Peregrine has developed an advanced thermal management design for high-altitude High Energy Laser (HEL) platforms capable of supporting a 1 MW laser operating above 55,000 ft, where approximately 3 MW of continuous waste heat—and up to 60–120 MJ of thermal energy during individual firing events—must be rapidly dissipated to maintain laser diode performance and reliability. The system combines the high efficiency of latent heat of vaporization with an Agitated Thin Film Evaporator (ATFE), which precisely controls the distribution and thickness of the working fluid on the heat exchange surface. This approach achieves heat transfer rates approximately ten times greater than conventional immersion, jet, or spray cooling while preventing insulating vapor layers and dry spots that can lead to overheating or diode failure. During Phase I, the concept was successfully demonstrated under the low-pressure, low-temperature conditions representative of high-altitude UAV operation. Water was identified as the preferred working fluid due to its favorable vaporization characteristics at altitude and its significant reductions in system weight and volume compared to alternative fluids. The resulting cooling architecture is compact, lightweight, scalable, and capable of supporting both continuous and intermittent laser operation, while Phase II will further optimize the system using water and evaluate additional working fluids for future applications.