Y. Izawa

Characterization of extreme ultraviolet emission from laser-produced spherical tin plasma generated with multiple laser beams

Spherical solid tin targets were illuminated uniformly with twelve beams from the Gekko XII laser system to create spherical plasmas, and the extreme ultraviolet (EUV) emission spectra from the plasmas were measured. The highest conversion efficiency of 3% to 13.5nm EUV light in 2% bandwidth was attained for an irradiance of around 5×1010W∕cm2. The experimental results were reproduced fairly well using a theoretical model taking the power balance in the plasma into consideration.

Progress of fast ignitor studies and Petawatt laser construction at Osaka University

100 TW light from the Petawatt Module (PWM) laser illuminated a preimploded spherical deuterated polystyrene(CD) shell target. The DD neutron yield increased from 2.5×105–106. Analysis indicates that hundreds of keV deuterons, generated around the critical density, collide with cold fuel deuterons and play the leading role in the enhancement of the neutron yield. A two-dimensional particle-in-cell (2D PIC) simulation predicted well the deuteron spectrum. A 60 TW laser was used for MeV proton emissions and megagauss magnetic fields generation on the rear surface of a Poly p-xylene(C8H8) plane target. The 2D PIC simulation explained well the results. The PWM laser was upgraded to one PW, making it the world biggest Petawatt laser (PW laser). An optically parametric chirped amplification was introduced in the front end. The pulse was synchronized to the GEKKO XII imploding beams to within 10 ps.

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.

Ultrathin amorphous Si layer formation by femtosecond laser pulse irradiation

Formation of ultrathin amorphized Si layer by femtosecond laser irradiation is reported in this letter. Below the fluence of ablation threshold, femtosecond laser irradiation induced an amorphization of crystalline Si. The authors confirmed the thickness of amorphous Si layer by transmission electron microscope. The thickness of the amorphized layer was found to be quite uniform and did not depend on the number of irradiated laser pulses and fluence, which was related to the effective light penetration depth.

Laser generated proton beam focusing and high temperature isochoric heating of solid matter

The results of laser-driven proton beam focusing and heating with a high energy (170J) short pulse are reported. Thin hemispherical aluminum shells are illuminated with the Gekko petawatt laser using 1μm light at intensities of ∼3×1018W∕cm2 and measured heating of thin Al slabs. The heating pattern is inferred by imaging visible and extreme-ultraviolet light Planckian emission from the rear surface. When Al slabs 100μm thick were placed at distances spanning the proton focus beam waist, the highest temperatures were produced at 0.94× the hemisphere radius beyond the equatorial plane. Isochoric heating temperatures reached 81eV in 15μm thick foils. The heating with a three-dimensional Monte Carlo model of proton transport with self-consistent heating and proton stopping in hot plasma was modeled.

Multiframe x-ray imaging system for temporally and spatially resolved measurements of imploding inertial confinement fusion targets

A compact and simple multiframe x‐ray imaging system was developed in order to monitor the implosion of spherical targets in inertial confinement fusion research. Time intervals between consecutive frames can be adjusted flexibly, and the maximum number of adjacent frames is 20 for an overall duration of 1.4 ns. Each frame is recorded with a temporal resolution of 83±20 ps, a spatial resolution of 10 lp/mm at a modulation transfer function of 20%, and an intensity dynamic range of 103. A proximity focused image intensifier with two microchannel plates allows to obtain a gain of 105. Measured temporal response and gain characteristics could be reproduced by a simple Monte Carlo calculation.

Characterization of density profile of laser-produced Sn plasma for 13.5nm extreme ultraviolet source

We investigated the electron density profile corresponding to the dominant extreme ultraviolet (EUV) emission from a laser-produced Sn plasma using a combination of a green and an UV interferometer. A comparison between experimental results and a one-dimensional radiation hydrodynamic simulation shows reasonable agreement, and the discrepancy could be attributed to three-dimensional plasma expansion. It was found that, due to opacity effects, most of the EUV light comes from an under-dense plasma region.

Monochromatic imaging and angular distribution measurements of extreme ultraviolet light from laser-produced Sn and SnO2 plasmas

Properties of extreme ultraviolet (EUV) emission from laser-produced Sn and SnO2 plasmas were investigated. EUV emission images were taken with a monochromatic imager for 13.5nm with 4% bandwidth. It was found that the EUV emitting plasma is not formed symmetrically with respect to the target surface normal but extends toward laser incident axis. This result is consistent with the angular distribution of EUV emission peaked toward the direction nearly perpendicular to the laser axis. The asymmetric plasma can be attributed to the interaction of a long laser pulse with expanding plasma along the path of laser incidence.

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.

Foam materials for cryogenic targets of fast ignition realization experiment (FIREX)

Development of foam materials for cryogenically cooled fuel targets is described in this paper. The fabrication development was initiated as a part of the fast ignition realization experiment (FIREX) project at the ILE, Osaka University under a bilateral collaboration between Osaka University and National Institute for Fusion Science (NIFS). For the first stage of FIREX (FIREX-I), a foam cryogenic target was designed in which low-density foam shells with a conical light guide will be fuelled through a narrow pipe and will be cooled down to the cryogenic temperature. Acrylic polymer, resorcinol–formaldehyde (RF) resin, poly(4-methyl-1-pentene) (PMP), and polystyrene-based crosslinking polymer have been investigated as supporting materials for cryogenic fuel. The properties of the material and the present status of the material development are summarized.