The world's largest peta watt (PW) laser LFEX, which delivers energy up to 2 kJ in a 1.5 ps pulse, has been constructed beside the GEKKO XII laser at the Institute of Laser Engineering, Osaka University. The GEKKO-LFEX laser facility enables the creation of materials having high-energy-density which do not exist naturally on the Earth and have an energy density comparable to that of stars. High-energy-density plasma is a source of safe, secure, environmentally sustainable fusion energy. Direct-drive fast-ignition laser fusion has been intensively studied at this facility under the auspices of the Fast Ignition Realization Experiment (FIREX) project.In this paper, we describe improvement of the LFEX laser and investigations of advanced target design to increase the energy coupling efficiency of the fast-ignition scheme. The pedestal of the LFEX pulse, which produces a long preformed plasma and results in the generation of electrons too energetic to heat the fuel core, was reduced by introducing an amplified optical parametric fluorescence quencher and saturable absorbers in the front-end system of the LFEX laser. Since fast electrons are scattered and stopped by the strong electric field of highly ionized high-Z (i.e. gold) ions, a low-Z cone was studied for reducing the energy loss of fast electrons in the cone tip region. A diamond-like carbon cone was fabricated for the fast-ignition experiment. An external magnetic field, which is demonstrated to be generated by a laser-driven capacitor-coil target, will be applied to the compression of the fuel capsule to form a strong magnetic field to guide the fast electrons to the fuel core. In addition, the facility offers a powerful means to test and validate astronomical models and computations in the laboratory. As well as demonstrating the ability to recreate extreme astronomical conditions by the facilities, our theoretical description of the laboratory experiment was compared with the generally accepted explanation for astronomical observations.
In Fast ignition (FI), an imploded core with a density of several hundred times the solid density is created by an imploding laser. The imploding core is auxiliary heated and ignited by hot electrons. The hot electrons are produced by the interaction between a chirped ultra-short pulse laser (heating laser) and the pre-formed plasma created by the pre-pulse of the heating laser at a tip of a guiding cone. According to the simulation, the maximum coupling efficiency between the core and the hot electrons can be expected at the typical hot electron energy of 2 MeV or less). Usually if the scale length (1/e-hold length) of the plasma density is long, the effective electron temperature T eff (=slope of the electron spectrum) is high and the coupling efficiency is reduced because the electron passes through the core with small energy deposition. Sunahara et al. proposed an hole cone shell target without the cone tip (hole-cone shell) to resolve these problems. The heating laser directly irradiates the core. The low energy electron, which had been lost in the cone, can be used for core heating. The efficiency reduction due to the divergence of the hot electrons can be also recovered because the distance from the hot-electron generation point to the core can be shorten. It is the most serious problem that the pre-formed plasma may be created because a part of the compressed plasma enters the cone through the hole during the imploding phase. T eff may increase due to the interaction between the heating laser and the plasma.Direct heating of the imploded core has been performed by using the hole-cone shell target in order to minimize the energy loss of the hot electrons in the cone itself. At the beginning, we had worried about the increase of T eff due to the exploding plasma with long scale length from the hole. However T eff is not so high and high neutron yield can be obtained. This reason may be that the pre-formed plasma is created in the inner side of the imploding shell (because there is no tip of the cone), the pre-formed plasma is shortened by the shell implosion and the scale length remains short because the spouting plasma from the hole is too dense. Actually the LFEX laser can reach the imploded core according to the MIXS image. Although the neutron yield increases, the neutron energy is slightly shifted from 2.45 MeV (thermal DD neutron) because the accelerated deuterium beam is also contained. The beam may be accelerated in the spouted plasma from the imploded core by a ponderomotive force of the LFEX. The effective core heating can be expected because the stopping range of the accelerated deuterium ion is too short.In the hole-cone shell, higher coupling efficiency can be expected because T eff is not so high and the range of accelerated ions is short. We summarize the results about the hole-cone shell as follows;(a) T eff in the hole-cone shell is lower than T eff in the standard cone shell.(b)The neutron yield in the hole-cone shell is larger than that in the standard cone shell. However much neutrons c...
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