Counterpropagating Light
The NIF Target Chamber is set up to avoid direct propagation of the 1ω laser past TCC into opposed beamlines. In addition, the design incorporates a wedged final focus lens so that the unconverted 1ω and 2ω light is offset from the 3ω focus at the target plane. Some targets have extended components that fall within the envelope of the 1ω light. This puts the laser at risk, as scattered 1ω light may be imaged back up other beamlines and potentially reach and damage the front end.
There are several specific examples where image relay of the 1ω light is a concern. For planar targets located horizontally in the chamber, there are conjugate beams on the same hemisphere. For planar targets located vertically in the chamber, there may be conjugate beams on the opposite hemisphere depending on the orientation of the target. Separately, the generation of large-scale plasmas from extended target components may result in the refraction of the 1ω light that propagates through these plasmas, steering the light into nearly opposed beamlines.
To mitigate 1ω scattered light reflecting off target structures and being imaged up the laser, target components that fall within the footprint of the unconverted 1ω light must meet the following surface shape requirements:
- Surfaces must be curved to disperse the 1ω reflected light. This can be a 1-dimensional curvature (cylindrical) with a radius not larger than 2 mm, or a 2-dimensional curvature (spherical) with a radius not larger than 5 mm.
- Larger extended surfaces may be created by dimpling flat surfaces with a curvature that meets these specifications, but with a periodicity that is not too short. For the 2-dimensional dimpled surfaces, the ratio of the half-side of the dimple to the radius cannot be smaller than 0.25 mm.
To field a target that has large flat surfaces, such as an ignition target with flat silicon cooling arms, unconverted light management is required. The flat cooling arms are covered with thin dimpled aluminum sheets that are patterned with a spherical dimple pattern. These dimpled sheets are attached on both the inner and outer surfaces of both cooling arms. There are additional beehive-shaped shields added over the wire connections between the cooling arms at the thermal mechanical package. For a VISAR (velocity interferometer system for any reflector) target, the cone shield that protects the VISAR line-of-sight to the physics package from closing due to plasma blowoff from unconverted light has a small radius of curvature in one dimension, and larger conical shape in the other.
