H. Azechi

Scalings of implosion experiments for high neutron yield

A series of experiments focused on high neutron yield has been performed with the Gekko‐XII green laser system [Nucl. Fusion 27, 19 (1987)]. Deuterium–tritium (DT) neutron yield of 1013 and pellet gain of 0.2% have been achieved. Based on the experimental data from more than 70 irradiations, the scaling laws of the neutron yield and the related physical quantities have been studied. Comparison of the experimental neutron yield with that obtained by using a one‐dimensional fluid code has led to the conclusion that most of the neutrons produced in the stagnation phase of the computation are not observed in the experiment because of fuel–pusher mixing, possibly induced by the Rayleigh–Taylor instability. The coupling efficiency and ablation pressure have been calculated using the ion temperature measured experimentally. A coupling efficiency of 5.5% and an ablation pressure of 50 Mbar have been obtained.

Kilotesla Magnetic Field due to a Capacitor-Coil Target Driven by High Power Laser

Laboratory generation of strong magnetic fields opens new frontiers in plasma and beam physics, astro- and solar-physics, materials science, and atomic and molecular physics. Although kilotesla magnetic fields have already been produced by magnetic flux compression using an imploding metal tube or plasma shell, accessibility at multiple points and better controlled shapes of the field are desirable. Here we have generated kilotesla magnetic fields using a capacitor-coil target, in which two nickel disks are connected by a U-turn coil. A magnetic flux density of 1.5 kT was measured using the Faraday effect 650 μm away from the coil, when the capacitor was driven by two beams from the GEKKO-XII laser (at 1 kJ (total), 1.3 ns, 0.53 or 1 μm, and 5 × 1016 W/cm2).

Direct-drive hydrodynamic instability experiments on the GEKKO XII laser

Hydrodynamic instabilities, such as the Rayleigh–Taylor (R–T) instability, play a critical role in inertial confinement fusion as they finally cause fuel-pusher mixing that potentially quenches thermonuclear ignition. Good understanding of the instabilities is necessary to limit the mixing within a tolerable level. A series of experiments has been conducted on the GEKKO XII laser facility [C. Yamanaka et al., IEEE J. Quantum Electron. QE-17, 1639 (1981)] to measure hydrodynamic instabilities in planar foils directly irradiated by 0.53 μm laser light. It has been found that (1) the imprint is reasonably explained by an imprint model based on the equation of motion with the pressure perturbation smoothed by the cloudy-day effect, and (2) the experimental R–T growth rate is significantly reduced from the classical growth rate due probably to ablative stabilization enhanced by nonlocal heat transport.

Spectrally dispersed amplified spontaneous emission for improving irradiation uniformity into high power Nd:glass laser system

An amplified spontaneous emission (ASE) from Nd:glass has been introduced into the high power twelve beam Nd:glass laser system, Gekko XII for obtaining smooth intensity distribution of a focused beam. The angular dispersion of an ASE spectrum with large beam divergence was adopted for efficient beam smoothing without significant reduction of the harmonic conversion efficiency. Temporal evolution of the beam smoothing was evaluated as a function of the beam divergence by using a statistical model of speckle. In Gekko XII, the spectral width and beam divergence of ASE were controlled in a range of 0.4 to 0.6 nm and 6 to 22 times diffraction limited, respectively. Final output energy of 1.3 kJ/beam in a 2.2 ns duration was demonstrated without significant gain reduction and spectral narrowing. The doubling efficiency of 50% was obtained at a low intensity region of around 0.3 GW/cm2 by matching the angular dispersion of spectrum to that of phase matching condition of a frequency conversion crystal. The spec...

Experimental determination of fuel density‐radius product of inertial confinement fusion targets using secondary nuclear fusion reactions

The first demonstration of a fuel density‐radius product measurement using secondary nuclear fusion reactions is presented. This technique involves using neutrons and protons generated by DT {T(d,n)α} and D3He {3He(d,p)α} fusion reactions, respectively, in a pure deuterium fuel.

Present status of the FIREX programme for the demonstration of ignition and burn

The recent fast heating of a compressed core to 0.8–1 keV temperature as well as the previous high-density compression of 600 times liquid density have provided proof-of-principle of the fast ignition (FI) concept. These results have significantly contributed to official approval of the first phase of the FI realization experiment (FIREX) project. The goal of FIREX-I is to demonstrate fast heating of a fusion fuel up to the ignition temperature of 5–10 keV. Although the fuel of FIREX-I is too small to be actually ignited, sufficient heating will provide the scientific viability of ignition and burn by increasing the laser energy, thereby increasing the fuel size. The FIREX programme is being carried out in collaboration with the Institute of Laser Engineering, Osaka University and the National Institute for Fusion Science, including the development of cryogenic targets, holistic simulation systems and diagnostic equipment.

Innovative ignition scheme for ICF—impact fast ignition

A new ignition scheme is proposed, in which the compressed DT main fuel is ignited by impact collision of another fraction of separately imploded DT fuel, which is accelerated in the hollow conical target to super high velocities of about (1–2) × 108 cm s−1. Its kinetic energy is directly converted into thermal energy corresponding to temperatures >5 keV on the collision with the main fuel, and this self-heated portion plays the role of ignitor. The ignitor shell is irradiated typically by nanosecond pulses at intensities well beyond 1015 W cm−2 at such a short laser wavelength as 0.25 µm to exert ablation pressures of 150–300 Mbar. A preliminary two-dimensional hydrodynamic simulation demonstrates substantial heating of 3–5 keV on the impact. Simple physics, potential for high gain designs and low cost—these are the crucial advantages of the present scheme.

Irradiation nonuniformity due to imperfections of laser beams

Intractable low‐mode nonuniformities caused by power imbalance or pointing error of beams are studied quasi‐analytically based on Skupsky’s axially symmetric model [S. Skupsky and K. Lee, J. Appl. Phys. 54, 3662 (1983)]. These nonuniformities can be improved by decreasing the imperfections σPΩ, or by increasing the number of laser beams NB: deteriorated irradiation uniformity is shown to be proportional to σPΩ/√NB. Criteria of these imperfections for high irradiation uniformity [≤1% root‐mean square (rms)] are then presented in terms of NB. The model is further extended to treat general asymmetric beam patterns. Optimization of beam pattern is also a central issue.

Pr3+-doped fluoro-oxide lithium glass as scintillator for nuclear fusion diagnostics

Experimental results are presented on the neutron scintillating properties of a custom-designed Pr3+ (praseodymium)-doped lithium (Li) glass. Luminescence was observed at 278 nm wavelength, originating from the 5d-4f transition. Time-resolved measurements yielded about 20 ns decay times for ultraviolet and x-ray excitation while much faster decay times of about 6 ns were observed for alpha particle and neutron excitation. Actual time-of-flight data in laser fusion experiments at the GEKKO XII facility of the Institute of Laser Engineering, Osaka University reveal that it can clearly discriminate fusion neutrons from the much stronger x-rays signals. This material can promise improved accuracy in future scattered neutron diagnostics.

Fast plasma heating in a cone-attached geometry - towards fusion ignition

We have developed a PW (0.5 ps/500 J) laser system to demonstrate fast heating of imploded core plasmas using a hollow cone shell target. Significant enhancement of thermal neutron yield has been realized with PW-laser heating, confirming that the high heating efficiency is maintained as the short-pulse laser power is substantially increased to a value nearly equivalent to the ignition condition. It appears that the efficient heating is realized by the guiding of the PW laser pulse energy within the hollow cone and by self-organized relativistic electron transport. Based on the experimental results, we are developing a 10 kJ-PW laser system to study the fast heating physics of high-density plasmas at an ignition-equivalent temperature.