A series of experiments to determine the optimum laser-beam radius by balancing the reduction of cross-beam energy transfer (CBET) with increased illumination nonuniformities shows that the hydrodynamic efficiency is increased by $\sim 35\%$, which leads to a factor of 2.6 increase in the neutron yield when the laser-spot size is reduced by $20\%$. Over this range, the absorption is measured to increase by $15\%$, resulting in a $17\%$ increase in the implosion velocity and a $10\%$ earlier bang time. When reducing the ratio of laser-spot size to a target radius below 0.8, the rms amplitudes of the nonuniformities imposed by the smaller laser spots are measured at a convergence ratio of 2.5 to exceed $8\mu \mathrm{m}$ and the neutron yield saturates despite increasing absorbed energy, implosion velocity, and decreasing bang time. The results agree well with hydrodynamic simulations that include both nonlocal and CBET models.

• Received 17 November 2011

DOI:https://doi.org/10.1103/PhysRevLett.108.125003