Short-wavelength-laser requirements for direct-drive ignition and gain

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Abstract
Inertial confinement fusion (ICF) requires high compression of fusion fuel to densities approaching 1000 times liquid density of deuterium-tritium (D–T) at central temperatures in excess of 5 keV. The goal of ICF is to achieve high gain (of the order of 100 or greater) in the laboratory. To meet this objective with minimum driver energy, a number of central issues must be addressed. Research in ICF with laser drivers has shown the importance of using short wavelength (λ < 0.5 µm). To achieve conditions for high gain at driver energies of a few megajoules or less, high intensities (>1014W/cm2) are required. The directdrive approach to ICF is more energy efficient than indirect drive if the stringent drive symmetry and hydrodynamic stability requirements can be met by a suitable laser irradiation scheme and target design. Experiments carried out at 351 nm on the 2-kJ, 24-beam OMEGA laser system at the Laboratory for Laser Energetics (LLE) at the University of Rochester, and future experiments to be performed on a 30-kJ upgrade of this laser, can resolve the remaining physics issues for direct drive: (1) energy coupling and transport scaling; (2) irradiation-uniformity requirements for high gain; (3) hydrodynamic stability constraints; and (4) hot-spot and main-fuel-layer physics. We review progress made on achieving uniform drive conditions with the OMEGA system and present results for direct-drive cryogenic-fuel-capsule and CD-shell, “surrogate” cryogenic-capsule implosion experiments that illustrate the constraints imposed by hydrodynamic instabilities and drive uniformity on the design of high-performance direct-drive targets. Target designs have been identified that will explore the ignition-scaling regime using the OMEGA Upgrade. Experiments on the OMEGA Upgrade will signal whether or not there is a high probability of achieving modest to high gain using direct drive on an upgrade of the NOVA facility.