Capsule illumination uniformity illuminated by direct laser-driven irradiation from several tens of directions

Abstract

Capsule illumination uniformity obtained by direct driving lasers from several tens of directions is studied systematically. The best polar angles of the three focal spot rings on the capsule are determined to be 22.4, 47.7, and 73.6by a spherical-harmonic mode analysis and a numerical simulation. Based on the configuration of indirect laser driven facility, we have optimized the beam re-directions and the focal spot distributions for polar direct drive, which smooth successfully the illumination distribution on the capsule.Laser driven inertial confinement fusion is an important way to achieve controllable nuclear fusion for human beings, which includes two laser-driven schemesdirectly driving and indirectly driving scheme. Since the indirect driving scheme considerably relaxes the strict requirements for laser performance and decreases the engineering difficulties, the main laser facilities around the world have adopted the indirect driving scheme, such as the National Ignition Facility in the U. S., the Laser Megajoule in France, and the SG series laser drivers in China.Meanwhile, scientists keep developing the key technologies for directly driving and have made great progress. For example, the fast ignition and shock ignition are two new methods to achieve fusion ignition in the direct driving scheme, which attracted lots of attention in the past few years. However, the main laser drivers for inertial confinement fusion research are configured as indirect drivers, which are not suitable for direct driving experiments. So a compromising suggestion was proposed that by redirecting the lasers, changing the laser energy distributions, designing new type of targets, and so on, a radiation field which is very close to a direct driving radiation field can be simulated in a laser facility that is configured as an indirect driver. This is the so called polar direct drive method that provides a feasible way for primary researches on direct driving technologies in an indirect laser driver. Such experiments have already been conducted in the National Ignition Facility.In China, the large indirect laser driver with an output capability in the level of hundreds kilojoule will finish its engineering construction and routinely operate for physical experiments soon. To achieve a good polar direct drive performance in this laser facility is much more difficult than in previous smaller laser drivers. In this paper, capsule illumination uniformity by directly driving laser from several tens of directions is studied systematically. The best polar angles of the three focal spot rings on the capsule are determined to be 22.4, 47.7, and 73.6 by a spherical-harmonic mode analysis and a numerical simulation. Based on the configuration of indirect driving laser facility, we have optimized the beam re-directions and the focal spot distributions for polar direct drive, which successfully smoothes the illumination distribution on the capsule.