Integrated simulations for FIREX-I fast ignition experiments

The FIREX-I project aims to demonstrate that the imploded core could be heated up to the ignition temperature, 5 keV, and incorporated experiments for FIREX-I, in which heating is combined with implosion, have started at ILE, Osaka University. Efficient heating mechanisms and achievement of such high temperature have not been, however, clarified yet, and we have been promoting the Fast Ignition Integrated Interconnecting code (FI3) project to boldly explore fast ignition frontiers. First series of the incorporated experiments have been performed in 2009, and only 30-fold enhancement in neutron yield, which was ∼ 1/30 smaller than that in 2002 experiments, was achieved and lower energy coupling from the heating laser to the imploded core was expected. According to integrated simulations, it is pointed out that low-density plasmas inside of the cone, which is generated by an unavoidable pre-pulse of the heating laser, can result in low coupling efficiency. A main pulse of the heating laser has to interact with these preformed plasmas and it results in increasing the distance from the generation point of fast electrons to the core and generating too energetic fast electrons, which in turn deceases low energy fast electrons suitable for core heating. To mitigate the preformed plasma effects, an aperture of the cone is suggested to be covered with an extremely thin film. The pre-pulse could be interrupted and absorbed by this film, and cannot irradiate the cone wall to produce the preformed plasmas. But inside the cone is filled up with rarefied plasmas, which are the residue of expanding thin film plasmas. The main pulse of the heating laser must propagate through very long (∼1mm) rarefied (‹‹n cr) plasmas, however there have been few researches using such long rarefied plasmas. Thus we have investigated effects of long rarefied plasmas on core heating with the use of FI3. In addition, an extended double cone and a pointed cone tip concepts are also proposed to enhance the energy coupling from the heating laser to the imploded core. The extended double cone can be expected to confine runaway fast electrons from the cone sidewall and improve a large divergence angle of fast electrons. The pointed cone tip can be used to delay the tip breakup-time and to survive the cone tip until the maximum compression. (author)