Hydrodynamic studies of high gain shock ignition targets: effect of low- to intermediate-mode asymmetries

Shock-ignition (SI) is a direct-drive laser fusion scheme, in which the fuel is imploded at velocity somewhat smaller than in conventional schemes. The hot spot required for ignition is attained thanks to a converging shock-wave generated by an intense final spike of the laser pulse. Earlier studies show potentials of SI for high gain at driver energy of the order of 1 MJ, provided that laser-plasma instabilities do not degrade laser absorption and do not preheat the fuel. However, also hydrodynamic aspects need investigation. A crucial issue is the design of target-pulse concepts with sufficiently large 1D safety margin (ITF, ignition threshold factor), capable of tolerating unavoidable departures from design specifications. In the present paper we use 2D simulations to investigate the effect of low- to intermediate-mode implosion asymmetries (modes l = 1 - 8) on the performance of targets with different ITF's. As expected, tolerance to perturbations increases with ITF. For given perturbation amplitude at beginning of deceleration, modes l=1-4 are substantially more damaging than mode l=8. By comparing simulations with and without alpha-particle heating we find a clear relation between tolerable perturbation amplitudes in simulations with alpha-particle heating and yield degradation in simulations without alpha heating. (authors)