Yasuyuki Nakao

Burn characteristics of inertially confined D-3He fuel

Many difficult technological problems encountered in fusion reactors could probably be solved if D-3He fuel would be used instead of D-T fuel. The paper examines the burn characteristics for inertial confinement fuel pellets, i.e. for pure D-3He fuel pellets and for D-3 ignitor/D-3He fuel pellets, using a hydrodynamics code modified to include neutron transport and charged particle transport. The results indicate that it is difficult to ignite pure D-3He fuel pellets with a reasonable driver energy. However, the ignition conditions can be relaxed by using D-T fuel as ignitor. It is found that a fuel gain of ~480 can be obtained with D-T/D-3He pellets having a spark temperature of 5 keV, a D-T ignitor ρR value of 3.0 g/cm2 and a total fuel ρR value of 15.5 g/cm2. When the density of these pellets is assumed to be 10 000 times the liquid density, the internal energy of the configuration at ignition is ~1.2 MJ, which is still more severe than that in the case of D-T fuel. It is also shown that the transport of 14.7 MeV protons produced by the D-3He reaction plays a significant role in determining the burn characteristics of the pellets. Nuclear elastic scattering (NES) of 14.7 MeV protons enhances the ion heating, leading to a fuel gain which is higher by about 20% than that in the case without NES. The effect of nuclear spin polarization is also examined for an ideally polarized fuel pellet. It is found that the burn characteristics are improved and that the required driver energy is possibly decreased.

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.

Two-dimensional relativistic Fokker-Planck model for core plasma heating in fast ignition targets

One of the key issues in the fast ignition scheme is a clarification of the imploded dense core heating by laser-produced fast electrons. To investigate the core heating process, a two-dimensional relativistic Fokker-Planck code “RFP-2D” for fast electron behavior in dense core plasmas has been developed. Energy loss of fast electrons due to Coulomb interactions is treated through not only usual short-range binary collisions but also long-range binary collisions, including a collective shielding effect. After describing the physics model, an examination is made of the energy deposition of fast electrons injected into a highly compressed D-T cylindrical plasma. The relative importance of the long-range Coulomb interaction and the influence of a self-generated electromagnetic field on the energy deposition profile are demonstrated.

Core heating properties in FIREX-I?influence of cone tip

On the basis of one-dimensional coupled PIC and Fokker?Planck simulations, the core heating properties of different cone materials for sub-ignition class experiments of the cone-guiding fast ignition have been studied. When Au is used as a material of the cone tip, the Au atoms ionize to a high charge state during the interaction with a heating pulse in a few hundreds of femtoseconds. Because of the extreme photon pressure, the pulse starts to interact directly with a solid-density cone tip after the density slope is steepened. In addition, the electrons in the return current are strongly scattered by the highly ionized Au ions. In such a situation, the energy coupling of the heating laser to the fast electrons could drop drastically. During the transport in the cone tip, the quality of the generated fast electron beam deteriorates due to the collisional and resistive drags and the scattering by the Au ions. As a result, the core heating gets saturated quickly and the energy coupling efficiency of the heating laser to the core decreases. We proposed CH as an alternative material of cone tip to reduce the collisional defects. It is found that in comparison with the Au cone tip, a twice higher rise in temperature of a compressed CD core has been achieved with the CH cone tip after 1?ps heating by a 1020?W?cm?2 intensity pulse.

Ion distribution function and radial profile of neutron production rate in spherical inertial electrostatic confinement plasmas

The radial profile of the neutron production rate in spherical inertial electrostatic confinement plasmas is investigated. The electrostatic potential is obtained by solving the Poisson equation, and by using the potential; the fuel ion velocity distribution function is determined at each radial point. From the velocity distribution function, the neutron production rate is accurately evaluated. Numerical results show that if it is assumed that fuel ions are confined keeping the total energy and angular momentum almost constant, the double radial peak in the neutron production rate can appear without creation of the deep double potential well.

Impact of Revised Thermal Neutron Capture Cross Section of Carbon Stored in JENDL-4.0 on HTTR Criticality Calculation

Recently, high-temperature gas-cooled reactors (HTGRs) have been receiving particular attention as one of the Generation IV nuclear reactor systems in the world, because of its excellence in safety, economical efficiency, and nuclear proliferation resistance, and applicability of nuclear power as a heat source for the thermochemical Iodine-Sulfur (IS) process, by which hydrogen is produced without the release of carbon dioxide. Many countries, then, have been performing design studies on HTGRs. Meanwhile, in Japan, the Japan Atomic Energy Agency (JAEA) has been conducting design studies on commercial HTGRs. JAEA has also been operating the High-Temperature Engineering Test Reactor (HTTR), which is a testing HTGR, and it has yielded useful data for conducting design studies on commercial HTGRs. The improvement of accuracy of the HTGR neutronics calculations allows the design of commercial HTGRs for low cost and high performance. Generally, the accuracy of neutronics calculation results depends on the nuclear data library used in the calculations; thus, the evaluation of the applicability of nuclear data libraries to HTGR neutronics calculations is one of the important subjects. In the past, the neutronics calculations for the HTTR critical approach were performed with the three major nuclear data libraries, namely, JENDL-3.3 (Japan), ENDF/BVI.5 (U.S.A), and JEFF-3.0 (Europe). As a result, JENDL3.3 yielded keff values that were in better agreement with the experimental results than the other libraries. Additionally, it was found that the discrepancies of the keff values between JENDL-3.3 and the other libraries are mainly caused by the slight difference in the neutron capture cross section of carbon at 0.0253 eV among the libraries, and we focused on the accuracy of this cross section as one of the important subjects for the improvement of the neutronics calculations for the HTGRs. JENDL-3.3 showed better applicability to the HTTR criticality calculations than the other libraries as mentioned above, but still overestimated the keff values by 0.5– 1.1% k. By overestimating the keff values, the calculation result of the loaded number of fuel columns achieving the first criticality did not agree with the experimental results. These problems were not resolved until today, despite our refinement efforts, such as the description of the core geometry and the concentration of the components. Meanwhile, the neutron capture cross section of carbon at 0.0253 eV stored in each nuclear data library had not been revised for a long time. Thus, we proposed that this cross section should be revised based on the latest measurement data, and also predicted that the problem of overestimating the keff values will be resolved by revising the cross section to be about 10% larger than that of JENDL-3.3. In May 2010, the latest JENDL, JENDL-4.0, was released by JAEA. In JENDL-4.0, our proposal with the prediction was applied, and the neutron capture cross section of carbon at 0.0253 eV was revised based on the latest measurement data. Accordingly, the problem of overestimating the keff values in the HTTR criticality calculations was expected to be addressed. This paper describes the investigation of the applicability of JENDL-4.0 to the HTTR criticality calculations.

Effect of nuclear elastic scattering on ion heating characteristics in deuterium-tritium thermonuclear plasmas

An effect of nuclear elastic scattering (NES) on the energy transfer to plasma ions and electrons during (a) neutral beam injection (NBI) and (b) α-particle heating operations is examined on the basis of the Boltzmann-Fokker-Planck (BFP) equation for a beam ion and an α-particle in deuterium-tritium thermonuclear plasmas. The BFP calculations show that the enhancement in the fraction of the NBI heating power deposited to ions due to NES becomes appreciable when the beam energy is larger than 1MeV. How the NES effect is influenced by the plasma condition is discussed.

Present Status of Fast Ignition Research and Prospects of FIREX Project

Abstract This is the review on the laser fusion research at Institute of Laser Engineering of Osaka University. Since 1996, we have concentrated our efforts on fast ignition laser fusion research. By constructing 100 TW and 1Peta watt lasers, experiments on relativistic laser plasma interactions related to fast ignition and pellet implosion and heating have been carried out. The results indicate that imploded core plasma is heated with relatively high coupling efficiency. According to the above results, we started the FIREX (Fast Ignition Realization Experiment) project for demonstrating ignition and burn with a multi 10kJ short pulse laser. The future prospects of the project are presented in this paper.

Effects of nuclear elastic scattering on energetic ion transport in hot dense plasmas

Effects of nuclear elastic scattering (NES) on the transport of energetic ions in plasma spheres at densities and temperatures of interest for inertial confinement fusion are investigated on the basis of a linear Boltzmann-Fokker-Planck equation. After presenting the adopted calculational model, numerical results are given for two particular problems: (1) the deposition of 14.7 MeV proton energy in a typical deuterium sphere and (2) the inflight fusion probability for energetic deuterons in a D-T pellet. It is shown that in both cases NES effects are significant. An adequate treatment of the slowing-down process, especially consideration of the transport of NES recoils, is essential for an accurate estimation of the energy deposition and of the fusion probability. Calculations neglecting NES considerably underestimate these quantities.

Effects of nuclear elastic scattering on ignition and thermal instability characteristics of D-D fusion reactor plasmas

Nuclear elastic scattering is included in the analysis of ignition condition and thermal instability in pure and catalysed D-D fusion reactor plasmas. Nuclear-elastic-scattering events take place, to some extent, during the slowing-down of energetic fusion products and enhance the heating of the background ions. The inclusion of this effect relaxes the minimum of the confinement requirement for self-sustained D-D plasmas by about 15%. It enhances, however, the thermal instability of the plasma, leading to an increase in the critical temperature.