Innovative, Lightweight Methods For Thermal Management of HEL Mirror Subsystems

The Air Force (AF) is interested in innovative, unique approaches for thermal management of HEL mirror subsystems to minimize optical aberrations under HEL load. These include active and passive methods for both web-based and foam-core lightweighted mirrors. In the Phase I program Optical Physics Company (OPC) investigated three (3) active methods for cooling a standard lightweight silicon carbide (SiC) foam-core mirror flowing helium, water and water with phase changing wax through the mirror. A standard foam-core mirror is optimized for minimum weight and maximum natural frequency, not thermal performance. The improved cooling performance was accomplished refining only the locations of the cooling ports via CFD. The performance of the standard mirror is very good showing minimal surface distortion for absorbed energy intensities of 0.5 W/cm2 for helium cooling and 1.0 W/cm2 for water cooling. There are five other parameters that can be varied to manufacture purpose-built SiC foam-core mirrors to improve the performance by a factor of 5-10. These are foam relative density, pore size, core thickness, foam cell elongation and mirror skin thickness. This is the focus of the Phase II program. If Phase II is successful this optical thermal management technology would achieve TRL 5-6. BENEFIT: A complete prototype closed-loop thermal management system would be demonstrated at the AFRL Laser Effects Testing Laboratory in Phase II. This system would include compact closed cycle COTS secondary cooling systems integrated with the gas and liquid cooled SiC foam-core mirrors. In Phase III and beyond, OPC’s compact, lightweight cooling system would be upgraded to be field deployable and modified as required for application to Air Force, Navy, and Army HEL programs. The near term military applications include cooled mirrors for Relay Mirror systems, Advanced Tactical Laser, mobile THEL, JHPSSL and follow-on programs, e.g. RELI, and FEL.