Ignition threshold in cylindrical fast ignition targets

Abstract

In this paper, the ignition stage of heavy ion driven fast ignition is evaluated numerically by using a quasi-1D model. Recently, the minimum value of the fuel areal density in cylindrical fast ignition was reported by Ramis and Meyer-ter-Vehn [Laser Part. Beams 32, 41–47(2014)]. Here, we intend to examine the impact of the initial fuel temperature on the ignition threshold. It is expected that the initial temperature and fuel areal density provide an acceptable practical framework to manage a successful ignition scenario. Assuming a precompressed DT fuel with temperature between 0.1 keV and 1.0 keV and the areal density ⟨ρr⟩DT ≥ 0.45 g/cm2, the minimum required ignition energy has been derived. It is found that as the ignition energies decrease, the burn wave propagation into the fuel layer is suppressed and the ignition condition becomes more sensitive to the initial fuel temperature. Moreover, at fixed fuel areal density, the ignition threshold energy reveals a weak dependence on the fuel radius smaller than rDT ≤ 40 μm. In order to attain the high energy gain, G > 30, the minimum ignition energy of 250 kJ corresponding to beam intensity higher than 1.6 × 1019 W/cm2 is recommended. While, in the burn strategy, fb ≥ 0.33, it has been found that the threshold ignition energy scales exponentially with the fuel radius, which becomes vivid for the critical areal density of ⟨ρr⟩crt= 0.45 g/cm2. Finally, it has been shown that the contribution of the released fast neutrons provides the additional plasma heating, which generally improves the burn stage.