Laser irradiation uniformity for polar direct drive on ShenGuang III facility

Abstract

Inertial confinement fusion utilizes sufficient laser beams to directly illuminate a spherical capsule, or convert the laser into thermal X-rays inside a high Z hohlraum to drive capsule implosion. The direct drive implosion is one of ways toward central ignition and similar to the indirect drive implosion, but has higher laser energy coupling efficiency and the potential for higher-gain implosion than indirect drive, and needs stringent laser condition. In order to develop and execute the direct drive experiment on the laser facility, which is configured initially for indirect drive, the polar direct drive has been proposed and validated on the Omega laser facility and the National Ignition Facility. The polar direct drive repoints some of the beams toward the polar and equator of the target, thus increasing the drive energy on the polar and equator of capsule and achieving the most uniform irradiation. The present article focuses on the laser irradiation uniformity of the target in polar direct drive on ShenGuangIII (SGIII) facility. Firstly, the laser beam configuration of the SGIII, the characteristics of laser spots, the laser beam repointing strategy and the principle of optimization are introduced. The 48 laser beams are distributed over four cones per hemisphere and the beam centerlines are repointed in polar direct drive. The continuous phase plates (CPPs) of the SGIII are designed to have unique shape to make the laser beam with a 250 m-radius circular section at the laser entrance hole in indirect drive, and thus the laser beams have ellipse cross sections with fixed major axis and different minor axes in different cones. Then, the irradiation uniformity of 540 m target is optimized by the three-dimensional (3D) view factor method on the assumption that the laser intensity distribution is super-Gaussian with three and five orders, and the energy deposition distributions are expressed as cos2 and cos . The irradiation nonuniformity of less than 5% on the polar direct drive capsule of 540 m in diameter is achieved. The pressure distribution of the hot spot at the neutron bang time with the optimized parameter is also simulated by Multi2D, and the results of 2D hydrodynamics simulation indicate that the hot spot under the assumption of cos distribution is more symmetric. Finally, the effects on irradiation uniformity of the beam-to-beam power imbalance, the repointing error and the target pointing error are estimated by the Monte Carlo method. According to the simulation results, the laser root mean square nonuniformity on the target will not become worse observably when the maximal beam-to-beam power imbalance is limited to a value of 5%, and the repointing error and the target pointing error are better than 7 m.