H. Homma

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.

Plasma physics and laser development for the Fast-Ignition Realization Experiment (FIREX) Project

Since the approval of the first phase of the Fast-Ignition Realization Experiment (FIREX-I), we have devoted our efforts to designing advanced targets and constructing a petawatt laser, which will be the most energetic petawatt laser in the world. Scientific and technological improvements are required to efficiently heat the core plasma. There are two methods that can be used to enhance the coupling efficiency of the heating laser to the thermal energy of the compressed core plasma: adding a low-Z foam layer to the inner surface of the cone and employing a double cone. The implosion performance can be improved in three ways: adding a low-Z plastic layer to the outer surface of the cone, using a Br-doped plastic ablator and evacuating the target centre. An advanced target for FIREX-I was introduced to suit these requirements. A new heating laser (LFEX) has been constructed that is capable of delivering an energy of 10 kJ in 10 ps with a 1 ps rise time. A fully integrated fast-ignition experiment is scheduled for 2009.

Enhanced Catalytic Activity of Gold Nanoparticles Doped in a Mesoporous Organic Gel Based on Polymeric Phloroglucinol Carboxylic Acid−Formaldehyde

Gold nanoparticles were supported by a phloroglucinolcarboxylic acid-formaldehyde (PF) gel, a new organic gel with a 30 nm spheroid-like structure. The surface area of the PF gel with gold nanoparticles was 550 m(2)/g. Gold nanoparticles supported on a PF gel exhibited catalytic activity in the reduction of 4-nitrophenol with a reaction rate constant of 7.4 x 10(-3) s(-1), which is high in the reported heterogeneous reaction system. The adsorption behavior of 4-nitrophenol into the gel support was observed by ultraviolet-visible absorption spectroscopy. Gold nanoparticles in the PF network were characterized by scanning electron microscopy, atomic force microscopy, and transmission electron microscopy observation. The high reduction rate would be attributed to the extraction and diffusion of the reactant through the pores of a PF gel support to encounter the highly dispersed gold nanoparticles on the surface and inside the material.

Fabrication of aerogel capsule, bromine-doped capsule, and modified gold cone in modified target for the Fast Ignition Realization Experiment (FIREX) Project

The development of target fabrication for the Fast Ignition Realization EXperiment (FIREX) Project is described in this paper. For the first stage of the FIREX Project (FIREX-I), the previously designed target has been modified by using a bromine-doped ablator and coating the inner gold cone with a low-density material. A high-quality bromine-doped capsule without vacuoles was fabricated from bromine-doped deuterated polystyrene. The gold surface was coated with a low-density material by electrochemical plating. For the cryogenic fuel target, a brand new type of aerogel material, phloroglucinol/formaldehyde (PF), was investigated and encapsulated to meet the specifications of 500 µm diameter and 20 µm thickness, with 30 nm nanopores. Polystyrene-based low-density materials were investigated and the relationship between the crosslinker content and the nanopore structure was observed.

Recent Developments in Fabrication of New Conceptual Gold Cone and Machining of Polystyrene Shell for Fast Ignition Target

Abstract Recent developments of several key issues for fabrication techniques of cone and shell target for the first phase of the Fast Ignition Realization Experiment (FIREX-I) project at the Institute of Laser Engineering, Osaka University, are described in this paper. The most important modification of the target design is a double cone, and a new fabrication technique has been developed. Although the error of assembling the cones is still several microns, the first prototype of a double-cone target with a vacuum gap of 20 μm was successfully provided for the preliminary experiment. Additionally, Ti:sapphire laser machining was used to bore a hole in the polystyrene shell.

Leakage Control of Tritium Through Heat Cycles of Conceptual-Design, Laser-Fusion Reactor KOYO-F

Abstract To reduce the tritium permeation from the primary liquid metal loop to the secondary water loop, a heat exchanger concept that incorporates small diameter tubes containing an oxidizer was proposed. An inert gas containing a small amount of oxidizer flows in the small tubes oxiding tritium that comes from the primary liquid metal coolant. The tritiated water is sent to a tritium recovery system minimizing leakage to the secondary water loop. Our evaluation results indicated that the tritium leakage through the heat exchanger was reduced to 1/105 with an acceptable increase in the size of the heat exchanger.

Laser machining for fabrication of targets used in the FIREX-I project

This paper reports on way to fabricate a gas-tight targets dedicated for the first stage of Fast Ignition Realization Experiment (FIREX-I) at the Institute of Laser Engineering (ILE), Osaka University. It was found that a Ti;sapphire laser machining can be used to fabricate the target. The performance of the laser machining using a fs Ti;sapphire laser was examined on shell materials. The conditions for accurate machining were determined. Michelson interferometer with two different wavelengths which imitates a white light interferometer is an excellent tool for confirming the gas-tightness of the target after assembly.

Present status and future prospect of Fast Ignition Realization Experiment (FIREX) Project at ILE, Osaka

Thermonuclear ignition and subsequent burn are key physics for achieving laser fusion. In fast ignition, a highly compressed fusion fuel generated with multiple ns‐laser beams is rapidly heated with a large energy, ps‐laser pulse in prior to core disassembly. This scheme has a high potential to achieve ignition and burn since driver energy required for high fusion gain is predicted to be about one tenth of that needed for the central ignition scheme. In Japan, Fast Ignition Realization Experiment (FIREX) project has been started to clarify the physics of energy transport and deposition in the core plasma and to demonstrate fuel temperature of above 5 keV. After the success, FIREX‐I will be followed by the second phase of the project (FIREX‐II) to demonstrate ignition and burn. LFEX laser, designed to deliver a laser pulse of 10 kJ in 10 ps, are operational and the first phase of FIREX experiments has been stated. A new target is proposed to attain dense compression of fuel and improve laser‐core coupling efficiency by adopting double‐cone structure, a low‐density inner liner, low‐Z outer coating, and Br‐doped fuel shell. In this paper, present status and near term prospects of the FIREX‐I project will be reported together with activities on target designing, laser development, and plasma diagnostics.

High-speed monochromatic x-ray imager for electron temperature mapping of fast igniter plasmas

We report on experiments aimed at obtaining laser-imploded core plasma temperature maps using two color, monochromatic x-ray imager. This instrument uses two toroidally bent Bragg crystals and a high-speed sampling imager to provide spatial resolution of 25 ?m, temporal resolution of 20 ps, and spectral resolution over 200, simultaneously. Using a deuterated plastic shell target filled with CHClF2 gas, time-resolved monochromatic x-ray images in the Cl-He? and Cl-Ly? were successfully obtained.

A model experiment of a double-cone target using a gap target

We have conducted preliminary experiments to prove the vacuum-shielding effect using an Al-Cu double-foil (both Al and Cu foils are 1 mm × 1 mm × 10 μmt) target with a vacuum-gap. Particle-in-cell simulation results have shown that the double-cone confines the electrons for hundreds of femtoseconds by the sheath electric field inside the vacuum-gap [T. Nakamura, et al., Phys. Plasmas 14, 103105 (2007)]. Cu-Kα intensity decreased with the vacuum-gap, i.e., the number of fast electrons that reached the Cu foil decreased with the gap. In the stack measurement, the number of electrons detected in the target surface direction increased with the gap. These results indicate that when the double-cone target is used, fast electrons created by an ignition laser can be reflected from the vacuum-gap, move along the cone surface, and be transferred towards the cone tip.