THGs for NIF operations (>2 MJ NIF operations)

Lawrence Livermore National Security’s (LLNS) National Ignition Facility (NIF) requires over 600 large crystals of KH2PO4 (KDP) and KH0.6D1.4PO4 (DKDP) for use in polarization rotation and frequency conversion assemblies. There are near term plans to increase the energy on NIF to 2.2 MJ and up to 3 MJ in future years. Managing optics’ damage is one critical aspect of achieving these aggressive goals. Frequency conversion crystals are being examined for readiness to meet the goals for higher energy operations. All aspects of the crystal fabrication process are being studied including growth, processing, laser conditioning, and mitigation to strengthen the crystals against damage. THG crystals have an approximately 10× lower exchange rate than fused silica optics, so unlike NIF fused silica optics which are micromachined to mitigate surface laser damage between recycle loops, THG crystals are refinished to remove surface laser damage. Given the long time to grow NIF size DKDP crystals (3 years) and the impact of THG thickness on frequency conversion, a good understanding of the evolution of the inventory is necessary. The number of damage sites that require laser blocking dictates the exchange rate of THG crystals. Understanding the difference between bulk and surface damage, is important to limit the number of blocked sites. Also, in preparation for higher power operations on NIF, the current KDP polarization rotators on half of the NIF beams are being exchanged with DKDP due to an ~5% reduction in absorption at 1053 nm thus reducing beam contrast in the NIF amplifiers.