Arresting Sound Waves to Mitigate Laser Imprint in Direct-Drive Implosions

Performance of direct-drive implosions can suffer from laser imprint, which introduces broadband modulations in implosion shells. The effects of imprint in OMEGA implosions were studied in three dimensions using the code ASTER. It is shown that the most destructive short-wavelength imprint modulations (with Legendre modes l > 30) are seeded at the outer edge of target shells during the first 100 ps of the laser pulse. Implosions driven by laser pulses with pickets produce aftershock rarefaction flows, which can arrest sound waves propagating inward and, therefore, can prevent the development of imprint modulations inside the shell by these waves. Implosions driven by continuous pulses do not produce such flows and sound waves can propagate inward and develop imprint modulations. ASTER simulations suggest that an optimum choice of laser pulses and target dimensions can reduce the development of short-wavelength modulations and, therefore, mitigate imprint because of arresting sound waves. These simulations show good agreement with OMEGA implosion experiments.