The ATLID (ATmospheric LIDar) laser head places severe demands on the thermal control system. The heat load of 230W is concentrated in less than 60 cm2, while heat rejection is via radiators of restricted size exposed to a highly varying external thermal environment. Interface temperature is to be regulated to 5 ±1°C, while lateral temperature gradient at the interface is to be held below 0.1°C/cm. The system is to be subjected to qualification level vibration testing, and must meet challenging stiffness and mass requirements.Design and testing of a capillary pumped two-phase heat transport system to meet these requirements is described. The results demonstrate significant margins in thermal performance even under adverse 1-g conditions, and illustrate several interesting two-phase phenomena which are discussed in detail.