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A key element of the fast ignitor scheme is the pulse of fast particles creating the igniting spark. In this paper, the dependence of the parameters (energy Ep, power Wp, intensity Ip) of such a pulse on the penetration depth ℛ of the fast particles is studied by two-dimensional simulations of the evolution of a deuterium–tritium fuel, precompressed at density ρ, and heated by a beam of particles with assigned ℛ. The ignition windows in the (Ep,Wp,Ip) space are found to depend very little on ℛ over the interval 0.15⩽R⩽1.2 g/cm2. At ρ=300 g/cm3, the minimum ignition energy is about 14 kJ; an optimal set of parameters (with energy and power about the required minimum, and intensity relatively close to the minimum) is Ep≅17 kJ, Wp≅0.85×1015 W, and Ip≅6.5×1019 W/cm2 (achieved at R=0.6 g/cm2). The optimal energy scales with the density as Ep∝ρ−1.85. Scaling laws are also presented for the other pulse parameters and for the limiting energy gain.

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Basic issues for the design of moderate-gain fast ignition targets at total laser energy of 200–400kJ (with less than 100kJ 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 100–150kJ, properly shaped laser pulses (with wavelength λc=0.35μm), and ignited by 80–100kJ 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 1.2–1.5g∕cm2. According to the present understanding of ultraintense laser-matter interaction, this limits the wavelength of the ignition beam to λig≤0.5μm. With the same assumptions, energy gain G=100 can be achieved by targets driven by a 250kJ compression laser pulse and an 80–100kJ ignition pulse.