Fuel Assembly for Fast Ignition Inertial Confinement Fusion

Large densities and areal densities (\textit{$\rho $R}) of compressed thermonuclear fuel lead to high gains and low ignition energies in fast-ignition inertial confinement fusion. It is well known that high densities and high \textit{$\rho $R} can be achieved by driving the imploding shell on a low adiabat. However, low adiabat pulses are difficult to realize in practice because of the extreme contrast ratio between peak and foot laser power. A recent advancement in direct-drive pulse design has provided a pulse shape that mitigates the power contrast ratio and enhances the ablative stabilization of the RT instability while driving the capsule on a very low adiabat. The laser pulse, referred to as the relaxation (RX) pulse design, consists of a short prepulse followed by a power shutoff and the main laser pulse. Massive cryogenic shells can be imploded with a low implosion velocity on a low inner adiabat with an RX pulse. The low velocity and the shaped adiabat also prevent a significant growth of the RT instability. Target designs for a 25-, 100-, and 750-kJ driver are presented. One-dimensional simulations show fuel assemblies with \textit{$\rho $R} of 0.7--3 g/cm$^{2}$ and peak densities of 400--700 g/cc obtained with implosion velocities of 1.7--2.5 $\times $ 10$^{7}$ cm/s This work has been supported by the U.S. DOE under Cooperative Agreements ER54789 and DE-FC03-92SF19460.