as a result of heating laser irradiation. Then we installed a total 15-cm thick lead shield as a measure against it. It was confirmed that the newly installed shield achieves x ray attenuation to 5.7±2.0×10−4 of previous level by Geant4 MC simulation. And the error for ion temperature measurement was confirmed to be smaller than 0.3 keV in an experiment whose neutron yield was 1.7±0.2×106.

Progress of impact ignition

Recent progress of impact ignition is reported: First, a maximum velocity ∼ 1000 km/s has been achieved under the operation of NIKE KrF laser at Naval Research Laboratory (laser wavelength = 0.25μm) in the use of a planar target made of plastic. Two-dimensional simulation have been performed for burn and ignition to show the feasibility of the impact ignition. Optimized direct illumination scheme is also addressed.

Observation of the non-local electron transport effect by using phase zone plate

Non-local electron transport effect plays a significant role in inertial confinement fusion because it potentially preheats the fusion fuel and lowers the target density. Non-local electron transport effect is more pronounced for longer laser wave-length and higher intensity. We measured the density of the plastic target irradiated with 0.53 μm laser by using a phase zone plate (PZP) that has spatial resolution of about 2 μm. The target density predicted by the ILESTA-1D simulation with Spitzer-Harm thermal conduction is 1.5 times as large as that predicted with Fokker-Planck thermal conduction. The measured density profile is close to the density profile predicted by the simulation with Fokker-Planck thermal conduction.