1-D Lasnex calculations show efficient (15 to 20percent) hydro-coupling of heavy ion beams to direct drive targets without hohlraums at less than 1 MJ drive energy. Beam symmetry studies show that 60 beams may suffice with rotated beam spots on the ablator. NIF scale capsules with low aspect ratio A < 2 for robust RT stability show 1-D gains ~; 50 at drive energies of 350 to 450 kJ. Application to small heavy ion fusion test reactors with ~; 10 MWe, and to small fission fusion hybrids @ ~; 30 MWe net power scale are considered.

We build upon recent work [1] that presented simulations of heavy ion beams' passive range lengthening in directly-driven DT targets by now raising the ion energy over the course of the drive to follow the ablation front inward. We have scaled the target from ref. [1] to reactor grade and have chosen a higher-energy driving ion species to reduce beam perveances. While an arbitrarily strong shock cannot compress the fuel by more than a finite factor (4X in a perfect monatomic gas), there is no bound on the entropy it can add. We present 1-D implosion calculations that demonstrate the approach to adiabatic compression with an increasing number of shocks tailored to keep the fuel nearly Fermi degenerate. We have studied capsule performance using either two discrete ion beam energies or a steadily ramped main pulse energy. These simulations show that a linear energy ramp reduces heating of the fuel early in the main pulse and improves coupling as ablated plasma accumulates. [1] B.G. Logan, L.J. Perkins, and J.J. Barnard, Phys. Plasmas 15, 072701 (2008).

Recent 1-D implosion calculations [1] have characterized pure-DT targets delivering gains of 50-90 with less than 0.5 MJ of heavy ion direct drive. With a payload fraction of 1/3, these low-aspect ratio targets operate near the peak of rocket efficiency and achieve ~;10percent overall coupling efficiencies (vs. the 15-20percent efficiencies analytically pre- dicted for less stable, higher-aspect ratio targets). In Ref. 1, the ion energy is ramped directly from a 50 MeV foot pulse to a 500 MeV main pulse. In this paper, we instead tune the ion energy throughout the drive to closely match the beam deposition with the inward progress of the ablation front. We will present the ion energy and intensity time histories that maximize drive efficiency and gain for a single target at constant integrated drive energy.