Targets for direct-drive fast ignition at total laser energy of 200-400 kJ

Basic issues for the design of moderate-gain fast ignition targets at total laser energy of 200–400 kJ (with less than 100 kJ for the igniting beams) are discussed by means of a simple integrated gain model. Gain curves are generated and their sensitivity to several parameters is analyzed. A family of scaled target is designed, based on 1D hydrodynamic simulations of the implosion stage and 2D model simulations of ignition and burn. It is found that ignition and propagating burn can be achieved by targets compressed by 100–150 kJ, properly shaped laser pulses (with wavelength λc = 0.35 μm), and ignited by 80–100 kJ pulses. This requires adiabat shaped implosions to limit Rayleigh-Taylor instability, at the same time keeping the fuel entropy at a very low level. In addition, the igniting beam should be coupled to the fuel with an efficiency of about 25%, and the hot-electron average penetration depth should be at most 1.2–1.5 g/cm2. According to the present understanding of ultraintense laser-matter interaction, this limits the wavelength of the ignition beam to λig ≤ 0.5 μm. With the same assumptions, energy gain G = 100 can be achieved by targets driven by a 250 kJ compression laser pulse and an 80–100 kJ ignition pulse.