Laser fusion research at Institute of Laser Engineering, Osaka

The primary goal of the Kongoh Project at the Institute of Laser Engineering, Osaka, is to demonstrate the scientific feasibility of laser fusion in the next ten years. In the first phase of the project the glass-laser systems Gekko IV and Gekko XII together with the CO2-laser systems Lekko II and Lekko VIII were used to test the appreciable implosion and to choose the most suitable type of 100-kJ driver. Wavelength scaling of the implosion process is the main objective. In the second phase, a 100-kJ class driver will perform a feasibility test experiment. Basic understanding of laser coupling with plasma has been established in such areas as absorption, stimulated Brillouin scattering, hot-electron and fast-ion generation, and energy transport, by using laser beams of 10.6μm, 1.06μm and 0.53μm wavelength. Implosion dynamics, including compression symmetry and stability of the laser fusion pellet, has been studied by means of time-resolved X-ray shadowgraphy. In ablative-mode compression of a high-aspect-ratio pellet target, an irregularity was observed on the ablation surface during compression. An opacity diagram of the pellet was obtained by time-resolved shadowgraphy and compared with a computer-simulation result. Quantitative determination of plasma parameters of laser-driven spherical targets was performed by X-ray spectroscopic techniques. In the ablative mode, using a thick-polymer-coated pellet, the emission came only from the inside of the pellet, and absorption lines due to resonant photoexcitation and ionization were observed in the outer corona region of the pellet. The core density could be derived reliably by the edge shift of radiative recombination spectrum. Using neon-filled glass microballoons of 26atm., 100 μm dia., with a 10-μm-thick polyethylene coat, the Gekko IV laser of 100ps, 2.5TM, has successfully compressed the core up to 5g.cm-3. The Kongoh Project is beginning to demonstrate the scientific feasibility of laser fusion within the next ten years. (author)