Fast ignitor target studies for the HiPER project

Target studies for the proposed High Power Laser Energy Research (HiPER) facility [M. Dunne, Nature Phys. 2, 2 (2006)] are outlined and discussed. HiPER will deliver a 3 omega (wavelength lambda=0.35 mu m), multibeam, multi-ns pulse of about 250 kJ and a 2 omega or 3 omega pulse of 70-100 kJ in about 15 ps. Its goal is the demonstration of laser driven inertial fusion via fast ignition. The baseline target concept is a direct-drive single shell capsule, ignited by hot electrons generated by a conically guided ultraintense laser beam. The paper first discusses ignition and compression requirements, and presents gain curves, based on an integrated model including ablative drive, compression, ignition and burn, and taking the coupling efficiency eta(ig) of the igniting beam as a parameter. It turns out that ignition and moderate gain (up to 100) can be achieved, provided that adiabat shaping is used in the compression, and the efficiency eta(ig) exceeds 20%. Using a standard ponderomotive scaling for the hot electron temperature, a 2 omega or 3 omega ignition beam is required to make the hot electron range comparable to the desired size of the hot spot. A reference target family is then presented, based on one-dimensional fluid simulation of compression, and two-dimensional fluid and hybrid simulations of fast electron transport, ignition, and burn. The sensitivity to compression pulse shape, as well as to hot electron source location, hot electron range, and beam divergence is also discussed. Rayleigh-Taylor instability at the ablation front has been addressed by a model and a perturbation code. Simplified simulations of code-guided target implosions have also been performed. (c) 2008 American Institute of Physics.