High-gain shock ignition of direct-drive ICF targets for the Laser Mégajoule

High-gain shock ignition is numerically investigated for specific marginally igniting targets-targets producing thermonuclear gain less than one-extracted from two distinct target families.Homothetic transformations keeping constant the implosion velocity, the adiabat, and the in-flight aspect ratio are used to draw target families.Conventional and fast ignition (FI) directly driven targets are considered for the Laser Mégajoule.It is shown that high gain can be achieved with shock ignition for designs, which do not ignite only from the laser compression.Shock ignition is achieved for different targets of the FI family, which are driven by an absorbed energy between 100 and 500 kJ and deliver thermonuclear energies between 10 and 130 MJ.Direct-drive (DD) inertial confinement fusion (ICF) has been studied for the Laser Mégajoule (LMJ) for a decade [1-10], leading to the definition of a DD baseline design in 2004 [3,4].This approach is an alternative to the indirect drive to reach high thermonuclear energy yield and gain.In the DD, laser beams overlap on the external side of a spherical target.The latter usually consists in an ablator layer where part of the laser light is absorbed.The absorber encloses a deuterium-tritium (DT) ice shell, and, in the central part, some DT gas.Laser heats the ablator and the plasma blow-off accelerates the target, which implodes to achieve the conditions of density and the temperatures required to ignite and burn the fuel (DT).In 2007, we showed [9] that it was possible to assemble a standard DD target on LMJ with the beam layout of the indirect drive.We have also shown [10] that the conventional polar direct drive (PDD) [11] is less interesting for LMJ since we have the possibility to do DD without using the repointing technique.However, only a part of the whole energy of the LMJ is accessible for DD.A new design was then proposed for a specific target design with the optical zooming [5].