Sadao Nakai

CsB3O5: A new nonlinear optical crystal

On the basis of the anionic group theory, a new nonlinear optical crystal cesium triborate, (CsB3O5, CBO) was discovered. Single crystals of CBO in centimeter size were grown from a stoichiometric melt. The transparent range of CBO covers from 170 to 3000 nm. The preliminary measurement of its second harmonic generation coefficient shows that its d14 is ∼0.468×d11 (β−BaB2O4). Its measured damage threshold is as high as 26 GW/cm2 at 1.053 μm, 1.0 ns.

Extremely high damage threshold of a new nonlinear crystal L‐arginine phosphate and its deuterium compound

L‐arginine phosphate monohydrate (LAP) and deuterated LAP (DLAP) are new organic nonlinear optical materials useful for higher harmonics of radiation from high‐power lasers. We measured the bulk laser damage threshold of these crystals using light from a 1.05 μm laser with 1 and 25 ns pulse widths and 0.53 μm laser light with 0.6 and 20 ns pulse widths. In every case, these crystals show much higher thresholds than potassium dihydrogen phosphate (KDP and fused silica. These crystals are very interesting not only as a frequency converter but also as other optical components of high‐power lasers, because of their extremely high damage threshold.

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.

Single-longitudinal-mode operation and second-harmonic generation of Nd:YVO(4) microchip lasers.

A single-mode output of 221 and 16 mW for the fundamental and second-harmonic light was obtained with the combination of a Nd:YVO(4) microchip laser and an intracavity KTP crystal. It was found that the beam size and laser power in a plano-plano cavity were dependent on the absorption coefficient of the laser medium and that the material that had a higher absorption coefficient produced higher output power and smaller beam size in the laser cavity. This means that Nd:YVO(4) is suitable not only for single-mode oscillation but also for efficient intracavity second-harmonic generation.

Nonlinear Optical Properties of Cesium Lithium Borate.

Nonlinear optical (NLO) properties of CsLiB6O10 (CLBO) have been investigated. Frequency quadrupling of the 1.064 µ m Nd:YAG laser output for type-I phase matching was observed by using CLBO. The walk-off angle and the effective NLO coefficient deff were estimated to be 1.83° and 1.01 pm/V for fourth harmonics generation of Nd:YAG laser output, respectively. Phase matching for sum frequency generation of 4ω (fourth harmonics of Nd:YAG laser)+ω (fundamental of Nd:YAG laser), resulting in fifth harmonics of Nd:YAG laser output, has been observed.

Laser driven inertial fusion energy: present and prospective

Recent progress in laser driven implosion is reviewed. Improvements in the uniformity of irradiation by laser beams on fuel pellets have achieved quantitative progress in implosion performance. The recent results of the direct drive–central ignition experiments give us confidence in achieving fusion ignition, burning and energy gain using a multi-beam megajoule laser with full implementation of beam smoothing techniques. Fast ignition research is also reviewed, which could give us a higher energy gain with lower laser energy. The science and technology of laser fusion power plants are beginning to attract wider attention, as forming the road map to achieve commercial power plants for cleaner, safer and abundant fusion energy.Corrections were made to this article on 28 April 2004

Growth and characterization of deuterated L-arginine phosphate monohydrate, a new nonlinear crystal, for efficient harmonic generation of fusion experiment lasers

L-arginine phosphate monohydrate (LAP) is a promising nonlinear organic crystal for higher harmonic generation in place of the conventional KDP. It has a large absorption at 1.06 μm. Deuterated LAP (DLAP) has been synthesized to decrease this absorption. The absorption coefficient was reduced from 0.09 to 0.02 cm -1 Microbes which easily multiply in the growth solution were successfully inhibited by Hadding H 2 O 2 . Damage thresholds of LAP and DLAP crystals are much higher than that of KDP and fused silica. We suggest that DLAP can be used in place of KDP in high power lasers.

Radiation conversion and related ablation behavior of a gold‐foil target irradiated by 0.35, 0.53, 1.06, and 10.6 μm lasers

Ultra‐soft x‐ray emission from a gold‐foil target was studied by irradiating with four different wavelength lasers. The experimental data were analyzed under the blackbody approximation to evaluate the radiation temperature and the radiation conversion efficiency as functions of laser intensity and wavelength. The results show that the shorter‐wavelength laser leads to a remarkable conversion of absorbed energy into radiation. The ablation behavior due to radiation and related energy transport in a gold foil is also studied.

Shack Hartmann wave-front measurement with a large F-number plastic microlens array

A new plastic microlens array, consisting of 900 lenslets, has been developed for the Shack Hartmann wave-front sensor.The individual lens is 300 µm × 300µm and has a focal length of 10 mm, which provides the same focal size, 60 µm in diameter, with a constant peak intensity. One can improve thewave-front measurement accuracy by reducing the spot centroiding error by averaging a few frame memories of an image processor. A deformable mirror for testing the wave-front sensor gives anappropriate defocus and astigmatism, and the laser wave front is measured with a Shack Hartmann wave-front sensor. The measurement accuracy and reproducibility of our wave-front sensor are better than λ/20 and λ/50 (λ = 632.8 nm),respectively, in rms.

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...