Takuya Kondo

1 Hz fast-heating fusion driver HAMA pumped by a 10 J green diode-pumped solid-state laser

A Ti : sapphire laser HAMA pumped by a diode-pumped solid-state laser (DPSSL) is developed to enable a high-repetitive inertial confinement fusion (ICF) experiment to be conducted. To demonstrate a counter-irradiation fast-heating fusion scheme, a 3.8 J, 0.4 ns amplified chirped pulse is divided into four beams: two counter-irradiate a target with intensities of 6 × 1013 W cm−2, and the remaining two are pulse-compressed to 110 fs for heating the imploded target with intensities of 2 × 1017 W cm−2. HAMA contributed to the first demonstration by showing that a 10 J class DPSSL is adaptable to ICF experiments and succeeded in DD neutron generation in the repetition mode. Based on HAMA, we can design and develop an integrated repetitive ICF experiment machine by including target injection and tracking.

Multilayered polycrystallization in single-crystal YSZ by laser-shock compression

A single shot of an ultra-intense laser with 0.8 J of energy and a pulse width of 110 fs (peak intensity of W cm−2) is divided into two beams and the two beams counter-irradiated onto a 0.5 mm-thick single crystal yttria-stabilized zirconia (YSZ), changing the YSZ into a multilayered polycrystalline state. The laser-driven shock wave of the intensity 7.6 Pa penetrated the crystal as deep as 96 m, causing formation of a four-layered structure (the first layer from the surface to 12 m, the second from 12 to 28 m, the third from 28 to 96 m, and the fourth from 96 to 130 m, respectively). The grain size of the first layer was 1 m, while that of the second layer was broken into a few tens nanometers. The grain size of the third layer was a few hundred nanometers to a few ten micrometers. The area deeper than 96 m remained as a single crystal. The plasma heat wave might remelt the first layer, resulting in the grain size becoming larger than that of the second layer. The surface polycrystallization seems to maintain the residual stresses frozen in the film thickness direction. Our experimentally observed spatial profile of the grain size can be explained by this shock and heat waves model.

Tailoring double‐wall carbon nanotubes?

Double‐wall carbon nanotubes (DWNTs) are interesting nano‐objects derived from single‐wall nanotubes (SWNTs) whose potentiality for some applications (such as composite reinforcement) are even more promising than for SWNTs. Depending on the respective chirality of the inner and outer SWNTs involved, electronic properties may also vary, with subsequent possible applications in nanotechnology. Up to now, DWNTs were prepared either from catalytically‐enhanced thermal cracking of gaseous hydrocarbons, electric arc, or thermal annealing of C60@SWNTs (peapods). A drawback of the former as opposed to the latter is that DWNTs hardly concern the whole nanotube production as far as they are always mixed with SWNTs and other MWNTs. Whatever, in any DWNT (or MWNT) from the literature so far, the graphene‐graphene distance looks about equal to the d002‐spacing in turbostratic, polyaromatic carbon, i.e., ∼0.34 nm. For peapod‐derived DWNTs prepared from regular 1.35 nm wide SWNTs, for instance, it means that the inner d...

Neutron Generator Using Spherical Targets on a Rotating Disk Irradiated with an Ultraintense Laser at 1.25 Hz

Abstract A neutron generator is developed using 1-mm-diam spherical deuterated polystyrene targets on a rotating disk irradiated with an ultrahigh-intensity (>1018 W/cm2) diode-pumped laser. It consists of a rotating disk supplier, the targets, and a control system to irradiate the targets at 1.25 Hz. We adjusted the laser focus and position on the target to obtain the maximum neutron yield.

Target Monitoring and Plasma Diagnosis using 2ω probe beam for CANDY

We have developed the shadowgraph and interferometer with second-harmonic of heating pulses laser to observe target and plasma in highly-repetitive fusion reaction experiments. In the deuterated polystyrene ((C8D8)n double foil experiment, we confirm implosion plasma and plasma collision. In target injection experiment at a 1 Hz rate, we measure the position of the flying deuterated polystyrene beads at the moment of laser pulse illumination and observe the plasma generation by counter-illumination by 0.63 J, 800 nm, and 104 fs laser pulses.

Progress toward a unified kJ-machine CANDY

To construct a unified experimental machine CANDY using a kJ DPSSL driver in the fast-ignition scheme, the Laser for Fast Ignition Experiment (LFEX) at Osaka is used, showing that the laser-driven ions heat the preimploded core of a deuterated polystyrene (CD) shell target from 0.8 keV to 2 keV, resulting in 5 x 108 DD neutrons best ever obtained in the scheme. 4-J/10-Hz DPSSL laser HAMA is for the first time applied to the CD shell implosion- core heating experiments in the fast ignition scheme to yield neutrons and also to a continuous target injection, which yields neutrons of 3 x 105 n/4πsr n/shot.

Upgrade of repetitive fast-heating fusion driver HAMA to implode a shell target by using diode pumped solid state laser

The HAMA is 1-Hz fast heating fusion driver pumped by a 10 J second-harmonic of diode-pumped Nd:glass laser: KURE-1. We have upgraded HAMA to realize an implosion of spherical shell target by using a remaining fundamental beam from KURE-1. This beam of 6 J/1 Hz is transported to the current counter irradiation system. The resulting beam includes three pulses in sequence: 2.2 J/15 ns and 0.7 J/300 ps for implosion, and 0.5 J/ 190 fs for heating. We estimate the implosion dynamics from 1-D radiation hydrodynamic code (START- 1D). It indicates a possibility of tailored-pulse implosion by optimizing the beam spot sizes of imploding beams on the target surface. This upgrade leads to a demonstration of repetitive implosion and additional heating of a spherical shell target in accordance with a repetition of laser operation and that of a target injection system.

Repetitive Solid Spherical Pellet Injection and Irradiation toward the Repetitive-mode Fast-Ignition Fusion miniReactor CANDY.

Pellet injection and repetitive laser illumination are key technologies for realizing inertial fusion energy[1-4]. Neutron generator using lasers also requires a repeating pellet target supplier. Here we present the first demonstration of target injection and neutron generation[5]. We injected more than 1300 spherical deuterated polystyrene(C8D8) bead pellet targets during 23 minutes at 1 Hz(Fig. 1). After the pellet targets fell for a distance of 18 cm, we applied the synchronized laser-diode-pumped ultra-intense laser HAMA. The laser intensity at the focal point is 5 x 1018 W/cm2, which is high enough to generate neutrons. As a result of the irradiation, we produced 2.45-MeV DD neutrons. Figure 2 shows the neutron time-of-flight signals detected by plastic scintillators coupled to photomultipliers. The neutron energy was calculated by the time-of-flight method. The maximum neutron yield was 9.5 x 104/4π sr. The result is a step toward fusion power and also suggests possible industrial neutron sources.

Automated Fatigue Crack Growth Tracking System with Image Processor.

An automatic fatigue crack tracking system with an image processor has been newly developed. This system consists of a microscope with a CCD camera, an image processor, a position measuring system, an automatic stage which moves the microscope, a personal computer and a fatigue testing machine. The tracking procedures fall into the following four distinct steps. Step 1 : When a maximum load is applied to the cracked specimen, the system obtains the image around the crack tip and stores the image to the frame memory. Step 2 : After appropriate stress cycles, the system obtains the image at the same position again. Step 3 : The image processor freezes the motion and subtracts the stored image from the image obtained in Step 2. Then the subtracted image is emphasized by adding the offset brightness level. Step 4 : The dark image which is obtained by preceding procedures, is tracked with a template by the position measuring system. The amount of shift of the template represents the length of fatigue crack extension. The system moves the microscope by a distance equal to the length of the crack extension. We applied this system to the fatigue crack propagation tests on steel and Si3N4 ceramics, and confirmed the reliability of our data.

EFFECT OF WATER VAPOR ON FATIGUE CRACK PROPAGATION OF A SINTERED SILICON NITRIDE

Effect of cyclic load on the crack propagation of a sintered silicon nitride is enhanced by the water vapor. Crack propagated cycle-dependently in vacuum over the entire range of stress intensity factor employed. These are well explained by the fatigue mechanism based upon the decrease in bridging effect by cyclic load. The only necessary condition for cyclic fatigue in ceramics is not stress corrosion cracking due to water vapor but load cycling.