Future manned space exploration missions will require high power electric propulsion. VASIMR thrusters are the most attractive option because they offer short transit times and low propellant usage. A key component in VASIMR thrusters is the cryogenic electromagnets. Cooling systems for the magnets do not currently exist. The innovation of the proposed project is a high-capacity turbo-Brayton cryocooler for thermal management of VASIMR electromagnets. The cryocooler has heritage in the space-qualified unit that was developed by Creare for the Hubble Space Telescope. Turbo-Brayton cryocoolers are ideal for space applications because they are lightweight, compact, efficient, highly reliable and have long maintenance-free lifetimes (>10 years). Furthermore, the technology scales well to high cooling capacities and is inherently simple to integrate with multiple cooling objects; attributes that are particularly beneficial for VASIMR systems. In Phase I we will design thermal management systems for VASIMR thrusters at power levels consistent with near-term flight experiments and future manned space exploration missions. During Phase II, we will demonstrate a prototype thermal management system. In Phase III we will develop the complete thermal management system, integrate the system with a VASIMR thruster, and demonstrate the system through tests in a thermal vacuum chamber.