Tadashi Kanabe

Prepulse-free petawatt laser for a fast ignitor

We have developed a prepulse-free short-pulse Nd:glass laser system of 0.9-PW peak power to heat a pre-imploded high-density plasma. An optical parametric chirped amplification system is introduced to reduce the prepulses to an amplitude (1.5/spl times/10/sup -8/) of that of the main pulse. The compressor is a double-path grating pair system 94 cm in diameter compressing the 50-cm-diameter laser beam to 470 fs. An off-axis parabolic mirror has focused the 420-J energy to an intensity of 2.5/spl times/10/sup 19/ W/spl middot/cm/sup -2/. Part of the front end of the chirped pulse is seeded into the preamplifier of the GEKKO XII laser, used to implode a pellet target, to enable the petawatt laser to irradiate the pre-imploded pellet during stagnation of a few tens of picoseconds.

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...

Bi-doped SiO2 as a new laser material for an intense laser

Abstract We discovered a new fluorescence from Bi in SiO2 glass. A broad fluorescence spectrum (FWHM; 150 nm, peak at 1150 nm) and a long lifetime (650 μm) were observed and the quantum yield of this material was 0.66 at 500 nm excitation. These properties are useful for ultra-short pulse amplification at high efficiency.

Rapid growth over 50 mm/day of water-soluble KDP crystal

The rapid growth of KDP crystals was demonstrated at growth rates of up to 54 mm/day. Thermal and acoustic energy were used in the saturated solution to delete clusters of molecules. High supersaturations larger than 120% were achieved by overheating of the mother solution by 25 K. The application of acoustic energy to the solution was also used to achieve high supersaturation. The dissolution of the clusters in the solvent was discussed, explaining the rapid growth rates.

Optical properties of rapidly grown KDP crystal improved by thermal conditioning

Abstract The medium-size crystal (64 × 63 × 43mm) of potassium dihydrogen phosphate (KDP) was grown for one day from the 8 mm cubic seed crystal with a growth rate over 30 mm/day. The spectral transmittance, second-harmonic generation (SHG) property and laser damage threshold of the rapidly grown crystals were measured. The comparison with those of conventionally grown KDP crystal (2 mm/day) showed the prismatic sector included more impurities resulting in UV absorption. No difference was detected between the SHG properties. The lower laser damage threshold of the rapidly grown crystal was improved by a thermal conditioning to be 17 J/cm 2 of 1 ns pulse at 1 mm wavelength.

Partially coherent light generated by using single and multimode optical fibers in a high‐power Nd:glass laser system

A simple and flexible method is presented for generating a partially coherent light which obtains the highly smooth focused beam pattern. The beam divergence of 32 times diffraction limited light having a spectral width of 1.6 nm has been easily and reproducibly achieved by injecting a laser pulse from an actively mode‐locked Nd:YLF oscillator to a single mode optical fiber, coupled to a multimode optical fiber. Temporal evolution of the beam smoothing due to the induced incoherency was examined with temporally resolved measurements of the beam pattern. The partially coherent light was focused through a random phase plate after the amplification. Small‐scale intensity perturbation in a focused beam pattern was greatly reduced.

Design and Performance of a Diode-Pumped Nd:Silica-Phosphate Glass Zig-Zag Slab Laser Amplifier for Inertial Fusion Energy

A high-energy, high beam quality, diode-pumped 1053-nm Nd:silica-phosphate glass laser amplifier has been designed, constructed and tested in order to verify the conceptual design of HALNA (high average-power laser for nuclear-fusion application): a diode-pumped solid-state laser based on a water-cooled zig-zag slab optical geometry. This amplifier yields an 8.5 J output energy per pulse at 0.5 Hz in a 20 ns pulse of two times the diffraction limit beam quality with an optical-to-optical conversion efficiency of 10.9%. This is the first demonstration of a diode-pumped solid-state laser amplifier for the inertial fusion energy (IFE) driver. The detailed considerations on the optical and thermal designs of the novel amplifier architecture are discussed. The experimental results revealed that the primary requirements for the IFE driver, such as diode-pumping, energy storage and extraction efficiencies, and beam quality have been fulfilled.

Conceptual Design Studies of a Laser Diode Pumped Solid State Laser System for the Laser Fusion Reactor Driver

We have developed a conceptual designing technique of a laser diode pumped solid state laser system having high peak power, high efficiency, good beam quality, and high repetition rate. Using this technique, we evaluated the feasibility of the systems with several solid state laser materials for the laser fusion reactor driver with 10 MJ blue output, 10% overall efficiency, and 10 Hz repetition rate. The systems with such as HAP4 glass, Nd:SiO2 glass, Nd:Y:CaF2 crystal, and Tm, Ho:YAG crystal are shown to be attractive candidates for the reactor driver while the systems with such as LHG8 glass, Nd:YAG crystal, and Tm:YAG crystal will not be selected as the candidates. For an economical laser fusion reactor driver, the solid state laser materials are required to have the following properties; \hbarω/σ~10 J/cm2 (\hbarω: the laser photon energy, σ: the stimulated emission cross section), a thermal shock parameter of ≥3 W/cm, a nonlinear index of refraction smaller than ~3×10-13 esu, and a fluorescence lifetime of ≥4 ms.

Suppression of interference speckles produced by a random phase plate, using a polarization control plate

Abstract The suppression of a speckle structure and the reduction of the irradiation nonuniformity on a spherical target using a polarization control plate are described. The irradiation nonuniformity was reduced 30% compared with case of only using the random phase plate from a numerical calculation. A preliminary experiment is also demonstrated.

Recent progress of implosion experiments with uniformity-improved GEKKO XII laser facility at the Institute of Laser Engineering, Osaka University

The irradiation uniformity of the GEKKO XII laser [C. Yamanaka et al., IEEE J. Quantum Electron. QE‐17 1639 (1981)] has been improved by using partially coherent light with angular spectral dispersion and by improving power balance among the beams. Implosion experiments with the uniformity‐improved GEKKO XII have been carried out in order to demonstrate stable formation of the hot spark. The isentrope of the compressed pellet shell is controlled by adding a prepulse to enhance the ablative stabilization of the Rayleigh–Taylor instability. The implosion dynamics has been investigated by x‐ray imaging and neutron diagnostics. The experimental results are compared with the mix model prediction based on the one‐dimensional hydrodynamic simulation. The degradation of neutron yield is attributed to the low‐mode asymmetry. Finally, recent results of planer target experiments on the ablative stabilization of the Rayleigh–Taylor instability are also presented. The observed linear growth rate was about 50% of the c...