A path to a hydro-equivalent ignition demonstration for laser direct-drive on the OMEGA laser*

Laser direct drive (LDD) offers significant advantages in terms of target simplicity, improved energy coupling and large fuel masses over indirect drive. However, performance degradations from hydrodynamic and laser-plasma instabilities seeded and driven by the direct illumination pose limitations on the parameter space available for achieving ignition. Recent advances in target design and prediction capability using statistical modeling led to significant improvements in performance to achieve a Lawson triple product that hydrodynamically scales to about 90% of the value required for ignition if equivalent laser-target coupling is achieved at the 2 MJ energy level of the NIF. New design improvements are identified to forge a path forward for a hydro-equivalent ignition demonstration. The first is related to controlling the mode-1 asymmetry through a novel analysis that quantifies the individual contributions from target offset, laser mispointing, power imbalance and a residual systematic mode. It uses flow measurements, mode-1 simulations and statistical regressions over a large database of shots to determine the optimum target offset to be applied in the experiments to minimize mode-1 amplitude. The second improvement comes from cooling the ice layer below the triple point right before shot time. This leads to lower DT vapor densities and higher convergence. While subcooling is an effective way to increase areal density, it requires mitigation of mode 1 due to greater sensitivity to low modes. One implosion with 2.5K subcooling achieved record hot spot pressure. The third improvement comes from a new formulation of the statistical model (SM) used to accurately predict target performance directly from input parameters such as laser pulse shape and target specifications. This new SM formulation provides direct guidance on target dimensions and laser beam to target radius to achieve the highest performance on the OMEGA laser.