Effects of the injected trigger pulse focusing and timing on the ignition and gain of dense static, or imploding DT fuel

We discuss two issues relevant for the feasibility of the scheme in which a heavy ion pulse is used to ignite a DT fuel spherically compressed, by laser induced ablation, along a low adiabat (no self-ignition). The discussed issues are (i) the degree of synchronism between the laser driven implosion and the trigger pulse; (ii) the requirements on focusing for the trigger beam. The numerical simulation have been made by using cylindrical heavy ion beams with Gaussian radial distribution, truncated where the intensity is 1/e-4 of the maximum. The parameter (dbeam), used to measure the focusing, is the diameter of the circle where the intensity is 1/e of the maximum (energy content ∼64% of the total energy). Requirements on focusing have been first explored by simulating (2D) the irradiation of static DT cylinders at 200 g/cm3 by coaxially impinging 15 GeV Bi ions. The ignition conditions have been studied for pulses having 10 ps or 50 ps duration. For both the cases, the ignition energy (Emin) is constant for spot radii smaller than 50 μm. In the range 50-140 μm the ignition energy increases linearly (3 x Emin at 140 μm, with Emin=40 kJ for 10 ps pulses, Emin=100 kJ for 50 ps pulses). The study on synchronism has been performed by simulating (2D) the irradiation, by a heavy ion beam, of a laser imploded spherical DT shell (initial aspect ratio 10). The trigger beam was started at different times near the stagnation, and the initial fuel state (field of velocity, density,temperature, etc.) was that computed by a 1D simulation. It has been found that ignition, and almost constant thermonuclear energy release, can be obtained by triggering within a temporal window of the order of 1 ns, around the stagnation. (orig.)