Adiabat shaping in direct-drive inertial confinement fusion implosions

Adiabat shaping in inertial confinement fusion (ICF) capsules has been proposed as a method for increasing the stability of ICF implosions without compromising capsule compressibility and one-dimensional yield. The optimal adiabat profile has a maximum on the outer, ablation surface for higher ablative mitigation of the Rayleigh-Taylor (RT) instability, and a minimum on the shell inner surface for high compressibility and high neutron yields. Ablative theory of the RT instability predicts that the increased ablation velocities of adiabat-shaped designs reduce the growth of capsule nonuniformities, potentially leading to higher experimental yields. One implementation of adiabat shaping currently under investigation for direct-drive implosions is the decaying shock (DS) method [Goncharov, Phys. Plasmas 10, 1906 (2003)], which implements a strong laser prepulse or picket at the beginning of the laser pulse to induce a decaying shock in the shell material, and thereby create a shaped adiabat profile. This thesis proposes another method for adiabat shaping, the relaxation (RX) method, which employs a weak laser prepulse designed to relax the shell density profile followed by a shaping strong shock driven by the main pulse.; Derivations of the analytic adiabat profiles generated in the shell material are presented here for both DS and RX designs from one-dimensional hydrodynamic theory, and the resulting profiles are shown to agree well with simulation. It is demonstrated that the adiabat gradients present in adiabat-shaped designs are destabilizing to the RT instability, yet this destabilizing effect is more than compensated for by the increase in ablation velocity, ultimately resulting in lower linear-phase RT growth rates. Numerical simulations of DS and RX targets designed for the 60-beam OMEGA laser system predict lower growth rates and less laser imprint than for standard "flat-adiabat" (no adiabat shaping) designs, in good agreement with the instability theory derived here. Results are presented from plastic-shell implosion experiments, in which higher neutron yields and better compression are obtained for an RX design than for a flat-adiabat design, indicating improved stability due to adiabat shaping.