Assess the key physics that underpins high-hydro coupling-efficiency in NDCX-II experiments and high-gain heavy ion direct drive target designs using proven hydro codes like HYDRA
The simulations provided in this milestone have solidified the theoretical underpinning of direct drive targets and also the ability to design experiments on NDCX II that will enhance our understanding of ion-beam hydrodynamic coupling, and thus be relevant to IFE. For the case of the IFE targets, we have studied hydro and implosion efficiency using HYDRA in ID, a starting point towards the goal of polar direct drive in geometry compatible with liquid wall chambers. Recent analysis of direct drive fusion energy targets using heavy ion beams has found high coupling efficiency of ion beam energy into implosion energy. However, to obtain optimal coupling, the ion energy must increase during the pulse in order to penetrate the outflowing ablated material, and deposit the energy close enough to the fuel so that the fuel achieves sufficient implosion velocity. We have computationally explored ID (radial) time dependent models of ion driven direct drive capsule implosions using the Arbitrary Lagrangian-Eulerian (ALE) code HYDRA, to help validate the theoretical analysis done so far, particularly exploring the effects of varying the ion energy and ion current over the course of the pulse. On NDCX II, experiments have been proposed to explore issues of ion penetration of the outflowing plasma over the course of the ion pulse. One possibility is to create a first pulse of ions that heats a planar target, and produces an outflow of material. A second pulse, {approx}10 ns after the first, of higher ion energy (and hence larger projected range) will interact with this outflow before reaching and further heating the target. We have investigated whether the change in range can be tailored to match the evolution of the ablation front. We have carried out simulations using the one-dimensional hydrodynamic code DISH and HYDRA to set parameters for this class of experiments. DISH was upgraded with an ion deposition algorithm, and we have carried out ID (planar) simulations. HYDRA was also used for ID (planar) and 2D (r,z) simulations of potential experiments. We have also explored whether similar physics could be studied using an energy ramp (i.e., a velocity tilt) rather than two separate pulses. We have shown that an optimum occurs in the macropulse duration (with fixed velocity tilt) that maximizes the shock strength. In the area of IFE target design we have continued to explore direct drive targets composed of deuterium-tritium fuel and ablator layers. We have extended our previous target designs at 0.44 MJ drive energy, gain 50, (50 MeV foot, 500 MeV main pulse, Rb ion, which requires a large number of beams due to a high beam space charge constraint) to a power plant scale 3.7 MJ drive energy, gain {approx}150 (220 MeV foot, 2.2 GeV main pulse, Hg ion) that eases requirements on the accelerator. We have studied the effects of two important design choices on ICF target performance. We have shown that increasing the number of foot pulses may reduce the target's in-flight adiabat and consequently improve its compressibility and fusion yield. As in the case of laser drive, the first three shocks are the most important to the target's performance, with additional shocks contributing only marginally to compression and burn. We have also demonstrated that ion range lengthening during the main pulse can further reduce the target adiabat and improve the efficiency with which beam energy is coupled into the target. (Ion range lengthening using two different kinetic energies for the foot and main pulse has previously proven effective in the design of high gain targets).