Integrated Experiments of Fast Ignition with Gekko-XII and LFEX Lasers

Full text: Based on the successful result of fast heating of a shell target with a cone for heating beam injection, FIREX-1 project has been started from 2004. Its goal is to demonstrate fuel heating up to 5 keV by using upgraded heating laser beam. For this purpose, LFEX laser, which can deliver, at the full spec, an energy up to 10 kJ in a 0.5 - 20 ps pulse, has been constructed beside Gekko-XII laser system at the Institute of Laser Engineering, Osaka University. It has been activated and became operational since 2009. Instead of the previous experiment with PW laser, upgraded integrated experiments of Fast Ignition have been started by using LFEX laser with an energy up to 1 kJ in a 1-5 ps 1.053-micron pulse. Initial experimental results including implosion of the shell target by Gekko-XII, heating of the imploded fuel core by LFEX laser injection, and up to 30 times increase of the neutron yield due to fast heating compared with no heating have been achieved. Results indicate that the heating efficiency is 3 - 5%, which is much lower than 20 - 30% expected from the previous data. It is attributed to the too hot electrons generated in a long scale length plasma in the cone preformed with a prepulse in the LFEX beam. By controlling the prepulse condition, 5-keV heating can be expected at full output of LFEX laser with improved heating efficiency. (author)