Inertial confinement fusion with advanced ignition schemes: Fast ignition and shock ignition

Essential ingredients of inertial confinement fusion (ICF) are fuel compression to very high density and hot spot ignition. In the conventional approach to ICF both fuel compression and hot spot formation are produced by the implosion of a suitable target driven by a time-tailored pulse of laser light or X-rays. This scheme requires an implosion velocity of 350-400 km/s. In advanced ignition schemes, instead, the stages of compression and hot spot heating are separated. First, implosion at somewhat smaller velocity produces a compressed fuel assembly. The hot spot is then generated by a separate mechanism in the pre-compressed fuel. The reduced implosion velocity relaxes issues concerning hydrodynamic instabilities, laser-plasma instabilities and preheat control. In addition, it can lead to higher target energy gain (ratio of fusion energy to driver energy). Fast ignition and shock ignition are promising advanced ignition schemes. In fast ignition the hot spot is created by either relativistic electrons or multi-MeV protons or light-ions, produced by a tightly focused ultra-intense laser beam. In shock ignition, intense laser pulses drive a converging shock wave that helps creating a hot spot at the centre of the fuel. These advanced schemes are illustrated in the present chapter. Motivation, potential advantages and issues are described. Research needs and perspective are also briefly discussed.