Laser wavelength dependence of laser imprint

A typical laser pulse in an inertial confinement fusion implosion includes a narrow-intensity pulse (picket) followed by the main drive pulse. The first picket launches a shock that sets the shell on a desired adiabat α. To minimize the asymmetry effects on implosion, laser beams typically have smoothing by spectral dispersion (SSD), which reduces single-beam imprint on a target surface seeded as the first shock travels through the shell. Current laser-direct-drive (LDD) experiments conducted on OMEGA show that imprint is a dominant performance-degradation mechanism for designs with α < 3, and that supplemental SSD techniques will be required to improve the performance of low-adiabat implosions. It is well understood that thermal-conduction smoothing plays a critical role in the smoothing of early-time perturbation seeds, and that increasing the size of the conduction zone should help in mitigating the laser imprint. Since longer-wavelength lasers deposit their energy at lower electron densities, the size of the conduction zone increases with the laser wavelength λL. This work will study the sensitivity of the laser imprint on λL, propose experiments on OMEGA to test the code predictions, and aid in the design of future laser facilities for high-yield LDD experiments.