Effectiveness of silicon as a laser shinethrough barrier for 351-nm light

Abstract

Many laser beams directly illuminate a spherical target in direct-drive inertial confinement fusion experiments, ionizing the outer surface and surrounding the target with a plasma containing an opaque critical surface. In the earliest stages of irradiation, before the plasma forms a critical-density surface, laser light can penetrate into the target. This “shinethrough” light can be sufficiently intense to undergo filamentation and damage the inside of the target, thereby seeding hydrodynamic instabilities. Laser shinethrough can be blocked by a thin coating of opaque material, such as, aluminum (Al). For cryogenic direct-drive targets, the shinethrough barrier material must also be compatible with cryogenic target fabrication procedures, which rules out Al layers since they would interfere with permeation filling and optical characterization of cryogenic targets. Silicon (Si) has been found to be a promising candidate for a direct-drive cryogenic target shinethrough barrier material. Several cryogenic targets have been coated with Si, successfully permeation filled with either deuterium (D2) or deuterium-tritium (DT), and subsequently layered and optically characterized. Various thicknesses of Si coatings have been applied to planar targets and tested under relevant irradiation conditions. Experiments have shown that 200Å of Si is sufficient to protect targets from laser shinethrough.